Category — Space Systems

Dust in our solar system experiences a surprising lifecycle. For very small particles, solar pressure and electrostatic forces can compete with gravity to create highly non-traditional orbits. Some dust finds a stable orbit in which to live out its existence; some dust calmly lands on the surface of planets like our own, and some dust is energetically ejected from our solar system altogether, embarking on interstellar trajectories.
Dust particles vary from a few molecules to 100 µm in size and have a mass smaller than a few micrograms. At these mass scales, the acceleration due to what would be considered perturbation forces on larger bodies can no longer be neglected. In fact, they can potentially be harnessed and controlled in order to enable new technologies and missions. [Read more →]

Divya Shankar

Divya Shankar is a student designer involved with Project Icarus. Divya is doing a Bachelor of Engineering in Electronics and Communication Engineering at Nitte Meenakshi Institute of Technology, Bangalore, India. And will be graduating in June 2012. She is a self-admitted space buff and loves space technology. She has recently been working in a project called STUDSAT (www.teamstudsat.com ), which is a Student satellite project in collaboration with ISRO (Indian Space Research Organization) and has been working on this since her 1st year of her engineering degree. Divya writes about her experience on STUDSAT. [Read more →]

One famous science-fictional nod to the hazards of high speed interstellar travel are the navigational deflectors used on the most famous line of SF starships of all, the “Enterprise” starships. All of them feature a glowing forward facing disk which, as made explicit in “The Next Generation” series, is a deflector shield against hazardous impacts when travelling through space-time at high sub-light speeds. Curiously the 0.2c quoted as the maximum impulse speed for the NCC1701-D model “Enterprise” is about right for “Icarus” too. Can “Icarus” be defended by something straight of “Star Trek”? [Read more →]

Introduction

In the thirty years since the original Daedalus papers were published, mankind has made huge strides in most areas of technology relevant to the project. We have more computing power in our cell phones now than even the biggest super-computers could muster back then; we have discovered new materials with exotic properties; we seem to be on the brink of controlled, exoenergetic fusion; and we’re discovering new extra-solar gas giants almost monthly. It’s easy to suppose that anything which could be envisioned by the Daedalus team in 1978 is now easily accomplished.

There is one aspect of the Daedalus design, though, that still seems confounding even by today’s standards: the requirement that the ship survive without human intervention for upwards of fifty years. The original Daedalus team calculated that the ship would experience a failure of some sort on average every hour. It was easy to see that such a failure rate could not be addressed via redundancy because the mass of the ship would become enormous. The Daedalus team instead proposed an advanced automated repair system which was capable of identifying problems, devising solutions, and implementing those solutions. Critical to this design was a pair of fully autonomous repair droids (“wardens”) which were able to access all parts of the ship, interior and exterior.

My colleague Philipp Reiss has recently published a paper on this blog discussing the motivation for automated repair and some of the challenges associated with its implementation. I wanted to take a step back and examine the requirements for such a system to succeed.

Mathematics of Failure

The original Daedalus team estimated that something on Daedalus would fail approximately every hour for the fifty-year duration of the journey. That’s 50*365*24 = 438,000 individual failures that have to be “handled” over the lifetime of the journey. Assuming that just half of those failures are “critical” — and that the wardens’ response to any single failure is 99.9% accurate — the probability of overall mission success is then:

.999 ^ (438,000/2) = 7E-94%.

That’s essentially zero. Assuming instead that the wardens have a six-sigma success rate (99.99966%) with all repairs, the probability of mission success is still just 47.5%.

I can’t even begin to imagine an intelligence (human or otherwise) that can handle hundreds of thousands of random, disparate failures with a six-sigma success rate. That really is extraordinary. (In fact, the percentage above is “six sigma” in the corporate parlance, which is actually only 4.5 sigma.) The inescapable conclusion is that we need a much lower mean time between failures (MTBF) system-wide for the mission to be possible.

To take it a step further, we can express a generic formula for the relationship among the MTBF (in days), a criticality factor (F), the wardens’ success rate in handling failures (WSR), the mission duration in years (D) and the overall mission success rate (MSR):

MSR = WSR ^ (F*D*365 / MTBF)

We can also invert this calculation. Given a target rate (MSR) for overall mission success, and given a certain mission duration (D) and a certain criticality rate (F), we can pick some target MTBF values and WSR values with which we’re comfortable:

WSR = e^ [ ln(MSR) / (F*D*365 / MTBF) ]

Given MSR = 99%, D=50 years, and F=50%, it seems to me that the only values that make sense involve a MTBF that’s really low, on the order of one failure per year, with only one critical failure every other year.

WSR = e^ [ ln(99%) / (50%*365*50 / 365) ]
WSR = 99.96% success rate

That is to say, for the overall mission to be 99% certain of success, the ship has to experience no more than 25 critical failures over the entire course of the mission, and the warden / repair system has to handle each failure with a 99.96% success rate. Reducing the desired success rate for the overall mission to 80% can reduce the required accuracy of the wardens by a factor of ten:

WSR = e^ [ ln(80%) / (50%*365*50 / 365) ]

WSR = 99.11% success rate

Nevertheless, this is still a rather daunting set of requirements. Firstly, we must construct a ship capable of surviving for over a year between failures. Then we must design a fully autonomous system that is capable of identifying problems, devising solutions, and implementing them with a 99% success rate. Even a human team with unlimited resources would have difficulty accomplishing 25 repairs over 50 years with only a 1% chance of failure on each one. Repairs aboard the International Space Station have demonstrated this.

Analogies

It is difficult to name existing man-made systems (mechanical, electronic, or computerized) that have run for decades without human intervention. The latter criterion is really the kicker — most machines that we build are either maintained ongoing or fall rapidly into disrepair. Most really aren’t designed to maintain themselves. Analogous systems are most likely found in environments that discourage human intervention.

Space:

- Previous Probes: As Philipp Reiss states in his recent blog article, “The maximum lifetime of the Pioneer probes is 31 years (Pioneer 10) and 22 years (Pioneer 11). For the Voyager probes 1 and 2 it is currently 33 years and expected to become up to 48 years until the end of their functionality in the 2020’s.” I don’t really know how much human interaction (in the form of uploaded code, new instructions, etc.) was involved with these probes post-launch, but several have clearly been “out there” for decades without any physical maintenance. That’s promising.

- Satellites: Many satellites have also been in orbit for decades, but again I don’t know how much human interaction they receive from the ground. Few receive physical repairs.

- Space Shuttle & International Space Station: These require constant human interaction, and they still seem to suffer a lot of problems that require new parts to be sent up from Earth. So they’re not really good examples at all.

Nautical:

- Lighthouses & Marks: Most remaining lighthouses are unattended, as are all of the lighted marks along the U.S. coasts. These devices generally consist of a light, a timer, a battery, a solar panel, and a support structure. Although these are unattended, the U.S. Coast Guard does maintain them — but I have no idea what the mean time between failures is. The newer lights use LEDs that last for decades without problems, but I reckon the batteries, at least, have to be replaced every few years.

- Underwater Sonar Arrays: The U.S. Government has set sonar sensors all over the Atlantic and Pacific ocean floors to track ship movement along America’s coasts. I have no idea what kind of maintenance this system requires, but it stands to reason that such maintenance would be very difficult, and thus is presumably rather infrequent.

Underground:

- Oil Wells: I remember driving through Texas when I was a teenager and seeing miles and miles of oil wells pumping away in an otherwise desolate landscape. The things certainly gave the impression that they were running unattended, but I imagine that someone must have been checking on them periodically. How often? Today, I’ll bet they’re all monitored 24×7 via remote sensors. Still, it’s an interesting analogy.

Archaeological:

- Roman Aqueducts: Some of these have been carrying water for close to 2000 years. I’m sure that the farmers who benefit from these aqueducts have maintained them over those years, but it’s certainly reasonable to assume that some aqueducts have gone for decades without any maintenance.

It’s difficult to come up with other archaeological examples because most of what we find are just structures rather than machines. The aqueducts, in fact, just barely qualify as simple machines (essentially just ramps).

Medical:

- Pacemakers routinely run for a decade or two without any intervention.

- There’s at least one case of a vascular assist device that lasted for 25 years before the patient died of unrelated causes.

Random Electronics:

- Casio Watch: I have a Casio digital diving watch that I bought back in 1985. Aside from replacing the band every few years, the only maintenance I’ve provided is a new battery every 7 years. So not counting the band, that’s three “incidents” in 25 years — not bad.

- Casio Calculator: I have a solar-assisted Casio calculator that I bought back in 1983. I have replaced the battery in it exactly once in 27 years, and that was maybe 7 years ago, so it went a good 20 years with no maintenance whatsoever.

- Adam Crowl noted on our forums that a lot of electronic components used by the U.S. Department of Defense have a MTBF of ~10 million hours or so. “The real trick is combining them and getting a MTBF for the whole to be a significant fraction of the individual.”

Those two Casios are the only self-contained electronic devices that I’ve had for decades and that still work. Most everything else plugs into a wall jack, meaning that they’ve been “down for maintenance” every time I’ve moved. Few of those items (toasters, microwaves, coffee makers, TVs, etc.) have even remained working for the intervening decades.

Conclusion

For the Icarus probe to succeed, it must be designed and constructed in such a way that failures happen on the order of years apart. Moreover, the probe must include a completely autonomous system that is capable of addressing these failures (whether via redundancy or repair) with a better than 99% success rate. These are extraordinary requirements that stretch the limits of what mankind has thus far achieved, and may in fact represent the single biggest hurdle to the actual launch of an Icarus-type mission.

In 1928 Oberth[1] suggested the option of a two-burn orbital maneuver that would, on the first burn, drop an orbiting spacecraft further down into the central body’s gravity well, then a second burn would be performed to accelerate the spacecraft allowing it to escape the gravity well.  This maneuver was not studied in detail by Oberth.  Later, a brief analysis was done by Levin[2].  A sketch of the maneuver is illustrated in Figure 1. 

Fig 1. In the two-burn maneuver, the first burn decelerates the spacecraft resulting in a trajectory that takes it further into the gravity well of the central mass (this is opposed to a single direct burn which accelerates the spacecraft). The second burn is performed at the periapse of the new elliptical orbit and accelerates the spacecraft out of the gravity well of the central mass. 

In two-body motion the summation of forces on the vehicle is simply:

Where:

From the above equation the gravitational force is a function of the inverse square of this radius vector.  In the Newtonian equation only the mass of the large body is considered since the mass of the vehicle is much smaller.  Taking the dot product of equation (1) with respect to   yields:

where   is referred to as the specific orbital energy of the vehicle.  The orbital energy is the sum of the specific kinetic energy (the first term) and the specific potential energy (the second term).  In deriving this equation it is convenient to assume that the reference line for potential energy is at infinity.  The right hand side of the equation expresses specific orbital energy as a function of the orbits semi-major axis. 

   The equations above define the major parameters of a spacecraft in a coasting trajectory around a central body.  In the operation of a spacecraft, changes in the orbit must be made to allow the spacecraft to travel to points of interest such as other planetary bodies.  Turning on the spacecraft propulsion system, or performing a ‘burn’ in industry parlance, will place the vehicle in a new coasting trajectory once the burn is completed.

   Due to the computationally intensive nature of trajectory design, there has been strong interest since the 1950’s to determine analytical approximations to low thrust trajectories that would give generally accurate results.  A prevalent argument in the derivation of these approximations is that the thrust vector should always be aligned with the spacecraft’s velocity vector.  The reason can be seen by taking the time derivative of the kinetic energy equation, which is:

Thus the instantaneous rate of change of kinetic energy is proportional to both acceleration and velocity.  The local maximum is found when the spacecraft velocity and acceleration are parallel.  However, as argued by Levin[3], the spacecraft could accelerate in a different direction, forcing the spacecraft into a different orbit with a point of closest approach to the central body, the periapse, being closer to the central body than the original orbit.  Examination of equation (2) shows that for a coasting orbit, the specific mechanical energy remains constant but the kinetic energy is traded for potential energy.  At periapse kinetic energy is at a maximum and potential energy is at minimum, just like for any gravitational force dominated problem like a swinging pendulum or a ball in free flight.  By equation (3), driving to a lower orbit and then accelerating at periapse would maximize the change in energy of the spacecraft for a given acceleration (and thrust).  The optimal method for increasing V to increase a would be to decelerate, coast to a lower orbit where V is maximized (at periapse) and accelerate again.  It has been shown that this maneuver will produce more specific orbital energy than a direct burn out when the total to be produced exceeds the initial circular velocity around the central body.  In equation form:

which is always greater than:

The discussed two-burn option is easily confused with a gravity assist maneuver. However, the gravity assist maneuver is based on a massive body such as a planet dragging the spacecraft along for part of the spacecraft’s trajectory.  Momentum (and specific orbital energy) will be exchanged between the planet and spacecraft.  The effect on the spacecraft is substantial, imparting in most cases a velocity change   that could not be easily duplicated with current propulsion systems.  The effect on the planet is minimal, due to its massive nature relative to the spacecraft.  This momentum exchange is between an external body and the spacecraft and the exchange will occur even if no burn is made by the spacecraft.  Conversely, a slingshot maneuver will not work unless there is a substantial burn at the periapse of the elliptical trajectory.  The additional energy gained by the spacecraft is represented by the additional loss in specific energy by the propellant expended at the periapse burn.  It does not represent a transfer of momentum from the central body to the spacecraft. 

The two-burn option can produce a greater specific mechanical energy for a given  budget than a direct burn but only when the total budget exceeds the initial velocity in the initial orbit.  So the  budget must be considerable before the slingshot maneuver is worthwhile.  For instance starting from a circular orbit around the sun at a distance equal to earth’s orbit, the  budget equal to the initial circular velocity is sufficient to completely escape the solar system with a   of about 17.5 km/sec.   However the   budget is well within the range of many missions of interest to NASA. For instance the interstellar precursor mission presents the challenge of traveling 1000 astronomical units (AU) within 50 years, the career lifetime of the average engineer or scientist.  The escape velocity above will deliver a spacecraft to the required distance in over 110 years so clearly a slingshot maneuver would be useful for this mission.  Other deep space missions to the outer planets, Kuiper Belt, Oort Cloud, and heliopause would similarly be enhanced by use of this maneuver.

                Finally a class of mission that has received attention by NASA in recent years is the deflection or fragmentation of asteroids and comets that are on a collision course with Earth.  The  imparted to an oncoming asteroid is very low, on the order of 1-100 cm/sec[4].  This   is sufficient to deflect most asteroids provided that the impulse is applied to the asteroid early enough.  Current deflection methods require 2-50 years between application of the impulse and the projected collision date.  Therefore the device that will impart the impulse to the asteroid must intercept or rendezvous with the asteroid with all haste.  Given the above the  requirement to intercept an incoming asteroid is generally on the order of 10-30 km/s[5].  The  requirement to rendezvous can be as high as 70 km/sec.  Both values are well within the range necessary to make the two-burn maneuver economical.

                This paper started by examining the concept that acceleration along the velocity vector would result in an optimal acceleration of the spacecraft.  While acceleration along the velocity vector is locally optimal it turns out that there is a special maneuver that in certain cases will outperform the “optimal” cross product acceleration by actually decelerating the vehicle and accelerating it when it reaches periapse.  It is presented here as an important maneuver to be considered for high   missions such as interstellar precursor or similar deep space missions and potentially crewed round trip missions to Mars and beyond.  This has profound implications for future space exploration.  Being able to use in-situ resources to create propellants or even construct vehicles on the moon has even greater importance now that the two-burn maneuver can be used to substantially reduce the propulsive requirements for deep space missions.

The  required to complete a mission to Mars and return is 12-13 km/sec for Hohmann transfers.  However, radiation exposure, crew supplies and crew mental health issues have forced vehicle designers to look at much higher  missions that can reduce trip times and mission risk.  Mission studies for crewed missions to Mars with a limiting total trip time of 2 years of less have   requirements above 20 km/sec.  Therefore it is possible that the new option could have application in orbit raising maneuvers where limiting mission time is critical.  A proof of principle calculation shows significant gains in performance for crewed missions to Mars using this maneuver.  This gain is predicated on the hope that water ice will be found on the moon and can successfully be turned into useable propellant.  Given this assumption the two-burn maneuver can reduce vehicle size by up to half, or decrease mission time by half.  The former dramatically reduces the cost of a Mars trip, while the latter reduces the risk to crew. 

                The two-burn maneuver shows considerable promise to enable a variety of scientific and exploration missions in deep space.  The authors believe that this two-burn maneuver could have as large of an impact on space exploration as the gravity assist.  Developed at the very beginning of the space program, the gravity assist enabled missions from Voyager to Cassini to visit the planets of the solar system using technologies that were then available.  Clearly without the gravity assist those technologies would have been inadequate to explore much of the solar system outside of the moon and Mars.  Similarly the two-burn maneuver provides a method for exploration of the boundary of the solar system and interstellar space using today’s technologies and technologies of the near future.  Such missions are difficult to conceive without considering the advantages of alternative maneuvers.

References:

[1] Oberth, Hermann, Ways to Spaceflight, translated from German in NASA TT F-622.
[2] Levin, D. F.., “Escape to Infinity from Circular Orbits”,Journal of the British Interplanetary Society, Vol. 12, No. 2, March 1953, pp.68-71.
[3] Levin, E., “Low Acceleration Transfer Orbits”, Section 9.1 in Handbook of Astronautical Engineering, edited by Heinz Hermann Koelle, McGraw-Hill book Company, New York, 1961.
[4] Near-Earth Object Survey and Deflection Analysis of Alternatives, Report to Congress, NASA, March, 2007.
[5] Adams R. B., Alexander, R., Bonometti, J., Chapman, j., Fincher, S., Hopkins, R., Kalkstein, M., Polsgrove, T., Statham, G., White, S., Survey of Technologies Relevant to Defense from Near-Earth Objects, NASA, TP-2004-213089.

The Project Icarus Team plan to follow on from Project Daedalus and design an interstellar spaceship. A few questions might spring to mind, such as, ‘amazing, how you gonna do that’? Or perhaps, ‘Why bother’. Well, that last question certainly deserves an answer and the Project Icarus Team have a subgroup of designers led by Dr Ian Crawford (of Birkbeck College, University of London) thinking exactly about the possible science that could be done on such a mission to make it worthwhile. In a recent preprint written by Dr Crawford (1) and submitted to the Journal of the British Interplanetary Society he discusses the different aspects of the science case for interstellar spaceflight. After my previous blog article on the navigation problem I had been mulling over a related science issue that he suggests in the preprint, “that it is unclear if it will be of benefit given the likely development of (local) space-based observations“. Nonetheless, he also says it’s worth keeping an open mind so let’s take a closer look at what I had been thinking about too – how do you measure accurate distances and scales in the universe?

After all you can’t run out a tape measure, or send out someone with a measuring wheel. It’s interesting that typically any direct measurement of distance in astronomy only works very close to home e.g. bounce a radio signal off the object and time the reply; great for objects in the solar system but it just doesn’t work further into space. This requires something called parallax and a bit of basic trigonometry.

So, in the spirit of this new Icarus, let us jump beyond the solar system where the first step in measuring interstellar distances is by parallax. If you’re not familiar with parallax this is simply the apparent visual movement of something when related to a much more distant background object when the observing position is changed. A classic effect is if you hold out a finger and line it up with a distant object. If you then look at the tip of your finger while alternatively switching between eyes, the position of your finger seems to leap between 2 different positions as compared to the stationary distant background. Finally, if you know the baseline of your observations (in this example the distance between your eyes) and can measure the angular movement of the image, you can simply calculate the distance using basic trigonometry.

Now, here’s the important bit, parallax is the gold standard for directly measuring interstellar distances and is the first rung on the ‘astronomical distance ladder’ and is traditionally the only direct method. Nevertheless, the distances to even the nearest stars are so great that parallax measurements are tiny and are only measurable for the nearest stars by using a baseline of observations taken 6 months apart and on opposite sides of the earth’s orbit. The angle of parallax measured in arc seconds (arc seconds are a small angular measurement, where there are 60 arc seconds in an arc minute and 60 arc minutes in an angular degree) relates simply to the distance (in parsecs, 3.26 light years), where for very small parallax angles; distance = 1/the angle.

In simplified terms, the greater the baseline, the better we can measure the parallax and the more accurate we’ll know the distances to the nearest stars and the better we will know the scale of the universe. The astronomical distance ladder is built step by step from the nearest objects to the furthest. Each step is dependent on the one before and any errors are magnified as we move up the ladder. For example, the next step on the distance ladder are the Cepheid Variables; a special class of star whose brightness varies in a regular periodical fashion. The really useful thing about Cepheid’s is that the period is related to the absolute luminosity and therefore they can be used as standard candles to measure distances. I recall when I was a student of astronomy at university there were no Cepheid variables that you could measure the distance to directly i.e. by parallax and I used to wonder how they calibrated this standard candle in the first place. Nowadays there are a handful of Cepheid’s that have had their parallax measured so at least now there is a direct link to calibrate.

Now what would be better than using a baseline of the earth’s orbit or other solar system based observations? Imagine the increase in accuracy of using a baseline at least 272,000 times greater than the radius of the Earth‘s orbit (if Icarus journeyed to the Centauri system) and using equipment of similar measuring ability. As Dr Crawford points out the costs of doing so would be marginal.

Interestingly in the original Daedalus Project the ability to do astrometric observations with long baselines was seen as an important part of their science yet in the ‘80s the Project Longshot Study (2) went further. That study made the observations a prime objective of sending an unmanned probe to Alpha Centauri realising the importance of knowing the distance to stars accurately. They pointed out that (at that time) measuring distances by parallax was only accurate to around 20 parsecs and estimated that the Longshot measurements could be made out to 1.2 million parsecs! Dr Crawford’s ambivalence is understandable, the technological developments of the last 30 years and the ingenuity of the scientists and engineers working on this problem from the confines of the solar system would have surpassed the imaginings of the Longshot team. This will be an issue to consider for Icarus; could an Icarus probe designed, built and launched say 50 years before arriving at it’s target compete with advances made back in the solar system over the same time period? (Now, that’s a story for another day and not just regarding the astrometric observations.)

Nevertheless, the potential for a greatly increased baseline for measuring parallax is one thing we can do amongst many others that are being weighed up by the Icarus science team. Perhaps some of you might think it is still not worth it, although if you follow this website I doubt it. At least the European Space Agency think measuring parallax(es) is important as following the success of the Hipparcos observation satellite they intend to launch Gaia – a mission to further increase our accurate knowledge of the positions and hence distances of the stars. Perhaps pottering around the solar system is your thing and that’s great too. But Project Icarus is bound for the stars and there are plenty of scientific reasons to go; helping to scale the universe may be just one more of them.

References:

(1) Crawford I. A., “The Astronomical, Astrobiological and Planetary Science Case for Interstellar Spaceflight”, Pre-print submitted to JBIS, Dec 09.

(2) Beals K. A. et al, “Project Longshot: An Unmanned Probe to Alpha Centauri”, NASA/USRA University Advanced Design Program Project Design 1987-1988

Since 1957 the era of space exploration has taken off with tremendous beginnings, eventually leading to men walking upon the surface of The Moon and astronauts permanently stationed in Earth orbit. With the sophisticated technology of satellite communications and the Space Shuttle it is very easy to forget that the history of early space exploration efforts start from much humbler beginnings. The development of the theory of rockets as a science can be traced back to the Russian School Teacher Konstantin Tsiolkovsky who published a paper in 1903 titled ‘The Exploration of Cosmic Space by Means of Reactions Devices’. In the United States Robert Goddard, also a teacher, wrote about the principles of rockets in his 1919 paper ‘A Method of Reaching Extreme Altitudes’. Then in 1930 David Lasser formed the American Interplanetary Society. Lasser had written an exciting book about space exploration titled ‘The conquest of Space’ which inspired many, including the science fiction writer Arthur C Clarke as a young man. In Germany a certain Hermann Oberth had written about rockets including his publication ‘The Rocket into Planetary Space’. He went on to form the German Society for Space Travel in 1927. Oberth would also inspire Werner von Braun to do rocket research leading to the V2 rocket used on London during World War 2 and the first rocket to be launched outside of the sensible atmosphere.

In Britain, people such as Arthur Clarke, Val Cleaver, Archibold Low and Phillip Cleator came together in excitement to discuss these rapid developments in rocketry. Then in 1933 Low and Cleator created The British Interplanetary Society (BIS) with Cleator serving as the first President. This was a society set up to explore interests in space exploration around the planets, stars and beyond. The thing that distinguished the BIS was the free flowing ideas which knew no bounds. Speculation on any field within astronautics was considered acceptable and speculative ideas were not drowned out, but debated. Of all the organisations formed at the outset of the space age, the BIS is the only one still in existence in its original form, and recently passed its 75th anniversary. The American Interplanetary
Society went on to become the American Institute of Aeronautics & Astronautics, but its main purpose today is to serve the professional career astronautics community, not the amateur. In this respect then, the BIS is unique. Indeed, the BIS went on to play an important role in founding the International Astronautical Federation in 1951. The IAF organises the annual International Astronautical Congress which is probably the biggest space event in the world today.

In the 1930′s through to the 1950′s America had the brilliant Chesney Bonestell who created the genre of Space Art. In Britain, Ralph Smith was a Fellow of the BIS and had worked closely with Arthur C Clarke on books such as ‘The Making of a Moon’ and ‘The Exploration of the Moon’. He had been designing spaceships since the age of 12 and after the war worked as a leading draughtsman for Westcott and become BIS Chairman between 1956-1957. Many of his original works still hang in the corridors of the BIS and are a thrill to the eye. Today, accomplished artists such as David Hardy follow on from the style created by Bonestell and Smith. One of Smith’s important early contributions was to produce beautiful drawings of the BIS moonship design in the 1930′s. This was an engineered vehicle which placed in the minds of men the idea that a fantasy once dreamed up by people such as Jules Verne in his stories could actually be a credible engineering project in future decades. The eventual US landings in 1969 showed that this foresight was correct and many of the elements of the Apollo Lunar Lander had a striking resemblance to the original BIS moonship drawings.

Arthur C Clarke himself is well known outside of the science fiction community. His discussions of current and future manned space exploration have inspired millions the world over. Clarke was a visionary who spent the latter half of his life living in Sri Lanka, but in his earlier days he was an active and important member of the BIS, serving as Chairman between 1946-1947 and again between 1951-1953. It is for people like Clarke that the BIS exists, to give a forum for those with ideas that may be considered speculative in other circles. It only takes one idea to make a difference. Clarke’s 1945 publication in Wireless World discussing a proposal for geosynchronous orbital satellites demonstrated that. Clarke was also a great admirer of the writer and philosopher Olaf Stapledon who wrote astonishing books like ‘Last & First Men’ and ‘Starmaker’, books which influenced many including the physicist Freeman Dyson who attributed his idea of Dyson Spheres to Stapledon’s works. In 1948 Clarke invited Stapledon to give a talk at the BIS on his paper ‘Interplanetary Man’. Providing a forum for visionary thinking about the future has always been one of the important roles of the BIS.

Another person important in the early years of the BIS was Val Cleaver, described by Clarke as the man who should have been the British von Braun. In his early years he worked for de Havilland and later rose to become a Chief Engineer. It was during this period that he began to learn about rocket propulsion. He served as BIS Chairman between 1948-1951 and in 1948 he published a paper with Les Shephard ‘On the Atomic Rocket’ which provided one of the first technical proposals for using nuclear powered engines with hydrogen as a working fluid. He later went on to to work on projects like the Sprite liquid propellant rocket and in 1956 took a job as Chief Engineer with Rolls Royce where he participated in the development of the British Blue Streak missile which never had a failure in its 13 launches.

Cleaver went on to mentor people like Alan Bond who in the 1970′s led the BIS Project Daedalus study, a landmark engineering vehicle design for an interstellar probe. Bond worked under the tutelage of Cleaver in the Blue Streak missile program and also worked on other projects such as JET at Culham, an experimental magnetic fusion reactor. Along with Bob Parkinson, the current President of The BIS, Bond worked on the HOTOL spaceplane project in the 1980′s. This has led directly to his current project, the development of the SABRE engine for the Skylon Spaceplane which is being developed under his company Reaction Engines Ltd. This vehicle design has the potential to revolutionise the cost of access to space.

Even today the BIS fosters new and exciting ideas for space travel. In 2007 this author organised a one day conference investigating the status of the speculative warp drive proposal, building upon the general relativity foundations which began with a paper by Miguel Alcubierre in 1994. One of the presenters, Richard Obousy, also an Icarus designer, demonstrated for the first time a physical mechanism by which the warping of space could be accomplished, linking the existence of the cosmological constant with the additional dimensions of space that have been hypothesised. The public interest in this event was enormous, illustrating that people are generally interested in the future possibilities of manned space exploration.

As we begin the start of a new century the BIS is still playing a vital role in promoting and supporting activities relevant to space exploration. It has an international reputation from its organised symposia to its regular high quality publications like Spaceflight, JBIS and Space Chronicle. In particular, the publication of JBIS special editions on specific themed topics is world renowned. This may be on electric propulsion, planetary terraforming, bases on the Moon or Mars, or the development of a Space Drive. It plays a key role in space exploration from a technical, popular
and educational level.

History has shown that members of the BIS have been willing to speculate about the future of space exploration, from the highly practical near-term space technologies to the futuristic ones. When such a climate of creativity and free thinking exists, there is no telling what ideas will spring forth, and potentially change the world for the improvement of our species. Project Icarus is also a bold initiative, requiring an open mind, free enquiry and the application of the scientific method. Project Icarus directly challenges the legacy of Greek mythology by demanding ‘we will reach the stars according to our destiny’. It’s highly probable that in 200 or 500 years from now when both interplanetary and interstellar travel are common, that history will record the important role of the BIS in continuing to remind us that if we are to succeed in the exploration of space then going from imagination to reality is a good principle upon which to work. This is precisely what Project Icarus aims to achieve.

If you have found this article to be of value then please consider donating a small amount to Project Icarus to assist us with our ambition of creating a credible starship design.

Alternatively take a look around the remainder of the Icarus site.

In a previous article we took a broad look at the problems involved in interstellar communications. In this article, we will take a closer look at one of the ‘exotic’ techniques mentioned: Gravitational Lensing.

Einstein published his General Theory of Relativity in 1915. This theory considered gravity to be the result of the curvature of space (or, more precisely, spacetime, though the distinction will not trouble us here). One of the consequences of this is that a massive object such as the Sun will bend the path of a light beam that passes it. Indeed, this effect predicted by Einstein provided one of the first verifications of General Relativity. During a total eclipse of the Sun it is possible to see the small distortion caused by the Sun’s gravity on the apparent location in the sky of distant stars.

Once it was realized that gravity could bend light this way it was also noted that massive objects can act as huge lenses, focusing light to a point, just as a glass lens does. The gravitational bending of light is much weaker than you can get from a glass lens, so despite using a huge lens like the Sun, the focus is still very far away. Interestingly there is an important difference between the optical focusing that we are familiar with and gravitational focusing. A normal glass lens has a specific focal distance. If you want to get a sharp image you have to hold the lens at exactly the right distance from your focal plane (which could be film or some electronic image detector). With a gravitational lens there is no specific focal length. Beyond a minimum focal distance at which the distant light rays are brought together, all the points in the line away from the Sun are foci.

The fact that we don’t have to be at a specific distance from the gravitational lens in order to use it has an interesting consequence for astronomy, We can use a distant galaxy as a lens and observe the bending of the light that it causes in the image. For example, in this image of Einstein’s Cross we are seeing the same distant quasar four times, brought to a focus by an intervening galaxy which is acting as a lens. The quasar is 8 billion light years from Earth, while the galaxy is only 400 million light years away.

So we know that this isn’t just some theoretical quirk of General Relativity with no practical effect. We can observe this lensing in reality. But can we exploit the Sun in a controlled way to focus on specific targets? First of all, we need to know how far away from the Sun our camera needs to be in order for the distant light to be focused. It turns out that if you consider the Sun to be a perfect sphere, then you need to get at least 550 AU from the Sun. (1 Astronomical Unit (AU) is the distance from the Earth to the Sun.)

However, there is a complication. The Sun is not a perfect sphere. There is a corona on the outside surface that tends to deflect light away from the Sun. This is working against the gravitational lensing effect, and serves to push the focus even further away from the Sun. The result is that we actually need to place our camera about 700 AU from the Sun.

So we fly our camera out to 700 AU, and start snapping pictures of the planets around Alpha Centauri. Is it that easy? No, of course not! First of all, 700 AU is an enormous distance. By comparison, Voyager 1 was launched in 1977 to take a tour of the gas giants of our solar system. This is one of the fastest craft mankind has ever produced, and yet it is (at time of writing) only 113 AU from Earth. There are options for getting craft out to these vast distances relatively quickly, but they don’t need to concern us here. Let’s stipulate that we can get our camera to 700 AU.

We can now use our camera to spy on the comings-and-goings on the surface of a planet at Alpha Centauri. (That’s no idle boast; the resolving power of a gravitational lens telescope is phenomenal.) However, we’ll leave such activities for the astronomers and xenobiologists. Here we are concerned with exploiting the gravitational lens for interstellar communications.

Well, it’s really quite simple… in principle! We have our receiver craft sitting at the Sun’s focus, and the distant probe at some point in interstellar space or around another star. We use the Sun to focus the transmissions from the distant probe onto the receiver. This gives us an enormous antenna gain compared to what we would be able to achieve if we were trying to receive the signals directly, without using the Sun. What this means is that we can use much lower transmitter power on the probe without impacting the bandwidth that we can transmit. And this means that we do not have such hefty power supply requirements on a probe that may have been flying for a hundred years before reaching its destination. These seemingly mundane resource constraints should not be underestimated. Like the old military saying goes: “amateurs study strategy; professionals study logistics.” It’s all very well having a gleaming state-of-the-art fusion drive to get your probe to the target, but it’s all for nothing if you don’t have the power to get any data back to Earth.

So we’ve got our receiver out to the focus, and the distant probe is beaming data back to us via the Sun’s gravitational lens. Have we solved all the problems? Unfortunately, no. The big problem remaining here is that the receiver has to stay very closely aligned with the transmissions from the probe to a phenomenal degree of accuracy. We are talking about a tolerance on the order of tens of metres, for a craft which is over 700 AU from the Sun. There is plenty more work to be done in this area, and the Icarus team is studying some ideas to see if they might help to make this a practical system in the near future.

Let’s assume that we have solved the positioning accuracy problem, and we can keep the receiver exactly in line with the distant probe. Is there anything else we can do with the system to improve communications even further? Indeed there is. We can exploit two gravitational lenses: the Sun, and the distant star. So we have our receiver craft at the Sun’s gravitational focus, and the distant probe at the focus of the target star. If we can keep both the probe and the receiver exactly in line with each other, then the antenna gain we achieve is beyond enormous; it’s simply phenomenal. Using trivial amounts of power, we can achieve perfect communications between the Sun and (say) Alpha Centauri. (Read the Centauri Dreams article referenced below for the fascinating details.)

Clearly the positioning accuracy problem is greater again when we are trying to keep two craft, separated by over four light years, exactly in line with each other. However, we also get another great bonus: we are now able to send decent amounts of data from Earth back to the probe. Remember that the probe may have left Earth up to a hundred years earlier. While it has been en route, our algorithms for data processing may have improved, and we may have obtained fresh data by other means that the probe could usefully take advantage of. It would be incredibly productive to be able to update the probe with the new information.

If we can make this system work, especially with the double-lens communications, we will have taken the first steps in creating an interstellar internet. Imagine the possibilities that a network of transceivers around all the local stars will open up for the next generation of probes. They will no longer need to send signals directly back to Earth. They can just “log on” to their local node which will relay data back to Earth for them.

With the establishment of an interstellar communications infrastructure, humanity will truly be a starfaring civilization.

If you have found this article to be of value then please consider donating a small amount to Project Icarus to assist us with our ambition of creating a credible starship design.

Alternatively take a look around the remainder of the Icarus site.

References:

General Relativity [Wikipedia]

Gravitational Lens [Wikipedia]

Einstein’s Cross [Wikipedia]

Voyager 1 distance from the Sun [Wolfram Alpha]

The Gravitational Lens and Communications [Centauri Dreams]

Antenna Gain [Wikipedia]

Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens, C. Maccone, Springer-Praxis, 2009 [Google Books Preview]

“Project Daedalus” was inspired by advances in nuclear fusion technology as well as the 1960s “Orion” nuclear pulse rocket. The apparently rapid developments in laser and electron beam initiation of fusion in the early 1970s caused many to believe that interstellar travel was now at least possible, no longer merely in principle, but using real near-term technology. Thirty years later nuclear fusion is edging closer to “break-even” – when sufficient energy is generated to power the triggering of the reaction, at least if energy conversion machines were in place – and the possibility of interstellar travel has become more tangible. “Project Icarus” carries the torch of “Daedalus” further forward.

Fusion will give us much more than interstellar travel, however. As the energy source of the Sun and the stars thus, indirectly of all life on Earth, fusion already has given us so much. Modern civilization became possible because of its use of chemical energy stored in the remains of living things once powered by the Sun, and in effect already uses (stored) fusion energy. While plenty of sunshine for our needs falls on planet Earth, it is spread very thinly and requires large areas to collect, and very large batteries or chemical fuels to store. Nuclear fusion can use the relatively abundant fuel of deuterium, found in any body of water in the Solar System, though some advocate rarer elements like boron and lithium, or even rarer isotopes like helium-3. Sufficient amounts of deuterium exist in the oceans to supply an energy-rich civilization on Earth for billions of years. With such abundant energy we could even mine the planet’s “rubbish” mountains for the resources we have discarded as “unusable” waste and begin reversing the damage wrought by 3 centuries of fossil-fuel powered industrial development.

High-performance fusion rockets would also open up the Solar System to speedy access via large space-vehicles, as well as enabling star-probes. Assuming a 1,000 ton space vehicle and constant acceleration at 0.722 m/s², which an exhaust velocity of 10,000 km/s with a mass-flow rate equal to “Daedalus’s” second-stage would produce, we get the following figures for travel times to various Solar System destinations.

Table 1. Travel Times for Mean, Minimum and Maximum distances. Mean distance is the difference between the mean orbital radii of the destination and Earth, minimum uses the distance of the destination at its perihelion and the maximum places the destination at its aphelion on the opposite side of the Sun to the Earth.

As can be easily seen even distant Eris, the former Tenth Planet, can be reached in a little over 3 months for just 960 tons propellant. The most difficult flight to Mars needs a mere 60 tons of propellant to deliver 500 tons of space-vehicle and its 500 ton payload. For comparison, the nuclear-propelled Integrated Manned Interplanetary Spacecraft (ref. Encyclopedia Astronautica), flying to Mars and back on 420 day missions were extensively studied in the 1960s. They typically required 1226 tons of materials in Earth orbit, used 873 tons propellant, and carried only 110 tons of payload, accommodating 6 crew persons. Since the shutdown of the NERVA program in the early 1970s nuclear rocket technology has essentially stagnated. More recent VASIMR plasma-rocket technology requires 476 tons of propellant to deliver 124 tons of space-vehicle in 39 days to Mars, while requiring 200 megawatts of power from a Magnetohydrodynamic generator fed by a gas-core reactor, itself a technology almost as difficult to achieve as the “Daedalus” fusion engine.

Another potential use for high-thrust, high-exhaust velocity fusion-rockets is the deflection and/or orbital management of comets and asteroids. Asteroids and comets more than 200 metres wide pose a long-term threat to human civilization, if not all life, on Earth. A fusion rocket using 100 tons of propellant and a 10,000 km/s exhaust velocity can produce a 0.1 km/s change in the orbital velocity of 200 metre wide asteroids that mass roughly 10 million tons. Even multi-billion ton asteroids, kilometres across, can be steered away from a planetary collision given a year or two of warning. Smaller asteroids can also be shepherded into more useful orbits and mined, their products returned to Earth-orbit via fusion rocket.

“Daedalus”, and its successor “Icarus”, extend our fusion-making abilities by requiring greater energy efficiencies and harder to achieve fusion reactions. Regular deuterium fusion, for example, produces ~40% of its energy as neutrons, which produces large amounts of heat. On Earth this is very useful and we have advanced steam-era technology for turning heat into power, but in space heat must be radiated away and this complicates the energy conversion process. “Daedalus” reduced the neutron heat-load by using the deuterium-helium3 reaction which produces far fewer neutrons and its end products are charged particles that can be steered via electromagnetic fields in useful directions. “Icarus” hopes to do the same, but with a clearer view of what such reactions require from our fusion technology. On Earth such low-neutron fusion-power would reduce the heat being ‘dumped’ to the environment and reduce safety concerns from neutron-activation of containment vessel structure, a source of low-level radioactivity.

The high-speed spaceflight enabled by high-efficiency, low-neutron fusion energy enables another desirable opening of possibilities – the mining of the gas-giants for helium-3. The interdependence of the two might seem a bottleneck, but there is a significant amount of helium-3 closer to Earth embedded in the crystal structure of the Moon’s regolith, deposited by the solar wind. As recently revealed the Moon’s regolith is also periodically coated by a thin layer of water, probably also produced by interaction with the solar wind. This may well provide a useful source of deuterium, thus allowing fusion rocket propellant to be extracted solely from the Moon, at least at first. The estimated helium-3 resource of the Moon is roughly 2.5 million tons, and the deuterium is probably similar. An estimated 4.1 million tons of fusion propellant as close as the Moon will be important in the early days of a developing solar economy. Long before we have disfigured the Moon we will have the means to mine the gas-giants, a resource many billions of times greater than the Moon’s.

“Project Daedalus” was the vanguard of practical fusion propulsion technology, and “Project Icarus” will carry us all closer to that goal. A goal that once achieved will give us far more than the stars, nothing less than security for our planet and access to many more as well. “Icarus” aims for the stars, but will lift up all of us with it as well.

If you have found this article to be of value then please consider donating a small amount to Project Icarus to assist us with our ambitions of creating a credible starship design.

Alternatively take a look around the remainder of the Icarus site.

You will probably have heard of the old tradition (before GPS!) of using the stars to help navigate around the earth especially at sea.  But when you’re actually travelling among the stars, and they are no longer ‘fixed’ in the sky, how would you navigate then?  This was the question asked by the Daedalus team in the 1970s as they planned their flight out of the solar system to Barnard’s star.  Despite noting that the interstellar probe presented problems of navigation far beyond those that had to be solved for the current interplanetary probes,  their original investigation concluded that there were no fundamental problems that could not be overcome through the normal process of technological development.  So one might ask what has happened over the last 30-40 years and what are the implications for the Icarus Team?

Those fundamental issues were discussed in the original Daedalus navigation paper but if you don’t want to search out your historical JBIS here are the main conclusions: they determined that an angular measuring accuracy of a microradian or so (0.2” – arc seconds) and a 5 metre telescope using sensors in the visible to thermal bands (0.5-100 micrometres) would do the job.  But there was a caveat that currently known stellar positions were not accurate enough for interstellar navigation although they supposed that this problem would be resolved by taking astrometric observations from Daedalus flight trails.

Well, in the first instance let’s check out one of the most recent missions that might offer some comparison …The NASA New Horizons Mission to Pluto and the Kuiper Belt.  The guidance and control system on New Horizons is capable of providing attitude knowledge to better than +/-471 microradians.  That relates to how accurately you can point your communications system back to earth; there’s more on this in the earlier blog by Pat Galea.  The Daedalus paper stated 250 microradians would suffice for this during its cruise phase so at least it is getting close to that requirement.  Interestingly, Galileo Avionica, the manufacturer of the star trackers often used for guidance and control in interplanetary missions including New Horizons, states the accuracy of their A-Str star tracker as better than 10” which is about 48 microradians.  So, still not that near the requirements set out by the Daedalus study.  Perhaps that was good enough for navigating around the outer solar system so hence there was no real need to go for greater accuracy.

Suppose we take a look at a mission specifically designed for accurate stellar measurements such as the ESA Hipparcos mission in the 1990s?  Hipparcos was designed to measure stellar parallax (and positions) using a main mirror of only 29cm…In this case the error was given as between +/-0.7 to 0.9” (for star magnitudes down to the 8^th magnitude).   That’s not a whole lot more than a microradian.  The follow-on ESA mission of Gaia promises even greater accuracy albeit with larger mirrors.  So it seems that the Daedalus team were correct in their assumption and sufficiently accurate systems are now possible.

You might recall that the Deadalus mission profile was to carry out a fly-by of the target system and the only complication was for the sub-probes to not fly too close to the targets (remember the fly past velocity was to be 12% of the speed of light – if you get too close the relative motion becomes too great).  Interestingly, it now looks possible that Icarus may include some deceleration or perhaps some elements might even go in to orbit which will remove some of the problems identified by the Daedalus team but will add a significant amount of complexity to a fully automated system.  In the near future and with the plans for exoplanet detection it is likely that there will be another advantage for Icarus: that of having prior knowledge of the planetary system before it sets off.  Indeed, there are some concerns that by the date of a possible Icarus launch/arrival at the target, future solar system based observations may well exceed the capability of the Icarus onboard systems until the probe gets quite close, but at least that will mean there would be a high level of knowledge about the target system.

This all sounds fairly promising for the Icarus study and there is more good news.  Recent studies have been made into using x-ray pulsars to help navigate space.  In effect, the suggestion is that by using 4 x-ray pulsars and their precise timing signals then you can effectively set up an interstellar GPS system.  You can find more information about this at http://www.centauri-dreams.org/?p=136. 

Finally, after all that promise from developments of the last 30-odd years there still seems to be one nagging thought that was also raised by the Daedalus team…how are we going to make the system reliable enough to last the anticipated mission time i.e. perhaps 50-70 years!  Modern star trackers are rated for around 15 years although some spacecraft out there, for example the Voyagers, are already over 30 years old and still under guidance and control and still expected to go on for another 10 years! 

So there is real hope and the Icarus team will be taking on the challenge of all these issues for their new interstellar design.  The review of the navigation problem will consider all the relevant assumptions of the Daedalus team and will be taking a closer look at the technology and the real missions that can be related to it.  Overall, it seems possible that rather than leaving it to any future technological development Icarus may be able to use current technology and meet the requirements for interstellar navigation.  But, as my partner, Sue always says, if all else fails we could always stop and ask for directions!

Next time I shall take a look at the scientific benefits of sending a probe to the nearest stars with specific reference to a related navigation issue raised by Project Daedalus: improving the accuracy of measuring stellar positions and distances and how it could help calibrate the scale of the universe.

If you have found this article to be of value then please consider donating a small amount to Project Icarus to assist us with our ambitions of creating a credible starship design.

Alternatively take a look around the remainder of the Icarus site.

References:

Fountain, Glen H., et al, ‘The New Horizons Spacecraft’, arXiv: 0709.4288v1 (26 Sep 07)

Perryman, M.A.C. et al, ‘The Hipparcos Catalogue’, Astron. Astrophysics, 323, L49-L52, (1997)

Richards, G.R.  ‘Daedalus Project: The Navigation Problem’, JBIS, ppS143-148, (1978)

“Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is.” – Douglas Adams, The Hitchhiker’s Guide to the Galaxy

If our current understanding of physics is correct, then the speed of light in a vacuum sets an upper limit to the speed of any matter or information transmission system. This speed of light (represented by scientists and engineers with the letter c) is about 300 million metres per second. That means a beam of light can go around the Earth’s equator seven times in one second. Yet despite this seemingly phenomenal speed, the time it takes for light to travel between the nearest stars to us is measured not in seconds, days, or even months, but years. That’s why for convenience we measure the distances to stars in units of “light years”. One light year is the distance that light will traverse in one year. The nearest star system to us, Centauri, is just over four light years from Earth.

When we send a craft to another star, we want to get the data from it back to Earth. There are several ways we could do this:

1. Send another small probe back to Earth carrying all the data.

2. Use radio signals.

3. Use optical signals.

4. Use an exotic technique.

Option 1 isn’t as absurd as it might seem. Although the message probe would take a long time to get back to Earth, it could carry a huge amount of data. In network engineering terms, it is “high latency, high bandwidth”. That is, you get a lot of data, but it takes ages to get it. (This is analogous to the often used example of sending a truck full of data DVDs to a recipient. In some circumstances, this may be the optimal way to send the data, if the initial waiting time isn’t a problem.)

However, the additional mass of an extra interstellar probe together with its propulsion and auxiliary systems probably rules it out as a practical solution for most missions.

Option 2 is an extension of the systems that we currently use on our space probes. Radio is just a longer wavelength of electromagnetic wave than light, and travels at the same speed as light. Radio is a great method for generating and receiving signals because it is easy to create using electronics, it can be piped around systems using wires (or for microwaves, tubes), and it can be easily reflected using metal dishes, allowing effective focusing of signals.

The drawback with radio is that the beam spreads significantly over longer distances, with the result that there is less signal power available at the receiving end, unless the receiving antenna is made very wide to catch as much of the signal as possible. The higher the frequency of the radio signal, the less the beam spreads with distance. Over the decades, space communications have indeed used higher and higher frequencies to reduce the beam spreading, with the result that the transmitter on the craft can use lower power while maintaining the same amount of power received at Earth.

For interstellar distances, the beam spreading at radio frequencies is enormous. The Project Daedalus team came up with two ways to make the use of radio signals viable over the terrific distances involved:

a. Make the engine’s reaction chamber be a parabolic dish shape. When the boost phase has ended, use this dish as an enormous reflector to focus the radio transmissions back to Earth.

 b. On Earth (or in near-Earth space), set up a huge array of parabolic dish receivers. (An array of receivers is almost as effective as a single receiver of the same size as the array.) This allows much more of the signal to be picked up than would be possible with just a single large dish. (The design was based on the proposed Project Cyclops, which was to be used to search for signals from extra-terrestrial intelligence.)

Option 3 is really just an extension of option 2, because light is just a much higher frequency radio wave. However, in practice the way it interacts with matter, and the way we create, manipulate and receive it is different enough from radio that we generally treat it separately.

As stated above, beam spreading decreases for higher frequency radio waves. This is particularly noticeable with laser light. If you’ve ever played with a laser pointer, you’ll know just how tight the beam is; shine the dot onto a distant wall, and the dot appears no bigger than if you’d shone it on your own hand, and the dot is about the same brightness.

Lasers are very likely to become more common on future deep space probes to distant parts of the solar system because they enable a much higher bandwidth of data to be sent back to Earth for the same amount of power as a radio system.

Over interstellar distances, despite the fact that lasers create a very tight beam, the beam spreading does cause a problem. The laser also has to be aimed very accurately, and this aim has to be maintained. The tiniest amount of jitter in the craft could cause the beam to miss the target completely. This would be a very tough engineering challenge, combining navigation (so that the craft knows exactly how it is oriented, and exactly where the target is) and control (so that it is actually able to point the laser accurately at the target).

Option 4 covers a range of suggested techniques which either use new methods of transmission altogether, or use existing systems in a new way to increase their effectiveness. 

Claudio Maccone has suggested the use of gravitational lenses to focus radio (and light), drastically increasing the amount of signal power received from the distant craft. The idea is to have two craft, one at the distant star, and one at our Sun. Each craft sits on the far side of its star on the line joining the two craft and the two stars. (So the system is arranged thus: Receiver Craft – Sun – Interstellar Space – Star – Transmitter Craft.)

The two craft have to be beyond a certain distance from their respective stars; this is the focal distance. In the Sun’s case, this distance is about 700AU (i.e. 700 times the distance from the Earth to the Sun). The transmitting craft sends its data, which is gravitationally focused by its star and sent across interstellar space. Years later, this signal arrives at our Sun, which similarly gravitationally focuses it, and is picked up by the receiving craft. This system is effectively using the two stars as lenses in an interstellar telescope.

There is nothing particularly outrageous about this suggestion. It is fully consistent with the laws of physics, but the big challenge here is in engineering. The two craft need to be kept exactly in line with each other and their stars to a very tight tolerance.

Other suggestions for ‘exotic’ techniques include using gravitational waves or neutrinos to send data. The problems here generally come with difficulties in generating and/or receiving the signals. Creating strong gravity waves would, as far as we know at the moment, require a feat of astrophysical engineering (manipulating large blobs of very dense matter). Neutrinos are easier to create, but they are very hard to detect. It is unlikely that any of these exotic techniques will be implemented in a practical system in the near future.

It is heartening to remember that back in 1977 the Project Daedalus team considered the problem of interstellar communications in detail, and concluded

“We do not need technological breakthroughs in this area. Development of the system could begin now.”

In the early 21st century, we can hope to do at least as well as Daedalus.

If you have found this article to be of value then please consider donating a small amount to Project Icarus to assist us with our ambitions of creating a credible starship design.

Alternatively take a look around the remainder of the Icarus site.

References:

The Hitchhiker’s Guide to the Galaxy, Douglas Adams, 1979 [Wikipedia]

Project Cyclops [Wikipedia]

Deep Space Optical Communications, Ed. Hamid Hemmati, Wiley-Interscience, 2006 [Google Books Preview]

Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens, C. Maccone, Springer-Praxis, 2009 [Google Books Preview]

Updating the Gravitational Focus Mission, P. Gilster [Centauri Dreams]

The Gravitational Lens and Communications, P. Gilster [Centauri Dreams]

Project Daedalus: The Vehicle Communications System, A.T. Lawton and P.P. Wright, JBIS, pp. S163-171, 1978. [CD-ROM]