Scaling the Universe
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
6 comments
It is said that the moon has 1 million tons of He-3. A significant space-based infrastructure will start with development of the moon. We would anticipate that He-3 mining activities will already be ongoing for nuclear power purposes on the Earth by the time that an Ircarus would be built.. So it really comes down to an assessment of whether it will cost more to purchase from the lunar suppliers or to develop an entirely new system to extract it from an outer planet. My bet is on purchasing from existing lunar suppliers. But, if the concentration is high in the uranian atmosphere perhaps that is a cheaper source. After all, He-3 would be a bulk commodity. When in free space transportation costs are a matter of time in which craft are tied up and loss due to boil off rather than fuel lost due to friction. It would be interesting (but unlikely in my opinion) to find that uranian He-3 was less expensive than lunar He-3.
Strange, I thought that I was responding to an Icarus post dealing with He-3. Not sure how that happened.
Anyhow, re: parallax, by itself it falls well short of a justification for the huge expense of Icarus. Icarus has to be justified for other reasons. The science return from a close-up view of another solar system will be far more valuable than improving parallax measurements. But, if Icarus goes for other, more valid reasons, then why not get improved parallax along the way.
More fundamentally, if a close-up view can be had in the same timeframe (i.e. launch date and travel time) but using a less expensive approach then this is what will determine the validity of the Icarus approach. I’m inclined to think that a far smaller craft using beamed propulsion will be less expensive and probably more doable within the timeframe. I just wish that a real assessment of alternatives was allowed as a part of the Icarus Project.
Hi John,
Thanks for your comments – can’t help with the He-3 mining – but you will see work done on that by the other guys on the team in the future.
Regarding the subject of my blog, you’re right, ‘if Icarus goes for other, more valid reasons, then why not get improved parallax along the way’. My sentiments exactly. In themselves, parallax measurements might not be a valid reason for a trip to one of the nearest star systems but maybe I didn’t convey too well how fundamental parallax measurements are in astronomy. I also hoped that I might illustrate just one task that could be carried out by an Icarus-type probe, perhaps for those readers who may not realise what can be done, along with close up views of a new solar system. If you would like to read more I would keep an eye on the Journal of the British Interplanetary Society as Dr Crawford’s paper must be near publication and covers the case for interstellar spaceflight, including other subjects such as intersteller medium studies which can pretty much only be done best by going there. We’re lucky that he has since joined the Icarus team to lead this work.
It’s a little early in the project to talk of costs of Icarus as we have no Icarus design yet, few other details other than terms of reference and some initial scoping work. I guess you’re a proponent of beamed propulsion, an alternative to schemes such as Daedalus, Icarus etc, and that is great. The more work going on with different options the better. In the end, if the propulsion system changes, some of the work that I and others will be involved in, such as Icarus navigation and communications systems, ground station and monitoring, materials and structure and the astronomical target, will not be wasted.
Rob Swinney
> but maybe I didn’t convey too well how fundamental parallax measurements are in astronomy.
Well, maybe you could educate me here. We have some limited parallax measurements on one end. On the other end we have red shift. So, perhaps there’s a middle ground which better parallax would help with. You can also probably figure out the relative positions of stars which orbit the galaxy’s black hole based upon their measured movements.
I can understand how fundamental parallax was when we were first figuring out the basic structure of the galaxy. But at this point, it’s not clear to me how Earth shattering the knowledge obtained from better parallax will be. Will it tell us the composition of exoplanets, the composition of stars, where black matter is…what fundamental thing will it tell us?
Hi John,
Thanks for taking the time to respond to my blog and comment. May I suggest you investigate how cosmological red shifts are calibrated (at an intrinsic level they are not perfect distance indicators and, in the end, actually depend on the ‘limited’ parallax measurements) and perhaps re-read my article. Of the three queries you raise two will be affected by accurate parallax measurements (the nature of stars, if not simply composition, and where the black (sic) matter is) and the third I would argue is not a fundamental issue in itself. The composition of exoplanets will be of interest, hardly fundamental, but that may just be my opinion.
Rob Swinney
I read with interet the article you wrote last year where you said ” ‘Helm, come to heading of 250 Mark 3.5″ says Captain James T. Kirk, and the USS Enterprise warps-off into the distance on another Star Trek adventure. Now, what ’250 Mark 3.5′ actually means is far from certain….” Actually, the Star Trek writers do their best to make sure all of what we heard on Star Treks makes some sort of sense. I quote from the Star Trek Technical Manual by R. Sternbach et al. “A flight vector can be specified as a azimuth/elevation relative to the current orienttion of the spacecraft. In such cases, 000-mark-o represents a flight vector straight ahead.” For a new heading, the Captain give a three digit number for the 360 degree circle on the plane of the ship’s current position (i.e., left of right from the current 000 heading) and the “mark” which is the number of degrees up or down relative to the current 000 position. Just thought you’d want to know (smile).
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