The nominal Daedalus mission profile would accelerate the probe for around 3.8 years with an exhaustive pulse frequency of 250 detonations per second. Assuming no major failure modes occurred during this time the vehicle would then attain its cruise velocity of 0.12c and coast to its target for another 46 years. It would whip through its target system Barnard’s star, 5.9 light years away, in a matter of days. Considering the likely cost of such a mission, perhaps in the hundreds of billions to trillions of dollars, and the energy expenditure required to accomplish it – is flyby worth the effort?

To address this issue the Project Icarus Study Group decided that we had to go that next step further and consider the difficult challenge of deceleration. With this in mind, this became part of our engineering requirement. Several options are being considered including the use of MagSail technology and Medusa sails – a derivative of Project Orion. Another option being examined is reverse engine thrust, but the problem with this is that if we assume an equal acceleration-deceleration profile then the mass ratio scales as squared compared to a flyby mission and so requires an enormous amount of propellant; definitely a turn-off for a design team seeking efficient solutions.

Let’s consider for a moment the pulse frequency of Daedalus, 250 Hz. Can we get away with less? In a recent paper presented at the 61^st IAC Prague (by this author) it was shown that in fact we can. The plot below shows the effect on the total mission time for a Barnard’s star target as a function of pulse frequency and with different exhaust velocity assumptions. The results clearly show that with low to moderate pulse frequencies total mission duration of less than a century are obtainable; and this is fortunate because the Project Icarus Terms of Reference state that the mission must be accomplished on this time scale.

But then we have to factor in deceleration and in order to accomplish this we possibly have to allow for several decades in the mission profile. A pulse frequency of somewhere between 50 – 100 Hz would perhaps then be more desirable, although we can get away with even less. This would have the added benefit of reducing the stress on the engines and as well as any fatigue delivered to the spacecraft structure over the boost phase through thermal loads.

So if Daedalus was approaching a solar system in flyby mode how long would the probe actually have to observe the target star and any associated planets? For this crude analysis we shall forget the angular line of sight effect on the approach and de-approach (e.g. ±45 degrees, say) and just assume that our probe happens to cross the path of an orbiting object at approximately ninety degrees to its orbital motion trajectory. We also assume that it passes within a close distance and so neglect the solid angle effect with distance from the object. Let’s just look at some actual numbers for how long the observation encounter will last between the probe and the object being observed.

Firstly, we look at flying past an object the size of the Earth which has an equatorial diameter of around 12,756 km, and the result for different probe speeds are shown below. Travelling at a Daedalus speed of 0.12c it will only have much less than one second to observe the target. Even if you can get decelerate the probe down to less than 0.01c you still only have seconds to observe the target; those biomarker instruments had best be quick.

What about a Jupiter target, which is over eleven times larger than the Earth with a diameter of 142,984 km. Approaching at a Daedalus speed the probe will only have much less than ten seconds to observe the object. This encounter time is increased to many tens of seconds if you can get your probe down to speeds approaching 0.01c; but no time for stopping for a Helium-3 refill.

So what about a star around the size of the Sun? Well the Sun is a lot larger than Jupiter with a diameter of 1391,000 km and so travelling at Daedalus speeds you will get over a minute at least. But if you can get that probe decelerated down to 0.01c your window starts to turn into minutes of observations time.

Let’s take an entire solar system, assuming that 100 AU is a typical size. How long will the encounter time be then? Again, travelling through the system at a Daedalus speed of 0.12c you will have an entire five days to get all your observations in. You best deploy those atmospheric re-entry probes quick though before you get out of communications range. Perhaps this may be worth the trip, but no time for any detailed examinations. If the probe is decelerated down to less than 0.01c the encounter time rapidly is measured in months – much more like what you need to do some real in situ science observations.

An original assumption of the Project Icarus Study Group when starting out (including this author) was to aim for a higher cruise speed than Daedalus and thereby get to the target much quicker; typically 0.15c was seen as a good figure to aim for and 0.2c was seen as our likely upper velocity bound – although some of the team still think 0.3c is possible. But if by going faster you increase the failure mode risk, make it harder to decelerate and only allow for reduced encounter time with the target – why go faster? Hence it may actually be more favourable to go slower than Daedalus in order to ensure reliability of the mission.

If you could bring the probe down to solar velocity that would be a huge benefit; circular velocity is given by the square root of GM/r, where G is the gravitational constant, M is the solar mass and r is the radius of the star. For the Sun this computes to around 437 km/s or around 0.15% of the speed of light in a vacuum.

The data in Table 1 shows the total cruise times for various missions to an Alpha Centauri target 4.3 light years away. It also shows the time remaining for deceleration, but subtracting a conservative 5 year acceleration time. Clearly at cruise speeds much lower than Daedalus there is a good deal of time remaining for deceleration.

So given the above data and the fact that deceleration is likely to prove a very difficult engineering challenge, it would be a prudent decision to go for a less risky mission profile than proposed for the Daedalus probe. Yes some will still say that it’s a challenge to allow a space probe to remain functioning for decades at a time, but there are examples where this has happed such as with the US Pioneer 6 probe which has been operating in space for many decades and should continue for another few decades. An interstellar probe travelling at say 0.08-0.09c will be slower than Daedalus, but it will also be able to operate in a less harsh engineering environment and using a pulse detonation frequency that is not so exposed to criticism. If you fold out some of the assumptions made in the crude analysis above the encounter times do increase, but not by significant amounts. Although we can’t ignore the benefits of long range observations as the probe approaches the target; it really depends on how you define the encounter period.

But all of this analysis goes to the heart of whether a flyby probe such as Daedalus is really useful given what it took to get there. The potential science return is massively amplified by performing a deceleration of the vehicle and although it is a significant engineering challenge this is why the Icarus team decided to address this problem; and it is a problem, even if you choose to just decelerate sub-probes. Coming up with a viable solution to the deceleration problem in itself would justify Project Icarus and the five years it took to complete the design process. Perhaps one option then for the Icarus mission is to adopt a cruise velocity that is a trade-off between getting to the mission target quickly in accordance with our Terms of Reference, whilst not going too quickly so that you place undue stress on the mission reliability and make it more difficult to effect a deceleration of the probe. In summary, there are times when ‘faster’ may not necessarily be ‘better’ and an Icarus profile that aims to decelerate at the target solar system may be one of them.