Interstellar flyby versus deceleration mission considerations

Icarus Project blog discusses the choice between choosing an interstellar flyby versus deceleration mission.

Is an interstellar flyby worth the effort ?

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.

Consider 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

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.

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.

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