# With current or near-future Cubesat propulsion technology, largest aphelion achievable?

Propulsion technology for cubesats and nanosatellites is in active development, but there are at least a few based on sound principles that are likely to be tested in the next few years. There are also a few time-tested thruster technologies used on hundreds of satellites now that could in principle be engineered to put in a cubesat.

Question: What would be the best way to design a (say) 3U cubesat, deployed from the ISS or similar altitude LEO vehicle, so that at least a 1U section of it could attain a heliocentric orbit with the highest possible aphelion (no longer bound to the Earth/Moon system) using current or near-future cubesat-comaptable propulsion technology?

If it's helpful for example, it could be assembled already in the microgravity environment of the ISS, and gently deployed from there, in order to save on structural mass that would have been needed to survive launch from earth. Parts could be 3D printed there as well. Budget for COTS-type items is large, and "the shelf" is a few years in the future to allow for things that are not quite ready yet but probably will be.

A clever use of the moon's gravity is of course both allowed and encouraged. I just ran across this answer, which links to this NASA page: http://stereo.gsfc.nasa.gov/orbit.shtml. The first video is a nice illustration of how handy the moon can be.

Here is a GIF made from the frames of that .mov file:

The 'at least 1U' part should have some minimal power system so it can measure something to verify its aphelion distance, a starcam snapshot of a few planet positions, or just a measurement of the 1/r^2 drop in solar intensity, or something different.

If spin-stabilization helps, assume that the method of deployment can set the attitude (aim it) and spin it if needed.

If you assemble it on the ISS, it should be considered safe to assemble on the ISS. Otherwise you'll have to launch from Earth the old fashioned way.

• Small ion thrusters suitable for cubesats exist; I think a 3u sat faced with solar panels can deliver about 40W, which is enough to drive a Busek BIT-1 busek.com/technologies__ion.htm so I'd think about going that route -- much better Isp and very safe to assemble compared to chemical reaction propellants. – Russell Borogove Aug 17 '16 at 21:03
• Engineering-maximum questions are generally impossible to answer without making restrictive assumptions. Consider "how fast is the fastest car that could possibly be built?" – Russell Borogove Aug 17 '16 at 23:50
• Looks like with an Isp of 2000s you only need 30% of your mass to escape Earth, the rest is is for fun! – uhoh Aug 18 '16 at 1:45
• @RussellBorogove Your point is well taken - the "at least 1U" and "COTS within the next few years" are meant to try to address the "as far as possible dilemma. I think there can be a reasonable answer, and I'll be sure to give one if nobody else posts one. – uhoh Aug 18 '16 at 3:36
• +1 for a focused engineering question instead of "is ____ possible? Why or why not?" – Erin Anne May 31 '18 at 21:03

If we take the "near future" route, there are a couple of light-sail projects that, in principle, have almost unlimited potential asymptotic aphelion:

• LightSail 2 is a 3U cubsat demonstrator. It will stay in earth-orbit.

• UltraSail is two 1U cubsats with a common ribbon sail, also staying in earth-orbit.

• Near-Earth Asteroid Scout is a planned NASA Mission to an asteroid, though it might not qualify for this question as it'll (probably) be 6U and it starts is cis-lunar space. It's got some of the most inspiring imagery, though:

None of those have flown yet, and none of those missions meet the exact "deployed from the ISS or similar altitude LEO vehicle, so that at least a 1U section of it could attain a heliocentric orbit with the highest possible aphelion" standard. But, pending success, they do indicate that there may be a technology that could (slowly) put a 3U vehicle on it's way.

• Wonderful! This isn't the solution I'd originally imagined, but using a solar sail avoids the need to cary and accelerate a reaction mass. Is there any way to estimate at roughly what altitude a solar sail would have more thrust than atmospheric drag? Would the ISS' ~400 km be sufficient, or would it have to be deployed at a higher altitude? – uhoh Jun 7 '18 at 5:01
• Light pressure and drag pressure on a perpendicular square meter are roughly equal at 700-800km, though the drag certainly varies due to various things. The Planetary Society has done some mission-planning work on ways to use the difference in light vs drag orientation to be able to leave from a true LEO deployment point. Not sure of the final status of those, but a higher deployment is definitely better. – Bob Jacobsen Jun 7 '18 at 5:20
• I see. Going from 400 to 800 km altitude only needs a Δv of about 220 m/s. At Isp of 200 seconds (just for example), that's only a 10% mass fraction, so getting to 800 km first would not be a large propulsion challenge. – uhoh Jun 7 '18 at 5:37
• Email with a colleague: seems I misremembered the Planetary Society work. That was about using sail attitude to optimize for use of light pressure vs solar wind, not drag. Sorry for the confusion. A higher deployment (maybe 1000km to be safe) seems necessary for an escape trajectory. – Bob Jacobsen Jun 7 '18 at 5:49