Hayabusa: A Round Trip of Six Billion Kilometers
Fly the asteroid probe Hayabusa — build up delta-v with coin-weight ion thrust, drop a ball in Itokawa's hundred-thousandth gravity, touch down at centimeters per second, rewire two broken ion engines into one, and bring the capsule home on June 13, 2010.
Into deep space on a coin-weight of thrust
In May 2003, the small probe Hayabusa set out atop an M-V rocket. Its hope rested on four μ10 ion engines — engines that accelerate xenon electrically and breathe it out. Even at full throttle, the thrust is about the weight of two one-yen coins. The craft hardly seems to accelerate at all. But try speeding up time. A few metres per second of velocity change per day, stacked for months on end. A weak force wins, as long as it never stops. That is the discovery of the ion engine.
The craft is a historically-informed model of the probe Hayabus, launched in 2003.
Raise the throttle, then speed up time, and watch the delta-v — the stacked change in velocity — grow.
How could an ion engine with only a coin-weight of thrust make a six-billion-kilometre round trip possible?