Space Exploration Tidbit

Just caught a story about the Japanese Hayabasu asteroid mission. It's just headed home from a near-Earth asteroid named Itokawa, where it may or may not have gotten a sample. Two interesting things about this story.

First, Hayabasu's chemical engines leaked out, losing absolutely all of its fuel. Luckily, the Japanese outfitted the Hayabasu craft with shiny new ion thrusters. Mostly used for low energy adjustments of Earth-orbit satellites' trajectories, this is only the second time an ion engine has been used by a craft exploring further out into the solar system, and the first time it's ever brought anything back. Way cool.

But even more intriguing is the fact that mission control doesn't know whether Hayabusa got the asteroid sample they were after. They know that it landed on the asteroid twice, and they think they know that the sample collection apparatus (a hunk of metal fired into the asteroid to kick up debris) didn't activate as planned. But waitaminute--

Shouldn't they be able to calculate how much mass the craft has picked up by measuring the change in thrust required to move it? If the satellite managed to grab anything at all, it would take the engines longer to accelerate the craft, especially if they're using weaker ion thrusters.

Simple Newtonian physics, right? F=MA. Presumably the force from the ion thrusters remains constant when the craft initiates a burn, so if you know the acceleration you can derive the mass.

Compare the mass on the return trip with the mass on the trip out and you can deduce whether you have any extra mass, which would presumably be a sample.

Any word on whether this has been done? Can't find it anywhere in the news stories.


EDIT: A little more research on the Hayabusa page on Wikipedia reveals that all Hayabusa was intended to collect was asteroid dust. It's possible the mass of such a tiny sample is negligible compared to the mass of the craft itself and thus doesn't factor into navigational calculations. The Apollo astronauts rode orbits calculated with only three digits of pi, so maybe interplanetary navigation isn't as precise as I'd assumed.