On Slashdot, I have been involved in a discussion about moving or torquing Philae using the thrusters on Rosetta.

I should say that this will be dangerous and would be unlikely to be attempted until we are close to the end of the mission. It would mean finding Philae, bringing Rosetta down close to the surface, and firing Rosetta’s thrusters right at Philae, all while some long round-trip time from Earth (so, Rosetta would have to do all of this autonomously).

I am also going to ignore for now any issues of damage from nearby thruster firings. That’s something the spacecraft designers would know, and I would only be guessing at.

Rosetta has 24 bipropellant 10 Newton thrusters and is 2.8 x 2 m, not counting solar panels. Philae is 1 x 1 x 0.8 m. Suppose Rosetta fires a thruster at Philae from 3 meters away – Philae is then 1/3 of a radian across, or about 0.1 steradians. Suppose the thruster has a exit angle of 2 pi steradian (i.e., the whole hemisphere away from the spacecraft, which is surely conservative). So, Philae would experience a force of ~ 10 N x 0.1 / 2 pi ~ 0.2 N. It has a mass of ~ 100 kg, so that would impart a thrust of 2 x 10^-3 m/sec^2. (I am assuming Rosetta has a thruster firing on the opposite side too, so it’s not moving away while it is doing this.) That is actually greater than the 67/P gravity, so Philae could move. If firing were done for say 10 seconds, Philae would have a velocity of ~ 1 cm/sec afterwards and maybe a total flight time of 30 seconds.

I would thus conclude that, while it wouldn’t be possible to move Philae **far**, it should be possible to move Philae **some**, maybe to torque to land on all three legs on a little flatter terrain and thus get it some more sunshine.

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