That Comet

[jonjel]

A fantastic bit of science and engineering to land the probe on the surface of the comet, even if it did fall over!

But I have been puzzling a bit. We are told that the 'weight' of the probe now it is on the surface is only about a gram. So three parts of bugger all gravity. Yet the space craft is in orbit around the comet, passing within I think about 10 miles.

I understand the principles of orbit, a bit like a rifle bullet falling to ground but in effect continually falling round the corner. But with so little gravity does that mean the orbit is remarkably slow, or what?

[principled]

Good point Jonjel. I thought "Let's do some research", and came across what I thought was an ideal research paper (calculating the orbit around Eros)....until I read the abstract. I am now completely confused! The problem seems to be not only the lack of gravity but also the irregular shape and motion of the body.

P

[alancalverd]

Orbital speed = (GM/r)^0.5 where M is the mass of the comet (assuming it is much larger than the mass of the orbiter). In this case M is about 10¹³kilogram and G = 6.7 × 10-11 m³ kg^-1 s^-2 so at a radius of 16 km the probe is travelling at about 0.2 m/s. That should make landing dead easy, even compared with landing a helicopter in a slight breeze!

[jonjel]

Alan. The maths is beyond my simple brain, but I am sure it is correct.

You said 'the probe' is travelling at 0.2M/sec.

Did you mean the probe, or the 'mother ship'? That would make each orbit very slow indeed, 1Km would take around 1.3 hours.