'Time for risks' with comet lander

[pErvinalia]

So, is it possible it will get sun again at some point as the comet gets closer to the sun?

[mistermack]

It's shut down altogether now, but there is a chance of waking it again next August, as the angle of orbit changes, and the sunlight comes in from a slightly different angle.

My idea for avoiding this in the future is to have the solar cells in a long coiled up string, rather than a panel. Either shoot the string sideways on a tiny rocket after landing, or just before.
You could then have the solar cells operating over hundreds of metres, so that even if some of it is in the shade, most of it will be in sunlight.

[Hermit]

Cheapest solution: Ship the thing along with a colony of glowworms. The only drawback is that they may be mistaken for stars on photographs taken by the lander.

[JimC]

The orbiting craft should have a laser of appropriate size (powered by some of the energy from its solar cells) that it can aim at the lander to at least trickle charge it...

[mistermack]

I'm watching the "sky at night" documentary about the landing.
One very surprising fact. The entire lander weighs just one gram, with the comet's low gravity.
On Earth, with our 1 g of gravity, it weighs 100 kg so that is it's actual mass.
Escape velocity from the comet is just 1.1 mph.

The orbiting craft should have a laser of appropriate size (powered by some of the energy from its solar cells) that it can aim at the lander to at least trickle charge it...

Or a pointable mirror.

[Xamonas Chegwé]

The mass of the Philae probe is 97.9kg. This is invariant.

Under Earth's gravity of 9.81ms-2, that translates into a weight of 960.399N. Weight is a force calculated by the simple Newtonian equation F=ma. 97.9 x 9.81 = 960.399N.
It's weight under the gravity of 67P (estimated to be 10-3ms-2 but varying due to the irregular shape of the comet) is 0.0979N. 97.9 x 0.001 = 0.0979N.
This would be the weight on Earth of an object with a mass of just under 10g. 0.0979 ÷ 9.81 = 0.00998kg = 9.98g.

So it looks like the Sky At Night is out by a factor of 10! Shoddy maffs.

[Hermit]

So it looks like the Sky At Night is out by a factor of 10!

Does it? How do you calculate the force of gravity that acts on Philae when the comet is not at all spherical?

[JimC]

So it looks like the Sky At Night is out by a factor of 10!

Does it? How do you calculate the force of gravity that acts on Philae when the comet is not at all spherical?

XC said the estimate was around 0.001 m/s2, but that it would be variable. I imagine one would calculate the surface gravity of a sphere with the same volume & density of the irregular comet, and accept the actual figure would vary to an extent. I would doubt it would vary by as much as a factor of 10...

[Xamonas Chegwé]

So it looks like the Sky At Night is out by a factor of 10!

Does it? How do you calculate the force of gravity that acts on Philae when the comet is not at all spherical?

The figure was quoted in the Wikipedia page for 67P. As Jim has said, I expect it is based upon a sphere with the same estimated mass as the comet and it will vary depending on the distance of any point on the comet's surface to its COG - however, this variance will be relatively slight, certainly far less than an order of magnitude! To be out by a factor of 10, the landing site would need to be √10 (~3.162) times further away from the COG than the surface of a sphere of equal mass - the comet is not that irregular.

[mistermack]

I thought they said one gram, I did record it but can't be bothered to check.
Even I can work out that if the gravity is one ten-thousandth of Earth's, and the lander weighs one hundred thousand grams, it's going to weigh ten grams on the comet.

Maybe I heard it wrong, or someone else did when writing the script.
Anyway, ten grams is fuck all. And the thing spins once every 12 hours which will reduce the weight a tiny bit more.

I wonder what proportion of water ice is in the comet? If this is an average comet, and it's only three miles long, imagine how many of them it would take to produce all of the water on Earth.
There must have been a hell of a lot of impacts at some time. It's no wonder the Moon is such a mess of craters. We're lucky the Moon is there. It probably mopped up enough comets to allow Earth to have some dry land. Otherwise, the Earth might have had nothing but sea life.

[klr]

I thought they said one gram, I did record it but can't be bothered to check.
Even I can work out that if the gravity is one ten-thousandth of Earth's, and the lander weighs one hundred thousand grams, it's going to weigh ten grams on the comet.

Maybe I heard it wrong, or someone else did when writing the script.
Anyway, ten grams is fuck all. And the thing spins once every 12 hours which will reduce the weight a tiny bit more.

I wonder what proportion of water ice is in the comet? If this is an average comet, and it's only three miles long, imagine how many of them it would take to produce all of the water on Earth.
There must have been a hell of a lot of impacts at some time. It's no wonder the Moon is such a mess of craters. We're lucky the Moon is there. It probably mopped up enough comets to allow Earth to have some dry land. Otherwise, the Earth might have had nothing but sea life.

I think that's one of the things they were trying to find out ...