Let down by solar panels

[Andy Burns]

Let down by solar panels

Should have had nuclear instead :-P

[Brian Gaff]

Actually its not dead. it had batteries for the prime mission, which are now exhausted, but there is still data in flash and a clock powered from a normal battery. The system is, according to the bod I heard, designed to hibernate and wait till there is sufficient charge before it is brought to life again, so one assumes its some kind of very low power mode. They always said that any landing site might be shaded and they expected it to not always be so as it headed in toward the sun, so it could well spring back to life when its got the correct charge. apparently embeded in the data is a signal from the charging system suggesting that there had been some charging but at a very low level. So I guess its a waiting game and anything else they get will be a bonus. If it had actually stayed where it first set down it would have been good they reckon, but trying to land on a body which has a the consistancy of something between rubbar and candy floss is never going to be easy. Brian

[polygonum]

Should have used grab adhesive on the lander's legs. Or a can of polyurethane foam. :-)

[nemo]

Did I really hear an interviewer on Radio 4's Today program refer to it as "putting their probe where the sun doesn't shine" ?

:-)

[Tim Streater]

You certainly heard John Humphries mentioning it being on Mars, and someone else talking about "charging the solar cells".

[Dave Liquorice]

Especially when you have a mass of ~100kg but sod all gravity. I think it was on BBC WS Discovery a while back that Philae weighs the same as a sheet of paper, so very little force on quite a large mass.

I've not looked all that hard about the go/no go decisions but wasn't there a problem with the thruster on the top designed to hold Philae down?

[Dave Liquorice]

I always thought Mr Humpries was reasonably on the ball for journo/presenter, oh well...

Then I didn't miss hear that one on BBC News 24 either. B-( It's not difficult for crying out loud.

[alan_m]

That WAS the problem - the No More Nails on the end of the legs failed.

The solar power is working exactly as it does on your average domestic roof in the UK - it doesn't work when you actually need the most power.

[Tim Streater]

True but you need to be in the vicinity of the Sun. No good for the outer reaches of the Solar System.

[Cursitor Doom]

By next summer it will be "in the vicinity of the sun" and hopefully start working again a good deal before then. Ridiculously over ambitious project, though, if it's for real. Landing on a comet! Bonkers!

[Dave Liquorice]

Have they lost any primary science? I haven't heard they have. Philea was only built to run off battery for a couple of days then go into a repeated sleep & charge / operate mode.

Er, It's somewhere between the distance of Earth and Mars from the sun hardly "the outer reaches of the solar system". Perihelion is

1.29 AU, an AU is the sun-earth distance.

It will be interesting to see what happens as the coment moves around the sun, it may well start to illuminate Philae for longer and it'll wake up more and more. Also I'd expect the flight crew to have worked out the crafts attitude and surroundings and come up with some means of either making it "hop" away from the cliff or orientate itself to get maximum benefit of sunlight.

[Andy Burns]

Maybe the media has been harsh on them (or badly briefed on what to expect?) the initial reports were "It's bounced off and they admit they've lost contact with it" when it was expected to only have periodic contact in any case.

"How long will the lander operate on the comet nucleus? The Rosetta lander, called Philae, will touch down on the comet's surface on 12 November 2014. The science observations will start immediately. During the first 2.5 days the first series of scientific measurements will be completed. During this phase the lander will operate on primary battery power. In a second phase that may last up to three months, a secondary set of observations will be conducted, using backup batteries that will be recharged by the energy from the solar cells on the lander. However, no one knows precisely how long the lander will survive on the comet."

I presume they expected to get more drilling done than they have been able to so far ...

[Tim Lamb]

I'm surprised that the rotation of the comet is apparently so stable. On that basis, the Sun will rise as the comet approaches.

>

[Andrew Gabriel]

It was originally projected to last until next March, after which it would be destroyed by the summer heat and the increasing attack by debris. It may be that the shadow will delay the death by overheating, whilst the approching summer will allow more power to it before it's killed by heat.

[Dave Liquorice]

Why? There is an awful lot of nothing out there, with nothing close by there are no significant forces acting on it so it just carries on as it is.

The comet has a rotational period of around 12 hours so the sun rises about every 12 hours, trouble is it sets about 1.5 hours later. B-( Which isn't long enough to put enough charge into the battery.

What may happen as the comet swings around is the "season" will change and just maybe the amount of sunlilght Philae sees will increase. Think of sitting in a chair a few feet from a tall wall to your south. In winter you may only sit in sun shine for an hour or two around midday. Come mid-summer you'll be in sunlight for much much longer. Of course Philae could be in "mid-summer" now...

[Tim Streater]

Oddly enough I know all that. I was referring to the notion that solar panels might be considered the answer for all probes.

[Tim Streater]

Yes. Objects that shape tend to undergo chaotic tumbling.

[Tim Streater]

I think the point here is that strangely shaped objects tend not to have nice constant rotational periods. They tend to tumble over and over chaotically and unpredictably.

[Tim Streater]

Not what I'm talking about.

[Dave Liquorice]

close

Millions of miles out in space where does the force come from to change the angular velocity of the object? It's basic phyics, objects don't change what they are doing unless a force acts upon them.

In our earth bound experience strangely shaped objects tumble chaotically because we have earths gravity pulling pretty hard in one direction all the time.

On earth a dumb bell with a 1 kg weight on one end and 2 kg weight on the other will try to rotate if held horizontally, but if held vertically it won't. Now have the dumb bell rotating (not along the axis of the bar) and the effective torque on the object, from earths gravity, will vary depending on how vertical or not the dumb bell is. It is this varying torque that induces the chaotic tumbling that we expect to see in an strangely shaped object. Take away earths gravity and you take away this varying torque and the object simply obeys Newtons First Law.