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Comet Probe Philae To Deploy Drill As Battery Life Wanes
An anonymous reader writes With less than a day of battery life left, The European Space Agency's Philae probe will begin to drill for samples even though the drilling may dislodge it. From the article: "Philae is sitting in the shadow of a cliff, and will not get enough sunlight to work beyond Saturday. Friday night's radio contact with the orbiting Rosetta satellite will be the last that engineers have a reasonable confidence will work. The team is still not sure where on the surface the probe came to rest after bouncing upon landing on Wednesday. Scientists have been examining radio transmissions between the orbiter and the lander to see if they can triangulate a position. This work has now produced a 'circle of uncertainty' within which Philae almost certainly lies."
Join us at 2 for #cometlanding briefing with @ESA_Rosetta and @Philae2014 experts from @esa @DLR_en @CNES @NASA
Radioisotope thermoelectric generators (RTGs) are big, heavy, and emit radiation that screws up some of the instruments.
The ones on Voyager are about the size of Philae. from Wikipedia: The GPHS-RTG has an overall diameter of 0.422 m and a length of 1.14 m.[1] Each GPHS-RTG has a mass of about 57 kg and generates about 300 Watts of electrical power.
Philae:
Launch mass 100 kg (220 lb)[1]
Payload mass 21 kg (46 lb)[1]
Dimensions 1 Ãf-- 1 Ãf-- 0.8 m (3.3 Ãf-- 3.3 Ãf-- 2.6 ft)[1]
Power 32 watts at 3 AU[2]
Unfortunately, RTGs don't come in multiple sizes, so you can't get a 1/10th scale RTG that weighs 6kg and is 15 liters in volume.
I don't know if ESA has access to RTGs in the first place, or if public policy would allow launching with a radiation source. The Cassini RTGs causes a huge public relations problem when it was launched in 1998.
There's also limited Pu240 available to make the RTGs.
An RTG is heavy. Solar panels are much lighter.
Also: An RTG is expensive. Solar panels are much cheaper.
RTGs are expensive to make, expensive to handle, and expensive to launch.
An RTG would have likely doubled the cost of the mission.
So if the budget is fixed, that means half as many missions, which is the same as a 50% failure rate, which is worse than solar panels.
Also: RTGs generate political opposition. Solar panels don't. If this was an American mission, that wouldn't matter so much, but this mission is from nuke-o-phobic Europe.
"It seems to me the design and/or planning of this mission were poorly thought out"
Is the funniest fucking thing I've heard all day. Do you have any idea how well thought out this mission was? FFS look at the trajectory it took 10 YEARS(!) to get to the comet. And you think they overlooked the fact that the comet is craggly?
Jesus-Dunning-Kruger-Christ.
http://www.esa.int/esatv/Video...
And Philae bounced twice, finally settling in two hours after first touching the comet, which is enough time for the comet to rotate almost 60 degrees. The two systems meant to prevent bouncing - the thruster and the harpoons - failed, so it ended up some kilometer away from the carefully chosen site. That we are getting any science at all after that potentially mission-killing news is just fantastic.
I'm hoping they make some last-ditch effort to have Philae try to jump over to another part of the comet to get more sunlight, though I'm not sure what kind of resources they have to try it. Can they command the drill and/or the legs to jab downward relatively quickly? Command the harpoons to fire? I don't know, but you can bet this will be part of the design on future missions. I actually did some work on this, which made hopping around a key part of the mission.
This blog pic shows just how far that bounce was... http://blog.wolfspelz.de/2014/...
It seems to me the design and/or planning of this mission were poorly thought out, it's obvious the comet has a rough surface, they knew there would be shadows.
The planning for this mission was started 30 years ago - in 1984; 5 years before the Berlin Wall came down, 7 years before the Soviet Union was dissolved, 3 years after the first IBM PC was released. This mission has been compared to "throwing a hammer from London and hitting a nail in New Delhi".
Imagine that - the IBM PC with its 16 KB of RAM was advanced, for gods' sakes! It may be that it is easy to sit in front of your top-of-the-bloody-range games PC and imagine that 'it can't be that hard', but the fact is that it would be very hard even today, and the fact that we actually have anything man-made touching the surface of an actual comet at this moment is mindblowing. It was only really designed to run a few measurements on its main battery, and even that was considered beyond what we could reasonably expect; the secondary battery and solarpanels were more of a "you never know, we might get lucky". This mission has already been a huge success.
According to Wikipedia, Philae's power system weighs 12.2 kg and generates 32 Watts @ 3 AU (approximately halfway between perihelion and aphelion).
A SNAP-19 (1970s-era RTG) weighs 13-15 kg and generates a constant 40+ Watts electrical.
The comet's perihelion is 1.2 AU, aphelion 5.7 AU. Generally, Mars (~1.5 AU) is about the point where solar ceases to be cost-effective. Orbiters sent to Mars are solar powered. But landers (which have to deal with longer nights) have used RTGs when possible (Viking landers, Curiosity rover), with solar powered landers having a life expectancy of weeks to years.
Given they were landing on a tumbling comet ((the comet has a 12.4h rotational period so the lander would experience a relatively lengthy "night"), and the perihelion being somewhere between Earth and Mars, this was probably a good candidate for a RTG. I suspect they weren't expecting the lander to survive past perihelion however (13 Aug 2015), which could have tipped it in favor of solar.
The science payload is ~25 kg. The lander itself (including frame, power systems, communications array, electronics, stabilizers, descent systems, etc) is about 100 kg.