Hacker News Reader

Comment by masklinn | Hacker News Reader

It did land, here's the next tweet:

> It looks like @Philae2014 made a fairly gentle touch down on #67P based on amount of landing gear damping #CometLanding

but the harpoons may have failed to fire.

View on HN

land =/= secure.

those harpoons are what is supposed to anchor Philae to the comet. the problem is the reaction thruster, the thruster that counteracts the force generated by the harpoon shot, (to satisfy Newton's 3rd law) is faulty.

So they're hoping that when they reshoot the harpoon, the foot screws will be anchored enough to hold it in place. If not, theyre hoping that Philae won't launch too far off the surface so that they can just "reel" Philae back in.

> land =/= secure.

And I replied to a comment which said "land".

Ah my bad. Nonetheless it seems like harpoons did not fire. Tweet from just 30 secs ago: https://twitter.com/ESA_Rosetta/status/532579871202238464

Can it stay on the surface without the harpoons?

> Can it stay on the surface without the harpoons?

technically yes. Philae has harpoons along with foot screws to secure it to the comet, but if harpoons are super glue, foot screws are tape.

Why does it need to anchor itself? It's not like it will blow away?

Newton’s Second (edit: Third) Law. It wants to extend a drill bit into the surface.

Third?

…oops (apropos username)

It could actually, since there's already outgassing on the comet.

Gravity there is so low that something like a light push may be enough to launch. If anything used to examine the surface makes contact and presses (drill, digger), good-bye lander.

Firing the harpoon would also be enough to launch, but at least the hook would grab the surface and pull the lander back down, giving enough pull to drill viable long-term anchors.

Are there any forces in effect as it gets closer and swings around the sun that might cause it to separate from the comet if there is no further attachment by then?

Absolutely! If its attached, its via surface ice. That ice will all melt on close approach to the sun. At some point the probe will drift away.

Of course, I should have deduced that. Is the harpoon expected to go deep enough to prevent this from happening?

It will dig into the surface anyway so it will anchor itself even without the harpoon, if it's on the surface of course.

See this video that explains how Philae foots work: https://www.youtube.com/watch?v=-77-Z_DHTlY#t=84

ESA says it landed gently on the surface: https://twitter.com/esaoperations/status/532575615527170048

Reply on news.ycombinator.com