
Rosetta: Earth waits for comet-chaser signal - yitchelle
http://www.bbc.co.uk/news/science-environment-25782249
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TallGuyShort
For the many of us in the Pacific Timezone, the signal is expected some time
between 9:30 AM and 10:30 AM today. Just over an hour from the time of this
comment.

edit: Live updates: [https://twitter.com/esa](https://twitter.com/esa)

edit: They've received the signal!

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welterde
Finally some audio on
[http://www.esa.int/spaceinvideos/esalive](http://www.esa.int/spaceinvideos/esalive)
:)

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Toenex
From personal experience I really hope this thing isn't Wake-on-Lan ;)

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eponeponepon
I'm fairly hopeful for this. When Curiosity landed on Mars, I was absolutely
_convinced_ the whole sky-crane thing was going to fail miserably, but this
really seems like a walk in the park by comparison.

...fingers crossed!

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swamp40
And... it just called in. A bit late - everyone was getting nervous.

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kitd
I'd be interested in what is involved in landing on something with effectively
no gravity. How does it differ from the usual planetary lander?

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yitchelle
pure speculating here...

Perhaps the lander could land on the front of it. Ie, it is moving just a
little bit slower than the comet, and uses the differential speed as a
"artificial" gravity for the landing.

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gus_massa
You don’t need to land in the front.

For the sake of simplicity let’s assume that your “ship” is very near (a few
miles?) the comet (or asteroid). Fist, you must use the engines to travel at
the same velocity than the comet, in the same direction. And let’s assume that
nearby there is an automatic camera in another spaceship that also travel with
the same velocity (because you must upload the video to YouTube to gain
founds).

The Sun’s gravity affect the three object equally, so you can “cancel” it,
i.e. from the camera point of view, there is no gravity and the object just
float around. (The technical term is no-inertial reference frame.) It’s like
the videos from the ISS, there is gravity, but the ISS is falling as fast as
the astronaut, so it looks like there is no gravity. (There are a little tidal
force, because all the object are at different distance from the Sun, so you
must add a little corrections.)

It’s not necessary that your ship goes at the same speed. It moves like there
is no gravity if you see that from the asteroid point of view, or from the
magical camera point of view. This approximation is useful when the ship is
near the comet.

So you can land as if there is no gravity. (With your ship in front of the
comet and going slightly slowly in Sun’s reference frame, or with your ship in
back of the comet and going slightly faster in Sun’s reference frame, or on
one side ...)

I still have more questions:

The comet is probably rotating? How fast? Is that a problem?

The comet produces jets that create the tail. These jets can modify the comet
speed. Is that a problem? Are they dangerous?

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netcraft
This is quite exciting. The fact that we are able to hit a moving target that
far away from 10 years ago is incredible. Congrats to the ESA and good luck.

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nmc
... then you will be amazed that Voyager I is still sending us data!

[http://voyager.jpl.nasa.gov](http://voyager.jpl.nasa.gov)

