1740: Rosetta

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I WONDERED why he kept asking whether we thought the impact speed was too low.
Title text: I WONDERED why he kept asking whether we thought the impact speed was too low.


On the day this comic was posted (September 30th 2016), the Rosetta mission ended with the final descent of Rosetta onto the comet 67P. Landing Rosetta on the comet gave the scientists a chance to collect extra data from very close to the comet, using the spacecraft's powerful sensors. Cueball however assumed that the landing was a "kinetic impact" mission to deflect a comet that was a collision course with Earth. A similar scenario (using a nuclear weapon implanted inside of the asteroid) was depicted in the 1998 film Armageddon, of which Cueball is apparently a fan.

In reality, at the time Rosetta landed, 67P was already leaving the inner solar system and was a long way past Earth. It will return to the inner solar system in around 5 years time, but its orbit is not expected to pose any threat to Earth any time soon. Plus, as the alt text hints, Rosetta's speed was only 90 cm per second relative to the surface at the moment of impact (or about 2 mph/3.25 km/h; the speed of a slow walk), while the comet was travelling at 14.39 km/s. Given that Rosetta only weighs a couple of tons (or six horses), and 67P weighs nearly 10 billion tons (or 22,200,624,370 horses), Rosetta's landing will have no actual measurable effect on the comet's momentum.

Rosetta (and its lander, Philae) were previously the subject of the comics 1402: Harpoons and 1446: Landing, and were mentioned in 1461: Payloads, 1547: Solar System Questions and possibly 1621: Fixion.


[A control room: Megan and Hairy are at computers, Ponytail is directing, and Cueball is watching]
Megan: Signal lost.
Megan: Rosetta has impacted the comet.
Ponytail: Good work everyone.
Hairy: Woooo!
[Zoom on Ponytail and Cueball]
Cueball: So.
Cueball: You think we deflected it?
[Ponytail, Cueball and Hairy are now visible. Hairy looks away from his computer in surprise]
Ponytail: Huh?
Cueball: Did we hit the comet hard enough to deflect it away from Earth?
[Back to Ponytail and Cueball]
Ponytail: That... Is that what you thought we were doing?
Cueball: I just assumed...
[Megan enters. Cueball has his arms up in celebration]
Megan: He's a huge Armageddon fan. Let him have this.
Ponytail: Okay, fine.
Ponytail: Yes! We did it! The Earth is saved!
Cueball: Wooo!

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Hairy's Woooo has four o's but Cueball's Wooo has only three o's. Rtanenbaum (talk) 14:41, 30 September 2016 (UTC)
Armegeddon actually had no kinetic impact. The plot device was a bomb drilled into the surface and detonated. This did deflect the comet by splitting it on half along a fault, causing the two pieces to miss Earth. 17:04, 30 September 2016 (UTC)
Agreed, and edited. Beret Guy (talk) 20:54, 30 September 2016 (UTC)Beret Guy

Hasn't the fact of simply being in orbit already caused a change in the trajectory? Does impact make any difference? (talk) (please sign your comments with ~~~~)

Technically, yes, but the effect is so small it may as well not exist. 01:56, 1 October 2016 (UTC)
Yes, the gas(?) expelled from Rosetta in order to accelerate it toward the comet leaves the comet/Rosetta system NotLock (talk) 02:33, 1 October 2016 (UTC)

Question: Why, if the purpose was to collect more data from the comet, was the signal lost, and why are the characters treating this as a good thing? Wouldn't we want a signal from Rosetta in order to transmit the data she gathers? NotLock (talk) 04:01, 1 October 2016 (UTC)

Rosetta has been in orbit around the comet since 2014, gathering data. It sent its lander Philae down in November 2014. This was always the intended ending for the orbiter itself. Miamiclay (talk) 04:38, 1 October 2016 (UTC)
Thanks for the response! So the "extra data from Rosetta's sensors" were transmitted as Rosetta was descending, but before she landed. After she landed, then communications were shut off. Is that correct? NotLock (talk) 05:28, 1 October 2016 (UTC)
I'm not sure whether comms were 'shut off,' or if they just knew that they'd get no more from the surface because of issues with solar power or line of sight once it was on the surface, but this was the plan IIRC. Miamiclay (talk) 23:36, 1 October 2016 (UTC)
I think the characters are celebrating because Rosetta impacting the comet (which coincides with the signal being lost) means the mission has been completed. 11:51, 5 October 2016 (UTC)

Just watched an ESA clip, someone did actually go woooo. https://youtu.be/GNoJz50YNJI?t=1m28s 10:25, 1 October 2016 (UTC)

You sure that was four 'o's? NotLock (talk) 07:11, 3 October 2016 (UTC)

The effect of impact may be small, but it will be multiplied with time. It might easily be measurable next orbit, and possibly be enough to "save Earth" after 10 orbits. If the 67P would actually be on trajectory colliding with Earth in 10 orbits, obviously. -- Hkmaly (talk) 00:30, 2 October 2016 (UTC)

Let's see. The impact was 2E-2m/sec and the masses are 3E6g and 1E13g, so delta V is 6E-9m/sec. Ten orbits is 2E9sec, so the change in position would be 12m. However, it might be in an orbit stabilized by Jupiter, or an in unstable orbit, in which case the change could be substantially different; impacts near aphelion or perihelion will also have nonproportional affects. I would guess that it is on the order of the uncertainty of the position and orbit.Matchups (talk) 11:31, 3 October 2016 (UTC)