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Explain xkcd: It's 'cause you're dumb.
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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 bomb instead of an impactor) was depicted in the 1998 film Armageddon.

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 kph; 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, 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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