# Is it possible that the Rosetta orbiter moved the comet when it crashed?

Rosetta Comet Orbiter (RCO) crashed into the surface of a comet after the comet passed near Jupiter, which would be out-of-range for its antenna to communicate with Earth. So, the ESA made the difficult decision to just let go and crash the darn thing. (Talk about going out with a bang! geez!) Anyway, I saw a mission called DART wreck into an asteroid on purpose in order to move it. It got me thinking, did Rosetta do the same thing? The DART main spacecraft was about the size of a refrigerator, with 8-meter-wide solar arrays. Rosetta was an aluminum box with two solar panels that extended out like wings. The box, which weighed about 6,600 lbs. (3,000 kilograms), measured about 9 by 6.8 by 6.5 feet (2.8 by 2.1 by 2 meters). It has a wingspan on 105 feet. Given Rosetta is MUCH heavier than DART, was it possible to move the comet with Rosetta? There is a slight factor that could mean all the difference, Rosetta's target was bigger than DART's.

I state my question one last time, is it possible that the Rosetta Orbiter might have moved the comet it crashed into?

• By the laws of physics, it definitely moved the comet when it crashed into it. Dec 13, 2022 at 17:57
• @RoryAlsop ok. I was just wondering, judging from the fact that the comet was already in motion when Rosetta came down on it. Dec 13, 2022 at 17:59
• Everything is moving, and every impact by anything makes a difference. The only thing that is important is the relative masses. A speck of dust will not impart much momentum to a comet or asteroid. Something massive will. Dec 13, 2022 at 18:02
• @RoryAlsop and Rosetta was like 6,600 pounds. Dec 13, 2022 at 18:03
• Relative speed is also important. DART's relative velocity on impact with Didymoon was at several kilometers per second. Rosetta's impact velocity on the much larger 67P/Churyumov–Gerasimenko was under a meter per second. Dec 13, 2022 at 18:04

Yes, it did. But not by much.

The comet has a mass of about $$10^{13}$$ kg. Rosetta had a mass (after fuel had been used up) of about 1300kg. The "impact" was at 0.9 m/s. This means that the spacecraft had a momentum of about 1200 kg m/s

After the impact, and in the frame of the comet before the impact, the combined body would have the same momentum: 1200 kg m/s. But with a large mass the velocity would be small: $$1200/10^{13}$$. That is (having converted units) about 0.01 mm per day (or about one foot per decade).

Now The comet would have had a velocity, relative to the sun of about 7 km per second. A change of 0.01 mm per day would be negligible.

• Nasa Horizons can generate the vector table for comet 67P at the time of impact. If I'm reading it correctly, the velocity was 14.4 km/s. The difference between 2013 and 2016 orbit estimates is 3 mm/s, so in comparison the 1 mm/day would be below even the detection accuracy of our instruments. Though Wikipedia gives 67P mass as 10^13 kg, which would yield even lower 0.01 mm/day.
– jpa
Dec 14, 2022 at 7:00
• My first thought was that Rosetta had already been in orbit, and entering orbit would affect the comet's velocity more than the impact. But looking at the figures it seems the escape velocity of the comet would be about 0.56 m/s and the velocity required to orbit is about 0.4 m/s. They must have really wanted to nail the comet by accelerating to 0.9 m/s instead of just slowing down to less than 0.4 m/s. The propellant that escaped the system during the 208-second thruster burn means there would be a change in momentum for the system. Dec 14, 2022 at 7:15
• @jpa the only problem is that when ESA decided to end the Rosetta mission, the craft was low on fuel and power, during the "death-dive" science was not their priority. But they did keep a very close eye on the speed of impact and the spacecraft's fuel and charge levels. Rosetta survived the initial impact, but the rockiness of the ground around it is what killed it. Dec 14, 2022 at 8:05
• @jpa, I think I might have got the mass out by a couple orders of magnitude. That would make the impact be 0.01 mm per day... or one foot per decade? Dec 14, 2022 at 18:41
• 0.9 m/s sounds more like a "landing" speed than a "crashing" speed. Dec 15, 2022 at 14:19

It had already moved the comet to whatever extent it was able, when the probe had gone into orbit around the comet. The future trajectory would be determined by the centre of mass of the comet and everything gravitationally bound to it (i.e. the probe). Becoming part of the surface of the comet isn't going to cause any further change.

• See Jason Goemaat's comment on the other answer - the system velocity changed, due to active thrust terminating the orbit. Dec 14, 2022 at 14:42
• @nigel222 and the Philae lander was (and still is) on the surface of the comet. Think of it as Rosetta going down to be with its little buddy again. Dec 14, 2022 at 15:58