# Why are meteors, asteroids and comets not used as propulsion for spacecrafts? [duplicate]

I know it would take a lengthy amount of time to reach the asteroids and meteors.

After hearing of the Europeans landing a probe on a comet, the first thing I could think of was: "Could this be used to increase the speed of a spacecraft?"

Think about it; a comet moves between 10-70 km/s.

If you can attach a spacecraft (safely)/probe to a 70 km/s object and increase the speed of your ship to 70 km/s then detach yourself from the comet, would that not steamtrain the time it would take for us to explore the universe?

• @Hobbes Its not a duplicate, its literally the same post....just look at the URL – Bernard Walters Jan 13 '17 at 15:55
• pasted the wrong thread, should have been this: space.stackexchange.com/questions/4606/… – Hobbes Jan 13 '17 at 16:33
• Yea i see, they are similar but different questions...but for the sake of peace i'd drop it – Bernard Walters Jan 13 '17 at 18:53

The problem is, to even reach the comet, you have to achieve that speed already by other means - but once you have reached that speed, the relative motion of the comet is 0 m/s.

There is actually something like what you might consider, only a little different. If you fly towards a large mass (like a planet) you can use some of the planet's momentum to accelerate your spacecraft if you plot your path carefully.

That is already being done for a long time and is called gravity assist - in fact the Rosetta mission you describe did it multiple times. The problem is, that you need a huge mass to assist you (a comet would likely be too little) and it needs to travel in the right distance at the right speed at the right time.

Oh and btw. Earth itself moves around the sun at 30 km/s. The speed noted in comets is either orbital speed (wich is similar to Earth's speed itself) or speed towards or from Earth. So the comets usually aren't that much faster or slower than planets anyways.

As for interstellar travel, 70 km/s might sound very fast, but compared to light speed, it is only a small fraction of 1/4285 c. So the nearest star system (Proxima Centauri) is still 18'425 years away at 70 km/s - and only that if those comet is rushing directly towards Proxima.

• I assume the OP has something like a tether or harpoon in mind (material or otherwise). And I bet there are some massively elliptically orbiting comets which are much faster than earth when crossing her orbit. – Peter - Reinstate Monica Jan 13 '17 at 17:30
• @PeterA.Schneider only comets that aren't bound to a solar orbit can be faster. The escape velocity for the sun at earth orbit is about 42 km/s. Since earth travels at about 30 km/s around the sun, the maximum approach speed would be ~ 70 km/s (earth's orbital velocity + the comet's orbital velocity) if the comet approaches exactly head on towards earth for all orbiting comets. – Adwaenyth Jan 17 '17 at 14:43

Rosetta achieved, by its own propulsion and gravity assists, the same speed and trajectory of the comet it entered orbit around. And it took it almost 10 years to do so. And it was the easiest to reach comet available at the time. One could send a probe to meet a comet at 70 km/s speed relative to it, but that would then be called an "impactor" for obvious reasons. The probe would be vaporized by the very violent collision. Some of the gas would indeed follow the comet back out to the Oort cloud, but no working structure could survive it.

But then there is the NIAC (NASA Innovative Advanced Concepts) that finances very small investigations into very futuristic space missions, almost none of which will actually ever be possible. So there's this guy who convinced them to pay him for a year or so to look into hitchhiking with a comet by using some kind of tether system, like a spider's web. Gaining up to 10 km/s speed, which would be alot. But it is still just a question being asked.

• Oh, here is the tether ;-). – Peter - Reinstate Monica Jan 13 '17 at 17:34
• I haven't read the links; but it's clear that using a tether for a meaningful acceleration (up to, say, dozens of km/s) is a far stretch. Even if, for a given delta-v of n km/s, you have n km of tether: That only gives you, iinm, 2 seconds to achieve that delta-v. For, say, 10 km/s, and a 10 km tether (not completely impossible, probably), you have an average acceleration of a = v/t = 10000m/s/(2s), or 5000 m/s/s, or about 500 times earth acceleration. A fairly light 10 kg probe would load the tether with F = m*a = 10*5000 = 50000N, or 5000 kg or 5 tons earth equivalent. – Peter - Reinstate Monica Jan 14 '17 at 13:12
• @PeterA.Schneider I have the same sentiment. Just the anchoring to a low mass low density object is a challenge, even without the stresses of huge velocity exchange. But because it is a pretty common public question, ever since baron von Münchhausen jumped onto a flying cannon ball, it is maybe the purpose of NIAC to once and for all give it a rest. Ina year or so there will be a paper I can link to. And it is healthy brain gymnastics for inventive rockets scientists anyway. – LocalFluff Jan 14 '17 at 13:21