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Luckmeister
2010-Jun-10, 02:19 AM
I watched a "popularized" TV science show on asteroids which discussed landing on one. They repeatedly stressed that one of the difficulty factors is that the asteroid is traveling as fast as a bullet (I assume they meant relative to Earth).

My question is this: Isn't it true that once you have matched speed with the object, that speed would not be a factor in executing a landing, assuming you are in open space where you're not strongly influenced by Earth's gravity?

Mike

Jeff Root
2010-Jun-10, 02:31 AM
Yes, of course.

Only the speed of a bullet??? Can you provide any more detail?

-- Jeff, in Minneapolis

Jens
2010-Jun-10, 02:55 AM
It's hard to get what they mean. Of course, what you say is true. The real problem is getting the probe to match the speed of the asteroid, but of course that's a problem with any landing, so I don't think it's any different with an asteroid.

I think the real problem with landing is that asteroids rotate. But again, this is true with other bodies as well. I suppose what's hard is that there is almost no gravity, so you really have to match the rotation as you come in for landing and don't have much time to do it, and can't rely on anything like aerobraking.

Luckmeister
2010-Jun-10, 03:45 AM
Thanks Jeff and Jens. I think a scriptwriter was just trying to make the landing sound more impressive when it was actually already impressive enough. It was the Eros landing they were referring to.

2010-Jun-10, 03:56 AM
we have already landed a probe on an asteroid once. it wasn't planned as a part of the mission, but it was a good way to end it.

Van Rijn
2010-Jun-10, 05:10 AM
There are issues with landing on an asteroid (which is more like docking): There is no aerobraking and there's no large mass (and gravity) for fancy trajectory tricks.

But, some asteroids are, in terms of energy, relatively easy to reach. See this list for a number of examples:

http://echo.jpl.nasa.gov/~lance/delta_v/delta_v.rendezvous.html

Van Rijn
2010-Jun-10, 05:26 AM
Thanks Jeff and Jens. I think a scriptwriter was just trying to make the landing sound more impressive when it was actually already impressive enough. It was the Eros landing they were referring to.

Yes, I think they were overstating things a bit. I think Eros was chosen for a visit because it is a good sized near earth asteroid, it was found quite some time ago (so astronomers have a decent amount of information on it), and from the list I linked to, takes roughly the same energy to reach as the Moon (it's number 1221 in the list).

JustAFriend
2010-Jun-10, 10:37 PM
Speed is relative.

If you have the guts, you can step from one car to another at 70mph on the freeway as long as you ignore everything else around you.

slang
2010-Jun-11, 10:56 AM
we have already landed a probe on an asteroid once. it wasn't planned as a part of the mission, but it was a good way to end it.

That was the Eros landing (http://en.wikipedia.org/wiki/NEAR_Shoemaker#Orbits_and_landing) that Luckmeister referred to as well.

jfribrg
2010-Jun-11, 04:50 PM
As mentioned above, the tiny gravity makes it hard to land. When we went to the moon, we slowed down to IIRC about 10,000 miles per hour and that put us in orbit. With Eros, the average escape velocity is 25 miles per hour, which means that the orbital velocity is in the neighborhood of 17-18 mph. I say average escape velocity because Eros is not spherical, and therefore the gravity varies greatly from place to place which makes orbiting even trickier. It has been done, but the approach and orbit insertion took several months. For a manned landing, a more direct approach would be preferable, but that would require far more fuel, a bigger rocket and therefore a far more expensive and risky mission.

Jens
2010-Jun-15, 04:14 AM
For a manned landing, a more direct approach would be preferable, but that would require far more fuel, a bigger rocket and therefore a far more expensive and risky mission.

I wonder if you could do it with a grappling hook and a fishing reel type contraption.

Van Rijn
2010-Jun-15, 04:37 AM
I wonder if you could do it with a grappling hook and a fishing reel type contraption.

That's been suggested for final landing and exploration, but a grappling hook might have problems if the surface isn't solidly bound. The surface might just be held by the minimal gravity, in which case it wouldn't do a good job of holding a hook. However, it might be useful to put ropes around the asteroid, to help people (or even robots) to get about. UT had this article a while back on this:

http://www.universetoday.com/2007/09/26/tether-to-keep-asteroid-explorers-grounded/

eburacum45
2010-Jun-15, 11:22 AM
Problems with landing on an asteroid include-

¬A lack of atmosphere for aerobraking; landing on Earth is made easier because of this useful deceleration aid.
¬Minimal Oberth Effect (http://en.wikipedia.org/wiki/Oberth_effect) making it expensive in delta-vee to decelerate into orbit round an asteroid,
¬Irregular shape, making any close orbit around an asteroid chaotic and potentially dangerous ( try plotting a close orbit around Kleopatra without hitting one or other of the protuding ends).
¬Rapid rotation- for asteroids smaller than 100m the rotation may be fast enough that you could be thrown off the surface.
¬Tumbling- many asteroids probably rotate around more than one axis, which also makes it difficult to land.

I'm not saying these problems make asteroid mining impossible- just a little more challenging...

Noclevername
2010-Jun-24, 01:47 AM
...And surface conditions, a rubble pile makes a poor landing surface.

Hungry4info
2010-Jun-24, 02:38 AM
¬Tumbling- many asteroids probably rotate around more than one axis, which also makes it difficult to land.
I wasn't aware of this. Could you name some example asteroids?

01101001
2010-Jun-24, 03:47 AM
I wasn't aware of this. Could you name some example asteroids?

Toutatis (http://en.wikipedia.org/wiki/4179_Toutatis) (named after a BAUT Forum member -- or vice versa) tumbles. NASA: TOUTATIS ONE OF THE STRANGEST OBJECTS IN THE SOLAR SYSTEM (http://impact.arc.nasa.gov/news_detail.cfm?ID=95)

"The vast majority of asteroids, and all the planets, spin about a single axis, like a [American] football thrown in a perfect spiral," Hudson said, "but Toutatis tumbles like a flubbed pass."

One consequence of this strange rotation is that Toutatis does not have a fixed north pole like the Earth. Instead, its north pole wanders along a curve on the asteroid about every 5.4 days. "The stars viewed from Toutatis wouldn't repeatedly follow circular paths, but would crisscross the sky, never following the same path twice," Hudson said.

Hungry4info
2010-Jun-24, 05:12 AM
Interesting! Thanks!

forrest noble
2010-Jun-25, 06:13 AM
Luckmeister,

........Isn't it true that once you have matched speed with the object, that speed would not be a factor in executing a landing.......

Your summation is correct. Speed would not be much of a factor. Exactly matching the speed and orbital path might be a little tricky but others have pointed out some other possible problems. As others have said, with a small asteroid its more like docking than like landing and you might need a tether to fasten the docking/ landing. Jumping around on a medium-size asteroid might be fun providing you don't use too much muscle and it would probably take practiced skill and effort to most always land on your feet after a jump. With a gentle impact you probably wouldn't care in most locations how you landed.