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View Full Version : How to land on an asteroid or comet?



jumpjack
2010-Jul-12, 09:44 AM
Recent asteroid flyby and upcoming (2014) comet landing made me think about it: how can landing on an asteroid be accomplished? It has not enough gravity to orbit around it without consuming fuel ("geostationary" orbit), but it rotates, so how can spacecraft speed match asteroid rotation speed, without using huge amounts of fuel?
Maybe you need to land on "poles", where "terrain" is just rotating under you rather than "sliding"? :eh: I think more than about "landing" we could talk about "rendez-vous and docking"! :eh:

Landing on an "active" comet is even harder: how is Rosetta supposed to survive to comet ejecta?!? Giotto was heavily damaged by Halley, and it didn't even get close to the comet!

Grashtel
2010-Jul-12, 10:25 AM
Recent asteroid flyby and upcoming (2014) comet landing made me think about it: how can landing on an asteroid can be accomplished? It has not enough gravity to orbit around it without consuming fuel ("geostationary" orbit), but it rotates, so how can spacecraft speed match asteroid rotation speed, without using huge amounts of fuel?
There is no lower limit on the size of an object that can be orbited around, many asteroids have moons and the NEAR Shoemaker (http://en.wikipedia.org/wiki/NEAR_Shoemaker) mission not only orbited one but even landed on it as well (despite not actually being designed to do so).

Maybe you need to land on "poles", where "terrain" is just rotating under you rather than "sliding"? :eh: I think more than about "landing" we could talk about "rendez-vous and docking"! :eh:
Its quite possible to land at almost any point, all that is needed is to match the rotational speed and let gravity pull you down.

Landing on an "active" comet is even harder: how is Rosetta supposed to survive to comet ejecta?!? Giotto was heavily damaged by Halley, and it didn't even get close to the comet!
Rosetta is headed for a much less active comet than Halley, 67P/Churyumov-Gerasimenko is a short period comet that has been being baked by the Sun for a long time resulting in it having lost much more of its volatiles than Halley and therefore being much less active making approaching and langing on it much easier and safer.

jumpjack
2010-Jul-12, 10:46 AM
Its quite possible to land at almost any point, all that is needed is to match the rotational speed and let gravity pull you down.
That's the point,as I said. How can you keep overing same point while it's rotating around center of mass of the asteroid? Landing on a planet, or even on a moon with no atmospehere, is just a matter of match linear speed of lander with approximately-linear speed of moon surface. Wouldn't you need continuous correction maneuvers, instead, to keep orbiting/hovering over same place AND to maintain legs towards surface? A "moon-lander-like" game/simulator would be cool to understand the mechanism... ;-)

Jason Thompson
2010-Jul-12, 11:50 AM
How can you keep overing same point while it's rotating around center of mass of the asteroid?

Why do you need to? There is no restriction for landing on anything that requires a perfectly vertical descent. You only have to match your speed to the ground in the very last moments, so that gives you a long time to alter it.


Wouldn't you need continuous correction maneuvers, instead, to keep orbiting/hovering over same place AND to maintain legs towards surface?

How fast is the object rotating? So far no asteroid has been seen to have a rotation period of less than 2.2 hours. Since most asteroids that are landing candidates are a few miles across the rotation rate means the ground is only moving at a few miles per hour. Since you can get into orbit of these asteroids there is no more problem landing on them than on any other body: it's still just a question of adjusting your speed to allow gravity to pull you down and making sure you can stop yourself smacking into it too quickly.

I'll repeat the salient points: many asteroids have smaller 'moons' orbiting them (look up Ida and Dactyl for the most famous example), and a landing on an asteroid has already been achieved with a spacecraft that wasn't even designed to do it.

jumpjack
2010-Jul-12, 12:34 PM
[double post deleted]

jumpjack
2010-Jul-12, 12:39 PM
How can you keep overing same point while it's rotating around center of mass of the asteroid?

Why do you need to? There is no restriction for landing on anything that requires a perfectly vertical descent. You only have to match your speed to the ground in the very last moments, so that gives you a long time to alter it.
My quote and your sentence are same phenomenon described in different fashions.



it's still just a question of adjusting your speed to allow gravity to pull you down and making sure you can stop yourself smacking into it too quickly.

I didn't know a 4-km asteroid had enough gravity to pull down a spacecraft (but I didn't perform any calculation actually).


I'll repeat the salient points: many asteroids have smaller 'moons' orbiting them (look up Ida and Dactyl for the most famous example),

It's easy for them they have no restrictions on speed! (as they're not going to land)


and a landing on an asteroid has already been achieved with a spacecraft that wasn't even designed to do it.
Easy for it, it didn't have to care about its orientation. Actually I can't even figure out how Hayabusa was able to even properly orienting itself to get its collecting instruments working properly.

AndreasJ
2010-Jul-12, 01:59 PM
I didn't know a 4-km asteroid had enough gravity to pull down a spacecraft (but I didn't perform any calculation actually).
It doesn't need to pull down the spacecraft - the later is canf thrust itself downward if need be.

Perhaps it would help you to think of a spacecraft docking with a space station. Landing on an asteroid small enough to have neglible gravity would be rather similar, only with lesser requirements for precision (as shouldn't matter if you're a meter off your intended spot).

Jason Thompson
2010-Jul-12, 02:11 PM
I didn't know a 4-km asteroid had enough gravity to pull down a spacecraft (but I didn't perform any calculation actually).

Any massive object can pull down a spacecraft provided you can slow it down enough to be captured.


It's easy for them they have no restrictions on speed! (as they're not going to land)

Of course they do. Too fast and they won't stay in orbit. Too slow and they'll collide. The ability to orbit anything depends absolutely on speed, whether it's a spacecraft or a rock.


Easy for it, it didn't have to care about its orientation.

Yes it did. It sent back pictures of the event, so it had to be oriented such that the camera was pointed at the surface the whole time.


Actually I can't even figure out how Hayabusa was able to even properly orienting itself to get its collecting instruments working properly.

So that means it can't be done?

jumpjack
2010-Jul-12, 02:13 PM
Perhaps it would help you to think of a spacecraft docking with a space station.
Yes, I thought about it. The only difference is that you don't have visual confirmation or realtime telemetry of what you are doing, and that asteroid is not flat and/or regular.

AndreasJ
2010-Jul-12, 02:23 PM
The only difference is that you don't have visual confirmation or realtime telemetry of what you are doing
Er? Asteroids are not invisible.

jumpjack
2010-Jul-12, 02:27 PM
Er? Asteroids are not invisible.

yes, but they're too far: Lutetia was 25 light-minutes away, visual confirmation of maneuvres for ground control would have been impossibile in case of landing. Only onboard SW could get advantage of visual aid.

Rhaedas
2010-Jul-12, 02:31 PM
How fast is the object rotating? So far no asteroid has been seen to have a rotation period of less than 2.2 hours. Since most asteroids that are landing candidates are a few miles across the rotation rate means the ground is only moving at a few miles per hour. Since you can get into orbit of these asteroids there is no more problem landing on them than on any other body: it's still just a question of adjusting your speed to allow gravity to pull you down and making sure you can stop yourself smacking into it too quickly.

I think this is an important thing to point out. Often times we get animations of asteroids, showing their rotation, and omitted is the time frame, so there's a sense of quick rotation. Such a rotation in actuality would have possibly torn apart or prevented the asteroid's formation in the first place. Instead, we have a very slow rotation, and while the gravity is very minor, it's enough to pull a probe towards the asteroid, and so it's just a matter of matching the surface movement. It was done with Eros.

jumpjack
2010-Jul-12, 08:27 PM
I found a way to simulate asteroid landing:
Download Phun (for free) (http://www.phunland.com/wiki/Download), and load this file into it:
http://win98.altervista.org/asteroid_lander.phz

Use cursor keys to move the spacecraft, Q, and W to rotate it. Beware of asteroid gravity and rotation, and try to land! :razz:

You can configure asteroid speed and gravity:
click on circle in the center of asteroid, then on HINGES menu item, and change speed and other parameters;
click on asteroid and then on MATERIAL menu, than change ATTRACTION parameter to change "gravity".

Click on spacecraft (rectangle) and then click on GEOMETRY CONTROLLER to determine how strong is the "pulse" you apply using cursor keys (unfortunately its direction is relative to screen, not to spacecraft).

It's just a game, but it allows understanding how difficult a task is to land on a small body! (although the game is not at scale).

13419

Rhaedas
2010-Jul-12, 09:00 PM
Again, it's all about rotation, and asteroids just don't usually rotate that fast. If you set everything up to scale in that simulation, you wouldn't be able to see any movement at all, so the only thing you'd have to do is decrease your relative speed to enable a safe landing, which given the low gravity isn't all that hard. NEAR Shoemaker landed on Eros at less than 4 mph, and continued to function afterward. What you're talking about is applicable to smaller scale things like stations, satellites, and small meteor debris, but not the stuff we'd actually land on.

AndreasJ
2010-Jul-13, 09:57 AM
yes, but they're too far: Lutetia was 25 light-minutes away, visual confirmation of maneuvres for ground control would have been impossibile in case of landing. Only onboard SW could get advantage of visual aid.

But there's no reason to run the landing by remote control - an onboard computer will handle it easily enough.

neilzero
2010-Jul-13, 11:42 AM
In these days of large scale integration, the probe managers have access to many programs, without adding much weight to the probe. Typically landing on a large asteroid is about as challenging as stepping onto an escalator or moving sidewalk. Medium and small asteroids are like stepping onto a very slow escalator or moving sidewalk = you may not even notice the movement. Very little docking energy is needed even if errors make the approach less than optimum.
If the net gravity is one millionth of the gravity at Earth's surface, the probe falls toward the asteroid at V = at = 0.001 feet per second after falling for 1000 seconds = 17 minutes, so we likely need to to use thrust except for the last inch before touchdown. There is no net gravity unless the probe is moving slower than orbital speed which may be less than one mile per hour. Neil

Grashtel
2010-Jul-13, 11:48 AM
yes, but they're too far: Lutetia was 25 light-minutes away, visual confirmation of maneuvres for ground control would have been impossibile in case of landing. Only onboard SW could get advantage of visual aid.
Why do you think that onboard software wouldn't be entirely in control during the landing with the people on the ground just being heavily time delayed spectators? Attempting to control something that needs realtime responses (eg landing) over a 50 minute delay (remeber that being 25 light minutes away means that it takes twenty five minutes for information from the probe to get to Earth and twenty five minutes for commands to get back to the probe) is just plain stupid, if anything goes wrong it will be too late to do anything about it by the time the command to do so gets there.

jumpjack
2010-Jul-13, 12:36 PM
I think Ultimate Answer to my initial question is "astroid rotate very slowly".