Asteroid question

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Rikkitic

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Topic # 243275 4-Dec-2018 10:02

Who here is an astrophysicist? I am reading about the OSIRIS-REx probe sent to Bennu. Because the gravity is so low, it is difficult to keep the probe in orbit. My question is, why bother? Would it not be simpler to just let the probe bump down onto the asteroid after doing a photo survey on approach? They have to match speeds anyway and I would think it would be easier just to let gravity join it up with the asteroid rather than try to stay in orbit. When it is time to leave, it shouldn't be difficult to achieve escape velocity. Can someone explain this please?

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surfisup1000

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Reply # 2139051 4-Dec-2018 10:16

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Brian May would be the coolest astrophysicist around... hopefully he reads gz :)

I don't think it is as easy to touch down as you think. Wildly undulating surfaces, caves, gas explosions and fissures, unpredictable gravity field would confuse the concept of up and down. Cos, you have to set down at a location and orientation where you can still get power and communicate back home.

Did you see armageddon? I like to think this is how it might be.

Rikkitic

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Reply # 2139057 4-Dec-2018 10:24

You are thinking of a comet. Asteroids are much quieter.

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MikeB4

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Reply # 2139063 4-Dec-2018 10:31

It makes soft contact with the surface and will return to with a sample circa 2023

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tdgeek

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Reply # 2139065 4-Dec-2018 10:35

surfisup1000:

Brian May would be the coolest astrophysicist around... hopefully he reads gz :)

I don't think it is as easy to touch down as you think. Wildly undulating surfaces, caves, gas explosions and fissures, unpredictable gravity field would confuse the concept of up and down. Cos, you have to set down at a location and orientation where you can still get power and communicate back home.

Did you see armageddon? I like to think this is how it might be.

Brian Cox you mean?

tdgeek

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Reply # 2139096 4-Dec-2018 10:40

Ah the Queen guy, and he got a PhD is Astrophysics as well! And one named after him, so yes, that pretty cool

KrazyKid

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Reply # 2139102 4-Dec-2018 10:43

Brian May from Queen (the Band) has to be much cooler than any other Brian :)

Rikkitic

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Reply # 2139152 4-Dec-2018 11:08

Way to stay on topic guys.

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wellygary

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Reply # 2139156 4-Dec-2018 11:17

Rikkitic:

Who here is an astrophysicist? I am reading about the OSIRIS-REx probe sent to Bennu. Because the gravity is so low, it is difficult to keep the probe in orbit. My question is, why bother? Would it not be simpler to just let the probe bump down onto the asteroid after doing a photo survey on approach? They have to match speeds anyway and I would think it would be easier just to let gravity join it up with the asteroid rather than try to stay in orbit. When it is time to leave, it shouldn't be difficult to achieve escape velocity. Can someone explain this please?

The fact they are going to spend 18 months orbiting the rock to determine the landing site, makes me think that a photo survey on approach would not get them anywhere near the level of detail that the survey team require to determine their sample site...

DarthKermit

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Reply # 2139165 4-Dec-2018 11:33

They need to watch The Empire Strikes Back. Han Solo managed to land on an asteroid no problemo. Just watch out for space ship eating slugs.

Fred99

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Reply # 2139213 4-Dec-2018 12:30

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Gravity is so low - ten millionths of the surface gravity of earth.

AFAIK, it's not "landing" in a conventional sense, but reducing orbit until it's close enough to move in and for a robotic arm to grab a sample, then it "bounces off". There's enough propellant for 3 attempts.

If the mass of the spacecraft is 1500kg (a guess with fuel), then the "weight" of the spacecraft on the surface of the asteroid is only about 15 grams, so if the contact with the arm on the surface was to hit an object and be deflected/lose control, it might possibly spin, bounce off the asteroid and not be able to complete the mission.

TwoSeven

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Reply # 2139215 4-Dec-2018 12:36

At a guess I would start with a 3 body differential equation.

My thinking is, one would need to calculate the orbit of the asteroid in reference to a known observation point using something like the gausian method.

One could then calculate the required orbit of the satelite and factor in the need to alow it down and adjust its approach angle I suspect to get it to fall into orbit.

I think to get it to land on the asteroid, one would need to something like the above, then add a rocket motor and a bunch of other equipment to get it to land (crash) in a controlled manner.

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Rikkitic

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Reply # 2139216 4-Dec-2018 12:40

That sounds fairly convincing. I was thinking that if they can control the probe precisely enough to keep it in orbit around such a low mass object (apparently that needs constant corrections), why not just match trajectories (surely no more difficult) until it just gently settles down. Gravity would be sufficient to draw it to the surface and keep it there if no other forces were acting on it. Once landed it wouldn't have to keep being adjusted since gravity would hold it in place. Still easy to take off again later. It doesn't land as such, it just eases in until contact is made. If that is done sufficiently gently, it won't bounce. But what you say also makes sense.

Edit: reply was to Fredd99

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