NEAR is now orbiting Eros

[Keeves]

See http://www.infobeat.com/stories/cgi/story.cgi?id=2564245818-c71 for more info about this NASA project which is now oribitng Eros. According to the article, Eros’ gravity is only 1/2000 that of Earth’s, and I’ve got to wonder: With such weak gravity, it must be orbiting at a very low altitude and very slow speed in order to avoid flying off into space. Does anyone have hard figures on what the orbit really does look like?

[Ursa_Major]

I just saw the pix on TV. Looks just like a baked potato.

[Rysdad]

I was wondering that, too. And, isn’t Eros tumbling? That’d make orbiting even more difficult, I’d think.

[UncleBeer]

Here’s the NEAR homepage from JPL.

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Here’s mud in yer eye,
UncleBeer

[DSYoungEsq]

With such weak gravity, it must be orbiting at a very low altitude and very slow speed in order to avoid flying off into space.

It wouldn’t have to be any specific altitude… an orbit can occur at any distance. But, yes, the velocity must be relatively low (easily calculatable).

I was wondering that, too. And, isn’t Eros tumbling? That’d make orbiting even more difficult, I’d think.

Why would the fact it tumbles affect the ability to orbit it? The attraction would still be the same I would think.

[Cooper]

an orbit can occur at any distance

As a practical matter however, the farther out you get the more other bodies will effectively make it impossible to do so.

[Tinker_Grey]

I saw one of the scientists on the McNeil-Lehrer News Hour (new name? oh well) last night.

I believe he said the satellite was moving at about 5 mph. (or 5 kph ;))

[frolix8]

Here’s a nice animated gif:
http://antwrp.gsfc.nasa.gov/apod/ap000210.html

[Vestal_Blue]

Originally posted by DSYoungEsq:
** Why would the fact it tumbles affect the ability to orbit it? The attraction would still be the same I would think. :)**

Yeah, but if it is shaped like a potato and is swapping ends, you’d have to have a large enough orbit to keep from getting smacked!

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VB

I could never eat a mouse raw…their little feet are probably real cold going down. :rolleyes:

[bantmof]

Why would the fact it tumbles affect the ability to orbit it? The attraction would still be the same I would think.

Not necessarily. It’s common for orbits around small, irregular objects to need to be “fine tuned” on a routine basis lest they become unstable. The closer you want to orbit, the worse the problem gets, and tumbling makes it a bit trickier to manage.

If the object was a homogeniously dense sphere, there is no difficulty, but if it isn’t, there can be. When close to an irregular object, you cannot treat it as a point mass for the purpose of computing orbits. You must integrate the gravitational equation (GM1M2/R^2) over the 3D mass distribution of the body, which can and often does produce a vector field that isn’t nicely oriented towards the same point in space due to the R^2 term. There’s no guarantee, if you work out the math, that an object moving in such a vector field will remain in a stable orbit, so one often has to manage this by hand. Even lunar orbiters such as Clementine have to do this - the moon is almost a sphere, but it has an uneven density distribution. IIRC, Clementine’s orbit was adjusted roughly once in each 1 to 2 weeks. (That’s just from memory - don’t quote me on that!)

As for the question raised in the OP, here are the numbers I have seen. The Feb 14th orbital insertion burn left NEAR in a 327x450 km elliptical orbit. The final mapping orbit will be 200 km spherical, and will be gradually reduced to less than 35 km over the life of the mission.

If you knew the mass of Eros, you could probably come up with a good approximation of the orbital velocity using those numbers.

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peas on earth

[Keeves]

Thank you Bantmof, for a very clear answer. But get a load of this: The original article I cited ( http://www.infobeat.com/stories/cgi/story.cgi?id=2564245818-c71 included the following:

During its mission, NEAR will descend slowly into lower and lower orbits. Late in the year, the craft may actually brush Eros’ surface with a solar panel and photograph the mark it leaves. “That’s one of several options the team is considering for the end of the mission,” said Farquhar, a scientist at the John Hopkins University Applied Physics Laboratory where NEAR was designed and built under NASA contract.

Wish I could figure out how they’re gonna do that on such an irregularly shaped thing without crashing.

[Triskadecamus]

This page gives an orbital distance of 200 miles, and lists the escape velocity as 22 mph. (Slow pitch!) The odd shape will have some miniscule effect on the orbital characteristics, since it does alter the pull of gravity some amount. The fact is that it is a small percentage of a very small force.
<P ALIGN=“CENTER”>           Tris </P> <HR>
          The ancient Chinese proverb says, “Better to light one small candle than to curse the darkness.”
I have never understood why these options are considered mutually exclusive.
<FONT FACE=“Webdings” SIZE=5 COLOR="#ff2400"> ** - ** </FONT>

[bantmof]

Wish I could figure out how they’re gonna do that on such an irregularly shaped thing without crashing.

I’m glad figuring that out is not my job. I suppose one has to have quite a lot of confidence in one’s rangefinder.
Trisk says:

The fact is that it is a small percentage of a very small force.

I think you might be surprised at how big an effect it can be for a mission like NEAR, or even for lunar orbiters. It’s possible for low orbits to be unstable over a period of mere hours or days. It definately is not something mission planners can ignore.

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peas on earth