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Is a gravity scanner possible?

airelibre

By airelibre
in Science & Spaceflight

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rtxoff

rtxoff

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Do you mean a satellite that tracks the gravitational field of Earth below it? We've had several of those

That only works because this field is uneven and they measure the difference in the field strengh. If you are in perfectly round orbit around a perfectly round body consisting only of one type of matter it would not be possible to measure gravity directly. However you could calculate it.

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sal_vager

sal_vager

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Like that joke about the physicist who came up with a way to double milk production from a cow, but it only worked for spherical cows in a vacuum. :)

Reminds me of something I found earlier today when dealing with that "zero is dangerous" thread.

Math+not+even+once_79dfa0_3820803.jpg
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airelibre

airelibre

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It is possible to measure gravity from orbit :)

GRACE, Gravity Recovery And Climate Experiment.

Ah, but that works by measuring the distance to within micrometers to two other satellites using lasers. Not by detecting gravioli :D

- - - Updated - - -

I was being sarcastic

Ok Im not sure why, but whatever.

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pincushionman

pincushionman

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Ah, but that works by measuring the distance to within micrometers to two other satellites using lasers. Not by detecting gravioli…
What exactly are you expecting to measure? It's not like there's some kind of particle we can (currently) detect that can tell us about a body's (distribution of) mass. As far as I know, the only thing we can do is measure the accelerations it causes on (usually) macroscopic objects.
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airelibre

airelibre

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What exactly are you expecting to measure? It's not like there's some kind of particle we can (currently) detect that can tell us about a body's (distribution of) mass. As far as I know, the only thing we can do is measure the accelerations it causes on (usually) macroscopic objects.

hmm, how would that work.

And btw, I'm not expecting anything. If we can't currently figure it out without working out the orbital calculations then we just have to wait until we discover gravioli!

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Neil1993

Neil1993

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There was also that mission to scan the Moon's gravitational field. Two satellites were placed far apart on the same orbit and one satellite shot a laser beam which reflected off the other satellite and returned to the first satellite. The deflections in the laser beam were measured.

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K^2

K^2

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That only works because this field is uneven and they measure the difference in the field strengh. If you are in perfectly round orbit around a perfectly round body consisting only of one type of matter it would not be possible to measure gravity directly. However you could calculate it.

That's not precisely right, either. The Equivalence Principle states that you cannot distinguish between gravity and acceleration locally. Even from a spherical body, gravitational field is uneven. You can measure gradients across all 3 directions.

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YNM

YNM

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Even from a spherical body, gravitational field is uneven.

Not sure what you mean? How is the field uneven? Explaination?

Had the field be even at all site, there'd be no acceleration to any preferred direction... Look up field theory.

It's pretty simple : take the Rosetta spacecraft as an example. Nobody knows precisely how heavy (uh, massy ?) 67P/C-G was back then. But accelerometers do exist (or even, the mission mentions about precise tracking), and you can tell, as the spacecraft moves around the comet's influence, which direction it's headed to and what's the magnitude of that acceleration. Distance to comet can be deduced by radar (or, worst case, angular separation), then you get the mass.

Edited by YNM
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K^2

K^2

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Not sure what you mean? How is the field uneven? Explaination?

Take perfectly spherical planet. Field is stronger as you get closer. This causes a tidal "stretching" force in the radial direction. What's interesting, it also "squishes" things in the two other directions. These sorts of things can actually be measured.

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rtxoff

rtxoff

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Take perfectly spherical planet. Field is stronger as you get closer. This causes a tidal "stretching" force in the radial direction. What's interesting, it also "squishes" things in the two other directions. These sorts of things can actually be measured.

So where exactly i was not precisely right?

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