# Why can't cubesats get gravity assists from space junk in Low Earth Orbit?

I have a simple question: Why isn't space junk orbiting Earth used for gravity boosts to launch small cubesats into higher orbits or to other planets?

I know the current gravity boost and gravity braking math is built from numerical solutions to the 3 and 4 body problem in astrodynamics, but couldn't something similar be done in Low Earth Orbit between space junk and a small satellite (maybe a cubesat or kicksat)?

From my understanding, if the cubesat is launched into orbit opposing the space junk, if it makes a close enough pass to the space junk, the cubesat can get a massive velocity boost (almost 8 km/s) as its orbit is changed by the space junk. Am I missing something here? (maybe overestimating the gravitational influence of the space junk?)

• Gravity of those small pieces of space junk is terribly small, just the gravity gradient of slightly different orbital altitudes across the height of them would be orders of magnitude larger than gravity between a few kilograms to a couple hundred kilograms pieces of junk flying millimeters past each other. Case in point: gravity of 1 ton, 1 meter in diameter object at 1 mm flyby, and gravity gradient along same object at ISS altitude. Did you perchance have something like this in mind instead? – TildalWave Aug 17 '15 at 15:58
• I plugged in 5500 tons (the estimated mass of All The Space Junk) and a 5.5m radius (~a solid steel sphere of 5500 tons) and got out 1.2 × 10^-5 m / s^2. – Russell Borogove Sep 16 '15 at 21:03

In a recent question about New Horizons, it was shown that the planet Pluto gave NH a gravity assist worth about 21 km/h. Now, Pluto weighs $10^{22}$ kg. If your space junk weighs 10 tons ($10^4$ kg) (just about the largest piece of junk out there today), its gravitational influence would be 22-4 = $10^{-18}$ times the size of Pluto's, i.e. minute.
$10^{-18}$ km/h is a femtometer per hour, or 1/10000 times the diameter of an atom.

• New Horizons went a very large way from Pluto, so this number could be quite a bit better. If you assume, say, a 100 m difference, the gravitational effect could be 5e9 times more powerful. Still not much, but something... – PearsonArtPhoto Sep 16 '15 at 20:29
• That would have to be within 100m of the center of mass of the assistor, not 100m of the surface. For any massive object, that means inside it. – Russell Borogove Sep 16 '15 at 20:38
• But 5e9 benefits greatly 10e-18. – Joshua Nov 3 '15 at 19:08
• @PearsonArtPhoto Had NH brushed the surface of Pluto it would have been 11.5 times closer to the center of Pluto. I don't know how much stronger of a gravity assist this would have given it. – Charles Apr 24 '18 at 15:27

It could theoretically be done, but would be very difficult to line up correctly, with very high consequences for failure (Collision of the two objects), and wouldn't do much if anything to change the speed. The mass of space junk is so small that it really wouldn't have much of an effect on the orbital speed of said space junk.

Basically, for this to work it would require a much denser satellite than is typically found, one that likely would survive reentry. This satellite would then have to be perfectly lined up with the receiving satellite. As there is almost no gravity from such a small object, any force impact would be minimal.

Even if you had some massively dense object, to line the two objects up you'd have to use fuel. Even the smallest thrust to accomplish this would almost certainly be more than you'd gain from the maneuver.

Gravity assists only work with really massive objects. Planets are the usually required size to have it work. Large moons will also work, but only slightly. Space junk, well, any effect would be extremely minimal.

• It's not even theoretically possible. If you got close enough to a piece of space junk for gravitational effects to be measurable, the gravitational attraction would be massively overwhelmed by electromagnetic repulsion. – Russell Borogove Sep 16 '15 at 19:55
• If you had a massive enough piece of an object, it could be done. Electromagnetic repulsion really wouldn't apply, I would think... – PearsonArtPhoto Sep 16 '15 at 20:25
• Electromagnetic repulsion gets pretty substantial when you try and force two objects to occupy the same space. Besides that, trace atmospheric effects in LEO, solar wind, and several other small effects will still dominate the gravitational. – Russell Borogove Sep 16 '15 at 20:46
• Sure, if you run them into each other, they will have some significantly negative effects. It could theoretically be done only if you had a really massive dense object, and passed very close to the surface. How dense of an object? It would have to be the densest substance known to man, and a whole lot of it. But yeah, I agree, it's basically negligible, and wouldn't really work in any practical sense. I've added a few details to make this clearer. – PearsonArtPhoto Sep 16 '15 at 20:50
• I think the OP had in mind real pieces of space junk. That is, things that are actually up there now. Not neutronium cores left over from Higgs Drive booster stages. – Oscar Bravo Sep 22 '15 at 7:30