More complicated than it looks (Image: SPL)

Satellites stay in their orbits thanks in part to the Earth’s squashed shape – something we have only just discovered.

Our planet is ringed with more than 1000 working satellites, plus thousands of tonnes of space junk, and for the most part they stay up there quite happily. But surprisingly, it is only now that we properly understand why.

Ideally, a tiny satellite orbiting a perfectly spherical planet will remain there forever, assuming nothing nearby disturbs it. But Earth is not a perfect sphere, and there are plenty of other objects that can disturb artificial satellites in low-Earth orbit – first and foremost, the moon. According to the laws of motion, the moon’s influence alone should cause satellites to crash into the Earth’s atmosphere, where they would burn up.


Saving grace

It turns out that Earth’s imperfections are a satellite’s saving grace. Because of its rotation, Earth is slightly flattened at the poles and bulges around the equator.

According to computer simulations and analysis by Scott Tremaine at the Institute for Advanced Study in Princeton, New Jersey, and Tomer Yavetz of Princeton University, the gravitational pull of that bulge shifts satellites’ orbits over time, preventing tugs from the moon and other sources from pulling them too far in one direction or another. If the Earth were closer to being a perfect sphere, many satellites would crash into the atmosphere and burn up in a matter of months or years.

“It’s interesting that there are lots of things that could destabilise low-Earth orbits, but that things happen to combine in such a way that we have a good environment for satellites,” says Gregory Laughlin, a physicist at the University of California, Santa Cruz, who was not involved with this research. “It makes you pause to think a little bit – when you look in detail at how things work, you can find surprises.”

Journal reference: arxiv.org/abs/1309.5244v1