Where can you park in space?

Storms on the Sun can knock out satellites, GPS and power grids. To warn the Earth in advance, a probe watches the Sun, staying between it and us all the time. But you can’t stop in space: anything that stands still falls into the Sun. So how does it stay there?

Balancing gravity isn’t enough

Between the Sun and the Earth there’s a spot where their pulls balance, about 260,000 km from the Earth. But a probe can’t stay there: nothing bends its path, so it flies straight on, while the Sun bends the Earth’s path into an orbit. It won’t stay put.

Closer means faster

Mercury goes round the Sun in 88 days, the Earth in a year and Mars in just under two years. The closer to the Sun, the harder the Sun pulls, the faster a body has to fly so as not to fall in, and the shorter its orbit.

If only the Sun pulled

For a probe to stay put relative to the Earth, it has to go round the Sun in a year, just like the Earth. But if only the Sun pulled it, a probe a little closer to the Sun would orbit faster and overtake the Earth. A probe a little further out would fall behind.

L1: the Earth holds it back

The Earth pulls the probe too, away from the Sun, weakening the Sun’s pull, so it orbits slower. At 260,000 km the Earth cancels it fully – too much. About 1.5 million km away is just right: one lap of the Sun a year, like the Earth. This is L1, first of five Lagrange points, named after the mathematician who computed them.

L2: the Earth gives it a push

Behind the Earth it’s the other way round: the Earth pulls the probe in the same direction as the Sun, so the probe orbits faster than if only the Sun pulled it. Again, about 1.5 million km from the Earth it goes round the Sun in exactly a year. That’s L2.

The James Webb Space Telescope

The James Webb Space Telescope orbits at L2. From there the Sun, the Earth and the Moon are always on the same side, so a single sunshield the size of a tennis court blocks them all. Behind it the telescope stays in the shade and stays below −220 °C – only that way can it pick up the faint warmth of distant galaxies.

Five points

There are five such places. L3 lies opposite the Earth, beyond the Sun; L4 and L5 sit on the Earth’s orbit, 60° ahead of it and behind it. There’s no simple way to show why exactly there, but the maths confirms that there, too, a probe goes round the Sun in exactly a year.

Hilltop or valley?

L1, L2 and L3 are like the top of a hill: if a probe drifts a little away from the Earth at L2, the Earth pulls it more weakly and it drifts even further. That’s why Webb fires its engine briefly every few weeks. L4 and L5 are like valleys: whatever gets caught there stays.

Asteroids near Jupiter

Every planet has the same points. Jupiter shows that the valleys hold: more than ten thousand asteroids have been caught at its L4 and L5, and they’ve been going round the Sun with it for billions of years.

So where can you park?

At the Lagrange points: there a probe goes round the Sun in exactly a year and doesn’t move relative to the Earth. So the probe from the first card watches the Sun from L1, and Webb hides from it at L2. At L1 to L3 the engine has to be fired now and then; at L4 and L5 a probe stays put unaided.