Space has no parking spaces — everything is falling around something else. But there are five spots in the Sun–Earth system where a spacecraft can sit still relative to Earth and stay there, year after year, on almost no fuel. The James Webb Space Telescope lives at one of them.
Anything circling the Earth keeps swinging round to the day side. For a telescope trying to stay colder than −220 °C, that's a disaster: the Sun, and then the warm Earth, keep coming back into view. Hubble copes because it works in visible light. An infrared telescope is looking for heat, so it has to hide from every warm thing at once — permanently.
The Sun pulls one way. The Earth pulls the other. Nearly everywhere those two pulls fight and a spacecraft drifts. But at five particular spots they combine so neatly that the spacecraft circles the Sun at exactly Earth's pace — so from Earth it never appears to move away. They're called Lagrange points, after the mathematician who worked out where they are.
L2 sits directly behind Earth as seen from the Sun, about 1.5 million kilometres further out — roughly four times as far as the Moon. That direction is the whole point: from there, the Sun, the Earth and the Moon are all bunched together in the same part of the sky. One shield, pointed one way, blocks all three at once. The cold side stares out at empty space forever.
Webb doesn't sit exactly on the point — it loops slowly around it in what's called a halo orbit, taking about six months per lap. Sitting precisely on L2 would mean sitting in Earth's shadow, which sounds ideal but would cut off its solar panels and its radio path home.
| Point | Where | Used for |
|---|---|---|
| L1 | Between Sun and Earth, ~1.5 million km sunward | Watching the Sun — it never gets eclipsed by Earth |
| L2 | Directly behind Earth, ~1.5 million km outward | Cold infrared telescopes: Webb, and Gaia before it |
| L3 | Right around the far side of the Sun | Nothing useful — the Sun blocks all contact |
| L4 / L5 | On Earth's orbit, 60° ahead and 60° behind | The genuinely stable pair; dust and asteroids collect at Jupiter's |
L1, L2 and L3 are balanced the way a pencil balances on its tip: nudge a spacecraft and it drifts off, so it must nudge itself back every few weeks. L4 and L5 are balanced the way a marble sits in a bowl — push it and it settles back on its own. That's why rocks pile up naturally at L4 and L5 but never at L1 or L2.
Stand a friend in the middle of a room as the Sun, and hold a ball at arm's length as the Earth. Now put a coin just behind the ball, on the far side from your friend. Walk slowly in a circle around them, keeping the coin behind the ball the whole way. Notice what you have to do: the coin travels a bigger circle than the ball, so it should fall behind — you have to keep dragging it along. Out at the real L2, Earth's gravity does that dragging for free. That's the whole idea.
Webb's sunshield is about the size of a tennis court, for a mirror only 6.5 m across. Hold a small object up and try to shade it completely with a card held at arm's length. Move the card closer and it shades easily; move it far away and you need a much bigger card. Webb's shield has to sit well clear of the mirror so its own warmth doesn't leak in — which is exactly why it has to be so much larger than the thing it protects.