Solar System Simulator · Sandbox · Lagrange points
Lagrange points
Five places near a planet where a small object can keep pace with it. Only two of them hold on. Drop a probe on each and find out which.
What happens at each point
| Point | Where | A probe dropped there |
|---|---|---|
| L1 | between the Sun and Jupiter | gone in about 4 years |
| L2 | just beyond Jupiter | gone in about 4 years |
| L3 | the far side of the Sun | gone in about 26 years |
| L4 | 60° ahead of Jupiter | still there after 100 years |
| L5 | 60° behind Jupiter | still there after 100 years |
Measured in the simulator: each probe starts exactly on its point, moving with Jupiter, and counts as gone once it strays half an astronomical unit.
Why only L4 and L5 hold
At every point the Sun's and the planet's pulls balance so an object can circle the Sun in step with the planet. At L1, L2 and L3 the balance is a knife edge: drift a little and you keep drifting. At L4 and L5 a drifting object gets swung into a slow loop around the point instead of falling away.
Jupiter's Trojans
That's why Jupiter shares its orbit with thousands of asteroids, bunched 60° ahead and behind it. They're called the Trojans, and NASA's Lucy spacecraft is on its way to visit several of them. The green dots in the simulator are Trojans, seeded near L4 and L5 and then left to gravity.
Telescopes at L1 and L2
Earth has Lagrange points too. The Webb telescope works near Sun–Earth L2, 1.5 million km out, and the SOHO solar observatory watches the Sun from L1. Those points don't hold, so both craft fire small thrusters every few weeks to stay put.
What the simulator does
- The view turns with Jupiter, so Jupiter and its five Lagrange points hold still while years pass in seconds.
- Tap a point, or its row in the card, to drop a probe there. It starts moving exactly with Jupiter.
- The card tracks how far each probe has strayed. Watch the L1, L2 and L3 probes leave and the L4 and L5 probes stay.
Questions people ask
What is a Lagrange point?
A place where a small object can orbit the Sun in step with a planet, because the two pulls on it balance just right. Every planet has five of them, named L1 to L5.
Where is the James Webb Space Telescope?
Near the Sun–Earth L2 point, about 1.5 million km from Earth on the side away from the Sun. It circles the point rather than sitting on it, and fires thrusters every few weeks to stay there.
Which Lagrange points are stable?
L4 and L5, the two that sit 60° ahead of and behind the planet. L1, L2 and L3 are unstable: anything there slowly drifts away unless it corrects its course.
Keep going
Every number on this page comes from the simulator's own physics or the real mission record it is checked against. Where the simulation and reality differ, the page says so.