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moon · moon of asteroid Didymos

Dimorphos

A little moon that people crashed a spacecraft into on purpose. It worked: Dimorphos now goes round its asteroid about half an hour quicker than it used to. It is the only thing in the sky people have ever moved.

🚀 Travel there
Dimorphos — Drawn, not photographed.

What it is

Small moon of Didymos, deliberately struck by NASA's DART spacecraft in 2022 to test asteroid deflection.

Moon: Orbits a planet rather than the star. Every kind of thing there is →

Can you see it?

No. No telescope on Earth will show you this one; it is here to be understood, not seen. It is here because it is worth knowing about — and because leaving it out would make the sky look emptier than it is.

Worth knowing about Dimorphos

Dimorphos is about as long as one and a half football pitches. In weight, about 5 billion kg — tiny even for an asteroid. Its day: it used to turn once for every trip round Didymos, keeping the same face towards it. The DART crash shortened the trip, so the two no longer match.

How it moves
Used to circle its parent asteroid in just under 12 hours.
The machines we sent
NASA's DART spacecraft deliberately crashed into Dimorphos in 2022, shortening its orbit around Didymos by about 32 minutes — the first-ever test of nudging an asteroid off course to protect Earth someday.
How did anyone measure the Earth in the first place?
The numbers
Across0 km
Distance from the planet it orbits1 km
Once round its orbit11.4 hours
One spin11.4 hours

How anyone knows this

Nobody flew a tape measure out there. Every number on this page was worked out from the ground:

What it belongs to

☀️ Everything the Sun holds, in one place📖 How the solar system fits together

Then go outside

Not this one. But the sky over your head is full of ones you can see, and the Planetarium knows which.

🌙 Plan an evening outside🌌 Everything else in the sky