Didymos

Rocky near-Earth asteroid orbited by its moon Dimorphos; target of NASA's DART mission.

What it is

Asteroid: Rock or metal left over from building the planets. 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 Didymos

Didymos is about as wide as eight football pitches laid end to end. In weight, far too light to meaningfully compare to Earth. A little farther out than Earth on average. Sunlight takes 14 minutes to reach Didymos — on average, because that distance changes as it goes round. Its year: about 2 years. Its day: 2.26 hours.

Its surface: Rocky, orbited by its own small moon, Dimorphos.
The machines we sent: Target of NASA's DART mission (see Dimorphos).
A near-Earth asteroid: Its orbit stays close to Earth's own, which is why it was reachable enough to be chosen for the first test of whether we could nudge one off course.
What makes it worth knowing: One of the few asteroids with a moon of its own -- little Dimorphos circles it every 11 hours and 55 minutes.

The numbers

Across	1 km
Distance from the Sun, on average	1.64 AU ~245 million km · sunlight takes 14 minutes
Once round its orbit	2.1 Earth years
One day there	2.3 hours

What it belongs to

☀️ Everything the Sun holds, in one place

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