ObserveTheSky › Everything in the sky › Deimos
moon · moon of Mars

Deimos compared to the Moon

Tiny lumpy outer moon of Mars, possibly a captured asteroid.

Can you see it?

No small telescope will show you this one; it is here to be understood, not spotted.

How big is Deimos compared to the Moon?

Deimos is roughly 1/277 as wide as the Moon.

How far away is Deimos from Mars compared to the Earth-to-Moon distance?

Deimos sits about one sixteenth of the Earth-to-Moon distance from Mars.

How long does Deimos take to go once round Mars?

Deimos takes about 30 hours to go once round Mars.

How long is a day on Deimos compared to Earth?

Deimos turns round once in about 30 hours; Earth takes about 24 hours. Its orbit takes exactly as long, so Deimos keeps the same face turned towards Mars the whole time.

Worth knowing about Deimos

Where it came from
Perhaps an asteroid that Mars captured, or a piece of Mars thrown up by a giant impact; JAXA’s MMX mission, which is to bring back a sample of Phobos and study Deimos from close by, is meant to settle it.
Deimos — A model of its measured shape, seen from the Earth and lit by the Sun: not a photograph, and not what a telescope shows.
The numbers
Mean diameter12.54 km
Distance from the centre of the planet it orbits23,459 km
Once round its orbit30.3 hours
One spin30.3 hours

How anyone knows this

Games it appears in

Show Deimos in the model☀️ Everything the Sun holds🌙 Plan an evening outside