Deimos
What is Deimos?
Moons· Diameter ~12 km. Mars's small outer moon — a smooth rock about 12 km across that takes 30 hours per orbit, so from the Martian surface it drifts slowly westward like a bright star over two or three days. Watch its lazy path outside Phobos in the orbit view.
📍 Visible from where you are right now — computed in your browser
bearing · altitude · rise time · best time tonight
The orbit of Deimos at this moment: https://nowsky.org/solar/orbit?fw=moon:mars:deimos&sp=0.0006 — replayed live, no login.
Orbit and size
| Size | 15 × 12 × 11 km |
|---|---|
| Distance from Mars | 23,460 km |
| Orbit | 30.3 h |
| Note | looks like a bright star from Mars |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Parent planet and sibling moons
Frequently asked
- How far is Deimos from Earth?
- 23,460 km.
- What is notable about Deimos?
- looks like a bright star from Mars.
- Which planet does Deimos orbit?
- Mars.
Captured or shattered — the origin debate
Deimos was long taken for an asteroid captured by Mars's gravity, its dark surface and low density resembling bodies from the outer asteroid belt. But a captured object should have a tilted, stretched orbit, and Deimos's is nearly circular and lies in Mars's equatorial plane — an arrangement capture struggles to explain.
In 2021 the UAE's Hope orbiter passed within 100 km of Deimos, and its spectra showed a surface closer in composition to the Martian crust than to an asteroid. The hypothesis that debris from a giant impact on Mars formed a ring and then coalesced is gaining ground, and if Phobos and Deimos were born from the same ring their compositions should match. The Phobos sample from MMX will be the benchmark for that comparison.
The moon in depth
Deimos, the outer moon of Mars, was discovered by Asaph Hall on 12 August 1877 and named for Dread, a son of the Greek war god Ares. Just 15 × 12 × 11 km — half the size of Phobos — it orbits 23,460 km from the planet's center every 30.3 hours. That is slightly slower than Mars's 24.6-hour rotation, so it creeps westward across the Martian sky at a snail's pace.
Unlike Phobos, Deimos looks smooth: craters exist, but a thick blanket of dust softens most of them, and its largest, Swift and Voltaire, are only about 3 km wide. Its low reflectivity resembles carbon-rich asteroids, feeding a long debate over whether it was captured from the asteroid belt or formed from the same giant-impact debris as Phobos.
From the surface of Mars, Deimos appears as little more than a bright star, and rovers have repeatedly photographed it crossing the face of the Sun. Track Mars in the orbit view to see Deimos idling outside the fast inner loop of Phobos. Japan's MMX spacecraft will observe it up close after its Phobos landing.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.