How many moons does Mars have? Phobos, Deimos and the moon that rises in the west

Mars has two moons: Phobos and Deimos, according to NASA. Both are tiny, lumpy and dark, and the American astronomer Asaph Hall discovered them in August 1877. Phobos, the bigger one, circles Mars about three times a day and rises in the west. Deimos is smaller and farther out.

Distances and orbit times from NASA JPL Horizons (our calculation from its orbit sizes); sizes and discovery dates from NASA.

How many moons does Mars have, and what are their names?

Two. Mars is the only planet with exactly two moons: Mercury "doesn't have moons", and NASA calls Venus "one of only two planets in our solar system that doesn't have a moon". Earth has one, and the giants have many more: see how many moons Jupiter has and how many moons Saturn has. In total NASA counts 461 officially recognised moons around the planets and the dwarf planet Pluto, as of October 2026.

Asaph Hall found both Martian moons in August 1877 at the US Naval Observatory in Washington, D.C.: Deimos on 11 August and Phobos on 17 August, according to NASA. He named them after the sons of Ares, the Greek god of war, whom the Romans called Mars. Phobos means fear and Deimos means dread, NASA explains. The largest crater on Phobos, Stickney, is named after Chloe Angeline Stickney Hall, a mathematician and Asaph Hall's wife.

The Hubble Space Telescope photo above shows how small Phobos is: even next to Mars it looks like a star.

Phobos and Deimos: how big are Mars' moons?

Very small. NASA gives Phobos as 27 by 22 by 18 km and Deimos as 15 by 12 by 11 km. Neither is round; both look like lumpy potatoes, because they do not have enough gravity to pull themselves into a ball. JPL says "Phobos is about 157 times smaller than Earth's Moon": its average width is about 1/157 of our Moon's.

Phobos is covered in craters and long grooves. Its biggest crater, Stickney, is about 9 km wide (NASA gives 9 to 9.7 km) and covers a large part of one side. NASA measured temperatures on Phobos from about −4 °C (25 °F) in sunlight to −112 °C (−170 °F) on the dark side, and the moon has no atmosphere.

Deimos is smoother. NASA says its craters are partly filled in by a thick layer of dust and broken rock, which hides much of its surface. Both moons are dark: NASA says they "appear to be composed of C-type rock, similar to blackish carbonaceous chondrite asteroids", which means their surfaces look like those of dark, carbon-rich asteroids.

What do Phobos and Deimos look like from Mars?

Phobos is so close to Mars, about 5,981 km above the equator, that it goes around faster than Mars spins. NASA says it "completes an orbit in just 7 hours and 39 minutes, which is faster than Mars rotates". So, unlike our Moon, Phobos rises in the west and sets in the east. Both moons go around Mars in the same direction as Mars spins; Phobos simply overtakes the turning planet, while Deimos falls slowly behind.

We used NASA JPL Horizons to predict where both moons will be in the sky above Jezero crater, where NASA's Perseverance rover is exploring, from 20 to 28 October 2026, for a flat horizon. In those predictions Phobos rises in the west every 11.0 to 11.2 hours and stays up for 4.1 to 4.3 hours each time, climbing at most 63° above the horizon. Deimos moves the other way, from east to west, but very slowly: it is predicted to rise in the east on 23 October (UTC) and stay above the horizon for about 60 hours, about two and a half Martian days. Because Mars turns only a little faster than Deimos goes round, Deimos rises about once every 5.3 Martian days, in our calculation from the orbit times.

Neither moon looks like our Moon. Seen straight overhead near Mars' equator, Phobos is about 0.21° wide, roughly 41% of the width of our Moon in Earth's sky, and Deimos only 0.035°, small enough to look almost like a bright star (see the chart above). These are approximate: we used the moons' mean diameters and average distances. Phobos is also so close that, according to NASA, "in some locations on Mars it cannot always be seen": far enough north or south, it stays below the horizon.

The animation below shows three predicted days over Jezero crater: watch the blue dot of Phobos dash across the sky while Deimos and the Sun crawl the other way.

Solar eclipses on Mars

Phobos regularly passes in front of the Sun as seen from the surface of Mars. On 2 April 2022 NASA's Perseverance rover filmed one: the eclipse "lasted a little over 40 seconds", JPL says (photo above). Because Phobos looks smaller than the Sun, it never covers it completely; it takes a bite out of the Sun instead. From Mars the Sun is only about 0.34° wide (in our calculation for October 2026), about a third smaller than it looks from Earth, and Phobos covers part of it.

Rovers have watched these eclipses since 2004, when Spirit and Opportunity made the first observations; Curiosity and Perseverance have recorded them on video since. They are useful for science. "Each time these eclipses are observed, they allow scientists to measure subtle shifts in Phobos' orbit over time," NASA explains. Deimos also crosses the Sun, but it is so small that it only makes a tiny dark spot.

Phobos is slowly falling toward Mars

Phobos is on a collision course with Mars. Right now it moves about 1.8 metres (six feet) closer every hundred years, NASA says. NASA expects it to crash into Mars or break up into a ring in about 50 million years; JPL puts it as "tens of millions of years". These are scientists' predictions, not today's speed simply multiplied out: dividing Phobos' height by 1.8 metres per century would give a far longer time.

The reason is where Phobos orbits. A moon at about 20,428 km from Mars' centre (17,031 km above the equator) would go round exactly once per Martian rotation, in our calculation from Horizons data: this is the synchronous orbit, the dashed ring in the animation below. Phobos is far inside it, at 9,377 km. The animation is a simplified model with circular orbits in one plane; the orbit sizes are to scale. Mars and Phobos pull on each other unevenly (scientists call these tidal forces), and JPL explains that these forces "also slowly change Phobos' orbit", pulling it closer. Deimos orbits outside the synchronous orbit.

Where did Mars' moons come from? The MMX mission

Nobody knows yet. NASA says both moons "are thought to be captured asteroids, or debris from early in the formation of our solar system", and that scientists suspect they are either "remnants of an ancient collision between Mars and a large impactor" or "asteroids captured by Mars' gravity". NASA's Hubble release about Phobos explains the puzzle: the moons are made of material like asteroids, but their "stable, nearly circular orbits" make some scientists doubt they were captured, because such orbits "are rare for captured objects".

Japan's space agency JAXA wants to settle it with MMX (Martian Moons eXploration). JAXA launched MMX on an H3 rocket from Tanegashima Space Center at 04:41 Japan time on 20 October 2026 (19 October, 19:41 UTC). According to JAXA, the rocket "flew as planned", the spacecraft separated about 71 minutes after liftoff and was placed into its planned orbit. The spacecraft is planned to reach Mars about a year later, orbit Mars while staying close to Phobos (a so-called quasi-satellite orbit) to study it, land on Phobos to collect at least 10 grams of material (a small rover is also planned to land), look at Deimos from a distance, and bring the sample to Earth in 2031. NASA contributes an instrument called MEGANE, which will measure gamma rays and neutrons coming from Phobos' surface to work out what it is made of. NASA says that measuring the elemental composition of the surface should "help peg the likely origin of the moon".

Can you see Mars' moons from Earth?

Not with your eyes, and even through a telescope they are hard to find. On 19 February 2027, the day of the next opposition of Mars (when Mars is opposite the Sun in our sky; see how far Mars is from Earth), NASA JPL Horizons gives Phobos a brightness of magnitude 12.1 and Deimos 13.2. Higher numbers mean fainter: these moons are hundreds of thousands of times fainter than Mars itself (magnitude −1.3), in our calculation.

They also stay very close to the planet. In our 10-minute samples for that day, Phobos reaches at most 19.3 arcseconds from Mars' centre and Deimos 47.7 arcseconds (an arcsecond is 1/3600 of a degree), while Mars itself is about 14 arcseconds wide (on 20 February, at its closest). Telescopes can show them, but their faintness and the glare of bright Mars make them difficult targets. Even to the Hubble Space Telescope, Phobos looked "star-like" in its 2016 photos, NASA says.

Mars moons FAQ

How many moons does Mars have? Two: Phobos and Deimos.

What are the names of Mars' moons, and what do they mean? Phobos (fear) and Deimos (dread), named by Asaph Hall after the sons of Ares, the Greek god of war, according to NASA.

Which moon of Mars is bigger? Phobos: 27 by 22 by 18 km, against 15 by 12 by 11 km for Deimos (NASA).

How close is Phobos to Mars? About 5,981 km (3,716 miles) above Mars' equator, or 9,377 km from the planet's centre (NASA JPL Horizons). NASA calls it closer to its planet than any other moon in the solar system.

Why is Phobos called a doomed moon? Because it is slowly spiralling inward, now about 1.8 metres per century: NASA expects it to crash into Mars or break up into a ring in about 50 million years.

Why are Mars' moons so small and lumpy? They are too small for their own gravity to pull them into a ball. Whether they are captured asteroids or debris from a giant impact is still being studied; MMX should help answer it.

Does Mars have rings? Not now. NASA says Phobos may one day break up into a ring around Mars.

Which planets have no moons? Mercury and Venus, according to NASA.

Can you see Phobos from Earth with a telescope? It can be observed through telescopes, but it is a difficult target: Phobos is about magnitude 12 and stays within about 20 arcseconds of brilliant Mars, even at the 2027 opposition.

Does Phobos rise in the west? Yes. It goes around Mars faster than Mars spins, so it rises in the west and sets in the east, roughly every 11 hours over Jezero crater in our calculation.

How long does it take Phobos and Deimos to orbit Mars? Phobos 7 h 39 min and Deimos 30 h 19 min (NASA JPL Horizons; NASA rounds them to "three times a day" and "every 30 hours"). Our own Moon needs 27 days, NASA says; see how far the Moon is from Earth.

How we checked

The orbit sizes, orbit times, moon radii and Mars' size, mass and rotation come from NASA JPL Horizons' physical data (revised 2 June 2025). We followed Phobos, Deimos and the Sun in the sky above Jezero crater (18.4447° N, 77.4508° E) every 5 minutes from 20 to 28 October 2026, and the moons as seen from Earth every 10 minutes on 19 February 2027, also with NASA JPL Horizons. From those numbers we calculated the rising and setting times, the synchronous orbit (with Kepler's third law), the brightness difference and the apparent sizes; the Sun's distance from Mars comes from Astronomy Engine 2.1.19.

The moon facts, names and discovery dates come from NASA's Mars moons, Phobos and Deimos pages (updated November 2024) and NASA's Hubble release about Phobos (2017); the eclipse facts from JPL's Perseverance news release (20 April 2022) and NASA's image pages; the moon counts from NASA's Moons, Mercury and Venus fact pages; the MMX plans from JAXA's MMX website and the launch from JAXA's launch report of 20 October 2026 (both checked 20 October 2026), and NASA's MEGANE article.

The photos are NASA images; NASA says its images are generally not subject to copyright in the United States, and we credit each one. The animations, charts and diagram are our own, made from these numbers. None of the images or videos is AI-generated.