Phobos (moon) facts for kids
A composite image of Phobos captured by the Mars Reconnaissance Orbiter in 2008
|
|
| Discovery | |
|---|---|
| Discovered by | Asaph Hall |
| Discovery date | 18 August 1877 |
| Designations | |
| MPC designation | Mars I |
| Pronunciation | or |
|
Named after
|
Φόβος |
| Adjectives | Phobian |
| Orbital characteristics | |
| Epoch J2000 | |
| Periapsis | 9,234.42 km |
| Apoapsis | 9,517.58 km |
| 9,376 km (2.76 Mars radii/1.472 Earth radii) | |
| Eccentricity | 0.0151 |
| 0.31891023 d (7 h 39 m 12 s) |
|
|
Average orbital speed
|
2.138 km/s |
| Inclination | 1.093° (to Mars's equator) 0.046° (to local Laplace plane) 26.04° (to the ecliptic) |
| Satellite of | Mars |
| Physical characteristics | |
| Dimensions | 25.90 km × 22.60 km × 18.32 km (± 0.08 km × 0.08 km × 0.06 km) |
|
Mean radius
|
11.08±0.04 km |
| 1,640±8 km2 | |
| Volume | 5,695±32 km3 |
| Mass | 1.060×1016 kg |
|
Mean density
|
1.861±0.011 g/cm3 |
| 0.0057 m/s2 (581.4 μ g) |
|
| 11.39 m/s (41 km/h) |
|
| Synchronous | |
|
Equatorial rotation velocity
|
11.0 km/h (6.8 mph) (at longest axis) |
| 0° | |
| Albedo | 0.071 ± 0.012 at 0.54 μm |
| Temperature | ≈ 233 K |
| 11.8 | |
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Phobos is the larger of the two natural satellites that orbit the planet Mars. Its twin moon is called Deimos. An American astronomer named Asaph Hall discovered both moons in 1877. Phobos gets its name from the Greek god of fear and panic. This god was the twin brother of Deimos and a son of Ares (the Greek name for Mars).
Phobos is a small, oddly shaped space rock. It has an average radius of about 11 km (7 mi). This moon orbits Mars very closely, only about 6,000 km (3,700 mi) from its surface. This is closer than any other known moon to its planet. Phobos moves around Mars much faster than Mars spins. It completes one orbit in just 7 hours and 39 minutes. This means that from the surface of Mars, Phobos appears to rise in the west. It then zooms across the sky in about 4 hours and 15 minutes. It sets in the east, doing this twice every Martian day. Phobos is also one of the darkest objects in our Solar System. Its surface reflects very little sunlight. Temperatures on Phobos can be around −4 °C (25 °F) on the sunny side. But they drop to about −112 °C (−170 °F) in the shadows. A huge impact crater called Stickney covers a large part of its surface. The moon's surface also has many mysterious grooves. Scientists have several ideas about how these grooves were formed.
Pictures and computer models suggest that Phobos might be a "rubble pile". This means it's a collection of broken rocks held together by a thin outer layer. Mars's gravity is slowly pulling Phobos closer. It moves about 2 centimetres (0.79 in) closer to Mars each year.
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Discovering Phobos
The American astronomer Asaph Hall found Phobos on August 18, 1877. He made this discovery at the United States Naval Observatory in Washington, D.C.. Hall had already found Deimos, Mars's other moon, a few days earlier. He used the world's largest refracting telescope at the time, called the "Great Equatorial."
The names Phobos and Deimos came from Greek mythology. A British teacher, Henry Madan, suggested them. In these myths, Phobos is a companion to the god Ares, who is like the Roman god Mars.
What Phobos is Like
Phobos is about 26 by 23 by 18 kilometres (16 mi × 14 mi × 11 mi) in size. It is not round because it doesn't have enough gravity to pull itself into a sphere. Phobos also has no atmosphere because it is so small and has weak gravity. It is one of the darkest objects in the Solar System. Its surface is made of material rich in carbon, similar to some meteorites. This material is also like what is found on Mars's surface. Phobos is not solid rock. It has a low density, meaning it has many empty spaces inside. Scientists think it might even have ice hidden beneath its surface. The surface temperatures range from about −4 °C (25 °F) in sunlight to −112 °C (−170 °F) in shadow.
Phobos has many craters, unlike Deimos. The most famous one is Stickney. This crater is 9 km (5.6 mi) wide and covers a huge part of the moon. The impact that created Stickney was so powerful it almost broke Phobos apart.
Many grooves and lines crisscross Phobos's surface. These grooves are usually less than 30 metres (98 ft) deep and up to 20 kilometres (12 mi) long. For a long time, people thought the impact that made Stickney crater also made these grooves. However, studies from the Mars Express spacecraft showed something different. The grooves seem to point towards a spot on Phobos that always faces forward in its orbit. Scientists now think these grooves were made by rocks and dust thrown off Mars by impacts. These pieces then hit Phobos, creating lines of small craters. In 2018, astronomers suggested that boulders from the Stickney impact itself rolled across the moon. These rolling boulders could have created the grooves.
Scientists have also looked for faint dust rings around Mars, made by Phobos and Deimos. So far, they haven't found any. Images from Mars Global Surveyor show that Phobos is covered in a layer of fine dust, at least 100 meters thick. This dust likely came from impacts by other space objects. It's a mystery how this dust stays on a moon with almost no gravity.
Named Places on Phobos
Places on Phobos are named after astronomers who studied the moon. They are also named after characters and places from Jonathan Swift's famous book, Gulliver's Travels.
Craters on Phobos
Some of the craters have special names. For example, the largest crater is called Stickney. It is named after Angeline Stickney, the wife of Phobos's discoverer, Asaph Hall. Other craters include Hall, named after Asaph Hall himself. There are also craters named Gulliver, Flimnap, and Limtoc, all characters from Gulliver's Travels.
How Phobos Orbits Mars
Scientists have studied Phobos's orbit very closely. It is one of the best-understood moons in our Solar System. Phobos orbits Mars at an altitude of about 5,989 km (3,721 mi). This is below the "synchronous orbit" height. This means Phobos moves around Mars faster than Mars spins on its own axis. So, if you were standing on Mars, Phobos would appear to rise in the west. It would then quickly cross the sky in about 4 hours and 15 minutes. It would set in the east, doing this about twice every Martian day.
Because Phobos orbits so low and close to Mars's equator, you can't see it from all parts of Mars. It's not visible from latitudes higher than 70.4 degrees. Also, because it's so close, its size in the sky changes as it moves. When it's near the horizon, it looks smaller. When it's directly overhead, it looks bigger. At its largest, Phobos appears about one-third the size of Earth's full Moon. The Sun, as seen from Mars, looks about twice as wide as Phobos.
Phobos Crossing the Sun
If you were on Mars, you would often see Phobos pass in front of the Sun. These events are called "transits." The Mars Rover Opportunity has taken many pictures of these transits. During a transit, Phobos casts a shadow on Mars's surface. Several spacecraft have photographed these shadows. Phobos is not big enough to completely block the Sun. So, it cannot cause a total solar eclipse on Mars.
Phobos's Future
Mars's gravity is slowly pulling Phobos closer. This process is called "tidal deceleration." Phobos's orbit is shrinking by about 2 m (6 ft 7 in) every 100 years. Scientists predict that in about 30 to 50 million years, Phobos will break apart. The strong tidal forces from Mars will tear it into pieces. When this happens, Phobos will likely form a beautiful planetary ring around Mars. This ring could last for 1 million to 100 million years.
The grooves on Phobos have been a mystery for a long time. They were once thought to be cracks from the Stickney impact. But later studies showed they might be "stretch marks" from Mars's tidal pull. However, recent models suggest the pull isn't strong enough to crack a solid moon. The latest idea, from 2018, is that boulders from the Stickney impact rolled and bounced across the moon. These rolling rocks compressed the soft dust on the surface, creating the grooves.
Where Did Phobos Come From?
Scientists have different ideas about how Phobos and Deimos formed. Both moons look a lot like C-type asteroids. These are dark, carbon-rich space rocks. One idea is that Mars's gravity captured these asteroids from the asteroid belt. But for this to happen, their orbits would need to become nearly circular and line up with Mars's equator. This would require a lot of energy to be lost, perhaps through atmospheric drag or tidal forces.
Another idea is that Phobos formed in orbit around Mars. It might have come together from leftover material after Mars itself formed. Or, Mars might have been hit by a large object. This impact could have thrown a lot of material into orbit, which then gathered to form Phobos and Deimos.
The inside of Phobos is very porous, meaning it has many empty spaces. This makes the asteroid capture idea less likely. Also, observations show that Phobos's surface material is similar to rocks found on Mars. This supports the idea that Phobos came from material ejected from Mars. This is similar to how Earth's Moon is thought to have formed from a giant impact.
Some parts of Phobos's surface look reddish, while others are bluish. Scientists think Mars's gravity might move the reddish dust around. This could expose fresher, bluish material underneath. This blue rock is different from known Martian rock. This difference could challenge the idea that Phobos formed from Martian impact debris.
In 2021, a new idea was proposed. Scientists looked at data from the Mars InSight Mission and other missions. They suggested that Phobos and Deimos came from a single parent body. This body might have been hit by another object 1 to 2.7 billion years ago. This collision would have shattered it, forming both moons.
Exploring Phobos
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Many spacecraft have taken close-up pictures of Phobos. Their main job was usually to photograph Mars. The first was Mariner 7 in 1969. Others include Mariner 9, Viking 1, and Mars Global Surveyor. The Mars Express and Mars Reconnaissance Orbiter have also sent back amazing images. In 2005, the Spirit rover on Mars even took pictures of Phobos at night.
The Soviet Union launched two probes, Phobos 1 and Phobos 2, in 1988. Phobos 1 was lost on its way to Mars. Phobos 2 reached Mars in 1989. It sent back some data and images before it stopped working. Other missions have gathered more information. However, no mission has successfully brought samples from Phobos back to Earth yet.
The Russian Space Agency tried a sample return mission called Fobos-Grunt in 2011. It also carried a life science experiment. But after reaching Earth orbit, the probe failed to travel to Mars. It crashed back to Earth in January 2012.
On July 1, 2020, the Mars orbiter from the Indian Space Research Organisation took photos of Phobos. It captured these images from 4,200 km away.
In March 2025, the ESA's Hera spacecraft flew past Mars. On its way to another asteroid, it observed Phobos. It took pictures of Phobos from less than 13,000 km away.
Future Missions to Phobos
The Japanese Aerospace Exploration Agency (JAXA) is planning a mission called Martian Moons eXploration (MMX). This mission aims to collect samples from Phobos. MMX is scheduled to launch in 2026. It will land on Phobos multiple times to gather samples. It will also observe Deimos and Mars's climate. The spacecraft plans to bring at least 10 grams of samples back to Earth in 2031. NASA, DLR, and CNES are helping with this project. They will provide scientific tools and a rover named Idefix.
Other missions have been proposed to study Phobos. These include Aladdin, Phobos Reconnaissance and International Mars Exploration (PRIME), and Phobos Surveyor. Russia also plans to try another sample return mission in the late 2020s. The European Space Agency is looking into a mission called Phootprint for 2024.
Human Missions to Phobos
Phobos has been suggested as a first stop for humans traveling to Mars. Landing on Phobos would be easier and cheaper than landing directly on Mars. A Phobos lander could use technology similar to lunar or asteroid landers. It would not need to deal with Mars's atmosphere. Also, Phobos's weak gravity means less fuel is needed to land and return.
Humans on Phobos could control robots on Mars without much delay. This would help explore Mars safely. Phobos itself is also a very interesting place for science.
See also
In Spanish: Fobos (satélite) para niños
- List of natural satellites
- List of missions to the moons of Mars
- Phobos monolith
- Transit of Phobos from Mars
