Phobos is a moon of Mars.
It is a small rock. It moves very fast in the sky. It looks like it rises in the west. This moon is very special. Do you want to see it?
Phobos is a moon of Mars.
It is a small, bumpy rock. It has a huge hole on it. This hole is called Stickney.
The moon moves very fast. It orbits Mars in just a few hours. Because it is so fast, it rises in the west. It sets in the east.
Phobos is not a solid rock. It might be a pile of rubble. It is held together by a thin crust.
This moon is very special. It is the closest moon to any planet.
Phobos is a moon of Mars. It is the larger of two moons. An astronomer named Asaph Hall found it in 1877.
Phobos is a small and bumpy object. It is not a solid rock. Scientists think it might be a rubble pile. This means it is a pile of rocks held by a thin crust. It also has many empty spaces inside. This is called porosity.
The moon has a very large crater. We call this crater Stickney. It is so big that it might have almost broken the moon. Phobos also has many long grooves on its surface. These grooves may have been made by rocks hitting the moon.
Phobos moves very fast around Mars. It finishes one trip in about 7 hours. Because it moves so fast, it looks different from our Moon. If you stood on Mars, you would see it rise in the west. It would move across the sky and set in the east. It does this twice every Martian day. Phobos is also the closest moon to any planet in our solar system.
Phobos is a very special moon of Mars. It is the larger of the two natural moons orbiting the red planet.
This moon is quite small and has a strange, bumpy shape. It is actually the closest moon to any planet in our entire solar system. Because it is so close to Mars, it behaves in a very unique way. It orbits Mars much faster than the planet itself rotates.
This makes Phobos a very interesting place for scientists to study.
Watching Phobos from the surface of Mars would be a wild sight. Most moons rise in the east and set in the west. However, Phobos moves so fast that it rises in the west instead. It travels across the sky in about 4 hours and 15 minutes. Then, it sets in the east. This happens twice during every single Martian day.
It is a fast-moving neighbor in the Martian sky.
An American astronomer named Asaph Hall discovered Phobos in 1877. He found it while working at the United States Naval Observatory in Washington, D.C.
Hall used a massive telescope called the 26-inch Great Equatorial. He discovered Phobos on August 18, 1877. Just a few days before that, he had also found Mars's other moon, Deimos. A man named Henry Madan suggested the names for both moons. He chose names from Greek mythology to honor the god Ares.
Phobos is not a solid, heavy ball of rock. Instead, scientists think it might be a rubble pile. This means it is a collection of rocks held together by a thin outer crust. It also has a lot of porosity, which means it has many empty spaces inside.
The surface is covered in a thick layer of fine dust called regolith. One of its most famous features is a huge crater named Stickney. This crater is 9 kilometers wide and covers a large part of the moon. The impact that made Stickney might have almost broken Phobos apart.
Many long grooves and streaks cover the surface of Phobos. Scientists once thought these were made by the Stickney impact. Now, they think boulders from that impact rolled across the moon in patterns.
Phobos is also very dark and does not reflect much light. It is one of the least reflective objects in our solar system. Even though it is small, it tells us a big story about how planets and moons form. It continues to move closer to Mars by about 2 meters every year.
Phobos is the larger and innermost of the two natural satellites orbiting Mars.
It is a small, irregularly shaped object with a mean radius of about 11 kilometers. This moon is unique because it orbits closer to its primary planet than any other known natural satellite in the solar system. Because of its proximity, Phobos experiences intense tidal interactions with Mars. These forces are slowly pulling the moon toward the planet. Scientists believe Phobos may be a rubble pile. This means it is a collection of rocks held together by a thin outer crust rather than being a solid chunk of stone.
The orbital mechanics of Phobos create a strange view from the Martian surface. Phobos orbits Mars much faster than the planet rotates on its axis. It completes a full orbit in only 7 hours and 39 minutes. Because of this speed, Phobos does not follow the typical path of most moons. Instead of rising in the east, it appears to rise in the west. It moves rapidly across the sky in 4 hours and 15 minutes or less. It then sets in the east, repeating this cycle twice during every Martian day.
The physical composition of Phobos is quite dark and porous. It is one of the least reflective bodies in the solar system. This level of reflectivity is measured as an albedo of 0.071. The moon has no atmosphere because its mass and gravity are too low to hold gases. Its density is also quite low, which suggests it is not solid rock. Researchers have calculated its porosity to be between 25% and 35%. This means a significant portion of its volume consists of empty spaces or voids. Infrared spectra show that the moon contains carbon-rich material. This material is similar to carbonaceous chondrites found in space.
Surface features on Phobos are dominated by massive impact craters and strange markings. The most prominent feature is the Stickney crater. This impact crater is 9 kilometers in diameter. It covers a substantial proportion of the moon's total surface area. The impact that created Stickney was likely powerful enough to almost shatter Phobos. The surface is also covered in many grooves and streaks. These grooves are typically less than 100 meters wide and up to 2 kilometers long. While some thought these were caused by the Stickney impact, newer research suggests a different cause. Astronomers concluded in 2018 that these grooves were formed by boulders ejected from the Stickney impact. These boulders rolled across the surface in predictable patterns.
History shows that our understanding of Phobos has changed over time. American astronomer Asaph Hall discovered Phobos on August 18, 1877. He made this discovery at the United States Naval Observatory in Washington, D.C. He used the 26-inch "Great Equatorial," which was the largest refracting telescope in the world at that time. Hall had also discovered Deimos, the other moon of Mars, just a few days earlier. The names Phobos and Deimos were suggested by Henry Madan. He chose these names from Greek mythology. Phobos is named after the Greek god of fear and panic. He is the son of Ares, who is the Greek name for the planet Mars.
In the mid-20th century, scientists debated whether Phobos was a natural object. In the late 1950s and 1960s, some researchers suggested Phobos might be hollow. Iosif Shklovsky, a Russian astrophysicist, proposed that it might be a thin metal shell. He based this on the way Phobos was accelerating in its orbit. At the time, some even speculated it could be an artificial object. However, later measurements proved these orbital calculations were based on errors. The acceleration is actually caused by tidal effects that create drag. Modern spacecraft, like the Viking probes and Mars Express, have since confirmed that Phobos is a natural, porous body.
Phobos remains a vital subject for planetary science and future exploration. Because it is so close to Mars, it is a candidate for future human missions.
The moon is currently spiraling inward toward Mars at a rate of about 2 meters per year. This process will eventually lead to the moon being torn apart by Martian gravity. Studying the regolith, which is a layer of fine dust at least 100 meters thick, helps scientists understand how impacts shape small worlds. The study of Phobos connects our knowledge of orbital physics, cratering history, and the composition of the early solar system.
Where will your curiosity go next?
🖼️ Images & Media (10)
More to explore
✨ What else?
Related topics you might enjoy
🔬 Go deeper
More advanced topics to explore
🪜 Step back
Simpler topics to build understanding
What is Nepedia?
A free, ad-free encyclopedia for children. Every article is written at five reading levels, so the same page works for a five-year-old and a fifteen-year-old — use the level switcher above to see this one change. No account needed to read.