How Big Is Phobos? Size of Mars’s Strange Moon Explained

How big is Phobos? Phobos is the largest moon of Mars, but it is very small compared to Earth’s Moon. Many people are surprised when they learn that Phobos is smaller than most cities on Earth. It has an irregular shape, so it does not look round like a normal moon.

The size of Phobos is about 27 kilometers long, making it one of the smallest moons in the solar system. This tiny moon of Mars orbits very close to the planet and looks huge in the Martian sky. Learning how large Phobos is helps us understand why this unusual Martian moon is so interesting to scientists.

Quick Facts Box

Size

The size of Phobos is very small compared to most moons. Phobos is about 27 kilometers long, 22 kilometers wide, and 18 kilometers tall. Because of its irregular shape, scientists do not measure it with just one diameter like a round moon.

Even though it is called a moon, Phobos is one of the smallest moons in the solar system. This tiny moon of Mars is so small that it could fit inside a large city on Earth. Understanding how big Phobos is helps explain why it has weak gravity and a rough, uneven surface.

Shape

The shape of Phobos is not round like most moons. It has an irregular shape and looks like a potato-shaped moon. This happens because Phobos is very small and does not have enough gravity to pull itself into a ball.

The uneven shape of Phobos is covered with craters, ridges, and grooves. These rough features formed after many asteroid impacts over time. Learning about the shape of Mars’s moon Phobos helps scientists understand how small moons form in the solar system.

Comparison to Earth Moon

When we compare Phobos to Earth’s Moon, the difference is huge. Phobos is much smaller than Earth’s Moon and is one of the smallest natural moons in the solar system. Earth’s Moon is wide and round, while Phobos is tiny and oddly shaped.

The Earth Moon is about 3,474 kilometers wide, but Phobos is only about 27 kilometers long. This means the size of Phobos is more than 100 times smaller than Earth’s Moon. This moon size comparison helps us see why Phobos has weak gravity and cannot be seen from Earth without powerful telescopes.

Why Phobos Isn’t Spherical (Irregular Shape)

Phobos isn’t spherical because it is very small. Small moons do not have enough gravity to pull their rocks into a round shape. This is why Phobos has an irregular shape instead of looking like a ball.

The weak gravity of Phobos lets its surface stay rough and uneven. Over time, asteroid impacts added craters and cracks. Because of this, the shape of Mars’s moon Phobos stayed lumpy and uneven, making it look more like a space rock than a normal moon.

Size Comparisons People Get

Phobos vs Earth’s Moon

When we look at Phobos vs Earth’s Moon, the size difference is easy to see. Phobos is much smaller than Earth’s Moon and has a rough, irregular shape. Earth’s Moon is big and round, while Phobos is tiny and uneven.

The Earth Moon is about 3,474 kilometers wide, but Phobos is only about 27 kilometers long. This big moon size comparison explains why Phobos has weak gravity and cannot hold an atmosphere. Learning about Phobos compared to Earth’s Moon helps us understand how different moons can be.

Phobos vs Manhattan and Chicago

When we compare Phobos to Manhattan, the size of Phobos is about the same length as the island of Manhattan. This means the moon of Mars is small enough to fit over a large city on Earth. It is surprising that something this small can be a moon.

Phobos compared to Chicago would cover only part of the city. Chicago is much wider, while Phobos is narrow and uneven. This city size comparison helps people understand how big Phobos is in a simple and clear way.

Phobos vs Deimos

Phobos is bigger than Deimos, the other small moon of Mars. While Phobos is about 27 kilometers long, Deimos is only about 15 kilometers long. This makes Phobos the larger and faster-moving moon of Mars.

Even though both are tiny, Phobos and Deimos have very different shapes and sizes. Phobos has more craters and grooves, while Deimos is smoother and smaller. Comparing Mars’s moons helps us understand why Phobos is more noticeable in the Martian sky.

What Its Size Means for Observers

Apparent size from surface of Mars

From the surface of Mars, Phobos looks much bigger than it really is. Even though it is tiny, it moves quickly across the sky because it orbits very close to Mars. People on Mars would see Phobos rise and set in just a few hours.

The apparent size of Phobos is larger than Deimos, even though both are small moons. Its fast movement and irregular shape make it easy to notice in the Martian sky. Understanding how Phobos appears from Mars helps scientists plan future missions to the tiny moon of Mars.

Orbit Proximity of Phobos

Phobos orbits very close to Mars—closer than any other moon in the solar system. It circles Mars at only about 6,000 kilometers above the surface, which is very near compared to Earth’s Moon.

Because of its tight orbit, Phobos moves quickly across the Martian sky, completing a full orbit in just about 7.5 hours. This close orbit is why Phobos looks so fast and big from the surface of Mars and why scientists are interested in its movement and future.

Future Fate of Phobos (Inward Spiral)

Phobos is slowly moving closer to Mars every year. Its inward spiral means that one day it might crash into the planet or break apart. Scientists think this could happen in about 30 to 50 million years.

The tiny moon of Mars is already feeling strong tidal forces from the planet. These forces make Phobos orbit lower and lower, changing its speed and shape slightly over time. To understand how moons like this behave, you can also read more about Mars’s moons and what they might become in the future here: https://astronive.com/2025/12/28/mars-as-moon/.

FAQ About Phobos

Q1: How big is Phobos?
Phobos is a small moon of Mars that measures about 27 × 22 × 18 kilometers. This makes it much smaller than Earth’s Moon and one of the smallest moons in the solar system. (NASA Science)

Q2: Why does Phobos look odd shaped?
Phobos has an irregular shape because it is so tiny that its gravity cannot pull it into a sphere. Instead, it looks like a potato‑shaped moon with bumps and curves. (Wikipedia)

Q3: How fast does Phobos orbit Mars?
Phobos orbits very close to Mars and completes a full orbit in about 7 hours and 39 minutes. This is much faster than Mars rotates, so Phobos appears to move quickly in the sky. (NASA Science)

Q4: What will happen to Phobos in the future?
Phobos is slowly moving closer to Mars and may crash into the planet or break apart in tens of millions of years. Tidal forces from Mars are pulling it inward over time. (NASA Science)

Q5: Where can I read more about Phobos?
You can find more official facts about Phobos, the larger moon of Mars on NASA’s science page here:  . (NASA Science)

Conclusion

Phobos is a tiny, irregular moon of Mars that is very different from Earth’s Moon. Its small size, unusual shape, and fast orbit make it one of the most interesting moons in the solar system.

Understanding how big Phobos is and how it moves helps scientists learn more about small moons, Mars, and space exploration. Even though it is tiny, Phobos has a big story to tell about the planets and moons of our solar system.