The moon Phobos snuck into photos the Hubble Space Telescope was taking of its parent planet, Mars. This composite puts together all 13 photos Hubble took. NASA

A moon has pulled off one of the solar system’s best photobombs, sneaking into the Hubble Space Telescope’s pictures of Mars.

NASA’s Goddard Space Flight Center released photos on Thursday of the sneaky moon creeping into the Martian shots, although Phobos is so small that it just looks like a star in the distance. Hubble took 13 images in a little more than 20 minutes, so the telescope was able to catch Phobos moving quote a bit around Mars. When the photos are stitched together, they look like a movie of its orbit.

It was just one small piece of the moon’s rapid loop around its host planet, which it orbits three times a day — faster than Mars can rotate on its axis. According to NASA, Phobos is the only moon in the solar system that orbits faster than the number of hours in its planet’s day.

But its quick orbit is not the only thing that makes Phobos unique. The moon, whose name means “fear” in Greek and comes from a son of Ares, the god of war, is also closer to its planet than any other moon, at just 3,700 miles from the planet’s surface. It is also one of the smallest moons in our solar system, measuring only 16.5 miles long.

“It is so tiny that it would fit comfortably inside the Washington, D.C., Beltway,” NASA said.

Read: 5 Moons to Explore in Our Solar System

Scientists aren’t exactly sure how Phobos got to be orbiting Mars. Some speculate that, with its oblong shape, it is an asteroid that originated in the asteroid belt between Mars and Jupiter — the ring that separates the inner and outer solar system — but got swept up in the Red Planet’s orbit. Others say it formed from debris that was thrust into space as other objects crashed into Mars while it was forming.

One theory that recently emerged plays upon the idea of those collisions, which would have been common when the solar system was younger. Researchers have suggested that an enormous asteroid the size of a dwarf planet crashed into Mars about 4.4 billion years ago. They say such a collision would have thrown rocky debris into orbit around Mars that would have eventually stuck together and formed Phobos and the other Martian moon, Deimos, which orbits farther out.

That model, the researchers say, would also explain why Mars’ northern hemisphere has such different geography from its southern hemisphere — smooth, flat land compared to crater-pocked highlands — and why Mars has so rare metals in such high quantities its mantle.

Read: Did a Dwarf Planet Crash into Mars? That Would Explain Its Weird Moons

For now, Phobos remains a bit of a mystery, although some scientists are planning a trip that could make its origin a little clearer. France and Japan announced earlier this year that they might work together to launch a space probe that would land on Phobos in 2024 and collect samples that would be brought back to Earth. If no other mission to another location in our solar system beats these nations to the punch, it would be the first time scientists have had samples on Earth from another otherworldly body besides our own planet’s moon. Experts from those countries suggested that having the samples would help them understand Phobos’ origin.

That could, in turn, help understand the origin of Deimos as well. And a space probe on Phobos would have an interesting view of Mars itself.

We have between 30 million and 50 million years to figure it out. After that time, Phobos is expected to crash into Mars or be torn into pieces. The moon has long grooves in it that are likely caused by the gravitational pull of Mars, NASA explained — they are basically planetary stretch marks. And it moves 6.5 feet closer to the planet every 100 years.