Scientists Say: Quasi-satellite

These tagalongs orbit outside a planet’s main sphere of gravitational influence

In this illustration, an asteroid orbits its planet. An asteroid — or other space object— that appears to orbit a planet but orbits beyond the planet’s main sphere of gravitational influence is called a quasi-satellite.

SCIEPRO/SCIENCE PHOTO LIBRARY / Getty Images Plus

By Katie Grace Carpenter

June 16, 2025 at 6:30 am

Quasi-satellite (noun, “QWA-see-SAT-el-lite”)

A quasi-satellite is a space object — such as an asteroid — that appears to orbit a planet like a true satellite. However, the object lies beyond the reach of much of that planet’s gravity.

The prefix quasi- means to resemble something. A quasi-satellite resembles a true satellite. But it’s not one.

Earth’s moon is a true satellite. The sun exerts more gravitational pull on the moon than Earth does. Even so, Earth controls much of the moon’s orbit. That control comes from the moon’s closeness to Earth. The moon orbits within the Earth’s Hill sphere — the space around a planet where the planet’s gravity dominates. The Earth, meanwhile, orbits well within the sun’s Hill sphere.

A quasi-satellite orbits outside a planet’s Hill sphere. That doesn’t mean the planet’s gravity can’t affect the object. But it does mean that other gravitational influences intervene more frequently. For example, the gravitational pull from the sun dominates the orbit of Earth’s quasi-satellites. For that reason, quasi-satellite orbital paths change over time. They will most likely eventually fall out of orbit around their planet.

A quasi-satellite orbits the sun with its planet. The object circles the planet while orbiting the sun. If you were to watch from space, the object would appear to loop around Earth as both objects travel around the sun. Those loops around Earth are usually oval-shaped, not circular. This stretched-out orbit can take the object far from its planet.

Earth has several quasi-satellites. Cardea is one. This asteroid is less than 300 meters (985 feet) in diameter. Astronomers predict that in about 600 years, Cardea will likely be flung out of orbit. Kamoʻoalewa is another example. Astronomers believe this small asteroid is a fragment of the moon. In May 2025, the China National Space Administration launched a mission to collect samples from the object.