Does Mercury Have Moons?

Why doesn't Mercury have any moons?

Mercury sits closer to the Sun than any other planet, and that closeness is the problem. According to NASA, Mercury has no moons. The Sun's gravity is so dominant at Mercury's distance that any object Mercury might capture or hold would be pulled away by the Sun's pull instead. There's a region around each planet called the Hill sphere, where the planet's gravity wins over the Sun's; Mercury's Hill sphere is too small and unstable to support a moon.

Could Mercury ever have had a moon?

Possibly. Some models suggest Mercury could have briefly captured small objects in the past, but they wouldn't have stayed long. Without a stable Hill sphere big enough to hold a satellite indefinitely, any moon would either crash into Mercury or get yanked into a solar orbit. There's no current evidence Mercury has lost a moon, but the geological history of the planet has plenty of impact craters, some of which could plausibly be from objects that came too close.

Which other planets have no moons?

Venus is the only other planet without moons. Like Mercury, it sits inside the Sun's strong gravitational reach, and although Venus has a Hill sphere wide enough in theory to host a small moon, none has ever been detected. From Earth outward, every planet has at least one moon: Earth has 1, Mars has 2, and the gas giants have dozens each. Jupiter and Saturn alone account for over 100 known moons combined.

Could a spacecraft act like a moon?

Sort of, briefly. Mercury has been orbited by spacecraft, including NASA's MESSENGER mission from 2011 to 2015, and is currently being studied by the ESA and JAXA BepiColombo mission. These spacecraft technically meet the definition of an artificial satellite while in orbit, but they require constant adjustments to stay in place because Mercury's environment is so gravitationally unstable. A natural moon couldn't make those adjustments and wouldn't survive.