Mercury is the smallest planet in the Solar System and the closest to the Sun. It is about 4,880 kilometers (3,030 miles) across, a little more than a third of Earth's width. That figure is its mean diameter.

Size

NASA gives Mercury's mean radius as 2,439.7 kilometers, so its mean diameter is about 4,880 kilometers. The mean diameter is the width of a perfect sphere with the same volume. Mercury is almost perfectly round: its radius at the equator is only about 2 kilometers more than its radius at the poles.

Mercury is small but dense. It is the second-densest planet, after Earth, and it has a large metal core. The core's radius is about 2,074 kilometers (1,289 miles), about 85% of the planet's radius. Its mantle and crust together are only about 400 kilometers (250 miles) thick.

Size comparisons

• Mercury is about 1.4 times as wide as the Moon.
• Two moons are wider than Mercury. Ganymede, which orbits Jupiter, is about 8% wider, and Saturn's moon Titan is about 6% wider. Mercury has more than twice the mass of either.
• Jupiter's moon Callisto is almost exactly as wide as Mercury, about 99% of its width.
• Earth has about 18 times Mercury's volume.
• If the Sun were shrunk to the size of a basketball, Mercury would be a speck about 0.8 millimeters wide, about 10 meters (33 feet) away.

Years, days and temperatures

Mercury orbits the Sun at an average distance of about 57.9 million kilometers (36 million miles). It is the fastest planet, going around the Sun every 88 Earth days. But it spins slowly, so one full cycle of day and night lasts 176 Earth days, about two Mercury years.

Mercury has almost no atmosphere to hold in heat. Daytime temperatures can reach 430 degrees Celsius (800 degrees Fahrenheit), and at night they can fall to about minus 180 degrees Celsius (minus 290 degrees Fahrenheit). Even so, Mercury is not the hottest planet. Venus is hotter, because of its thick atmosphere.

Visiting Mercury

NASA's MESSENGER was the first spacecraft to orbit Mercury. It arrived in 2011 and studied the planet for more than four years, until it crashed into the surface on April 30, 2015. As of September 2026, BepiColombo, a mission of the European and Japanese space agencies, is due to enter orbit on November 21, 2026. It carries two spacecraft that will study Mercury at the same time.

Sources

• Mercury Fact Sheet. NASA, 2024.
• Mercury Facts. NASA.
• MESSENGER. NASA.
• Latest updates: BepiColombo's arrival at Mercury. European Space Agency, 2026.
• Jovian Satellite Fact Sheet. NASA, 2023.
• Saturnian Satellite Fact Sheet. NASA, 2025.
• Planetary Satellite Physical Parameters. NASA Jet Propulsion Laboratory.
• Moon Fact Sheet. NASA, 2024.
• Earth Fact Sheet. NASA, 2024.
• Sun Fact Sheet. NASA, 2024.
• Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.