Mercury is the smallest and innermost planet in the Solar System.[a] Its orbital period (about 88 Earth days) is less than any other planet in the Solar System. Seen from Earth, it appears to move around its orbit in about 116 days. It has no known natural satellites. It is named after the Roman deity Mercury, the messenger to the gods.
Partly because it has almost no atmosphere to retain heat, Mercury's surface temperature varies diurnally more than any other planet in the Solar System, ranging from 100 K (−173 °C; −280 °F) at night to 700 K (427 °C; 800 °F) during the day in some equatorial regions. The poles are constantly below 180 K (−93 °C; −136 °F). Mercury's axis has the smallest tilt of any of the Solar System's planets (about 1⁄30 of a degree), and its orbital eccentricity is the largest of all known planets in the Solar System.[a] At aphelion, Mercury is about 1.5 times as far from the Sun as it is at perihelion. Mercury's surface is heavily cratered and similar in appearance to the Moon, indicating that it has been geologically inactive for billions of years.
Mercury is tidally or gravitationally locked with the Sun in a 3:2 resonance,[14] and rotates in a way that is unique in the Solar System. As seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun.[b][15] As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years. An observer on Mercury would therefore see only one day every two years.
Because Mercury orbits the Sun within Earth's orbit (as does Venus), it can appear in Earth's sky in the morning or the evening, but not in the middle of the night. Also, like Venus and the Moon, it displays a complete range of phases as it moves around its orbit relative to Earth. Although Mercury can appear as a bright object when viewed from Earth, its proximity to the Sun makes it more difficult to see than Venus. Two spacecraft have visited Mercury: Mariner 10 flew by in 1974 and 1975; and MESSENGER, launched in 2004, orbited Mercury over 4,000 times in four years, before exhausting its fuel and crashing into the planet's surface on April 30, 2015.[16][17][18]
Mercury is a planet in our solar system. It is the smallest of the eight planets. It is also the closest to the sun. Mercury goes around the sun the fastest of all the planets. Mercury has no moons.
How Did Mercury Get Its Name?
The Romans believed that gods and goddesses were in charge of everything on Earth. Mercury is named after the messenger for their gods. The Roman Mercury had wings on his helmet and shoes. He could travel very quickly from place to place. The planet Mercury moves quickly around the sun. That is how it got its name.
How Big Is Mercury?
Mercury is a little bigger than Earth's moon. It is made of heavier materials, like iron. But if you could weigh Mercury and the moon, Mercury would weigh a lot more. Mercury is heavy, but it is small. It would take more than 18 Mercurys to be as big as Earth.
Where Is Mercury?
Mercury is the closest planet to the sun. Mercury goes around the sun once every 88 Earth days. A day on Mercury lasts a lot longer than a day on Earth. One day on Mercury lasts 59 Earth days.
What Is Mercury Like?
The surface of Mercury looks like Earth's moon. It is covered with holes. The holes are called impact craters. The craters were made by rocks falling from space. The rocks are going very fast when they hit Mercury. A hole is made where the rock hits. Earth has a blanket of air around it. Mercury does not. The blanket is what helps keep Earth from getting too hot or cold. Because it is so close to the sun, Mercury can be very hot. At night, Mercury gets very cold. We could not live on Mercury!
How Has NASA Studied Mercury?
Mercury is hard to study because it is so close to the sun. People have never gone to Mercury. Spacecraft without people have gone. Mariner 10 was the first to visit Mercury. It flew by in 1974 and 1975. Not even half of Mercury was seen then. After that, nothing was sent to Mercury for more than 30 years. NASA's MESSENGER spacecraft flew by Mercury in 2008 and 2009. In March 2011, it began to orbit Mercury. MESSENGER will study parts of Mercury that have not been seen before. It will let scientists learn many new things about the planet.