Astro Glossary

Mercury

The first planet from the Sun in the solar system. Mercury orbits the Sun at an average distance of 0.4 times Earth’s distance. The planet is just five percent as massive as Earth, and it about 50 percent larger than the Moon. It is made of solid, rocky material that is heavily cratered, and it has no significant atmosphere. Mercury has no moons.

Mercury Project

Project Mercury placed the first American astronauts in space, in the 1960s. The project’s objectives were to successfully place a manned spacecraft in orbit, investigate an astronaut’s performance capabilities and his ability to function in the environment of space, and recover the man and the spacecraft safely. The project successfully launched six astronauts into space, beginning with Alan Shepard on May 5, 1961.

Meso-American Astronomy and Folklore

Messenger to Mercury

NASA’s Messenger spacecraft entered orbit around Mercury in 2011 for a one-year mission to this least-explored of the solar system’s inner planets. It flew by Mercury three times before settling into orbit, with each encounter reducing its speed. Messenger will map the planet’s entire surface, measure the chemical and mineral composition of its rocks and soil, measuring its gravitational and magnetic fields, and conduct other observations.

Messier Objects

A list of about 110 astronomical objects compiled by French astronomer Charles Messier at the end of the 18th century. The catalog includes objects such as nebulae, star clusters, and galaxies, and the objects are denoted by an M followed by a number from 1 to 110.

Meteor Shower

A display of meteors streaking through the sky at rates of several to hundreds per hour. Whenever Earth intercepts a stream of comet debris in orbit around the Sun, some of the debris falls into Earth’s atmosphere, producing the meteor shower. Two of the best annual meteor showers are the Perseids in August and the Leonids in November. See our list of this year’s meteor showers for more information.

Meteors and Meteorites

A meteor is a “shooting star”; a bit of interplanetary ice or rock that vaporizes in Earth’s upper atmosphere as a burst of incandescent gas. A meteorite is a space rock that strikes the surface of Earth. Most meteorites are classified as stony because of their mineral composition, which may include olivine, pyroxene, serpentine, sulfates, organic compounds, iron, and nickel. See our list of this year’s meteor showers for more information.

Middle Eastern Astronomy and Folklore

Milky Way Galaxy

Our home galaxy. The Milky Way is a spiral galaxy with four major arms containing young bright stars, gas, and dust. The mass of the Milky Way is estimated to be between 400 billion and one trillion times the mass of the Sun. The Milky Way’s diameter is about 100,000 light-years, with the Sun orbiting about 27,000 light-years from the galactic center. However, most of the galaxy’s mass consists of “dark matter” which encircles the Milky Way’s disk in a vast halo. Dark matter may account for 90 percent of the Milky Way’s mass.

Mir Space Station

A Russian space station that remained in low Earth orbit from 1986-2001. It served as a research laboratory where crews conducted experiments in biology, physics, astronomy, meteorology and spacecraft systems, with an aim to develop technologies required for the permanent occupation of space.