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Mons
Mass
A way of describing the amount
of matter in something. The
mass of an object gives rise
to its gravitational force. The gravitational force created by its mass gives an object its weight. Objects in space are often weightless, meaning they are not acted on by any significant external gravitational force, but they still have mass.
 Mercury—This small planet consists mostly of a rocky mantle and an iron core.
Mercury project
The first NASA project to send people into space. The first American sent into orbit was John Glenn, in 1962.
Meteor
A brief streak of light seen when a meteoroid enters the atmosphere. It is commonly called a shooting star.
Meteoroid
A small rock in space, often no more than a few centimeters across. The majority of them are fragments of asteroids, but others come
from sources scattered randomly
in space. Because they mostly come from asteroids, they are
made of mixtures of rock, carbon, and water (dirty snowballs). They cross the orbit of the Earth all
the time and can be seen burning up in the Earth’s atmosphere at night, making the faint streaks of light called shooting stars. The larger meteoroids sometimes reach the Earth before they burn up completely (see: Meteorite).
Meteoroids are the oldest known rocks, with an average age of
4.6 billion years. Their age is one reason why scientists suggest a similar age for the Earth and the Solar System.
Meteorite
A piece of a meteoroid that survives the trip through the Earth’s atmosphere and hits the surface of the Earth. The most famous of them formed Meteor Crater in Arizona. (See also: Chondrite and Crater.)
Milky Way
The band of shimmery stars and glowing gas that is seen across the sky on a clear night. It marks the disk region of our galaxy.
Minor planets
Another term used for asteroids.
Mir
A Soviet (Russian) space station launched in 1986 and crewed continuously until 2000. It was the longest surviving manned orbiting spacecraft. From 1995 it was operated with the cooperation of NASA and was visited by the Space Shuttle.
Miranda
The fifth largest moon of Uranus (see: Moons), some 480km across and 130,000km from the planet’s surface. It is an extraordinary place, made up of very different types of geological activity. It so resembles a patchwork of different worlds that scientists believe it formed from a giant collision that first blew it apart before most of the parts were pulled together again by gravity.
Mons
The Greek word for mountain, for example, Olympus Mons on Mars.
 Mons—Spectacular Olympus Mons is the largest volcano in the Solar System. It has a summit 24km above the surrounding plains.
Rocky mantle
Mercury
The crust is extremely thin.
Very thin crust Iron core
Mercury (diameter 4,900km) is
the closest planet to the Sun. It follows the most elliptical (oval) path of all the planets except Pluto. It appears gray in telescopes and has a reflectivity similar to the Moon (see: Albedo).
Because its path keeps it
close to the Sun, it is difficult
to see Mercury with the naked
eye. Mercury has just a trace of atmosphere—mainly made up of sodium, helium, and hydrogen. It has a cratered surface (see: Crater).
Without a significant atmosphere, its surface temperature changes violently each day. The temperature soars to over 470°C by day, nearly five times the boiling point of water, but plummets to -183°C at night.
Mercury is just under half the diameter of the Earth and has a large iron core, possibly molten, responsible for weak magnetic fields. The planet orbits the Sun at about 60 million km once every
88 days and spins on its axis once every 58 days. (See also: Mariner.)
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