Glossary of terms


The NASA space program that took the first astronauts to the moon and safely back. The program was split into several different missions, with Apollo 11 being the first to accomplish a successful human lunar landing on July 20, 1969. Apollo 17 was the last mission of the Apollo program to the moon, landing on December 11, 1972.
asteroid belt
A zone between the orbits of Mars and Jupiter at about 1.7 and 4.0 AU containing the majority of asteroids.
Small rocky bodies orbiting the sun. The majority of asteroids are contained within the asteroid belt. Some asteroids have different orbits, and these include the near earth asteroids, whose orbits may have a chance of colliding with the earth.
Short for astronomical unit. A unit of length used to measure distances within the solar system and is equal to the mean distance between the Earth and the Sun, or about 149.5 million km.





Our planet. The third planet of the Solar System and the largest of the inner planets. It orbits the sun at a mean distance of 1 AU in 365.26 days. The Earth's equatorial diameter is 12 756 km and it rotates every 23 hours and 56 minutes on its axis, which intersects the north and south poles, at a tilt of 23.45°. It is this tilt of its axis that is responsible for the seasons and the variation in daily sunlight throughout the year. The Earth has one natural and stable satellite, the Moon, orbiting the Earth at a distance of 384 400 km. It has a diameter of 3476 km and rotates synchronously to its orbit on its north-south pole axis, keeping always the same side towards the earth, the nearside. It was explored by human astronauts from 1969 to 1972.




A vast region that surrounds the Sun, where the solar wind plays a fundamental role.



The largest planet in the solar system. It is the 5th planet from the Sun and the first of the outer planets, orbiting the Sun at a mean distance of 5.2 AU in 11.86 years. It has an equatorial diameter of 143 985 km, about 9 000 km more than its polar diameter and has a rotation period of less than 10 hours.


Short for kiloparsec (1000 pc). See also parsec



Also known as Whirlpool Galaxy. The 51st object of Messier's list of diffuse objects.
The fourth planet from the Sun, with a diameter of 6795 km. It orbits the sun every 686.98 days at a mean distance of 1.53 AU. It rotates every 24 hours and 37.45 minutes on its north-south pole axis, which is tilted at 25.19°. This makes it more like Earth than any other planet. Viewed from Earth, Mars presents a reddish disk. It has been visited by several robotic probes, including the Viking lander missions in 1976 and the Pathfinder mission in 1997, which explored from the surface of Mars.
The first planet from the Sun, with a diameter of 4878 km. It orbits the sun every 88 days at a distance of 0.39 AU.


The eighth planet, orbiting the sun every 164.79 years at a distance of 30 AU. It has a diameter of 50 538 km. Neptune was visited in 1989 by Voyager 2, which showed the planet to possess a rich blue colour due mainly to methane in its atmosphere. It has 8 known satellites, among them Triton, which was shown by Voyager to have a multi coloured intricate surface.



Short for parallax second. The distance at which the semimajor axis of the Earth's orbit appears to be one arc second long. One parsec equals 30.857×1012km, or 3.2616 light years.
The ninth planet from the Sun, discovered in 1930. It orbits the Sun at a mean distance of 39 AU, but its orbit is highly eccentric, making it closer to the Sun than Neptune for about 20 years of its orbital period of 248.59 years. It has, relative to its size, a large satellite, Charon, orbiting Pluto at a distance of about 20 000 km every 6.39 days. It's the only planet not yet visited by any human probe.




Saturn is the sixth planet, orbiting the Sun at a distance of 10 AU every 29.46 years. It is the second largest planet, with an equatorial diameter of 120 537 km and it is also the most oblate with a polar diameter of 107 519 km. It also has the most prominent system of rings, easily visible in telescopes from Earth. Saturn has 18 known satellites, including the largest in the solar system, Titan. It was visited by Pioneer 11 and Voyagers 1 and 2, with the Cassini mission bound to arrive at Saturn in 2004. The moon Titan is enshrouded in a thick orange hued atmosphere, hiding its surface.



Uranus is the seventh planet, beyond Saturn, orbiting the Sun at a distance of 20 AU every 84 years. It is slightly bigger than Neptune with a diameter of 51 119 km. Its atmosphere looks greenish and featureless, as clearly revealed by images from Voyager 2 in 1986.


Venus is Earth's sister planet, more due to their similar sizes and density than anything else. It orbits the Sun beyond Mercury and inside Earth's orbit at a distance of 0.72 AU every 224.7 days. Venus has a diameter of 12 104 km and is entirely covered by a thick atmosphere of carbon dioxide, water, sulfuric acid, nitrogen and sulfur. The surface temperature reaches a hellish 730 K (~ 460°C), the result of a runaway greenhouse effect. The atmospheric pressure is also 90 times the pressure on Earth, thus from an environmental sense, Venus is very different from Earth. The Soviet Venera probes were able to show the surface of the planet in th 1970s and the 1980s for the first time.
Voyager 1 and 2 were two US space probes launched in 1977 towards the outer solar system, beyond the asteroid belt. Voyager 1 approached both Jupiter and Saturn, in 1979 and 1980 respectively, after which it continued to leave our solar system. Voyager 2 explored Jupiter, Saturn, Uranus and Neptune. It then continued to move beyond our solar system. As of 2003, both probes still have power to be able to make observations of the heliosphere.