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
- 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
- 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
- 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