A flat or plane surface has just two dimensions at right angles to each other.
If something is moving, three dimensions are not enough to locate it. You need a fourth dimension – time – to describe where the object is at a length and width.
Any point on a flat surface can be pinpointed exactly with just two figures: one particular time, showing how far along it is and the other how far across.
There are three dimensions of space at right angles to each other: length, width and height.
A box can be described completely with just one figure for each dimension.
A point in space can be pinpointed with three figures: one shows how far along it is, one how far across it is and a third how far up or down it is.
In the early 1900s, mathematician Hermann Minkowski realized that for Einstein’s relativity theory you had to think of the Universe in terms of four dimensions, including time.
Four-dimensional space is now called space-time.
An eclipse of the Sun in 1919 showed Einstein’s general relativity shows suggestion that gravity can bend light is true, that in bending light, gravity is also bending actually curved.
After Minkowski’s ideas, German mathematician Hermann Minkowski used geometrical methods to solve difficult problems in number theory, in mathematical physics and the theory of relativity, mathematicians began to develop special geometry to describe four or even more dimensions.
Space is explored in two ways – by studying it from Earth using powerful telescopes, and by launching spacecraft to get a closer view.
Most space exploration is by unmanned space probes.
The first pictures of the far side of the Moon were sent back by the Luna 3 space probe in October 1959.
Manned missions have only reached as far as the Moon, but there may be a manned mission to Mars in 2020.
Apollo astronauts took three days to reach the Moon.
No space probe has ever come back from another planet.
Travel to the stars would take hundreds of years, but one idea is that humans might go there inside gigantic spaceships made from hollowed-out asteroids.
Another idea is that spacecraft on long voyages of exploration may be driven along by pulses of laser light.
The Pioneer 10 and 11 probes carry metal plaques with messages for aliens telling them about us.
Apollo 11, the US spacecraft that made the famous journey to the Moon, in 1969.
Most space exploration is by unmanned probes, guided by on-board computers and equipped with various devices which feed data back to Earth via radio signals.
Space probes are automatic, computer-controlled unmanned spacecraft sent to explore space.
The first successful planetary probe was the USA’s Mariner 2, which flew past Venus in 1962.
Mariner 10 reached Mercury in 1974.
Vikings 1 and 2 landed on Mars in 1976.
Voyager 2 has flown over six billion km and is heading out of the Solar System after passing close to Jupiter (1979), Saturn (1980), Uranus (1986) and Neptune (1989).
Most probes are ‘fly-bys’ which spend just a few days passing their target and beaming back data to Earth.
To save fuel on journeys to distant planets, space probes may use a nearby planet’s gravity to catapult them on their way. This is called a slingshot.
In the first ten years of the 21st century, more than 50 space probes will be sent off to visit planets, asteroids and comets, as well as to observe the Moon and the Sun.
Space probes will bring back samples from Mars, comets and asteroids early in the 21st century.
Space suits protect astronauts when they go outside their spacecraft. The suits are also called EMUs (Extra-vehicular Mobility Units).
The outer layers of a space suit protect against harmful radiation from the Sun and bullet-fast particles of space dust called micrometeoroids.
The clear, plastic helmet also protects against radiation and micrometeoroids.
Oxygen is circulated around the helmet to stop the visor misting.
The middle layers of a space suit are blown up like a balloon to gently hold the astronaut’s body. Small astronauts actually have room to ‘float’ inside their space suits.
The soft inner lining of a space suit has tubes of water in it to cool the astronaut’s body or warm it up.
The backpack supplies pure oxygen for the astronaut to breathe, and gets rid of the carbon dioxide he or she gives out. The oxygen comes from tanks which hold enough for up to seven hours.
The gloves have silicone-rubber fingertips which allow the astronaut some sense of touch.
Various different gadgets in the suit deal with liquids – including a tube for drinks and another for collecting urine.
The full cost of a spacesuit is about $11 million although 70 percent of this is for the backpack and control module.
The first artificial satellite, the Soviet Sputnik 1, was launched into space in 1957.
The first living creature in space was the dog Laika, on-board Sputnik 2 in 1957. Sadly, she died when the spacecraft’s oxygen supply ran out.
The first manned space flight was made in April 1961 by the Soviet cosmonaut Yuri Gagarin, in Vostok 1.
The first controlled Moon landing was made by the Soviet Luna 9, in February 1966.
In 1970, the Soviet Venera 7 was the first probe to touch down on another planet.
The Soviet robot vehicles, the Lunokhods, were driven 47 km across the Moon in the early 1970s.
The coming of the space shuttle in 1981 made working in orbit much easier.
Some cosmonauts have spent over 12 continuous months in space on board the Mir space station.
Cosmonaut Valeriy Poliyakov spent 437 days in space including the longest ever stay aboard space station Mir.