A spacecraft that escapes from the Earth is called a space probe. The
first probes were sent to the Moon. Russia first achieved success in 1959,
when its probe Luna 1 crashlanded there. Later, Russian and US lunar probes
went into orbit around the Moon and landed on it. Exploration of the planets
from space began in 1962, when the US probe Mariner 2 flew close (35,000
km.) to Venus. Since then probes have visited all the planets in our Solar
System except Pluto. They have also flown to meet a regular visitor to
Earth's skies, Halley's comet.
There are all kinds of problems involved in sending probes to the planets. One is distance. Even our nearest planetary neighbour, Venus, never comes closer to us than 42 million km. And the most distant planet at the moment, Neptune, lies more than 4,000 million km away. Using the rockets we have at present, it takes a probe several months to reach even Venus and our next nearest neighbour, Mars. It takes years to reach the more distant planets: Jupiter, Saturn, Uranus, and so on.
Another problem is aiming the probe. It must be aimed at a point in
space in the target planet's orbit so that it will arrive at the same time
as the planet. If the probe leaves the Earth in slightly the wrong direction
or at slightly the wrong speed, then it could miss its target by hundreds
of thousands of kilometres.
Maintaining communications with a probe over many millions of kilometres is also a major problem. The probe must be tracked precisely so that radio signals can be beamed in the right direction in space. To send and receive signals, large dish aerials are used. NASA, the National Aeronautics and Space Administration of the United States, communicates with US space probes through its Deep Space Network. It comprises tracking stations at Goldstone in California, at Madrid in Spain and at Canberra in Australia. They use dish aerials up to 70 m across.
Because the probes are so far away, there is a time-lag between the
sending and receiving of signals between the tracking station and the probes.
When Voyager 2 sent radio signals back from Neptune in 1989, they took
over four hours to reach the Earth. The transmitter on the probe has a
power output about the same as the bulb in a refrigerator. Yet NASA scientists
were able to convert the signals into remarkably clear pictures.
A
false-colour image (left) of Halley's comet, produced from data returned
by the European space probe Giotto in March 1986. At the time the probe
was less than 1,000 km away. It took a beating from the rocky debris around
the comet, but managed to survive.
A Two Viking probes landed on the Red Planet Mars in 1976, and took close-up pictures of its surface (above). The pictures revealed that the surface is rust-red in colour, and even the sky has a reddish tinge. The landing probes carried automatic soil samplers, which tested the soil for signs of life, but in vain. They reported temperatures up to - 300C and winds gusting up to 120 km/h.
