The belief that a tenth planet exists that is Planet X has been around since 1841. Many astronomers such as Percival Lowell had searched for many years for this planet with no success.
The reason as to why astronomers believe planet X exists is because it is believed that comets go on orbits around the whole of our galaxy (milky way) but while observing some comets it seemed to go much further than the orbit of Pluto. If a comet passes a large object the gravitational pull of that object will pull the comet away from its orbit. This is why many astronomers think that a tenth planet exists.
Galileo: This space probe was launched by the USA to observe Jupiter its moon and rings. It was launched by NASA on October 18th 1989 and started its orbit on December 7th 1995. Galileo was named after the Italian astronomer who discovered Jupiters four largest moons in 1610.
Galileo (the space probe) discovered that Jupiters chemical Compositions were similar to the original composition of our sun. Other discoveries were that the lava from volcanoes on Jupiter is hotter than lava on earth. Callistos surface is covered in a dark smooth material. Ganymede has a dense core and a magnetic field. Europas icy surface has criss crossed grooves and ridges.
Voyager: There were two missions launched by the USA in 1977 to Jupiter and beyond.. NASA launched Voyager 1 on September 5th 1977 and Voyager 2 was launched on August 20th 1977. The information that was gathered was studies of Jupiter, Saturn, Uranus and Neptunes satellites, rings and magnetic fields. THE Voyager mission also discovered activity on Io and Triton.
Sputnik: This satellite was launched by the Soviet Union on October 4th 1957. The Sputnik 1 weighed 83.6 kg. The Sputnik 2 carried the first ever living creature in space, Laika the dog. After these missions 8 other Sputnik missions were carried out.
Rover Spirit: The rover spirit was launched by NASA in June 2003 and was set for arrival in January 3rd this year. The rover spirit was launched to discover more about Mars surface and climate. Perhaps to see whether Mars may be suitable for human landing.
COST OF SPACE MISSIONS
The cost of missions to Space can cost up to millions of dollars. For example when NASA launched the Voyager 1 mission it cost them up to an estimated price of $250 million US dollars. This satellite was worthwhile investment as the Voyager 1 is still floating around in space for almost 27 years. While other satellites that cost twice that amount have disappeared, crashed into other planets or they are not able to send information back to Earth.