Mars is only a small planet.It's about half as big as the Earth.How ever scientists and very interested in it.The first spacecraft to visit Mars was. Mariner 4 in 1965. Since then,more than thirty spacecraft have been sent to learn more about the planet and its two small moons.The most sucesful of these until now has been the USA's Phoenix,which landed on Mars on 25 May 2008.Phoenix took more than 30,000 pictures of the planet.Scientist have leaned a lot about Mars from these picturces.