This week, the news that MESSENGER has become the first space probe ever to be placed into orbit around the planet Mercury has been widely reported. This marks a historic milestone in our exploration of the Solar System, thirty years after the Voyager probes returned superb images of its outer planets – Jupiter, Saturn, Uranus and Neptune. Whilst our understanding of the other planets has steadily increased over the intervening years, Mercury has always remained an elusive planet.

The problem is that it's very difficult to put a spacecraft into orbit around Mercury, because it's so close to the Sun. Any probe sent from Earth has to swoop in towards the Sun, and the effect is rather like rolling down a hill: the spacecraft ends up travelling so fast at the bottom of the hill, when it gets to Mercury, that it shoots past the planet rather than going into orbit.

The only spacecraft to have made that journey before MESSENGER was Mariner 10 in 1974, which was never designed to go into orbit around Mercury, and was only able to map one side of the planet as it flew past. MESSENGER has been able to map the other side of the planet, meaning that for the first time, we now have complete maps of the surfaces of all of the planets of our Solar System.

Mercury is interesting because it's one of the Solar System's four rocky planets. All four of them appear to have begun from very similar starting points, but to have evolved down very different paths. The Earth, of course, has turned out to be well suited for life. Mars has no magnetic field and a very thin atmosphere, exposing its surface to the Sun's ultraviolet and ionising radiation. Venus' thick atmosphere of CO₂ has lead to a runaway greenhouse effect and surface temperatures of 480°C.

Mercury is the twin that we know rather little about. It actually has a fairly strong magnetic field, protecting its surface from the worst of the Sun's radiation, but also suggesting that it has a surprisingly large iron core at its centre. Current thinking is that it might have once been an Earth-sized planet, but that it lost most of its outer mantle in a violent collision with another planet, leaving only the core with a thin outer layer. We will undoubtedly learn much more about Mercury's history over the next few years, and that will help us to understand not only our own Solar System, but also what we might expect to find around other stars.

The sky on 27 Apr 2024

Source

MESSENGER Mission Page at NASA

Transcript of a news story presented on the Naked Scientists, 20th March 2011.

Image credit

© NASA

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