Moons of planets ,comets,lumps of rock,asteroids,minor planets dust and gases present in the solar system.
Mercury
Mercury is the closest planet to the Sun. It orbits in a highly elliptical orbit ranging from 46 million km (29 million miles) from the Sun out to 70 million km (43.5 million miles).
It takes about 88 Earth days to orbit the Sun but rotates on its axis once every 59 Earth days. Because of the slow rotation, a single day on Mercury (mid day to mid day) takes 176 Earth days. Its axial tilt is remarkably small at 3/100ths of a degree. Much smaller than any other planet.
Mercury is quite small with a diameter of 4,878km, (2/5ths that of Earth) and only 5% of Earth’s mass. Its gravity on the surface is 1/3rd of Earth’s.
Mercury has almost no atmosphere and is blasted by the Sun during the day and exposed to cold space during the night. This means that it undergoes some of the widest temperature swings of any body in the Solar System with temperatures reaching +430 C and dipping down to -180 C.
It has a highly cratered rocky surface and is known to have an iron core. However its magnetic field is much weaker than the Earth’s (1% as strong). Initially RADAR waves reflected from the surface of Mercury indicated that water ice might be present at the poles. It has recently been confirmed by the Messenger Spacecraft that ice water does indeed exist in deep craters at the poles the interiors of which are permanently shrouded in shadow.
Because Mercury is so close to the Sun, it is only ever seen (with the naked eye) just before sunrise and just after sunset. At all other times it is masked by the brightness of the Sun.
Mercury and Man
The Greeks had two names for Mercury, "Apollo" when it appeared in the morning and "Hermes" when it appeared in the evening.
In Roman mythology Mercury is the god of commerce, travel and thievery, the Roman counterpart of the Greek god Hermes, the messenger of the Gods. The planet probably received this name because it moves so quickly across the sky.
Before 2011 it had only been visited by 1 spacecraft - the Mariner 10 spacecraft which performed 3 fly-pasts in 1974/75 mapping about 45% of its surface.
Mercury was recently being studied by the Messenger Spacecraft. Messenger entered Mercury's Orbit on 18th March 2011, the first man made object ever to do so. Messenger was impacted into the surface of Mercury on April 30, 2015, at a speed of more than 3.91 kilometres per second (8,750 miles per hour), marking the end of operations for the hugely successful Mercury orbiter.
Amongst other things, Messenger discovered that:
1. Mercury's weak magnetic field is not symmetrical which allows more solar radiation to hit the south pole than the north.
2. Water ice exists at the poles where it is hidden from the Sun in deep craters.
3. The iron core of Mercury is much larger than expected meaning it has a much thinner rocky crust.
4. There is a lot of sulphur on Mercury, 10 times as much as we see on Earth or Mars, giving rise to suggestions of past volcanic activity.
5. Tectonic features on the surface suggest the core has shrunk as it cooled reducing the planets diameter by 7km in radius - this is much more than expected.
All of the discoveries help us to understand the formation processes of Mercury and thus the formation of the solar system.
Because of its inhospitable environment, Mercury has been one of the least explored of the inner solar system planets.
The next mission at Mercury will be ESAs Bepicolumbo mission launched on October 20th, 2018 to arrive in late 2025.