The Closest Planet To The Sun - But Not The Hottest!
Mercury is the closest planet to the Sun. However, it is not the hottest planet in the solar system (which is Venus) because it has virtually no atmosphere which means the heat from the Sun cannot travel from the daylight side to the night time side. So the sunny side gets very hot (+430 degrees C) and the shaded side gets very cold (-180 degrees C). This is the wildest temperature range of any planet in the solar system. The average temperature is measured at about +167 degrees C.
Most Non-Circular Orbit
Planets move around the Sun in elliptical orbits which means the distance to the Sun changes throughout each orbit. Mercury's orbit is the most eccentric of the any planets which means that it's orbit is the most non-circular. It changes from 46 million km (29 million miles) from the Sun out to 70 million km (43.5 million miles).
Because all planets are held to the Sun by the force of gravity, the closer an object is to the Sun, the faster it must move in order to stay in orbit. This means that Mercury is the fastest planet (orbital velocity of 47.4km per second = 106,000 miles per hour) and orbits the Sun every 87.9 earth days.
Before 1965 It was thought that Mercury's rotation was the same as it's spin and that one side of the the planet always faced the Sun. However when Mercury was studied using radar it was discovered that the planet spun on its axis exactly three times in every two orbits. It's thought that this effect is caused by the eccentric orbit which means that the Sun exerts a strong tidal "tug" on the planet every orbit which has resulted in this lock.
Longest Day - Mid Day to Mid Day
Mercury takes about 88 Earth days to orbit the Sun but rotates on its axis once every 59 Earth days. Because of the slow rotation and the fast orbit, a single day on Mercury (mid day to mid day) takes exactly 2 orbits which is abut 176 Earth days. This means that Mercury has the longest day of any planet in the solar system (and not Venus which is the slowest rotating planet). For a full explanation of this, visit our "Which planet has the longest day?" page.
The Sun Moves Backwards For A While Every Day!
Another unusual aspect of this planet is that the Sun can rise, go past mid-day then go backwards for a while and then forwards again to dusk. This craziness is caused by the two facts: 1. A day (mid-day to mid-day) is twice as long as the planet's orbital period and 2. the planet's orbit is very eccentric, which means that the planet moves about the Sun much faster when it's closer than it does when it's further away.
To explain: When the planet is at it's furthest from the Sun, the planet is spinning on its axis faster than it's rotating about the Sun so the spin wins out and the Sun appears to move in the normal direction in the sky. Later, when the planet is closest to the Sun it's still spinning at the same rate but it's moving faster and closer to the Sun and so it's rotating about the Sun at a much quicker rate. This results in the orbital rotation being faster than the planet's spin and so the Sun appears to move in the opposite direction in the sky.
Because the day is exactly twice as long as the year, this backward movement occurs twice a day every day. However some locations will only see it once where it occurs about mid-day, because the next time the planet is closest to the Sun will be about mid-night when the Sun is not visible.
Other locations will witness a stranger phenomenon when the Sun rises in the morning before dipping back below the horizon and then reappearing. It will then arc across the sky before setting, popping back up again and setting again.
Virtually No Axial Tilt
Its axial tilt (the angle between the planet's rotational pole and the plane of the ecliptic) is remarkably small at 3/100ths of a degree. Much smaller than any other planet. Officially Jupiter has the next smallest tilt at 3.1 degrees, although one could argue that Venus's axis, with a tilt of 177.4 degrees, is the next smallest since it is really only 2.6 degrees, but spinning backwards!
Mercury is the smallest planet 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. However it is larger than any of the dwarf planets (Pluto at 2,374km) and our Moon (at 3,475km), but it is smaller that the largest moon in the solar system which is Jupiter's Ganymede at 5,268 km.
Amazingly, even though the planet is roasted by the Sun, water ice has been discovered on the planet. Because of the low axial tilt of the planet, the interiors of deep craters at the poles remain in perpetual shadow and contain large quantities of water ice. If ever man decided to setup a base on Mercury, locating it within one of these craters would seem to be the best location. Not only is there a source of water, but the clever use of mirrors to bounce light and heat into the crater could provide a perfect temperature control system.
Iron Core But A Weak Magnetic Field
Mercury is a rocky planet (much like the Earth) with a highly cratered rocky surface and a molten iron core. However, it's iron core is very large for the planets size and results in Mercury being the second most dense planet in the Solar System (Earth being the densest). Surprisingly, this large iron core generates a much weaker magnetic field than the Earth’s and is only 1% as strong.
The Sunrise/Sunset Planet
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.