Mercury - closest to the Sun

Mercury is the closest planet to the Sun. Its orbit is said to be "inferior", as it lies between the Sun and the Earth. Consequently, it always appears in the sky near to the Sun, being visible a little before sunrise or a little after sunset. It takes 87·9 days to complete its orbit.

Blown Away

The planet has no appreciable atmosphere, although a small amount of hydrogen, helium and other elements are surprisingly present. The temperature on the Sun-facing side is as high as 430°C. The planet rotates once every 58·6 Earth days, so its entire surface sees the Sun as a result. This surface is well cratered, and has some mountains and valleys. Interestingly, its rotation is tidally coupled to the Sun, rotating 1·5 times every orbit.

Frozen Poles!

Perhaps more unexpected than its ultra-thin atmosphere (perhaps provided by passing comets?), is that Mercury has ice caps. These can exist because they are in permanent shadow due to the low position of the Sun on the horizon and mountainous terrain.

Mercury is the densest of all the planets, with a large iron core. This also gives it an appreciable magnetic field.

Sole Visitor

Top Right ^^

Mercury Transit!
In May 2003, Mercury was seen from Earth to move over the face of the sun. This is called a Transit. Venus also does this, but much less often.	Mercury transit of 15th November 1999 © A.Wong

A book on transits of Venus and Mercury is available here.

Future Messenger

Due for launch in 2004 is MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging). It will enter Mercury orbit on 30^th September 2009, and complete its mission one year later.

This raises the question: why take so long getting there? In order to enter a permanent orbit, it needs to approach Mercury at slow speed, or it must carry enough fuel to slow down. The latter is impossible - tonnes of fuel could not be carried, let alone launched from Earth. Instead, the probe will use the gravity of Earth, Venus and Mercury to orientate itself appropriately to allow a smooth orbital insertion around the planet.

Mercury Re-visited

The probe (shown above right) will flyby Mercury in January and October 2008, giving humans the first close-up images of the planet for more than 30 years. Amazingly, it will be the first time that we will have viewed the entire surface in this way - Mariner 10 only saw one half.

The probe has a large collection of science instruments. They are designed to help answer the following questions:

1. Why is Mercury so dense?
2. What is the geologic history of Mercury?
3. What is the structure of Mercury's core?
4. What causes Mercury's magnetic field?
5. What are the unusual materials at its poles?
6. Where does its atmosphere come from?