1. Half of Mercury had never been seen.
Until NASA's MESSENGER spacecraft made its first flyby of Mercury in 2008, NASA only had close up pictures of one half of Mercury's surface. These first pictures were captured by NASA's Mariner 10 spacecraft, which made a series of three flybys in 1974-75. During its closest flyby, Mariner came within 327 km of the surface of Mercury, and detected its magnetic field. Just after its final close approach, Mariner 10 ran out of fuel, and has probably been orbiting the region ever since.
It wasn't until MESSENGER arrived in 2008, and captured the first close-up images of Mercury that planetary scientists finally got to see the other half of Mercury, up close for the first time. The resolution and details captured by MESSENGER are incredible, and have given scientists data to study for years.
Want more Mercury (the planet) facts? Read on
2. Mercury has a magnetic field
During its closest flyby, Mariner 10 detected a faint magnetic field around Mercury. This magnetic field is very similar to the one we have on the Earth, which protects are planet from the Sun's solar wind. Since Mercury cooled down a long time ago, it can't have the kind of dynamo running inside the Earth. So where's the magnetic field coming from? This is one of the big questions that NASA's MESSENGER spacecraft will try to answer.
Need more interesting facts on Mercury? Here's #3.
3. There might be ice on Mercury
I know, this one's hard to believe, since Mercury is so close to the Sun, and it's baking hot. But the reality is that there are regions on the surface of Mercury which are never heated by the Sun. These are craters around the poles of the planet which are eternally in shadow. And because they're in shadow, they can be hundreds of degrees below zero. Water could form ice in these craters that could last for millions of years. Once again, finding out if there's ice on Mercury will be one of MESSENGER's missions.
4. You can see Mercury with your own eyes
Mercury is actually a difficult planet to observe. Because it orbits closer to the Sun than the Earth, it appears to stick close to the Sun in the sky. The only times you can actually observe it is when the Sun has just set, or is about to rise. You only have a short amount of time to spot it, and you need a place that has a clear view to the horizon. When you hear Mercury is in the sky, find a clear view to the horizon in either the East for the morning, or the West in the evening. It will be a very bright object that will either set after the Sun, or fade away as the Sun rises.
5. We have known about Mercury for millennia
Unlike Uranus, Neptune, and Pluto, which were discovered in the last few hundred years, ancient peoples have known about Mercury for thousands of years. There are recorded observations of Mercury made by the ancient Greeks and Romans; they named it after the god Hermes, who pulled the Sun across the sky.
6. Mercury has an atmosphere
Mercury is so small that it has too little gravity to hold an atmosphere like Earth or Venus. But it does have a tenuous atmosphere of hydrogen, helium, oxygen, sodium, calcium and potassium around it. They don't form a stable atmosphere. Instead, there's a constant flow of these atoms into orbit around Mercury, and then the Sun's solar wind blasts it away into space.
7. It has the most eccentric orbit of all the planets
Now that Pluto is no longer a planet, Mercury takes the record for the most eccentric orbit. This means that its orbit is an ellipse, varying its distance to the Sun. At its closest point, Mercury gets to within 46 million km, and then it ranges out to 70 million km from the Sun.
8. The orbit of Mercury helped prove Einstein's theories of relativity
As astronomers got more and more accurate instruments, and a way to mathematically describe the motions of the planets, they realized there was something wrong with Mercury's movements around the Earth. They noticed that the closest point of Mercury's orbit (its perihelion) was slowly moving around the Sun. And they didn't know why. It finally took Einstein's calculations of general relativity to predict the motions of Mercury exactly.
9. Hubble can't look at Mercury at all
The Hubble Space Telescope has never been used to observe Mercury, and it never will be. The planet is so close to the Sun that the light from the Sun would overwhelm Hubble, and could permanently damage its optics and electronics.
10. Spacecraft are on their way
As we mentioned above, NASA's MESSENGER spacecraft is already on its way to Mercury. If all goes well, it'll pass the planet several more times, and finally go into orbit around the planet in 2011. The European Space Agency is working on a mission with Japan called BepiColumbo. This spacecraft will orbit Mercury with two probes; one to map its surface, and the other to study its magnetic field. Unfortunately, the concept of a lander was shelved.
