NASA's spacecraft MESSENGER is running out of fuel, so scientists will purposely crash it into Mercury later this week.
Along the way, they will gather data about this mysterious planet.
MESSENGER was launched in 2004, performed a series of flybys in 2008 and 2009 before entering Mercury's orbit in 2011.
Prior to this, data on Mercury came from telescopes and a flyby performed by Mariner 10 in the mid 70s.
MESSENGER has greatly improved our knowledge of the planet closest to the sun. Here is some of what it has taught us:
1. Mercury has ice caps
Mercury flies closer to the sun than any other planet. At its nearest pass it is a mere 29 million miles away from the big star.
Despite this, the planet somehow has ice caps at its poles.
Scientists believe the ice is able to stay frozen because there are places at Mercury's poles that are permanently in shadow.
2. It has a lopsided magnetic field
Mercury is the smallest planet in the solar system, only slightly larger than Earth's moon, but also has a large-scale magnetic field which isn’t typical for something that size.
There are only four other celestial bodies in the solar system with such a field; Earth, Saturn, Jupiter and its moon Ganymede.
Odder still, Mercury’s magnetic field isn’t balanced between north and south, like Earth’s is.
Instead, the planet's magnetic field is three times stronger in the north than it is in the south.
Scientists are still debating what causes this to occur. Some think it has to do with the planet's slow rotation and the way its core forms.
Others hypothesize there may be variations in the mantle of the north and south causing the imbalance.
3. Days there are really long
You think your day sometimes feels long? Try a day on Mercury.
One day there takes 59 Earth days, meaning it takes about two months for Mercury to complete one rotation.
However, years on Mercury are relatively short. It only takes 88 Earth days for the planet to complete an orbit around the sun.
4. Mercury is mostly core
Earth has lots of layers, and only about half the distance of its radius is taken up by the core.
Mercury's radius however is 85 percent core, which is totally hardcore.
The inside is mostly iron, says Caltech researcher Hao Cao.
He says scientists are still figuring out why Mercury is made up of so much core.
One theory is that the planet once had more mantle to balance things out, but that layer was stripped away over time by repeated impacts from space rocks.
5. Its orbit is very elliptical
Mercury has a highly oval-shaped orbit around the sun.
If you could visit Mercury when it's closest to our sun, it would look three times larger in the sky than it does on Earth, so you'd need some serious sun block and shades.