The MAVEN orbiter was successfully launched yesterday afternoon. If all goes well, it will reach Mars in ten months.
The purpose? To work out what happened to Mars' atmosphere. This is important data - it will help us understand climate and planet formation better. The lead theory is that Mars, lacking a magnetic field, was exposed to the solar wind, which slowly blew the atmosphere away. If true, it might mean that a significant magnetic field is an essential element of a habitable planet. Maven might tell us the answer.
Once its mission is over, MAVEN will be parked in orbit to act as a communications relay for present and future surface rovers.
A few days after arrival, it will be joined by India's Mangalyaan mission, which launched sooner but is on a different flight path.
So. To Mars!