It's taken the spaceprobe six and a half years to get to the right place at the right time and at the right velocity to make a major 15 minute engine burn for capture into Mercurian orbit.
This is, frankly, an astonishing achievement. Hopefully all will go well and the prime mission will return very large amounts of new and detailed knowledge about the innermost planet.
Read more on these web pages...
- NASA's MESSENGER website
- MESSENGER - the spacecraft (Wikipedia)
- Mercury - the planet (Wikipedia)
- APL information on the orbit insertion burn