NASA puts an eye in the sky to unlock Mercury's secrets
NASA scientists have put in place an orbiting eye-in-the-sky to spy on the solar system's smallest and strangest planet, Mercury.
Nasa's spacecraft Messenger successfully veered into a pinpoint orbit last night after a six-and-a-half-year trip, 4.9 billion miles and tricky manoeuvring to fend off the gravitational pull of the sun.
It is the fifth planet in our solar system that Nasa has orbited, in addition to the Earth and the moon.
A NASA Twitter account under Messenger's name gave play-by-play accounts as it arrived and "exchanged tweets" with Voyager 2.
Voyager 2, launched in 1977 and now at the edge of the solar system, tweeted good luck and Messenger answered: "Many thanks! Cold out there? Kinda warm where I am."
Messenger will investigate Mercury's mysterious magnetic field and unusual density.
Messenger is in orbit that brings it as close as 120 miles above the planet's surface. Its chief engineer, Eric Finnegan, said: "This is as close you can possibly get to being perfect. There's a lot of work left to be done, but we are there."