- MESSENGER was 28 million miles from the sun and 96 million miles from Earth when it headed into Mercury's orbit.
- The spacecraft began its journey more than six years ago, traveling through the inner solar system and embarking on flybys of Earth, Venus and Mercury.
A NASA spacecraft began orbiting Mercury, becoming the first to fly around the solar system's innermost planet, the space agency said.
The craft, known as MESSENGER, began the orbit around 9:00 p.m. (0200 GMT) on a mission to circle the planet for one Earth year in an unprecedented study of the tiny, hot planet.
The spacecraft began its journey more than six years ago, traveling through the inner solar system and embarking on flybys of Earth, Venus and Mercury.
NASA said that by achieving orbit, "this marks the first time a spacecraft has accomplished this engineering and scientific milestone at our solar system's innermost planet."
For the next several weeks, engineers will be focused on ensuring the spacecraft's systems are all working well in Mercury's harsh thermal environment.
Starting on March 23, the instruments will be turned on and checked out, and on April 4 the mission's primary science phase will begin.
MESSENGER was 28 million miles (46 million kilometers) from the sun and 96 million miles (155 million kilometers) from Earth when it headed into Mercury's orbit, NASA said.
The first NASA craft to study Mercury since the Mariner mission more than three decades ago, MESSENGER has already been able to return a partial map of the planet's crater-filled surface after a handful of flybys.
The craft is carrying seven science instruments, including a Mercury Dual Imaging System (MDIS), the Mercury Atmospheric and Surface Composition Spectrometer (MASCS), and the Energetic Particle and Plasma Spectrometer (EPPS).
It was first launched in August 2004 and has since traveled 4.9 billion miles (about 7.9 billion kilometers) through "a range of extreme conditions," NASA said.