A NASA spacecraft is poised to enter the orbit of Mercury later Thursday, and it will circle for one Earth year in an unprecedented study of the tiny, hot planet that is closest to the Sun.

The craft, known as MESSENGER, began its journey more than six years ago, traveling though the inner solar system and embarking on flybys of Earth, Venus and Mercury.

It will be the first ever spacecraft to orbit Mercury when it fires its largest thruster at 8:45 pm (0045 GMT Friday) in a 14-minute maneuver to slow down so it can enter its trajectory around Mercury, NASA said.

"The orbit insertion will place the spacecraft into a 12-hour orbit about Mercury with a 200-kilometer (124-mile) minimum altitude," the US space agency said.

MESSENGER will be 46.14 million kilometers (28.67 million miles) from the Sun and 155.06 million kilometers (96.35 million miles) from Earth when it heads 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.

"Orbit insertion is the last hurdle to a new game level, operation of the first spacecraft in orbit about the solar system's innermost planet," said principal investigator Sean Solomon of the Carnegie Institution of Washington.

"The MESSENGER team is ready and eager for orbital operations to begin."

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.