After a seven-year journey, a NASA spacecraft has become the first to orbit Saturn, the second-largest planet in the solar system. On Wednesday night, Cassini began a four-year tour of the planet.

During orbit insertion, it passed approximately 20,000 kilometers (approximately 12,500 miles) above Saturn's cloud tops, closer than any other spacecraft in history, and also flew about 10 times closer to the rings than it will at any other point in the mission.

NASA says the mission provides an unprecedented opportunity to study the planet and rings at close range. For this purpose, the $3.3 billion craft is armed with 12 scientific instruments.

Also, the spacecraft is carrying a probe, called Huygens. In December, it will be deployed on Saturn's largest moon, Titan, which is believed to have an atmosphere similar to the one Earth had four billion years ago.

How Cassini entered Saturn's orbit:
It approached the planet from below the ring plane and crossed the large gap between the F Ring and G Ring, passing approximately 158,500 km (about 98,500 miles) above the centre of the planet.

In order to slow the spacecraft enough to be captured by Saturn's gravity, Cassini's main engine will be turned to face the direction of travel. The resulting thrust from the engine will act as a braking device, which will slow down the spacecraft as it enters Saturn's orbit. The burn will place Cassini in a highly elliptical orbit.

The main engine burn began shortly after Cassini crossed the rings. It was scheduled to last 96 minutes.

Cassini's closest approach to Saturn during the entire four-year tour will occur during this period. Its distance from Saturn will be about 18,000 km (about 11,187 miles), which is less then a sixth of Saturn's diameter.

Cassini will continue to coast above the rings for approximately one hour and 44 minutes before its descent back through the ring plane.

External Link: Keep track of Cassini