BOULDER, COLORADO -- The Cassini tour bus to Saturn has arrived. After a nearly seven year journey, the spacecraft swung into an orbit around the giant gas globe tonight, ready to spend the next four years performing scientific investigations of the Saturnian system.

Beginning Wednesday evening, an engine on the nearly six-ton spacecraft throttled up and fired for more than 90 minutes, slowing Cassini down and placing it into an initial orbit around Saturn. That maneuver is called the Saturn Orbit Insertion, or SOI.

The SOI burn was critical to the success of the mission at Saturn. Cassini approached Saturn from below the planet’s ring plane, crossing through the large gap between the F Ring and G Ring.

Saturn's ring system is divided up into 7 major divisions, with the innermost ring to the outermost ring designated as D, C, B, A, F, G and E Ring. Each major ring division is further subdivided into thousands of individual "ringlets". These are made of ice particles and rocky material.

The spacecraft’s main engine was turned to face the direction of travel, and the resulting thrust from the engine acted as a braking device, slowing down the spacecraft as it entered Saturn's orbit.

Price tagged at over $3 billion, the mission is the most ambitious planetary mission ever conducted.

Cassini’s engine burn lasted 96 minutes, placing the spacecraft into a highly elliptical orbit around Saturn.

The spacecraft’s closest approach to Saturn during its basic four-year tour occurred during the engine firing. The spacecraft's distance from Saturn was about 11,184 miles (18,000 kilometers), or less than a sixth of Saturn's diameter.

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

As Cassini begins surveying the Saturnian system, ahead for the spacecraft is at least 76 orbits around the ringed planet, including 52 close encounters with seven of Saturn's 31 known moons. Scientists speculate that more moons orbiting the planet may still await discovery.

Program managers and scientists have already begun to discuss an extended Cassini mission, beyond the four-year primary mission. That extended mission might last 4 to 6 years, perhaps as much as 8 years if onboard fuel holds out.

Along with an array of science instruments, Cassini is toting the European Space Agency’s Huygens probe. The probe’s task is to parachute into the thick atmosphere of Titan -- Saturn's largest moon -- in mid-January of next year.

The Cassini-Huygens mission is an international undertaking led by three space agencies: NASA, the European Space Agency and the Italian space agency, Agenzia Spaziale Italiana (ASI). Seventeen nations contributed to building the spacecraft. The project is managed by the Jet Propulsion Laboratory (JPL) in Pasadena, California.

Numbers of anxious space scientists have gathered here tonight at the Colorado University’s Laboratory for Atmospheric and Space Physics (LASP). And for good reason.

Onboard Cassini is a $12 million CU-Boulder instrument -- LASP's Ultraviolet Imaging Spectrograph, or UVIS. It is one of the 12 scientific instruments that the craft has hauled to Saturn.

The UVIS instrument package has a set of telescopes to measure UV light reflected by or emitted from Saturn's atmosphere, its rings and its moon atmospheres and surfaces. The data collected can determine their compositions, distribution, aerosol content and temperatures.

Kip Denhalter, an electronics engineer at LASP helped build the UVIS instrument.

"There’s a sense of accomplishment. In the case of UVIS, an added feature on our flight instrument is that there’s a plate on the back to cover all the cables. All the people that worked on it, our names are nicely engraved on that plate. So we’ve got our names in orbit around Saturn, Denhalter said. "It’s kind of neat to point to Saturn and say I’ve got something I worked on out there," he told SPACE.com.

Alain Jouchoux, operations team leader for the UVIS, said he expects the device to keep working for many, many years – far beyond Cassini’s initial exploration goal of four years. "It can work forever," he confidently added.

"Officially, we’re in orbit," said Jim Crocker, Vice President, Civil Space for Lockheed Martin Space Systems in neighboring Denver, Colorado. "We really needed to nail it and it looks like we did. This will give us a lot of science," he told SPACE.com in a phone interview.

Lockheed Martin built the spacecraft’s propulsion system, 16 thrusters, the nuclear power generators, and assembled a camera that is onboard the Huygens probe.

Crocker said imagery from Cassini is forthcoming. "These will be the best images of the rings that we’ll see in our lifetime," he added. "We’re very happy. Of course this is only the beginning. We’ve got four years to go and we’ll be firing up the engine a number of times."