After a six-and-a-half-year fall into the inner solar system, NASA's compact Messenger probe fired its main engine for 15 tense minutes today to brake into a looping orbit around hellish Mercury, becoming the first spacecraft to take up long-term residence around the solar system's hard-to-reach, innermost planet.

A computer graphic showing NASA&39's Messenger spacecraft in orbit around Mercury, the first probe to make a long-term study of the innermost planet.

Following pre-programmed instructions, Messenger's main engine ignited on time at 5:54 p.m. PT, beginning the job of slowing the spacecraft enough for capture by Mercury's gravity. Engineers in the Messenger control center at Johns Hopkins University monitored subtle changes in a radio beacon from the spacecraft as the probe's velocity changed to track the progress of the rocket firing.

"The Messenger spacecraft went into orbit around Mercury tonight' we can confirm that through the Doppler signals we've seen," said Eric Finnegan, the systems engineer. "It is extremely exciting in the MOCC (Mission Operations Control Center), everybody was whooping and hollering, we were elated. We are in orbit! Now, there's a lot of work left to be done, but we are there."

The rocket firing was designed to slow Messenger by about 1,930 mph to put it into an elliptical 12-hour orbit tilted 80 degrees to the planet's equator with a low-point of around 124 miles and a high point of 9,400 miles.

While a precise determination of the actual orbit will depend on analysis of telemetry from the spacecraft, Finnegan said, "We hit the trajectory to within a half a sigma. For my engineering friends in the crowd, it was right on the money."

The Mercury orbit insertion, or MOI, rocket firing is the most critical "burn" in Messenger's 4.9 billion-mile journey, a voyage that included six trajectory warping planetary flybys, five major maneuvers, and 12 smaller corrective maneuvers during 15 trips around the sun.

Launched August 3, 2004, atop a Delta 2 rocket, Messenger is the first mission to Mercury since Mariner 10 flew past the scorched world three times in 1974 and 1975.

Messenger, too, flew past its target three times in 2008 and 2009, collecting a wealth of data, photographing the entire planet and using the planet's gravity to set up a fourth and final encounter today. Unlike Mariner 10, humanity's second robotic emissary to Mercury will not end its mission with yet another flyby. Instead, the craft will orbit for a full Earth year of close-up investigation.

"The [recent] flybys indeed have been wonderful, but they're really dress rehearsals for what this mission was designed to do," said principal investigator Sean Solomon of the Carnegie Institution in Washington. "All along, we have been aiming to be the first spacecraft to orbit Mercury and to take continuous global observations.

Mission managers and engineers at the Johns Hopkins University Applied Physics Laboratory burst into applause after confirmation the Messenger probe had achieved orbit around Mercury.

"This is what we designed this mission to do... We are extremely excited to begin that mapping."

Among the questions the $446 million Messenger (short for Mercury Surface, Space Environment, Geochemistry and Ranging) mission hopes to answer:

How did Mercury, believed to be 60 percent iron, end up with an oversize core, a thin shell of a crust and the highest density in the solar system Was its crust blasted away in the distant past by a cataclysmic impact Was it boiled away in the extreme heat of the young, nearby sun Or were metals for some reason concentrated in the inner region of the solar nebula that coalesced to form the sun and planets

What is the nature of Mercury's crust What elements are present and in what concentrations

Is Mercury's magnetic field, the only one in the inner solar system similar to Earth's, the result of a dynamo in the planet's still-molten outer core or is the core solid and the field "frozen" in place If the field is active, is it driven by a fluid outer core like Earth's Or is the core solid and the field the result of some other process

How does Mercury's magnetosphere interact with the solar wind and the tenuous, ultra-thin "atmosphere" of the planet What is the nature of that atmosphere, more properly known as an exosphere, and what are its constituents

Does water ice, the result of comet impacts, exist today in the basins of permanently shadowed craters near the planet's poles as radar data suggests

Figuring out how Mercury ended up so different from the other three terrestrial planets--Venus, Earth, and Mars--will provide valuable insights into how the solar system formed 4.5 billion years ago.

It also will help astronomers establish benchmarks for use when studying other solar systems, where many planets have been found orbiting as close or closer than Mercury to their parent suns.

"Mariner 10 was a mission that was designed as a reconnaissance of Mercury in order to characterize it to plan a Mercury orbiter," Robert Strom, a member of the Mariner 10 team and a co-investigator with the Messenger project, said before launch. "That orbiter was supposed to be planned and launched by about 1980. Well, it's been 30 years and until now, nothing has happened.

"So now we've got not only a mission to Mercury, an orbiter to Mercury, but we have a world-class orbiter of Mercury. This is a super mission, never in my wildest imagination did I think that we would get a spacecraft like this for a Mercury orbiter. It has the instruments on board to answer the questions that were raised by Mariner 10 and its going to do that in spades."

Equipped with seven miniaturized instruments, including two cameras, four spectrometers, a magnetometer, and a laser altimeter, Messenger will photograph the entire planet in color and stereo, map its magnetic field, the mineralogy of its crust and probe the nature of its hidden core.

The mission is expected to last for at least one year.

Editor's note: Portions of this story were included in CBS News' pre-launch coverage of the Messenger mission.