The Cassini spacecraft is heading toward its closest encounter with the mysterious world of Titan, the largest moon of Saturn, during a daring flyby Sunday night that scientists hope will answer a key question.

An artist's concept shows Cassini's flyby of Titan. Credit: NASA/JPL-Caltech

The joint NASA and European mission has been touring the Saturn system since June 2004, performing dozens of barnstorming passes around the moons and rings of the planet.

This weekend's rendezvous with Titan, however, has been years in the making to ensure the Cassini's health and safety won't be jeopardized.

The craft's magnetometer will be used to discover whether Titan has its own magnetic field, a feature that would unlock the unknown about the moon's interior.

But to perform the experiment, Cassini has to dive deeper into the outer fringes of Titan's atmosphere than ever attempted before.

"Flying at this low altitude will mark the first time Cassini will be below the moon's ionosphere, a shell of electrons and other charged particles that make up the upper part of the atmosphere. As a result, the spacecraft will find itself in a region almost entirely shielded from Saturn's magnetic field and will be able to detect any magnetic signature originating from within Titan," said Cesar Bertucci, a space physicist and Titan expert on the Cassini magnetometer team.

Titan is place that fascinates scientists because of its similarities to a young Earth. The moon has lakes and rivers of liquid methane shaping its surface, a thick atmosphere and complex organic chemistry.

Sunday night at 9:31 p.m. EDT (0131 GMT Monday), Cassini's controls will switch from the reaction wheel devices to its thrusters to manage the flyby. The spacecraft makes the turn to the proper orientation for the encounter at 10:15 p.m. EDT.

The moment of closest approach happens at 10:44 p.m. EDT (0244 GMT) some 547 miles above Titan's surface with Cassini traveling at 13,200 miles per hour.

The craft's previous closest flyby altitude was 590 miles.

Todd Barber, Cassini lead propulsion engineer, wrote on his NASA blog that the team began working on plans for this unique flyby nearly three-and-a-half years ago.

"The team decided to look at an 880 kilometers (547 miles) altitude flyby: not too much of a change, but enough to make our magnetometer scientists salivate, since their instrument's sensitivity for understanding the Titan subsurface structure increases as the inverse fourth power of distance to the center of Titan," he wrote.

The torques experienced by Cassini from its flight through the atmosphere were analyzed and deemed low enough for the spacecraft to handle. Its thrusters will be fired throughout the flyby to keep the orbiter pointed properly, officials said.

In addition to the Cassini team's assessment of the risk, the NASA Engineering and Safety Center also reviewed the flyby altitude's aerodynamics to ensure the torques and heating levels were within limits.

"I think I was most astounded to hear about the amount of work that went into assessing this flyby," Barber wrote, adding that more than 5,000 hours had been spent on the effort.

Why is getting Cassini so close to Titan worth it? Bertucci says this 71st flyby of the moon is one of scientists' most anticipated encounters.

"Titan orbits within the confines of the magnetic bubble around Saturn and is permanently exposed to the planet's magnetic disturbances. Previous measurements by NASA's Voyager spacecraft and Cassini at altitudes above 950 kilometers (590 miles) have shown that Titan does not possess an appreciable magnetic field capable of counterbalancing Saturn's. However, this does not imply that Titan's field is zero. We'd like to know what the internal field might be, no matter how small."

If a magnetic field is found, scientists then expect to determine if the moon has enough energy to power a dynamo at its heart.

"Planets with a magnetic field - like Titan's parent Saturn or our Earth - are believed to generate their global-scale magnetic fields from a mechanism called a dynamo. Dynamo magnetic fields are generated from currents in a molten core where charge-conducting materials such as metals are flowing around each other and also undergoing other stresses because of the planet's rotation," Bertucci said.

Cassini will be tracked live during the flyby and science data downlink sessions are scheduled using the giant communications antennas in Spain and California on Monday.

"We're looking forward to poring through the data coming down, especially after all the negotiations we had to make for them!" Bertucci said.