Spaceflight Now

Cassini makes dramatic dive in the name of science

Posted: June 21, 2010

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The Cassini spacecraft pulled off its latest drama-packed performance Sunday night, braving to skim deeper into the outer atmosphere of Saturn's largest moon than it had ever attempted before in hopes of discovering a magnetic field around Titan.

Credit: NASA/JPL-Caltech
Scientists and engineers worked together for more than three years to design the flyby, balancing the risks and rewards before ultimately picking a close encounter altitude of just 547 miles.

That trajectory would send Cassini below Titan's ionosphere, enabling the instrument-laden spacecraft to detect the magnetic signature from the moon without being fooled by Saturn's own magnetic field.

But going that low meant the aerodynamic forces and heating that the craft would experience as it flew 13,200 mph through the outer parts of the atmosphere had to be thoroughly analyzed.

The team deemed it safe, yet there was tension and anxiety in Mission Control as the events unfolded Sunday. And with Cassini nearly a billion miles away, it'd take 78 minutes for communications to travel just one-way.

"On Sunday evening, my eyes were glued to eight windows on my computer screen, watching data pop up every few seconds. NASA's Cassini spacecraft was making its lowest swing through the atmosphere of Saturn's moon Titan and I was on the edge of my seat," Julie Webster, spacecraft operations team manager, wrote on her blog Monday.

"It was a nervous time for me - the previous night we had been at JPL to send some other real-time commands to the spacecraft when an alarm came in indicating that the magnetometer, the prime instrument taking data for the T70 flyby, needed a reset. Fortunately, the controller on duty immediately called the magnetometer instrument operations team lead in England. Within 90 minutes, the commands were on their way to do a computer reset and clear the alarm."

The craft's magnetometer would be used to discover whether Titan has its own magnetic field. Earlier efforts by Cassini and the Voyager spacecraft flying farther away from Titan hadn't found one.

"So here we were, past one hurdle, hoping nothing else would come up. We had run hundreds of simulations over the past three-and-a-half years, so I knew we had done everything we could think to do. We did more training for this event than anything else we had done since we dropped off the Huygens probe in January 2005 for a descent through the moon's hazy atmosphere," Webster said.

Data from Cassini began flowing to the Jet Propulsion Laboratory via the Deep Space Network tracking system at 10:26 p.m. EDT (0226 GMT) as planned, some 19 minutes before the moment of closest approach. Cassini had turned itself to the correct orientation for the flyby and would fire onboard thrusters to stay pointed properly as it flew through the atmosphere.

"I was focused on the data for spacecraft pointing. As long as we stayed within an eighth of a degree of the expected pointing, everything would be fine," Webster said.

"Over the vocabox, a cross between a telephone and walkie-talkie, the attitude control team reported that the thrusters were firing about twice as much as we expected. The Titan atmosphere appeared to be a little thicker than we expected, even though we had fed about 40 previous low Titan flybys by Cassini and the descent data from Huygens into our modeling.

"But spacecraft control was right on the money, keeping the pointing within our predicted limits. Even with the extra thrusting, we stayed well within our safety margin," Webster said.

As Cassini began to move away from Titan, it safely turned to conduct other observations. The daring encounter was a success.

"Another first for the Cassini mission!" Webster said.

"I let out a sigh of relief, happy that everything during closest approach had gone just as we planned. Five attitude control guys crowded into my office with smiles on their faces."

The work to sort through the science data is only beginning. If a magnetic field is found, scientists could unravel details about the moon's interior and determine if Titan possesses enough energy to power a dynamo.

"We have data playbacks today at two different Deep Space Network stations to make sure we have - as we say here - both belts and suspenders. Engineers will also go back to analyze the data with the scientists to see just how dense the Titan atmosphere turned out to be at our flyby altitude."