Comet-bound probe enjoys close encounter with Mars
BY STEPHEN CLARK
Posted: February 25, 2007
Europe's Rosetta comet probe shot past Mars early Sunday to line up for another swing by Earth later this year, putting the craft on course to reach its icy target in 2014.
Launched three years ago, Rosetta zoomed just 150 miles above the Martian surface at about 22,500 miles per hour relative to the Red Planet. The probe made its closest approach at 0215 GMT Sunday (9:15 p.m. EST Saturday) as it flew behind Mars and temporarily broke off communications with Earth.
The Mars flyby created a slingshot-like effect for Rosetta, putting the spacecraft on a path to reach Earth on Nov. 13 for a similar maneuver.
"We actually use this flyby to slow down the spacecraft to optimize the Earth gravity assist," said Gerhard Schwehm, Rosetta project scientist.
Martian gravity was to have naturally slowed Rosetta by 4,900 miles per hour relative to the Sun, according to a European Space Agency statement.
Rosetta already completed a pass by Earth in 2005, and the craft will fly past Earth two more times later this year and in 2009 to set up for a rendezvous with comet 67P/Churyumov-Gerasimenko in early 2014.
By bouncing across the solar system to utilize natural gravity assistance, Rosetta saves precious fuel that would be necessary to send the probe on a direct route to its destination.
A suite of imaging instruments aboard Rosetta was commanded to observe the planet as the probe moved toward Mars, but the craft's payload systems had to be turned off about one hour prior to the flyby. Controllers were concerned Rosetta's batteries would be drained as the spacecraft passed behind Mars, which blocked the Sun's rays for about 25 minutes.
"Unfortunately, we will have to switch off the payload at closest approach as we will pass through an eclipse, a scenario Rosetta isn't built for," Schwehm said in an interview last week.
The changes meant Rosetta would be subjected to more hazardous conditions during its voyage, including an unplanned eclipse during its swing by Mars, according to ESA.
Rosetta's huge solar panels span nearly 100 feet tip-to-tip to gain efficiencies in power production in the outer solar system. The craft will be the first to fly beyond the asteroid belt and rely entirely on solar power.
Schwehm, also head of ESA's Solar System Science Operations Division, said Rosetta's primary optical and infrared camera could have achieved an image resolution of about 12 feet as the craft swooped in for its closest approach.
Despite the need to shut down Rosetta's powerful instruments, scientists still expected to get unique science opportunities from the flyby. The probe was to have downlinked the first ultraviolet measurements from near Mars, Schwehm said.
The instruments were turned back on to gather more data as Rosetta sped away from Mars Sunday.
The flyby was also the first opportunity for Rosetta's 220-pound lander to conduct independent science observations. Imaging systems and a plasma monitor were kept on during the eclipse because the lander has its own power system, according to an ESA statement.
The lander's camera captured spectacular images of Rosetta and Mars as the probe skimmed just above the planet's atmosphere.
Called Philae, the tiny craft will be dropped onto the nucleus of Churyumov-Gerasimenko in November 2014 for several weeks of experiments. Philae carries nine science instruments and a small drill to retrieve samples from below the comet's surface.
Rosetta will orbit Churyumov-Gerasimenko for more than a year of extensive science operations to observe the comet's changes as it approaches the Sun.