Rosetta zooming by Earth on way to comet

The European Rosetta spacecraft, which is aiming to put a lander on a comet in 2014, is nearing Earth Tuesdayin the third of its four "swing-bys," approaches to planets so it can take advantage of their gravityto pick up speed for its 7.1-billion kilometre journey.

"This is the only way to reach the comet 67P/Churyumov-Gerasimenko, given the launch vehicle capabilities and the propulsion systems we have," the European Space Agency (ESA) said on its website.

The mission is planning to put the 100-kilogram Philae lander on the comet, where it will be the first to gather data physicallyon thestructure of a comet, which should throw light on the origins of the universe.

Comets are the oldest "building blocks" of the solar system, and so the mission"will allow scientists to look back 4,600 million years to an epoch when no planets existed and only a vast swarm of asteroids and comets surrounded the sun," the ESA said.

The billion-euro probe is named for the Rosetta Stone tablet that helped researchers decipher Egyptian hieroglyphic writing. "Similarly, scientists hope that Rosetta will unlock the mysteries of how the solar system evolved," the ESA said.

Rosetta's closest approach to Earth isexpectedatabout 4 p.m. ET,when itwill be 5,301 kilometres away, travelling at 45,000 km/h. Although observing the Earth and moonis not one of Rosettas scientific priorities, a few instruments on both the orbiter and lander will be activated for two weeks while the craft is in the area.

Before arriving at the comet, Rosetta willincrease itsspeed by passingaround Earth and Mars four times; this is the third swing-by, with the fourth set for 2009.

After picking up enough speed to reach the comet, the probe will go into "deep-space hibernation" fromMay 2011to January 2014, aiming to map 67P/Churyumov-Gerasimenko in August that year, and landPhilae inNovember.

Rosetta willtrack thecomet around the sun, which will allow it to measure the effect of heat, until the mission ends in December 2015.

Rosetta is a large black box, 2.8 by 2.1 by2.0 metres, that weighed 3,000 kilograms (including fuel) when launched in March 2004. It carries165 kilograms of instruments.

The Philae lander carries nine experiments anda drilling system to take samples of sub-surface material. Philae is an island in theNile River that contained an obeliskwith Greek and Egyptian words thathelped decipher the Rosetta Stone.