The European Rosetta spacecraft, which is aiming to put a lander on a comet in 2014, is nearing Earth Tuesday in the third of its four "swing-bys," approaches to planets so it can take advantage of their gravity to 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.

An artist's impression of the Rosetta lander, which is on its way to performing the first physical analysis of a comet.
(European Space Agency)

The mission is planning to put the 100-kilogram Philae lander on the comet, where it will be the first to gather data physically on the structure 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 is expected at about 4 p.m. ET, when it will be 5,301 kilometres away, travelling at 45,000 km/h. Although observing the Earth and moon is not one of Rosetta’s 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 will increase its speed by passing around 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" from  May 2011 to January 2014, aiming to map 67P/Churyumov-Gerasimenko in August that year, and land Philae in November.

Rosetta will track the comet 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 by 2.0 metres, that weighed 3,000 kilograms (including fuel) when launched in March 2004. It carries 165 kilograms of instruments.

The Philae lander carries nine experiments and a drilling system to take samples of sub-surface material. Philae is an island in the Nile River that contained an obelisk with Greek and Egyptian words that helped decipher the Rosetta Stone.