BEIJING - China's second lunar probe, Chang'e-2, has reached an orbit 1.5 million kilometers from Earth for an additional mission of deep space exploration, the State Administration for Science, Technology and Industry for National Defense announced on Tuesday.

It arrived at what is called a Lagrangian point, where it will stay till the end of next year to conduct scientific observations and test deep space tracking and control capability for future possible explorations of Jupiter and the poles of the Sun, experts said.

Lagrangian points are where gravitational forces and the orbital motion of a body balance each other. There are five Lagrangian points in the Sun-Earth system, and the Chang'e-2 has reached one of those nearest to Earth. Lying in the Earth's shadow, this Lagrangian point is exposed to less radiation from the sun than other Lagrangian points, and it is an ideal place for scientists to put space telescopes when they want to observe the universe.

After traveling for 77 days, Chang'e-2 entered the orbit on Aug 25 and has flown stably in the orbit for five days.

"It makes China the third to visit the Lagrangian point," the administration said. Before this, only the European Space Agency and the United States have sent probes there.

It is also "the farthest place a Chinese satellite has ever reached", said Zhou Jianliang, deputy chief designer of the moon probe's telemetry, tracking and command system.

This is an additional task for Chang'e-2, which was launched on Oct 1 last year with a designed life span of six months and a chief mission of testing technologies needed to enable an unmanned craft to land on the moon in 2013.

After it finished its chief mission, scientists designed new tasks to make better use of the lunar probe. On June 9, it set off from a moon orbit to the Lagrangian point.

Liu Tongjie, deputy director of the overall department of the second phase project of China Lunar Exploration Program, said that Chang'e-2 is expected to fulfill two scientific tasks in the orbit: observing the Earth's magnetic field and any solar storms.

Next October, the satellite will help test the capability of two large antennas being built for deep space exploration, he said.

According to earlier reports, an antenna with an aperture of 35 meters is being built at Kashgar in Northwest China and another antenna with an aperture of 64 meters is being built at Jiamusi in Northeast China. They are part of China's deep space network that will take shape in 2016.

"This test will be very meaningful for China's future deep space exploration, as scientists are now discussing ideas of exploring Jupiter and the two poles of the Sun in the future," he said.

Tracking and control experts said that maneuvering the moon probe to reach that far is a big challenge. The new orbit it now flies in is an ellipse with the farthest point 1.7 million kilometers away from the Earth's surface.

"The current tracking and control system was developed for lunar explorations 400,000 km away from the Earth's surface. So signals being sent to and from 1.7 million km away have weakened a lot," Zhou said.

The satellite has only limited fuel left for this additional mission, he said, adding that in order to economize on the remaining fuel, Chang'e-2 was maneuvered to travel slowly from the moon orbit to the Lagrangian point, with the lowest speed at only 31.7 meters per second.

"It was even slower than a car on the highway," he said. "Otherwise it is hard to say if Chang'e-2 can survive till the end of next year."

China Daily

(China Daily 08/31/2011 page2)