The U.S. space probe MESSENGER is poised to enter orbit around Mercury（水银） early Friday, Universal time, capping a nearly seven-year journey to the solar system’s innermost planet. Scientists with the U.S. space agency, NASA, hope the orbiting probe will greatly expand our knowledge of Mercury, and provide news clues to the origins(起源) of Earth, as well.
MESSENGER’s orbital rendezvous with Mercury is the final stage of a breathtaking journey that has taken it billions of kilometers through the inner solar system, including half dozen photographic fly-bys of Earth, Venus and Mercury itself for gravitational assists.
"Those were all hor d’oeuvres for the banquet we’re about to enjoy," said Sean Solomon of the Carnegie Institution of Washington D.C. , who is the MESSENGER mission’s principal investigator. "Once MESSENGER is in orbit around Mercury, it will be the first spacecraft to do so, we will have the first global observatory at the innermost planet of the solar system."The last close-up data on Mercury was collected by NASA’s Mariner 10 in a fly-by mission in the mid-1970s.
Fifteen minutes before insertion into Mercury orbit, MESSENGER will begin firing its main thruster engine, burning off one-third of its propellant. That burn will slow the spacecraft down by 862 meters per second for the orbit insertion maneuver.
Scientists say it will be an hour before they are able to confirm whether MESSENGER is securely in orbit around Mercury.
MESSENGER will circle the tiny planet, roughly the size of Earth’s Moon, every 12 hours, traveling over the planet’s polar regions just 200 kilometers above the rocky surface.
MESSENGER will be a mere 46 million kilometers from the Sun. Built to withstand the blistering heat, the spacecraft is carrying seven scientific instruments that mission investigator（研究者） Solomon hopes will help answer some intriguing questions about the solar system’s inner-most planet.
For example, Solomon says there appears to be ice in the floors of impact craters at Mercury’s poles, which are in permanent shadow.
"That’s pretty astonishing because at the equator of Mercury, the temperature varies between day and night by 600 degrees centigrade," he said. "So, the notion that there might be permanent repositories of water ice at the poles of Mercury seems to fly in the face of the fact that this is a planet closest to the Sun and has extraordinarily high daytime temperatures."Solomon says MESSENGER won’t begin returning any images of Mercury until April.
MESSENGER’s orbital mission is scheduled to last one year. But Solomon is hopeful there will be enough fuel to extend its observations of Mercury into a second year.(本文由在线英语听力室整理编辑) |