Dawn Spacecraft Nears Rendezvous With Dwarf Planet Ceres

NASA’s Dawn spacecraft is set to reach the dwarf planet Ceres in March and begin gathering clues about whether the icy body may have once had conditions conducive to supporting life.

Dawn left Earth more than seven years ago, on a course to explore two alien worlds in the solar system: Ceres and Vesta, both in the main asteroid belt between Mars and Jupiter.

“This is the first time in more than 57 years of space exploration that we have a spacecraft targeted to orbit any two extraterrestrial destinations,” said Dawn chief engineer and mission director Marc Rayman.

Vesta and Ceres are considered protoplanets. They were "in the process of growing to become full-size planets when their growth was terminated,” Rayman said.

At the Dawn mission control center in Los Angeles, the flight team sends second-by-second instructions to the spacecraft, directing its every move, including the images it takes of what it sees.

Scientists believe Ceres is made of rock covered with a mantle of ice. Dawn will take images and make a comprehensive map of this alien world. It will also examine Ceres’ composition to provide data on how its surface has evolved, said Carol Raymond, the mission's deputy principal investigator.

“One of the interests in bodies like Ceres and other icy bodies in the outer solar system is that when objects like that impacted the terrestrial planets, they brought a lot of water," she said. "So there’s a very vigorous debate at the current time as to whether wet asteroids like Ceres — and we know there are other wet asteroids in the outer edge of the main belt — whether they were the dominant source of water in the Earth’s ocean.”

Earlier in its mission, Dawn made an unexpected discovery while orbiting the protoplanet Vesta.

“Instead of being bone dry, which we expected, there were patches on the surface of Vesta that showed significant amounts of water bound in the rocks and possibly even free water that could have flowed on the surface in the past,” Raymond said.

Raymond said that if Earth’s water came from wet worlds like these, then life could have come with them as well.

If Dawn reveals that the environment on Ceres could be conducive to life, the finding may spur further explorations with other types of instruments to test that hypothesis.

As for Dawn, it will gather information on Ceres until the summer of 2016. Then it will continue to orbit the dwarf planet indefinitely.

美国航天（NASA）的“曙光号”（Dawn）太空探测器很快就要抵达矮行星谷神了。一旦“曙光号”开始围绕谷神星运转，就将开始搜集线索，了解它的演化历史以及这颗覆盖着冰壳的矮行星是否存在过能够支持生命的条件。

“曙光号”七年多前离开地球，踏上漫漫征途，探索太阳系内的两个陌生世界。

在洛杉矶的喷气推进实验室，马克•雷曼说：“在超过57年的太空探索中，我们第一次有一艘太空飞船围绕地球外的两个星体轨道运转。”

这两处目的地是灶神星（Vesta）和谷神星（Ceres），它们位于火星和木星之间的主小行星带。

在“曙光号”任务控制中心，飞行控制团队每秒钟都在向这艘太空飞船发送指令，指导它的每一个步骤，包括拍摄什么照片。

“曙光号”向谷神星靠拢之际向地球发回了最新图像。科学家相信，这颗矮行星由岩石组成，外表覆盖着冰壳。一旦飞船在3月间进入轨道，就将拍摄图像并对这处陌生的世界进行综合制图。“曙光号”将检查谷神星的构成，看看它的表面是否发生过演变以及是否存在过适合生命的条件。

卡罗尔•雷蒙德是这项探索任务的副首席调查员。她说：“人们对太阳系外围的谷神星和其它冰壳覆盖的天体的兴趣之一是，当这类物体撞到类地行星时，会带来很多水分，所以目前争论很激烈的是，像谷神星这样的含水分的小行星，---我们知道在主小行星带外围边缘还有很多其它含水分的小行星，它们是否曾经是地球海洋的主要水源。”

“曙光号”出发后，意外发现了属于原行星的灶神星。

雷蒙德说：“让我们颇感意外的是，我们本以为灶神星是干枯的，但却发现它的表面有斑点，显示岩石中有大量的水，可能过去甚至有地表水流动。”

雷蒙德说，如果地球上的水来自类似谷神星这样的含水世界，那么，生命也有可能来自它们。

如果“曙光号”发现谷神星上曾有适合生命的环境，这将推动人们进一步探索，使用恰当的工具来检验这项假说。

对“曙光号”来说，它搜集谷神星信息的任务将持续到2016年的夏天。然后，它将继续围绕这颗矮行星的轨道运转，直到永远。