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发表于 2005-7-12 13:49 | 显示全部楼层 |阅读模式
How Much Material Was Blasted Off By Deep Impact?

X-ray detections from Tempel 1 after Deep Impact collision. Image credit: Swift.

Here come the X-rays, on cue. Scientists studying the Deep Impact collision using NASA';s Swift satellite report that comet Tempel 1 is getting brighter and brighter in X-ray light with each passing day.
The X-rays provide a direct measurement of how much material was kicked up in the impact. This is because the X-rays are created by the newly liberated material lifted into the comet';s thin atmosphere and illuminated by the high-energy solar wind from the Sun. The more material liberated, the more X-rays are produced.
Swift data of the water evaporation on comet Tempel 1 also may provide new insights into how solar wind can strip water from planets such as Mars.
"rior to its rendezvous with the Deep Impact probe, the comet was a rather dim X-ray source," said Dr. Paul O';Brien of the Swift team at the University of Leicester. "How things change when you ram a comet with a copper probe traveling over 20,000 miles per hour. Most of the X-ray light we detect now is generated by debris created by the collision. We can get a solid measurement of the amount of material released."
"It takes several days after an impact for surface and sub-surface material to reach the comet';s upper atmosphere, or coma," said Dr. Dick Willingale, also of the University of Leicester. "We expect the X-ray production to peak this weekend. Then we will be able to assess how much comet material was released from the impact."
Based on preliminary X-ray analysis, O';Brien estimates that several tens of thousands of tons of material were released, enough to bury Penn State';s football field under 30 feet of comet dust. Observations and analysis are ongoing at the Swift Mission Operations Center at Penn State University as well as in Italy and the United Kingdom.
Swift is providing the only simultaneous multi-wavelength observation of this rare event, with a suite of instruments capable of detecting visible light, ultraviolet light, X-rays, and gamma rays. Different wavelengths reveal different secrets about the comet.
The Swift team hopes to compare the satellite';s ultraviolet data, collected hours after the collision, with the X-ray data. The ultraviolet light was created by material entering into the lower region of the comet';s atmosphere; the X-rays come from the upper regions. Swift is a nearly ideal observatory for making these comet studies, as it combines both a rapidly responsive scheduling system with both X-ray and optical/UV instruments in the same satellite.
"For the first time, we can see how material liberated from a comet';s surface migrates to the upper reaches of its atmosphere," said Prof. John Nousek, Director of Mission Operations at Penn State. "This will provide fascinating information about a comet';s atmosphere and how it interacts with the solar wind. This is all virgin territory."
Nousek said Deep Impact';s collision with comet Tempel 1 is like a controlled laboratory experiment of the type of slow evaporation process from solar wind that took place on Mars. The Earth has a magnetic field that shields us from solar wind, a particle wind composed mostly of protons and electrons moving at nearly light speed. Mars lost its magnetic field billions of years ago, and the solar wind stripped the planet of water.
Comets, like Mars and Venus, have no magnetic fields. Comets become visible largely because ice is evaporated from their surface with each close passage around the Sun. Water is dissociated into its component atoms by the bright sunlight and swept away by the fast-moving and energetic solar wind. Scientists hope to learn about this evaporation process on Tempel 1 now occurring quickly -- over the course of a few weeks instead of a billion years -- as the result of a planned, human intervention.
Swift';s "day job" is detecting distant, natural explosions called gamma-ray bursts and creating a map of X-ray sources in the universe. Swift';s extraordinary speed and agility enable scientists to follow Tempel 1 day by day to see the full effect from the Deep Impact collision.
The Deep Impact mission is managed by NASA';s Jet Propulsion Laboratory, Pasadena, California. Swift is a medium-class NASA explorer mission in partnership with the Italian Space Agency and the Particle Physics and Astronomy Research Council in the United Kingdom, and is managed by NASA Goddard. Penn State controls science and flight operations from the Mission Operations Center in University Park, Pennsylvania. The spacecraft was built in collaboration with national laboratories, universities and international partners, including Penn State University; Los Alamos National Laboratory, New Mexico; Sonoma State University, Rohnert Park, Calif.; Mullard Space Science Laboratory in Dorking, Surrey, England; the University of Leicester, England; Brera Observatory in Milan; and ASI Science Data Center in Frascati, Italy.
 楼主| 发表于 2005-7-12 14:17 | 显示全部楼层


[这个贴子最后由Steed在 2005/07/12 02:18pm 第 1 次编辑]

“在与深度撞击探测器遭遇以上,这颗彗星是个相当暗淡的X射线源,”雨燕小组成员Paul O';Brien博士说。“当你将一颗铜制探测器以每小时超过20,000英里的速度撞击一颗彗星时,一切都改变了。现在我们检测到的大部分X射线都是来自于撞击产生的碎屑。我们能够对被释放物质的总量做出一个可靠的测量。”
“撞击后,表面和表面以下的物质需要花费几天时间才能达到彗星的上层大气,即彗发,”Dick Willingale博士说。“我们预期X射线亮度将在本周末达到峰值。然后我们就能够估计撞击一共释放了多少彗星物质。”
“这是第一次,我们能够目睹从彗星表面被释放的物质是如何迁移到它的上层大气层中的,”任务操控主管John Nousek教授说。“这将能提供关于彗星大气以及它与太阳风相互作用过程的美妙信息。这完全是个全新的领域。”


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