2020年9月16日

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Gravel Ejected from Asteroid Bennu
Image Credit:
NASA’s GSFC, U. Arizona, OSIRIS-REx Lockheed Martin

Explanation: Why does asteroid Bennu eject gravel into space? No one is sure. The discovery, occurring during several episodes by NASA’s visiting OSIRIS-REx spacecraft, was unexpected. Leading ejection hypotheses include impacts by Sun-orbiting meteoroids, sudden thermal fractures of internal structures, and the sudden release of a water vapor jet. The featured two-image composite shows an ejection event that occurred in early 2019, with sun-reflecting ejecta seen on the right. Data and simulations show that large gravel typically falls right back to the rotating 500-meter asteroid, while smaller rocks skip around the surface, and the smallest rocks completely escape the low gravity of the Earth approaching, diamond-shaped asteroid. Jets and surface ejection events were thought to be predominantly the domain of comets, responsible for their tails, comas, and later meteor showers on Earth. Robotic OSIRIS-REx arrived at 101955 Bennu in late 2018, and is planned to touchdown to collect a surface sample in October 2020. If all goes well, this sample will then be returned to Earth for a detailed analysis during 2023. Bennu was chosen as the destination for OSIRIS-REx in part because its surface shows potential to reveal organic compounds from the early days of our Solar System, compounds that could have been the building blocks for life on Earth.

Experts Debate: How will humanity first discover extraterrestrial life?

Tomorrow’s picture: cycle 25


贝努小行星抛出的碎石
影像提供:
NASA’s GSFC, U. Arizona, OSIRIS-REx Lockheed Martin

说明: 贝努小行星为何会抛出碎石呢?没人能给个明确的答案。美国航太总署目前在造访贝努小行星的ORISIS-REx探测船,数次发现这种预期之外的现象。较主流的抛射假说有:绕日流星体的撞击、内部结构的突发性热破裂,和突然释出的蒸汽喷流。这幅由二张照片拼合的主题影像,右侧可清楚看见正在反射阳光,发生于2019年初的一例喷发事件之碎片。观测数据和模拟指出,碎石通常会掉回这颗不停自转的500米宽小行星,较小的碎石掉回后会在表面弹跳,而最小的则完全脱离这颗正在向地球靠近的菱形小行星。喷流和表面抛射事件,一般认为是彗星这类天体较可能出现的行为,从而造就了彗星的彗尾、彗发及后来洒向地球的流星雨群。无人OSIRIS-REx探测船,在2018年底抵达贝努小行星(101955 Bennu),并规划在2020年10月降落收表面样本。如果一切顺利,这份样本将在2023年送返地球进一步详细分析。之所以选贝努作为OSIRIS-REx探测船的标的,部分原因是它的表面可能存有太阳系 早期留下的有机化合物,而这些化合物是地球生命的基础。

专家辩论人类将如何首先发现外星生命?

明日的图片: cycle 25

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