揭秘创世之谜——OSIRIS-REx首次近距离观测贝努

上周，在美國地球物理學會 (AGU) 秋季会议上，NASA 的 OSIRIS-REx 任务科学团队展示了他们从小行星贝努获得的首批发现。正如预期的那样，该探测器发现了贝努过去存在液态水的证据。“这些发现对我们来说真的非常令人兴奋，”任务首席研究员 Dante Lauretta 在新闻发布会上公布早期结果时表示。“我们选择贝努作为目标，正是因为我们认为它含有含水矿物。” Lauretta 和其他团队成员表示，贝努上的水表明该小行星还含有富含碳的有机物质，这些物质原始地保存了太阳系诞生之初的状态。

贝努是否真的蕴藏着此类有机物还有待观察。但羟基（与氢和氧键合的分子）的发现是该任务科学阶段的一个有希望的开始。18 个月后，OSIRIS-REx 将尝试从贝努表面采集样本并运回地球，尽管小行星崎岖不平、环境恶劣的初始图像让人怀疑这一目标的可实现性。在此之前，该探测器将广泛绘制小行星地图，以便找到一个可访问的采样点，并揭示这个由比太阳系本身更古老的物质组成的天体的详细历史。完成这项工作的主要工具将是两个 onboard 光谱仪：OTES，用于研究小行星的热辐射；OVIRS，用于在可见光和红外光下检查这块太空岩石。

饱经风霜的岩石

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在 AGU 会议前一个月，我正在与 Lauretta 和 OSIRIS-REx 科学团队的其他成员一起，进行该任务对贝努的首次光谱观测。当时，关于远古水的未知证据仅以 0 和 1 的细束存在——一条 1.2 亿公里的 tether 连接着 OSIRIS-REx 和 54 号站，后者是马德里巨型射电天线，是深空网用于行星际通信的一部分。来自遥远小行星的数据流将进一步从西班牙传输到位于图森的亚利桑那大学月球与行星实验室的 Michael J. Drake 大楼。

Drake 大楼以已故的行星科学和 OSIRIS-REx 奠基人之父的名字命名，它朴实无华，隐蔽在图森北部安静的住宅区 off-campus。然而，在 2018 年 11 月 5 日的早晨，它很可能成为地球行星科学活动的 epicenter，数十名 frenzied 科学家正在仔细研究刚刚从贝努传回的新鲜图像。

在 Lauretta 的办公室里，他惊叹于这颗小行星及其 myriad features，外星地质显示在大型 wall-mounted 屏幕上。从一百公里的高度看，贝努大致呈核桃状，表面像两栖动物的皮肤一样 speckled，光斑和暗斑的 chaotic assemblage，在小行星 rugged terrain 中点缀着 fine-grained 的平滑区域。那里有盆地和洼地，贝努表面 strewn with boulders。团队预计会发现一个 order of 10 米的 boulder；但实际上，有数百个。整个上午，OSIRIS-REx 科学团队的成员 crowded 他的办公室，参加会议或 informal sneak peeks 刚 unveiled 的遥远世界。当该任务首次提出时，贝努只是 telescope 中的一个光点，多年来，这就是团队的 reality。然而，现在，他们看到了一些真实的东西，一些具有 three dimensions 和 personality 的东西——一些比预期的更 grittier 和更 rugged 的东西。这令人兴奋。这令人恐惧。

贝努不像龙宫那么 harsh，龙宫是另一个有 robotic visitor 的小行星——日本探测器隼鸟 2 号将在任务规划人员确信可以在不 scuttling 在龙宫 unwelcoming surface 上的情况下 retrieve samples 时尝试 retrieve samples。贝努看起来更年轻；impact craters 很少见。

Lauretta 也是隼鸟 2 号团队的成员，一项任务的 tribulations inform 另一项任务。他为他的小行星感到骄傲。“从 albedo、color 和 spectral perspective 来看，龙宫 featureless，”他说。“贝努很有趣。” 他说，它的 dark patches 特别有希望。OSIRIS-REx 科学团队希望在物体上找到 dark, carbonaceous material，因为这种 stuff 可能曾是 planets coagulating from the disk of gas and dust 的 feedstock，而后者是 embryonic 太阳系的 celestial womb。似乎它就在那里，haphazardly scattered 在小行星表面。darkness 不是阴影；在图像中，太阳几乎直接位于小行星后面。但这很可能只是 mirage，一种 compositional chimera，由表面 fine grains 对光的 scattering just so 产生。目前尚无人知晓。

尽管这可能看起来很 tedious，但值得 worry about 贝努表面有多 granular，因为整个任务的 success 可能取决于此。目前尚无 insufficient data 用于 confirmation，但 OSIRIS-REx 团队曾 predicted 贝努表面将由 centimeter-scale particles dominate。因此，centimeter-scale particles 是其 sample-gathering robotic arm TAGSAM 设计用于 collect 的东西。无法 collect samples 的 sample-collection mission 将令人失望。

时间旅行者

Niels Bohr 曾说过，prediction 很困难，尤其是在预测未来时——或者在这种情况下，当它 concern something no one has ever really seen 时。先前关于贝努成分的一切已知信息都来自 telescopes，这些 telescopes incapable of spatially resolving 小行星。到目前为止，它充其量就像 looking at 图森 ridge-ringed horizon 上的 peak。您看到 outline——mountain 存在——但仅此而已。您必须 venture much closer——stand on it，see its rocks、cacti、desert lavenders 和 velvet mesquites。在贝努，OSIRIS-REx 是 humanity 的 proxy mountaineer——其 onboard camera instrument 和 spectrometers 是我们的眼睛，向科学家们 revealing 这个来自 deep time 的 relic 的 true nature。

随着 data whispers in，1s 和 0s 通过深空网，科学家们可以从 telescopic observation 中说的是：贝努 most closely resembles meteorites called carbonaceous chondrites——特别是 “CI” 或 “CM” varieties。这些是 meteorites，它们在 history 的某个时候与水发生过 interactions。（I 和 M 分别指 Ivuna 和 Mighei——用于建立这些更大的 carbonaceous chondrites groupings 的 individual meteorites 的名称。carbonaceous chondrites 总共有八个 groups。）

这一切可能看起来并不 important——old rock、ancient water、maybe organics… so what？但在 studying 这个太阳系的 history 时，这些 ingredients 是 everything。Organic material 和 water 对地球上的生命至关重要。但 nobody knows for sure 它们来自哪里，如何来到这里的。它们是与我们的太阳形成同时出现的，还是仅在地球及其行星 siblings 形成后才由 comets 和 asteroids 运送而来？贝努及其 brethren hold the answers，因为它们被认为在化学上与我们太阳系的 genesis 保持 unaltered。OSIRIS-REx 今天 studying 的小行星贝努，据信基本上与四十亿年前存在的贝努相同。NASA 不是 flying a spacecraft，而是在 flying a time machine，其 purpose 是 figure out 生命的 stuff——bacteria 到 brontosauruses——来自哪里。

热与光

NASA 从未 sampled 过小行星，而且可能在很长一段时间内都不会再次 sample 小行星。rashly grab any old scoop of rock and rocket back home 是不够的。该机构需要 learn everything humanly and robotically possible about 贝努——需要 understandevery square centimeter of it——以便探测器 samples 的 spot 是 scientifically 最好的 place，可以 answering 关于 under the sun 的一切开端的 so many crucial questions。Long after everyone involved with OSIRIS-REx is dead，taken 的 samples 将 continue to be studied。随着 humanity 对太阳系的 understanding matures，a little bit of 贝努可以 be revisited endlessly，只需一张 plane ticket 的 price。

“您可以从 orbit 中 learn a lot about a planetary object，”Apollo sample curator 兼 NASA 约翰逊太空中心 Astromaterials Acquisition and Curation Office manager Ryan Zeigler 说，贝努 samples 如果一切顺利，应该在 2023 年到达那里。“当 remote-sensing mission is over 时，however，you’re done。You don’t get any new data。” 他说，Sample-return missions 是 everlasting。“Smart people will one day come along，look at the initial sample science and say ‘That’s great，but we can use those samples to answer questions we didn’t even think about 20 years ago.’ Getting samples in hand on Earth gives you the long-term ability to study a planet in an infinite amount of detail，with massive instruments——some of which haven’t been invented yet。”

因此，sample-site selection 是 generations 之间几乎 Confucian 的 correspondence。探测器的 instruments work together in service of choosing a worthy sample target，但只有其 spectrometers，OVIRS 和 OTES，可以 answer 关于 composition 的 critical questions。Each is designed to complement and affirm the findings of the other。Pictures reveal extraordinary geologic features，but what，exactly，are they made of？Is this the material we need to bring home and crack open in a laboratory？So important are spectrometers to this interplanetary mission that it is the first-ever to carry not one，but two onboard。

OVIRS measures light from the sun reflected from 贝努。OTES measures thermal energy radiated by 贝努。（The asteroid is like a hot rock on a summer day；put your hand near it and you feel the heat——that emitted heat is what OTES is measuring。）In both cases，the resultant spectral measurements，plotted as waves on graphs，suggest which minerals are present。OVIRS is better with small particles and organic molecules——grimy films，grains of sand and minerals——in large part because the relatively short wavelengths of visible light yield higher resolutions。OTES is better with bigger，bulkier things that can absorb and re-radiate lavish levels of longer-wavelength infrared light——again，think of hot rocks。If OTES sees a certain mineral，OVIRS should see it，too；a disparity between the two can reveal subtleties of particle size and composition useful for diagnosing the true state of things on 贝努’s surface。

The spectrometers are not penetrating deeply into 贝努’s heart；OTES can thermally probe only to a depth about five times the width of a human hair。OVIRS can see only to a single human hair width。During the sampling event in 2022，OTES will be active as the spacecraft approaches 贝努 and presses into it，and will remain in operation as the spacecraft backs away。“Assuming we don’t get dust all over our telescope，we should see what’s underneath that surface，”Southwest Research Institute in Boulder，Colo. 的 Vicky Hamilton 和 OSIRIS-Rex Spectral Analysis Working Group 的 leader 说。“It will tell us something about the uppermost surface of 贝努 and the timescales of change，”she says。“We know asteroid surfaces are space weathered。But is 贝努 space weathered all the way in，which implies particles are constantly moving around，constantly exposing new things to the sun and micrometeorites？Or is it relatively stable，with only the outermost surface seeing the effects？” she notes。This，again，tells scientists something about the goings-on billions of years before there was such a thing as Earth，or Mars，or the sun as we know it。

抵达

当天下午，随着 data done trickling 从 heavens 以 light-speed 进入深空网的 dishes，OSIRIS-REx 团队举行了 all-hands “Daily Downlink” 会议。11 月 5 日有 43 人出席，不包括来自全国各地的 substantial numbers 的 remote participants virtually participating。

“我知道整栋大楼都在 buzzing，”Lauretta 告诉大家。“我昨晚没睡好，因为我不得不 keep getting up and looking at the images。” 在一个 standout slide 上，贝努被 rendered 成 Magic-Eye-style，这样，只需稍微 eye-crossing focus，它就会 suddenly spring into three dimensions。spectroscopy data——line graphs 上的 arcane squiggles——在被 ogled 以揭示任何 beautiful mysteries 之前，还有很长的路要走。No one will ever make spectroscopic Magic Eyes。For now，the signal-to-noise ratio is excruciatingly low。The spectrometers were designed to observe 贝努 from a full field of view，the way the moon might take up the entirety of a backyard telescope’s viewfinder。But because the spacecraft is still so far away，贝努 is filling only about 40 percent of the frame；in addition to the asteroid，the spectrometers are taking readings from deep space。

Even having teased tantalizing results from the data，the month following that first full-frame spectroscopy download involved not “eureka！” moments concerning 贝努’s makeup but rather laborious calibration of the data sets returned。The spectrometers are sensitive：they get too warm and the results are skewed；some voltage is off and the results are skewed；they suffer weathering effects of deep space and the results are skewed。“You don’t want to just run your data blindly and start interpreting science out other end，”Hamilton told me later by phone。Calibration and the blessing of data are aimed，she says，“at making sure nobody，in overexcitement and not knowing everything about how an instrument behaves，goes off and interprets something that is not real。” The instruments are pointing either at 贝努，deep space or，in the case of OTES，an internal calibration target。Known signal variations reveal the state of the instruments。So far，both spectrometers have proved nominal。

12 月 2 日，贝努 at last filled OTES 的 full frame，并且，according to Lauretta，“those data were beautiful。” So far，every spectra taken has revealed hydrated minerals as the team begins the 18-month challenge of assembling a compositional map of every square centimeter of 贝努’s surface in preparation for sample collection in 2020。Hydrated minerals mean 贝努 matches to CI and CM carbonaceous chondrites，which is what the team wanted desperately to get back into laboratories on Earth。It was one relief in advance of a much larger one：The spacecraft’s formal arrival at 贝努 the next day，moving the mission to its operations phase。“I wear a Fitbit，”Lauretta says。“My resting heart rate dropped nine beats per minute when I slept the night after arrival。I didn’t realize how much tension and anxiety I had been carrying。”

But there are still those boulders。No matter how spectacular the spectra or impressive the images，贝努’s unsettling surface terrain presents problems。“We are uncertain that there is a sample-able site right now，”Lauretta says。“We have work to do。I haven’t given up hope。I think we’re going to figure it out。But you never know。There’s still that question mark hanging out there。”

And yet it entices。When OSIRIS-REx presses TAGSAM into 贝努，it will be tilling and taking the soil of a celestial Garden of Eden。The mission itself is practically written in verse；humans dug up the metals and minerals of this world and fashioned a tool to dig up the materials of another。Far into the future scientists will slice into the spacecraft’s quarry and look deeply into our past。And in the spacecraft studying the composition of an asteroid，we see most of all the stuff that humanity is made of。