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发表于 2009-5-31 21:46:39
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Tuesday, May 26, 2009
2009年5月26日 星期二
What has been happening.....
过去所发生的事......
We want to share with you some new developments in the search for interstellar dust that we are cautiously excited about. We want to say from the beginning that we don’t understand everything yet, and that this may lead to a dead end. This is a bit like detective work – we’re sorting through the clues, and there is always a stage of confusion before things become clear. It is important to follow every lead. This is often how science works. The confusing parts (we hope) become the exciting parts, because we learn from what we don’t know. We’re sharing our confusion and our excitement with you, even though the story is far from complete.
我们打算与大家分享一些搜索星际尘埃的新进展，它们令我们感到谨慎的兴奋。话说在头里，我们目前还不理解所有的事，这有可能把我们领进死胡同。这有点儿像侦探的工作——我们在线索中不断排序，在事情明朗前总会有一段混乱的阶段。追踪每一条线索都非常重要。科学工作经常是这样的。令人困扰的部分（我们希望）变成令人激动的部分，因为我们从那些不了解的事情中学到了东西。我们将与大家分享我们的困惑和兴奋，即使离故事结束还远着呢。
First, a quick status report on the project. Last year we focused on extracting candidates, so we were not able to do any additional scanning. We have good scan data on 247 cm2 of the collector – about 30% of the scannable area. 30 extractions have been done since the beginning of the Interstellar Preliminary Examination. 16 of these were interstellar candidates. The extraction process has evolved somewhat during the first year of the ISPE, but not in a substantial way. All candidates are now mounted between silicon nitride windows by default. 14 of these have been analyzed at synchrotrons all over the world. 12 do not appear to be consistent in their composition with interstellar dust. Two are sufficiently interesting that we want to do further study on them. One was lost due to a pre-existing fracture in the keystone. Unfortunate as this is, it emphasizes the need for a cautious approach to sample prep and extractions. Rather than rush, it is better to move carefully through the extraction phase in order to minimize sample loss and damage. One additional candidate is still under analysis.
首先是项目现状简报。去年我们集中在做候选物的提取，因此没能做任何进一步的扫描。我们有收集器247平方厘米扫描较好的数据——大约是可扫描面积的30%。在星际尘埃预查（ISPE）开始后完成了30个提取工作，其中有16个是星际尘埃的候选者。在ISPE的头一年里提取的过程发生了一些演化，但不是很本质的。所有的候选者通常情况下都安放在氮化硅窗体中。其中有14个已经被世界各地的同步加速器分析过了。12个在其成分上未表现出与星际尘埃相一致。2个十分的有趣，我们打算进一步的研究它们。有一个由于楔块儿中已存在的一条裂缝而丢失了。这真太不幸了，这着重表明准备样品和提取时需要非常小心的处理。与其仓促完成，还是在提取阶段小心的移动以减少样品的损失和破坏更好些。还有一个额外的候选者仍在分析中。
Based on the early estimates of the number of interstellar dust particles that we are likely to have collected, we have about ten times more candidates than we expect to have interstellar dust particles – in other words, we expect to extract and analyze about 10 candidates for every real interstellar dust particle that we find. We are refining this estimate now – so far it appears that the number of interstellar dust particles collected was likely lower than the original estimate. If this turns out to be correct, this means we will have more work to do than we initially anticipated to find the interstellar dust particles. We have done no extractions since last Fall, since the Cosmic Dust Lab (CDL) has been switched over to doing work on Interplanetary Dust Particles (IDPs). We expect to resume extractions when CDL is switched back over to interstellar dust work, nominally at the end of May. We also expect that some foils will be extracted so that the foils subteam can begin work on searching for impact craters of interstellar dust.
基于早先对我们可能收集到的星际尘埃数量的估计，候选者的数量是我们预期获得星际尘埃数量的十倍——换句话说，我们预计每找到一个星际尘埃颗粒就要大约提取和分析10个候选者。我们正在改进这一估计——到目前它似乎表明收集到的尘埃数量可能比原先预计的少。如果这一结果是正确的，那就意味着与最初预计相比要发现星际尘埃我们有更多的工作要做了。自从彗星尘埃实验室(CDL)转换到做行星际尘埃工作以及上一次提取失败以来我们没有再做提取工作。我们希望当CDL再次转换到做星际尘埃工作时继续进行提取工作，名义上是在5月末。我们还希望能取出一些衬片，以便衬片分队能寻找星际尘埃的撞击坑。
Now to the recent developments that we’re cautiously excited about. Stardust@home dusters have found 29 so-called “high-angle” tracks. These are tracks of particles that didn’t come straight into the collector, but entered at a significant angle – typically about 45 degrees. I want to emphasize that these were identified by Stardust@home dusters, not by us. The fact that these were found by our amateur colleagues is a tremendous demonstration of the value of the Stardust@home approach. Some of these high-angle tracks were discovered in the first days of the project. We had not thought of these as being likely interstellar candidates, for three reasons. First, the projectiles came in at a large zenith angle with respect to the aerogel surface. This was not expected for interstellar dust, because the interstellar dust collector was intentionally oriented during the exposures so that the interstellar dust would come more or less straight into the detector. Second, the azimuth angles were such that the tracks seemed to be coming not from space, but from the solar panels of the spacecraft. Third, the tracks did not appear to us to be high-velocity tracks, such as those seen on the cometary side of the collector, where particles were captured at 6 km/sec. High-velocity tracks, from projectiles at >>1 km/sec, make carrot-shaped tracks in aerogel. The tops are wide because the projectile produces an energetic shock wave in the aerogel that expands dramatically and blows out a relatively large cavity in the aerogel. As the projectile slows, the shock weakens and eventually disappears when the projectile drops below the speed of sound. The result is a carrot-shaped track. But the high-angle tracks were thin and smooth, which in our experience looked more like slow. (<< 1 km/sec) projectiles. Finally, we extracted five of these tracks and examined them using synchrotron x-ray fluorescence microscopy (SXRF) and by scanning transmission x-ray microscopy (STXM). We found that three of them contained significant amounts of cerium, which is a rare element and is not expected in any significant quantities in any primitive extraterrestrial material. All of these lines of evidence led us to conclude that the high-angle tracks were highly likely to be ejecta from impacts on the aft solar panels of the spacecraft.
对于最近的进展我们谨慎的兴奋。Stardust@home 的搜尘者发现了29个被称作“大角度”的轨迹。这是那些没有笔直的进入收集器的颗粒轨迹，而是以一个有意义的角度——典型的约45度进入的。我想强调，这些是 Stardust@home 搜尘者识别出来的，不是我们。实际上我们的爱好者的这些发现是 Stardust@home 这种途径价值的极佳证明。其中一些大角度轨迹是在项目的第一天被发现的。我们不认为这些像是星际尘埃候选者，有三点原因。首先，弹丸是以相对于凝胶表面大的天顶角度进入的。这不被认为是星际尘埃，因为星际尘埃收集器暴露在外期间是有意定向的因此星际尘埃差不多会是笔直的进入。第二，从方位角看这些轨迹不像是来自太空，而是来自飞船的太阳能电池板。第三，在我们看来这些好像不是那种在彗星尘埃收集器里可以看到的高速轨迹，那里捕获的颗粒速度是 6km/sec。高速轨迹来自于远大于 1km/sec 的弹丸，在凝胶中形成胡罗卜的形状。宽的顶部是由于弹丸在凝胶中产生了一个高能的冲击波，冲击波在凝胶中显著的扩展并炸出一个相对大的空腔。随着弹丸减速激波变弱，当弹丸降到音速之下最终消失。结果就是一个萝卜形的轨迹。但是大角度轨迹是细而平坦的，根据我们的经验看上去像是很慢的（<<1km/sec）弹丸。最终，我们取出了5个这样的轨迹并用同步X射线荧光显微镜和X射线扫描隧道显微镜进行了仔细检查。我们发现其中3个含有可观的铈，这是种稀有元素不被认为会以任何可观的量出现在任何原始的地外物质中。所有这些证据使我们得出结论，大角度轨迹极有可能是飞船尾部太阳能电池板上撞击溅射出来的。
But not so fast. Three developments have led us to reconsider seven of these 29 tracks.
但是别太急。三个发明引导我们重新考虑这29个中的7个。
First, we recently did some experiments with our colleagues Frank Postberg, Mario Treiloff, Ralf Srama, Sebastian Bugiel, and Eberhard Grün at the University of Heidelberg. They have cleverly adapted a particle accelerator, a van de Graaf accelerator, to accelerate small dust particles. For very small dust particles, they can achieve speeds up to 100 km/sec! This is the same accelerator that produced the calibration track in the phase I Stardust@home images. The dust accelerator works as follows. A bowl of the dust that you want to shoot is placed at the high-voltage end of a long evacuated pipe. The voltage is about 2 million volts. A sharp needle sticks up from the bowl. A high frequency voltage is applied to the bowl, which occasionally causes one of the dust particles to jump out of the bowl onto the tip of the needle, where it acquires a large charge and is accelerated down the 2 megavolt potential into the target. (Who thought of that??) However, it gets even more clever. Because this is a stochastic (random) process, the dust particles acquire wildly varying charges, so the accelerated dust comes out with a wide range of speeds. But to get unambiguous results, we need to have the particles coming out in only a narrow range of speeds, say 15-16 km/sec. So they use three detectors located at different positions along the flight path of the dust. These detectors can detect the passage of the dust particle, and by measuring the time delay between the passage of the dust grain past the three stations, they can measure the speed of the dust grain. If the speed isn’t right, the particle is rejected using a pair of electrostatic plates near the end of the pipe, to deflect the dust grain away from the target. Of course, all of this happens so fast that no person actually makes the decision – it is all done by electronics.
首先，最近我们和海登堡大学的同事 Frank Postberg, Mario Treiloff, Ralf Srama, Sebastian Bugiel, Eberhard Grün 一起做了一些实验。他们聪明的改造了一种范德格拉夫粒子加速器，来加速微小的尘埃颗粒。对于微小的尘埃颗粒，它们能获得达到 100km/sec 的速度！就是在 Stardust@home 第一阶段里用来产生校准轨迹的那台加速器。尘埃加速器是这样工作的。一碗准备射击用的尘埃放置在一条抽空的高电压管一端。电压大约是2百万伏。一个尖针从碗里伸出来。一个高频电压作用在碗上，会随机的使某个尘埃颗粒从碗里跳到针尖上，并在那里获得大量电荷并被2百万电势加速进入目标。（是谁想出来的？）真是很聪明啊。因为这是一个随机的过程，尘埃颗粒获得电荷数量范围很宽，所以被加速的尘埃获得的速度范围很宽。但是为了得到明确的结果，我们需要使颗粒在一个窄的速度范围内出来，比方说 15-16km/sec。于是他们在尘埃飞行路线的不同位置放了三个探测器。这些探测器能探测到尘埃颗粒的经过，并通过测量器通过三个探测器的时间差得出尘埃颗粒的速度。如果速度不合适，在管道末端附近一对静电板会使尘埃偏离射击目标。当然这一切发生的非常快，没人能做出决定——一切由电子设备完成。
Using the dust accelerator, Zack Gainsforth and our German colleagues shot small particles of latex, aluminum, a polymer called PMMA, orthopyroxene, and iron, into aerogel and aluminum foils at speeds from 10-20 km/sec. The latex particles produced very short tracks in the aerogel, which was not a surprise. However, the iron and the orthopyroxene particles appeared to produce very long, thin tracks, reminiscent of the high-angle tracks in the Stardust collector. The reason that we emphasize the word appeared is that there was a complication in the experiment. Sometimes the dust source in the accelerator “spits” -- that is, it produces rare bursts of particles, that can confuse the triggering system and allow slow, large particles to get through while the gate is open to allow a legitimate high-velocity particle through. We saw evidence of these in some of the shots, but not in the shots of the orthopyroxene or of the iron. In the iron shots, we saw about as many particles in the aerogel as we expected based on the number of times that the filter triggered. So there are two questions: (1) if these particles were actually slow, where are the fast ones that should also be in the target aerogels? (2) if the tracks we see are made by particles that are truly traveling fast, why do they make whisker-shaped tracks instead of carrot-shaped ones? We still don’t know the answer to either of these questions. We’re doing some additional shots in the next months with a new, improved filter that we expect to do better at rejecting “spits”.
Zack Gainsforth 和我们的德国同事用尘埃加速器把一些橡胶、铝、一种叫PMMA（聚甲基丙烯酸甲酯）的聚合体、斜方辉石、铁的小颗粒以 10-20km/sec 的速度射进凝胶和铝箔片中。橡胶颗粒在凝胶中产生了非常短的轨迹，这并不奇怪。不过，铁和斜方辉石颗粒显现出造成了非常长而细的轨迹，令人想起星尘收集器中的大角度轨迹。我们强调“显现”一词的原因是在实验中出现了一个难题。某些时候加速器的尘埃源会“吐出”，就是产生了少量粒子的喷发，这会扰乱触发系统使速度慢的大颗粒通过为合法粒子通过而开启的闸门。在一些射击中我们看到了这一现象的证据，但在斜方辉石或铁的射击中却没有。在铁的射击中，我们在凝胶中看到了同触发过滤器计数一样多的颗粒。那么就出现了两个问题：（1）这些颗粒是否确实很慢，那些应该出现在目标凝胶中的快速颗粒跑哪儿了？（2）我们看到的这些轨迹是否真是由快速颗粒制造的，为什么它们形成须状轨迹而不是萝卜形轨迹？我们还不知道这些问题的答案。我们准备在后面几个月里用改进的过滤器做更多射击，希望能更好的拒绝“呕吐”。
铁
斜方辉石
Second, Ryan Ogliore in our group at SSL has been modeling the trajectories of interstellar dust particles in the solar system. He has found that the largest interstellar dust particles can come into the detector at a substantial angle. This is because the interstellar collector was oriented to collect dust with β = 1. What is β? β is a ratio -- it is the ratio of the outward force that sunlight exerts on dust particles to the inward gravitational force exerted by the sun. For particles much larger than 1 micron (like the Earth!) β is essentially zero. But for small particles, around the wavelength of light (a few hundred nanometers) β can be 1 or even greater, depending on its size, shape and density. Large particles from interstellar space travel along hyperbolic orbits that curve inward toward the sun as they go through the solar system. Small particles with β<1 also travel along hyperbolic orbits, but ones that curve outward from the sun. Particles with β exactly equal to one just travel in straight lines through the solar system. Because it was not known what the average value of β was for the interstellar dust, the spacecraft controllers oriented the spacecraft and the collector to track the β = 1 particles. When you do the simulation, you find that the largest particles (those with small β) would come into the collector by flying between the solar panels of the spacecraft, and so in the collector you would expect them to produce tracks that have an orientation of "north", or, if you think about the hands of a clock, about 12 o'clock. There is a complication in that the Sample Return Capsule lid, during some phases of the collections, could actually block particles with small β and keep them from reaching the collector.
第二，我们组里在SSL的成员 Ryan Ogliore 在做星际尘埃颗粒在太阳系中弹道模型。他发现最大的星际尘埃颗粒能以基本一致的角度进入探测器。这是由于星尘收集器定向的收集 β=1 的尘埃。β是什么？β是一个比值，它是作用在尘埃上的阳光产生的外部力与太阳施加的引力的比值。对于比1微米大得多的粒子（比如地球）来说β基本上是0。但是对于尺度在光波波长（几百个纳米）附近的小颗粒β能达到甚至超过1，这取决于它的大小、形状和密度。来自星际空间的大颗粒在穿越太阳系时是沿一条向着太阳内弯的双曲线轨道。β>1(注：我怀疑他手滑了- -) 的小颗粒同样是双曲线的轨道，不过是向外弯的。那些β恰好等于1的星际尘埃以直线穿越太阳系。因为不知道星际尘埃β均值是多少，所以飞船控制人员将飞船和收集器定向到β=1的粒子轨道方向。如果你做一下模拟，你会发现那些最大的粒子（小β值）将从飞船的太阳能板之间飞入收集器，于是你会期望它们在收集器中产生有特定指“北”的轨迹，或者可以当作手表的12点方向。样品返回舱（RSC）盖子方面还存在一个问题，在某些收集阶段中，它实际上有可能阻碍了小β值的颗粒接触收集器。
Third, intriguingly, among these 29 tracks, there are seven that have an azimuth between 11:00 and 12:00. We have assumed that these are ejecta from an impact or impacts on the SRC lid, based on their similarity to the others that really do appear to be secondary ejecta based on the Ce content. And in fact, they all do point back to the SRC lid, but only for some of the collection times. The point is that the collector was on a hinged “wrist” and was constantly being rotated to track the β=1 dust stream. For other collection times, these tracks appear to point over the lid and into space. We don’t know when each track was collected, so the situation is ambiguous: for some collection times these events point into the small-β interstellar dust stream, for others they point into the SRC lid. Obviously, this is a critical question, and is not one that is resolvable just by looking at the track trajectories. We have to do more. We have to extract and analyze them.
第三，有趣的是，在这29个轨迹中有7个的方位角在11：00-12：00之间。我们猜测这些来自SRC盖子上的一个或几个撞击的溅射，根据它们相似且与其他不同的铈含量，它们似乎真是二次溅射物。并且实际上，它们的确都指向SRC盖子，不过仅是在收集的某段时间里。关键是收集器是在一个铰链关节上，会经常的转动来追踪 β=1 的尘埃流。在其他的收集时间里这些轨迹的指向似乎是越过了盖子指向太空。我们不清楚每一条轨迹是何时收集到的，于是情况变得模糊了：在某些收集时间里这些事件指向小β尘埃流；其他时间它们指向了SRC盖子。明显，这是个很重要的问题，而且不是仅通过查看轨迹弹道就能解决的问题。我们还要做更多。我们不得不将其取出分析。
It turns out that we have extracted one of these “midnight” tracks. This is sample, I1004,1,2, VM number VM number 862370V1. Last year we examined it using beamline 11.0.2 at the Advanced Light Source. This is the most powerful Scanning Transmission X-ray Microscope (STXM) in the world. We have been working with the beamline scientist, Tolek Tyliszczak, to analyze interstellar candidates on 11.0.2. Unfortunately, it turns out that we had inadvertently analyzed not the track but a feature that was very similar to the track but turned out to be a machining artifact left over from the “keystone” extraction of the track from the collector. We didn’t find any detectable elements in it, which in retrospect is no surprise. So recently George Flynn and Steve Sutton reexamined this track on beamline 2-ID-D at the Advanced Photon Source at Argonne National Laboratory. They did find detectable elements. They are still analyzing the data from this track, so stay tuned. We will also re-examine this track on 11.0.2 at the end of May.
我们已经取出了这些“午夜”轨迹中的一个。这就是样本 I1004,1,2 VM 编号862370V1。去年我们用先进光源的 11.0.2 光束对其进行了检查。这是世界上最强大的扫描透射X射线显微镜(STXM)。我们曾同光束科学家 Tolek Tyliszczak 一同工作在11.0.2上分析星际尘埃候选者。遗憾的是我们分析的不是轨迹而是一个很像轨迹的构造，是从收集器中提取“楔块”过程中人为加工的遗留物。我们没有在其中发现任何可探测的元素，回想起来这也不算意外。因此最近 George Flynn 和 Steve Sutton 在阿冈国家实验室的先进光子源（Advanced Photon Source） 光束2-ID-D 上重新检查了这条轨迹。他们的确发现了可探测的元素。他们依然在分析这条轨迹的数据，我们也会在5月末重新在11.0.2上再次检查该轨迹。
Even if I1004,1,2 turns out to be secondary ejecta, there are still six more tracks that could turn out to be interstellar. When the Cosmic Dust Lab re-opens in May, the first order of business will be to very carefully re-measure the trajectories of these particles, then extract them for analysis.
即便假设 I1004,1,2 原本就是二次溅射物，还有6个可能成为星际尘埃的轨迹。一旦彗星尘埃实验室5月再次开放，第一个议程就是非常仔细的重新测量这些颗粒的弹道，然后提取出来加以分析。
It’s possible that you have collectively identified real bona-fide interstellar dust particles in the Stardust collectors, the first contemporary particles from outside the Solar System ever identified. But it’s also possible that we are following a false lead. We’re still not sure, so we’re trying to gather as much evidence about the particles as we can. I expect we’ll know more in the weeks and months ahead.
有可能你在星尘收集器中共同鉴别出货真价实的星际尘埃颗粒，这可是当代首批来自太阳系之外的粒子被鉴别出来。但同样有可能我们跟着一条错误的线索。现在还不确定，所以我们尽量收集关于颗粒尽可能多的证据。我期望在随后的数周及数月里我们能了解的更多。
I want to express again my gratitude to you. Without your tremendous efforts, we would not have found these tracks. If it turns out that any one of these is likely to be interstellar dust, after our big celebration, we will redo the calibrations for Stardust@home to include these high-angle tracks. Meanwhile, keep up the fantastic work!
我想再次表达我对大家的感谢。没有你们巨大的贡献，我们可能找不到这些轨迹。假如其中有任何一个最终可能是星际尘埃的话，在我们的重大庆祝之后，我们将为Stardust@home重新制作包括这些大角度轨迹的校准片。在此期间，继续这奇妙的工作吧！
Andrew and the Stardust@home team |
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发表于 2009-1-15 22:27:43
听力好的去试试吧
不是第一次了。
发表于 2019-6-22 14:27:23