Shape-shifting mirrors on NASA’s new space telescope could unveil Jupiters like our own
Shape-shifting mirrors on NASA’s new space telescope could unveil Jupiters like our own
NASA 新型太空望远镜上的“变形镜”或将揭开类木行星的神秘面纱
EXECUTIVE SUMMARY 执行摘要
When NASA’s Nancy Grace Roman Space Telescope launches, as early as the end of next month, it will attempt one of astronomy’s most precise disappearing acts to date. The telescope will carry the first space-bound “active” coronagraph, an instrument that effectively erases most of the light from a star during photography. It will allow astronomers to take the first pictures of planets orbiting other stars that are similar to those in our solar system. Ultimately, it could pave the way for a future mission that could snap the first photos of Earth-like worlds. 当 NASA 的南希·格雷斯·罗曼太空望远镜（Nancy Grace Roman Space Telescope）最早于下个月底发射时，它将尝试天文学迄今为止最精确的“消失术”之一。该望远镜将携带首个进入太空的“主动式”日冕仪，这是一种能在拍摄过程中有效消除恒星大部分光线的仪器。它将使天文学家能够首次拍摄到围绕其他恒星运行的行星，这些行星与我们太阳系中的行星类似。最终，这可能为未来拍摄类地行星照片的任务铺平道路。
“I hope it’s remembered for it being that critical stepping stone for … finding Earth 2.0,” says Brandon Creager, the instrument’s lead mechanical engineer at NASA’s Jet Propulsion Laboratory (JPL). “我希望它能因成为寻找‘地球 2.0’的关键垫脚石而被铭记，”NASA 喷气推进实验室（JPL）该仪器的首席机械工程师布兰登·克里格（Brandon Creager）说道。
Named after Nancy Grace Roman, NASA’s first chief of astronomy, this new telescope will carry a roughly 300-megapixel wide-field camera that will enable it to capture images about 100 times larger than the Hubble Space Telescope’s widest exposures at a similar resolution. These capabilities will help astronomers unpack the mysterious identities of dark matter and dark energy—and to detect around 100,000 new exoplanets, planets outside our solar system, whose presence can be inferred from the way they distort the starlight of more distant stars. 这台新望远镜以 NASA 首位首席天文学家南希·格雷斯·罗曼的名字命名，它将携带一台约 3 亿像素的广角相机，使其能够以相似的分辨率捕捉到比哈勃太空望远镜最宽视野大 100 倍的图像。这些能力将帮助天文学家揭开暗物质和暗能量的神秘面纱，并探测约 10 万颗新的系外行星（太阳系外的行星），这些行星的存在可以通过它们扭曲更遥远恒星光线的方式来推断。
Javier Viaña, a research scientist at Harvard who has had two projects selected for Roman’s highly competitive first year of observing, compares the leap to moving from “interviewing a handful of people” to “conducting a global census.” 哈佛大学的研究科学家哈维尔·维亚尼亚（Javier Viaña）有两个项目入选了罗曼望远镜竞争激烈的首年观测计划，他将这一飞跃比作从“采访少数几个人”到“进行全球人口普查”。
Another camera will use the coronagraph, blocking out a star’s light as it observes one stellar system at a time. The instrument will allow astronomers an unprecedented look at the space around stars, enabling them to see smaller, dimmer, and more close-in exoplanets. “It’s giving us the ability to see planets that we haven’t been able to physically see before,” says Creager. 另一台相机将使用日冕仪，在观测单个恒星系统时遮挡恒星的光芒。该仪器将使天文学家能够以前所未有的视角观察恒星周围的空间，从而看到更小、更暗、距离恒星更近的系外行星。“它赋予了我们观察以前无法直接看到的行星的能力，”克里格说。
The anatomy of a vanishing trick “消失术”的解剖学
Coronagraphs in space aren’t new. But earlier incarnations, such as those currently aboard Hubble and the James Webb Space Telescope, use a stationary system to block a star’s blinding light. The approach does help, but it’s a bit like putting your thumb over a flashlight while searching a dark room for a firefly. Though the bulb vanishes, stray glare can still escape and overwhelm the light of the insect. Inside a telescope, that glare can come from light leaking around the edges of machinery or from minuscule imperfections in mirrors and coatings that can scatter starlight into speckles. All this can hide, or even impersonate, a planet. 太空日冕仪并非新鲜事物。但早期的版本，例如目前在哈勃和詹姆斯·韦伯太空望远镜上的那些，使用的是固定系统来遮挡恒星刺眼的光芒。这种方法确实有帮助，但就像在黑暗房间里寻找萤火虫时用拇指遮住手电筒一样。虽然灯泡消失了，但杂散的眩光仍可能逃逸并淹没昆虫的光芒。在望远镜内部，这种眩光可能来自机械边缘的漏光，或来自镜面和涂层的微小瑕疵，这些瑕疵会将星光散射成斑点。所有这些都可能掩盖甚至伪装成一颗行星。
Roman’s coronagraph, however, will attempt something completely unseen in space telescopes until this year: Before each observation, it will measure that leftover light and try to suppress it, a technique known as active wavefront control. The telescope is able to do this because it contains two deformable mirrors. Each has a 48-by-48 checkerboard of actuators (tiny pistons) beneath a thin, deformable sheet of glass. Applying a small amount of voltage makes the actuators contract and tug their patches of mirror slightly backward, like thousands of microscopic fingers delicately sculpting a surface. 然而，罗曼望远镜的日冕仪将尝试一种在今年之前太空望远镜中从未出现过的技术：在每次观测前，它会测量残留的光线并试图将其抑制，这种技术被称为“主动波前控制”。望远镜之所以能做到这一点，是因为它包含两面可变形镜。每一面镜子下方都有一个 48x48 的执行器（微型活塞）棋盘阵列，位于一层薄薄的可变形玻璃片之下。施加少量电压会使执行器收缩并向后轻微拉动镜面，就像成千上万根微小的手指在精细地雕刻表面一样。
The effect is very subtle: Each patch of mirror can deform by up to 0.5 micrometers, or about one-fourth the size of an E. coli bacterium, and in increments as small as approximately 10 picometers. That’s about a tenth the diameter of a hydrogen atom, says Ilya Poberezhskiy, the instrument’s project systems engineer at JPL. 这种效果非常微妙：每一块镜面最多可变形 0.5 微米（约为大肠杆菌大小的四分之一），增量小至约 10 皮米。JPL 该仪器的项目系统工程师伊利亚·波别列日斯基（Ilya Poberezhskiy）表示，这大约是氢原子直径的十分之一。
The actuators allow the mirrors to create an “active wavefront,” where each component is moved to the perfect position to cancel out incoming waves of unwanted light—a bit like a pair of noise-canceling headphones, but for light instead of sound. The “canceled-out” light creates a “doughnut-shaped region around the star where we suppress starlight and where we’re hoping to see exoplanets,” says Poberezhskiy. 执行器使镜面能够产生一个“主动波前”，每个组件都被移动到完美的位置以抵消传入的不必要光波——这有点像降噪耳机，只不过是针对光线而非声音。波别列日斯基说，这种“被抵消”的光线在恒星周围形成了一个“甜甜圈状的区域，我们在那里抑制星光，并希望在那里看到系外行星”。
Compared with current space-based coronagraphs, the system is expected to improve sensitivity to exoplanets against the glare of their host stars by a factor of up to 1,000, revealing planets that would have been far too faint to detect before. 与目前的太空日冕仪相比，该系统预计将对系外行星相对于宿主恒星眩光的灵敏度提高多达 1000 倍，从而揭示出以前因太暗而无法探测到的行星。
Like Hubble and JWST, Roman also uses masks, patterned plates placed in the path of the light that are designed to block the photons that run into them. One tool in Roman’s mask arsenal is “silicon grass,” a thicket of microscopic spikes on some masks that can be used in certain configurations to absorb photons so they don’t bounce around the telescope and accidentally reach a detector. Light entering the forest bounces deeper and bounces deeper between the blades and gets trapped instead of reflecting back toward the camera. “Once the light gets into there, it never gets out,” Poberezhskiy says. The mirrors and masks form a succession of gates and hedges to guide as much of the preserved planetary light as possible toward the final detector. 像哈勃和韦伯望远镜一样，罗曼望远镜也使用掩模（masks），即放置在光路中的图案板，旨在阻挡撞击它们的各种光子。罗曼望远镜掩模库中的一个工具是“硅草”（silicon grass），这是一种在某些掩模上由微小尖刺组成的丛林，可以在特定配置下吸收光子，防止它们在望远镜内反弹并意外到达探测器。进入这片“森林”的光线会在叶片之间不断深入反弹并被困住，而不是反射回相机。“一旦光线进入那里，就再也出不来了，”波别列日斯基说。镜子和掩模形成了一系列闸门和树篱，引导尽可能多的保留下来的行星光线射向最终的探测器。
Alien Jupiters 外星木星
This elaborate setup could open a new chapter in the direct imaging of exoplanets. Nearly all exoplanets photographed so far are oversize youngsters that are nothing like the residents of our solar system: several times the mass of Jupiter, still glowing with the heat left over from their birth, and orbiting tens or hundreds of times farther from their star than the Earth is from the sun. This is because they are relatively easy to see. Their size, warmth, and distance from their parent star makes them shine brightly in infrared light, far away from the worst of the stellar glare. 这种精密的装置可能会开启系外行星直接成像的新篇章。迄今为止拍摄到的几乎所有系外行星都是超大尺寸的“幼年”行星，与我们太阳系的行星完全不同：它们的质量是木星的数倍，仍散发着诞生时残留的热量，且轨道距离恒星的距离比地球到太阳的距离远几十倍甚至几百倍。这是因为它们相对容易被观测到。它们的大小、温度以及与母恒星的距离使它们在红外光下显得格外明亮，远离了恒星最严重的眩光干扰。
Roman, however, could directly image a true Jupiter analogue—a planet similar to Jupiter in mass and circling a sunlike star a few times farther out than Earth is from our sun. Unlike the hot Jupiters we can see now, this one would be a much more mature gas giant like ours, primarily reflecting its parent star’s light after billions of years of cooling instead of heavily emitting its own. 然而，罗曼望远镜可以直接拍摄到一颗真正的“类木行星”——一颗质量与木星相似、围绕类太阳恒星运行、距离比地球到太阳距离远几倍的行星。与我们现在能看到的“热木星”不同，这颗行星将是一颗像我们太阳系中那样更成熟的气态巨行星，在经过数十亿年的冷却后，它主要反射母恒星的光，而不是大量自身发光。