詹姆斯·韦布太空望远镜（JWST）于 2021 年 12 月 25 日发射，在第四个周年之际由「探索模式」走向更长期的研究阶段。它历时一代打造、成本约 97 亿美元，被描述为史上最强望远镜，能以前所未有的距离与细节观测，核心问题是：我们究竟能看见宇宙多远、多少。

相较于 1990 年发射、主要观测可见光的哈伯望远镜，JWST 以红外为主，得以穿透宇宙尘埃、观测更冷的天体，并追索被宇宙膨胀「拉红」的遥远光源。文中指出 JWST 可寻找大爆炸后约 1 亿年出现的第一批光源，并延伸对太阳系与系外行星系的形成史理解，例如研究海王星外「数十个」冰冷天体的表面化学，与在小行星带发现水的线索。

在更大尺度上，JWST 用于抽样观测不同演化阶段的星系与恒星死亡事件（如超新星），以解释半个世纪来的疑问：宇宙尘埃似乎多于可被核算的来源，超新星可能是来源之一。它的终极目标之一是找到几乎只含氢与氦的「原初」星系；可见光望远镜在大爆炸后约 10 亿年附近有观测极限，而 JWST 已能回溯到约 3 亿年。放在约 137 亿年的宇宙年龄中，这仍只是短暂一瞬；计划预期持续到 2040 年代。

The James Webb Space Telescope (JWST) launched on Dec. 25, 2021, and its fourth anniversary marks a shift from early discovery to longer-term research. Built over a generation at about $9.7 billion, it is described as the most powerful telescope yet, enabling unprecedented distance and detail. A central quantitative question emerges: how much of the universe is actually observable with this new instrument?

JWST extends Hubble’s legacy (Hubble launched in 1990) by observing primarily in infrared rather than visible light. Infrared viewing helps pierce dust, detect cooler objects, and see earlier epochs because cosmic expansion shifts distant light into longer wavelengths. The article highlights targets as early as ~100 million years after the Big Bang and connects JWST’s measurements to solar-system history (chemistry of scores of icy bodies beyond Neptune) and exoplanet diversity (mini-Neptunes and super-Earths).

On galactic scales, JWST samples galaxies and supernovae to address a long-running mismatch: the universe appears dustier than accounted for, with supernovae proposed as a source. Its longer-range aim is to find “pristine” galaxies dominated by hydrogen and helium; visible-light limits sit around ~1 billion years after the Big Bang, while JWST has already observed objects back to ~300 million years. Against a ~13.7-billion-year-old universe, that reach is brief, and operations are expected into the 2040s.