XRAY
A FIELD GUIDE TO THE CHANDRA X-RAY OBSERVATORY
Ten chapters on the observatory that sees the sky the way it looks at millions of degrees — the gas between galaxies, the debris of exploded stars, matter spiralling into black holes — using mirrors so shallow that X-rays skip off them like stones across a pond, from an orbit that swings a third of the way to the Moon. Scroll to begin.
BEGIN — CHAPTER 01 →The hot, violent sky is invisible from the ground
X-rays are given off by matter heated to millions of degrees or accelerated to enormous energies — supernova blast waves, gas swirling toward black holes, the multi-million-degree plasma that fills galaxy clusters. None of it can be seen in ordinary light.[1]
Earth's atmosphere absorbs cosmic X-rays completely — which is fortunate for life and inconvenient for astronomy. To study this side of the universe at all, the telescope has to be above the air.
Carried up by Columbia, commanded by Eileen Collins
Chandra rode to orbit in the payload bay of Space Shuttle Columbia on mission STS-93, launched July 23, 1999 — with Eileen Collins as commander, the first woman to command a Space Shuttle flight.[1]
With its attached Inertial Upper Stage booster, the observatory was the heaviest payload the Space Shuttle ever carried. The shuttle released it in low orbit; the booster then fired to fling Chandra out onto its towering elliptical path.
Above the radiation belts for 55 hours out of every 64
Chandra does not circle close to Earth like Hubble. Its orbit is a stretched ellipse — roughly 16,000 km at its nearest and 134,000 km at its farthest, about a third of the distance to the Moon — taking around 64 hours to go around once.[2]
That shape keeps the telescope above the Van Allen radiation belts, which would swamp its detectors, for most of each orbit — giving it long, uninterrupted stretches of observing time no low-orbit telescope can match.
One mirror assembly, and a wheel of detectors behind it
Chandra is about 14 metres long with its sunshade deployed, and almost all of that length is the optical bench holding the mirrors at the front apart from the instruments at the back. Engineers grouped the hardware into a few big assemblies — select one.
Sharp enough to read a stop sign from twelve miles away
An X-ray striking a mirror head-on is simply absorbed. But at a grazing angle — a degree or less — it reflects, the way a flat stone skips off water. Chandra's mirrors are near-cylindrical barrels that the X-rays travel almost parallel to, glancing off the inner surface twice on the way to focus.[2] The result is an angular resolution of about 0.5 arcseconds, the sharpest of any X-ray telescope — enough to pick a single star out of a crowded remnant.
The debut image found an object no one had confirmed was there
Chandra's first released image, in 1999, was of Cassiopeia A — the expanding shell of a star that exploded around 340 years ago. At the very centre sat a bright point of light: the compact object left behind by the collapse, most likely a neutron star, revealed clearly for the first time.[3] It was an early, direct demonstration of what a sharp X-ray eye could add to a much-studied object.
Two galaxy clusters collided and left their mass behind
In the Bullet Cluster, two clusters of galaxies have passed through each other. Chandra mapped the hot gas — most of the ordinary matter — and found it dragged and slowed in the middle by the collision. But gravitational lensing showed most of the mass sitting out ahead, with the galaxies, where the gas is not.[3]
The clean separation of where the matter is from where the mass is remains one of the strongest direct arguments that dark matter is real and not a mistake in our theory of gravity.
A black hole pressing sound waves into a cluster of gas
Chandra's images of the Perseus galaxy cluster showed concentric ripples in the multi-million-degree gas, pressure waves driven outward by eruptions from the giant black hole at the cluster's centre. Translated into a pitch, the waves correspond to a B-flat roughly 57 octaves below middle C — far too low to hear, but a real, measurable sound the black hole has been sounding for millions of years.[3] The same hot cluster gas, mapped across the sky, is how Chandra weighs the largest structures that exist.
Built for five years, still returning data after twenty-five
Chandra was designed for a five-year mission and passed twenty-five years of operation in 2024, still the only telescope that can produce sub-arcsecond X-ray images.[1] Its detectors have slowly warmed and its insulation has degraded, forcing careful thermal management, but the science output remains high. Proposed deep budget cuts in 2024 drew heavy pushback from astronomers; as of 2026 the observatory continues to operate while its long-term funding is debated.
One of four eyes on the same sky
Chandra is the X-ray member of NASA's Great Observatories — four large telescopes, each tuned to a different band of light, designed so their views could be laid over one another. Three have ended; Chandra is the last still working.
Sources cited throughout this guide
Bracketed numbers in the text above correspond to the sources below.