Scorpius X-1 (1962): Giacconi and the Birth of X-Ray Astronomy

Riccardo Giacconi and colleagues discovered the first X-ray source outside the Solar System in 1962, using detectors lifted above the atmosphere by a rocket. The bright source in Scorpius, later named Scorpius X-1, opened X-ray astronomy and helped earn Giacconi half of the 2002 Nobel Prize in Physics.

The question

Earth’s atmosphere blocks X-rays from space, which protects life on the ground but blinds ground-based telescopes to that part of the spectrum. To see the X-ray sky, instruments have to be lifted above the air. The team’s original target was modest and close to home: they hoped to detect X-rays from the Sun striking the Moon and making its surface fluoresce, that is, glow in X-rays of its own. Beyond the Sun, the X-ray sky was essentially unexplored.

What the paper found

• A bright source in Scorpius. Instead of the Moon, the detectors picked up a strong X-ray source in the constellation Scorpius, now called Sco X-1.
• The first of its kind. It was the first X-ray source found outside the Solar System.
• A glow everywhere. The flight also detected a diffuse X-ray background spread across the sky.
• A careful claim. The title says “evidence for” X-rays from sources outside the Solar System, a measured first step from a single short flight.

How they did it

On 18 June 1962 an Aerobee rocket launched from the New Mexico desert and carried X-ray detectors above the atmosphere for about 350 seconds, less than six minutes. The detectors were crude by later standards and could not make images, only count X-rays from the direction they faced. In that brief window, the strongest signal came not from the Moon but from Scorpius, with a fainter glow from all over the sky.

Why it mattered

This short flight opened X-ray astronomy, the window that later revealed accreting black holes, neutron stars and the hot gas that dominates galaxy clusters. Neutron stars had been proposed decades earlier, in Baade and Zwicky’s 1934 work on supernovae. The hot gas in clusters became key evidence in the Bullet Cluster study of dark matter, where the X-ray-glowing gas sits apart from most of the mass.

Giacconi received half of the 2002 Nobel Prize in Physics for pioneering contributions to astrophysics that led to the discovery of cosmic X-ray sources.

What we know now

Giacconi went on to lead the Uhuru and Einstein X-ray missions and to propose the Chandra X-ray Observatory. The Chandra X-ray Observatory, launched on 23 July 1999, and ESA’s XMM-Newton continue the work today. Chandra’s future is under budget pressure: for fiscal year 2026, Congress appropriated not less than $63 million, below the cost of the full mission, and the Chandra team reported in May 2026 that the 2027 presidential budget request makes no mention of it (as of October 2026).

Who wrote it

Riccardo Giacconi, Herbert Gursky, Frank R. Paolini and Bruno B. Rossi wrote the letter, published in Physical Review Letters on 1 December 1962. Semantic Scholar lists about 625 citations (October 2026).

Read the original

Giacconi, R., Gursky, H., Paolini, F. R. and Rossi, B. B. (1962). “Evidence for X Rays From Sources Outside the Solar System.” Physical Review Letters 9, 439-443. doi:10.1103/PhysRevLett.9.439

Free to read: publisher PDF from the American Physical Society.