Henry's Hot Star Without a Star, Apollo 17

Brief

Dick Henry, co-investigator on the Apollo 17 ultraviolet experiment, presented findings at NASA's Manned Spacecraft Center on January 8, 1973. Surveys at high northern and southern galactic latitudes returned a UV spectrum indistinguishable from that of a hot star, yet no hot stars were within the instrument's field of view. Henry offered two candidate explanations — galactic-plane starlight reflecting off interstellar dust, or extragalactic radiation — while acknowledging that spectral characteristics undermine the first. He also reported that a Lyman-alpha search for ionized hydrogen in the Coma cluster found nothing, leaving its gravitational binding mechanism unresolved.

Metadata

Agency

NASA

Release

5/8/26

Incident

1973

Type

PDF • .pdf

Length

3 pages

Programs

Apollo 17, OGO-5

Tags

UV background radiation, galactic pole survey, hot star spectrum without source, possible extragalactic radiation, Coma cluster binding mystery, Apollo 17, 1973, lunar UV observation

Key points

• UV surveys at high galactic latitudes (north and south) returned a spectrum consistent with a hot star, despite no hot stars being present in the field of view.p.119
• The abnormally high dark current in the instrument was confirmed not to have contaminated the UV measurement — the detected spectrum sits above the dark count.p.119
• Henry's conservative interpretation attributes the signal to light from hot stars in the galactic plane reflecting off interstellar dust.p.120
• Certain spectral characteristics are inconsistent with the dust-reflection theory, leaving extragalactic radiation as a live alternative explanation.p.120
• A Lyman-alpha search for ionized hydrogen in the Coma cluster returned no detection, leaving the cluster's gravitational binding mechanism unexplained.p.119
• Lyman-alpha observations indicate interstellar hydrogen streaming through the solar system, as theorized by Gary Thomas at the University of Colorado.p.120
• Apollo 17 produced an unusually large Lyman-alpha data set expected to substantially clarify the solar-system hydrogen picture.p.120
• Definitive computer analysis of the UV background data was pending at the time of the debriefing and expected to require significant time.p.120

Most interesting

• This document was released under a UAP disclosure initiative despite containing no reference to unidentified craft — the anomaly is a purely astronomical UV signal, illustrating how broadly the disclosure scope was drawn.
• The Coma cluster gravitational mystery Henry flagged in 1973 — what holds the cluster together if not ionized hydrogen — became a foundational piece of evidence for dark matter, a framework not yet dominant at the time of the briefing.
• The Apollo 17 UV instrument operated from cislunar space and the lunar surface, giving it access to UV wavelengths entirely blocked by Earth's atmosphere — a capability that would not be routinely available again until dedicated space-based UV observatories in subsequent decades.
• Henry used the phrase 'nobody had expected' to characterize the X-ray background discovery, then applied the same framing to his UV finding — signaling awareness that the UV result belonged to the same emerging category of anomalous cosmological backgrounds.
• The debriefing took place fewer than three weeks after Apollo 17's splashdown on December 19, 1972, meaning Henry was reporting on data that was still in its earliest stages of analysis; the 'long time' he anticipated proved accurate.