A narrowband radio burst recorded on 15 August 1977 by the Big Ear telescope at the Ohio State University Radio Observatory in Delaware, Ohio. It lasted 72 seconds, exactly the time the fixed dish’s beam takes to sweep a point in the sky, and rose to about thirty times the background. Reviewing the printout days later, Jerry Ehman circled the intensity string 6EQUJ5 and wrote one word beside it.
What was actually recorded
The frequency was close to 1420 megahertz, the hydrogen line, which is the band a transmitting civilisation would be expected to choose and the band terrestrial transmitters are forbidden to use. The direction was in Sagittarius. The profile matched a point source at celestial distance rather than anything local, and no aircraft, satellite or spacecraft was found to account for it. One detail complicates the clean reading: Big Ear observed through two feed horns, so a celestial source crossing the beam should have registered twice a few minutes apart, and this registered once.
Why it has never been resolved
It has never repeated. Ehman himself looked for it again, Robert Gray spent decades on targeted searches with larger instruments, and nothing has returned. A 2017 proposal attributing it to hydrogen clouds around a passing comet was rejected by the observatory’s own people and by most of the field, on the grounds that the comet was not in the beam and that comets do not produce that profile. The honest statement is that it is one unrepeated detection of unknown origin, which is the weakest possible evidential category and still the strongest thing SETI has ever recorded.
Connections
SETI · Arecibo Message · METI · Drake Equation · Fermi Paradox · The Three-Body Problem