The night sky is no longer as quiet as it used to be. Thousands of satellites are whizzing overhead, and each one can accidentally emit radio waves, interfering with astronomers trying to listen to distant galaxies. To make sense of this noise, scientists pointed the 21CMA radio telescope in China at Starlink satellite passes.
The 21CMA telescope consists of many small antennas spread across a field. It picks up radio waves at the frequency where hydrogen, the most abundant element in the universe, emits. The sensitivity of a single group of antennas is low, so astronomers built an entire pipeline: first, they used orbital data to predict when a satellite would enter the field of view, then they recorded the signal and looked for anomalies.
In the end, instead of broadband noise from Starlink, they caught a clear signal from ORBCOMM satellites: those beam data to Earth in narrow cones. This confirmed the method works: the predicted satellite matched the decoded identifier. As for the bright spikes initially thought to be satellite interference, they turned out to be sparks from power lines near the antennas.
Such a "listener" helps astronomers separate cosmic noise from earthly interference and keep radio observations clean in the era of megaconstellations.
The 21CMA radio telescope is so sensitive it could pick up a cell phone signal from the Moon, if the phone were transmitting on the right frequency.
In Carl Sagan's novel "Contact," radio telescopes pick up an alien signal, but first, astronomers have to filter out interference from earthly technology.