A radio telescope on its way to the Moon captured a “selfie” of our planet’s radio waves, updating a vintage 1990s science experiment by Carl Sagan.
The ROLSES instrument rode to the Moon aboard Intuitive Machines’ Odysseus lander in February 2024, which arrived in a crater near the lunar South Pole in February. Along the way, the team measured the spectrum of radio waves rippling out into space from Earth. It’s not possible yet to decipher all that radio traffic, but it provides an electronic fingerprint of a civilization busily chattering over the airwaves — and leaking that chatter out into space. University of Colorado physicist Jack Burns and his colleagues hope to compare Earth’s radio fingerprint to their observations of other worlds from a telescope they plan to build on the far side of the Moon.
Everything Gets a Reboot These Days
Along the way to the Moon, ROLSES repeated a famous Carl Sagan experiment from 1993: It captured the spectrum of radio waves pouring from our planet out into space. What they measured was an electronic fingerprint of all the wavelengths of radio broadcasts that passing aliens could see if they, too, pointed a radio spectrometer at Earth. They hope the data will give them a more detailed idea of what to look for in radio data from planets around nearby stars.
In 1993, Carl Sagan used the Galileo spacecraft — then on its way to Jupiter — to measure Earth’s atmospheric composition and radio signature.
“The presence of narrow-band, pulsed, amplitude-modulated radio transmission seems uniquely attributable to intelligence,” wrote Sagan in his 1993 paper in the journal Nature. “These observations constitute a control experiment for the search for extraterrestrial life by modern interplanetary spacecraft.” In other words, Sagan hoped that an alien’s-eye view of Earth from Galileo could help astronomers interpret what they saw in observations of planets orbiting other stars. If we could see what the signs of life on Earth looked like from a distance, then we might recognize them if we ever saw them on a distant world.
Astronomical instruments have come a long way since 1993, both literally and figuratively. With the James Webb Space Telescope (JWST), astronomers are finally able to measure how light filters through the atmospheres of planets like the TRAPPIST-1 worlds as they pass in front of their stars. And the radio spectrometer on ROLSES could measure Earth’s radio output in much more detail than the Galileo spacecraft’s instruments could.
“We can re-do the Sagan experiment from 1993 with a much improved radio spectrum,” says Burns in a presentation at the 244th American Astronomical Society conference on Monday.
Making the Most of an Awkward Situation
ROLSES was meant to spend about 8 days gathering data about how much radio energy from Earth, and the Sun, reaches the southern pole of the Moon. But Intuitive Machines’ Odysseus Lander had a difficult landing in Malapert A crater, and it came to rest with one broken leg, leaning on a nearby slope at about a 30-degree angle. That left the lander’s high-gain antenna pointed at the lunar surface, instead of back toward Earth.
“We were supposed to have 8 days worth of data. We ended up with an hour and a half to 2 hours worth of data,” says Burns.
ROLSES spent only about 20 minutes doing radio astronomy from the surface of the Moon — still a major first and (Burns and others hope) a precursor to full-scale radio observatories on the far side of the Moon. Because the observation time was so short, the data is more noise than signal. The rest, about an hour and a half of much clearer data, was collected on the way to the Moon, when ROLSES wasn’t supposed to have been up and running at all, which underscores how much of spaceflight still relies on being able to improvise quickly.
What’s Next?
Burns and his colleagues have ambitious plans for their hour and a half of data from ROLSES.
“As a control experiment, what we're looking forward to is comparing this Earth spectrum to ones of nearby exoplanets that we hope to observe with our FARSIDE array of radio telescopes,” says Burns. FARSIDE would be an array of 128 radio antennas arranged over about 6 square miles of the Moon and connected to a base station. It’s received some design funding from NASA so far.
The FARSIDE antennas will listen for faint signals from the Cosmic Dark Ages, but Burns also wants to use them to scan about 2,000 nearby star systems for signs of intelligent, radio-using life — and compare those signals with the ones Galileo and ROLSES measured coming from Earth.
Burns and his colleagues are also working with Texas-based Lunar Resources, INc. to design a 5-square-mile array of radio antennas sprawling across a lunar plain. Picture something like an old-fashioned television antenna; now picture roughly 100,000 of them, lined up end-to-end in a series of zig-zags. Combined, those antennas will act as one giant radio receiver.
Picture something like an old-fashioned television antenna; now picture roughly 100,000 of them, lined up end-to-end in a series of zig-zags.
The concept competing with Farview is basically a lunar version of the now-defunct Arecibo Observatory (RIP) called the Lunar Crater Radio Telescope, a semicircle of wire mesh about a third of a mile wide, lining the bottom of a 1.9-mile-wide crater on the Moon. Both projects have received some funding from NASA for development and design, but neither is fully funded for actually building or launch.
Before either of those ambitious projects actually break lunar ground, two more small radio telescopes will launch to the Moon in 2026. One is a second, upgraded version of ROLSES. The second is LuSEE-Night, which, if everything goes to schedule, will land on the far side of the Moon in January 2026 and deploy a pair of spring-loaded, 20-foot-long radio antennas. The little telescope will test whether the far side of the Moon is really as good a vantage point for radio astronomy as everyone hopes. It will also give us our first, albeit low-resolution, glimpse of the galaxy at the low radio frequencies we can't see from Earth.
