For the last decade, scientists have been shooting powerful laser beams toward a reflector on the Lunar Reconnaissance Orbiter (LRO), a spacecraft that orbits our moon about 240,000 miles from Earth. They were hoping to hit the reflector and bounce part of the laser beam back to Earth, thus learning key information about our nearest natural satellite.
It’s a difficult shot: the reflector in question is the size of a paperback novel, and it moves over the moon’s surface at 3,600 miles per hour, reports Katherine Kornei for the New York Times.
NASA scientists have tried and failed “dozens” of times to pull this off since the LRO launched in 2009, according to a statement. Finally, in 2018, a team from NASA and the Université Côte d’Azur in France succeeded. The team announced their exciting results in a study published in Earth, Planets and Space last week, reports Ashley Strickland reports for CNN.
As the Times reports, this new development could help scientists understand what’s happening to a data-collection project on the moon that dates back to the Apollo program—the only remaining science experiment from the Apollo era, per CNN.
In the Apollo 11, 14 and 15 landings on the moon, astronauts dropped off small reflector panels made up of the corners of glass cubes. These devices act as a powerful mirror that can reflect lasers between Earth and the moon, per the NASA statement. Subsequent Soviet missions to the moon in 1972 and 1973 also left behind two more reflectors on the moon’s surface, bringing the total number of reflectors on the moon’s surface up to five.
The “retroreflectors” work like “really long yardsticks,” explains Kornei for the Times. Scientists can measure the time it takes a laser beam to travel from Earth, bounce off the reflector, and return, and then calculate the precise distance between the Earth and the moon.
“Now that we’ve been collecting data for 50 years, we can see trends that we wouldn’t have been able to see otherwise,” explains Erwan Mazarico, NASA scientist and co-author on the study, in a statement. He adds: “Laser-ranging science is a long game.”
For instance, scientists found that the moon and Earth are drifting apart, “at the rate that fingernails grow,” or about 1.5 inches per year, per the NASA statement.
However, the retroreflectors on the moon’s surface have become less accurate in the last 50 years since their installment. Scientists aren’t yet sure why, but they hypothesize that the dust from the moon’s atmosphere might be obscuring the reflectors’ surfaces, reports Rafi Letzter for Live Science.
To test their theory and troubleshoot the problem, NASA scientists decided to fire lasers at a similar reflector on the LRO. The feat was difficult in part because the laser photons have to travel through Earth’s atmosphere, which tends to scatter the photons in many different directions and decreases the chances that scientists will receive a signal back on Earth.
The team finally achieved success thanks to the innovation of scientists in the Géoazur team at the Université Côte d’Azur, who developed a method to shoot infrared light up at the LRO. Previously, scientists had used a visible green wavelength of light, but infrared light does a better job of penetrating Earth’s atmosphere, per the NASA statement.
As Michelle Starr reports for Science Alert, this marks the first time that photons have been successfully reflected back to Earth from a lunar orbiter. Scientists say that this leap in innovation could help them determine if lunar dust is interfering with the retroreflectors on the moon’s surface, although they still need to conduct more research.
Still, some scientists are cautiously optimistic. As Tom Murphy, a physicist with at the University of California, San Diego not involved in the study, tells the Times: “For me, the dusty reflector idea is more supported than refuted by these results.”