In 1964, NASA first tracked a satellite with lasers; the method later helped measure sea-level rise and ice loss

A clear night late in October 1964, six months before the Beacon Explorer C mission made laser tracking almost routine, saw NASA researchers shoot up a red beam of a laser into the sky. According to NASA's Goddard Space Flight Center, the beam reflected off a satellite hundreds of miles above the Earth and then returned to the ground, making this the first time anyone had tracked a spacecraft with a laser. It was a rough setup, assembled from spare radar parts and a lot of patience. Sixty years on, that same idea is helping scientists measure how fast our oceans are rising and how fast polar ice is disappearing.

A red beam, a very late night, and some borrowed radar parts

The experiment was carried out at the Goddard Geophysical and Astronomical Observatory in Maryland. Physicist Henry Plotkin's team used a ruby laser system called GODLAS, which stands for "Goddard laser," mounted on a stand borrowed from missile-tracking radar. Lasers were still new; the first working laser had been built only four years earlier. The scientists aimed the GODLAS laser at Explorer 22, which had 360 small glass reflectors, or "cube corners," on nine panels. These corner-cube reflectors send incoming light back to its source, regardless of the angle at which it arrives. On the night of 31 October 1964, the pulses hit their mark, and faint return signals appeared as blips on an oscilloscope screen. NASA announced the success on November 13, 1964, saying the range accuracy was about 10 feet, roughly 25 times better than the radar of the day. Five ground stations in the US and France were doing the same thing by the end of the decade.

Then came Beacon Explorer C, and hundreds of hits within days

The real turning point came in April 1965 with the launch of Beacon Explorer C. By then, laser tracking was almost routine. Within days, they were getting hundreds of range measurements per orbit, Plotkin recalled at a 50th anniversary talk on the milestone. Engineers even mounted a laser tracking station in a mobile trailer that could travel between radio stations and calibrate them. More than 60 years after its 1965 launch, ground stations still track Beacon Explorer C for laser-ranging data.

How does bouncing a laser off a satellite work?

A ground station fires a short laser pulse at a satellite with cube-corner reflectors, and the light bounces straight back. Because the speed of light was known, scientists timed the round trip and calculated the satellite's distance. Distances that could be measured to within 10 feet can now be measured to within a few millimeters.

From tracking orbits to tracking the planet

In a pioneering prediction, Plotkin said the technique could one day tell the exact shape of the planet, and permit communication with spacecraft. That vision stood strong. Today, satellite laser ranging is used to map the Earth's gravity field, find the planet's center of mass, and measure the slow creep of tectonic plates. More than 40 ground stations are in operation on all continents except Antarctica, though the technique's reach extends well beyond Earth itself. By the same principle, and as NASA's own program records show, lasers have been bounced off a single 46-centimetre panel holding 100 corner-cube reflectors left by Apollo 11 astronauts on the moon in 1969, off the MESSENGER spacecraft en route to Mercury in 2005, off a Mars-orbiting spacecraft that same year, and off NASA's Lunar Reconnaissance Orbiter since 2009.

Watching the oceans rise

Since satellites have a tendency to shift due to changes in the distribution of masses on Earth, accurate orbital tracking is an effective way of monitoring the movement of water and ice on Earth. The study, 'Barystatic sea level change observed by satellite gravimetry: 1993–2022,' published in PNAS in July 2025, used nearly three decades of laser-ranging and gravity data to estimate how much global sea-level rise came from added ocean mass rather than thermal expansion. The researchers, led by Yufeng Nie, report that this mass-driven part of sea level rise averaged about 1.75 millimeters a year from 1993 to 2022, accelerating to about 2.16 millimeters a year after 2003, mostly because Greenland's ice sheet started melting faster. These estimates agreed with independent measurements from NASA's GRACE missions, supporting the team's conclusion that both methods are reliable.

Seeing the ice disappear

Satellite laser ranging can play a role in measuring ice loss in Greenland and Antarctica, which is a major cause of rising seas. Jennifer Bonin and colleagues published research in The Cryosphere testing whether laser ranging data could track ice mass change in both regions. The study suggests that this could extend the ice-loss record back to the early 1990s, before the existence of dedicated gravity-mapping satellites like GRACE, and could help bridge any gap if a future mission is delayed. Satellite laser ranging has come a long way from a single ruby laser on a converted missile tracker. Today, its biggest role may be closer to home: measuring, millimeter by millimeter, how Earth's oceans and ice sheets are changing.