'Shooting the Moon' with Satellite Laser Ranging
- This story is the second in a series celebrating the 50th anniversary of satellite laser ranging.
Laser ranging from Earth to NASA's Lunar Reconnaissance Orbiter (LRO) was a milestone in the 50-year history of satellite laser ranging.
Launched in 2009 and still orbiting the moon in 2014, LRO was the first spacecraft beyond Earth orbit to be routinely tracked using lasers, which were located at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and other facilities.
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“The advantage of laser ranging is its accuracy, and that was apparent even in the earliest experiments,” said John Degnan a former Goddard researcher who has been involved in satellite laser ranging since its earliest days. Degnan is now Chief Scientist at Sigma Space Corporation.
For Earth-orbiting satellites, laser ranging dates back to 1964, when the first successful experiments were conducted at the Goddard Optical Research Facility, now called the Goddard Geophysical and Astronomical Observatory. A team of scientists fired a ruby laser – still quite novel at the time – at the Explorer 22 spacecraft orbiting high in Earth’s atmosphere. The satellite, also known as Beacon Explorer B, was equipped with an array of reflectors designed specifically to send a laser signal back to the point of origin. The researchers measured a range of 600 miles (roughly 966 kilometers) to the spacecraft, with an accuracy up to about 10 feet (3 meters) – about 25 times better than the best microwave radars at the time could provide.
Laser ranging quickly became a standard technique. Throughout its 50-year history, it has been used to track more than 150 satellites and has ranged to five arrays of reflectors on the surface of the moon.
For laser ranging to LRO, laser pulses were transmitted by Goddard’s Next Generation Satellite Laser Ranging System and other global stations. Pulses of green laser light traveled about 240,000 miles to be received by the spacecraft, moving at 3,600 miles per hour. These measurements were carried out with a range accuracy of about four inches (10 centimeters). (Radio telemetry was used to communicate the arrival time of the laser pulses from the spacecraft back to Earth.)
The high degree of accuracy provided by laser ranging makes it possible for LRO to carry out one of its primary mission goals: creating detailed maps of the topography, revealing the altitudes of features on the lunar surface.
To make these maps, the spacecraft’s Lunar Orbiter Laser Altimeter (LOLA) sends 28 laser pulses per second to the lunar surface and determines how long it takes the return signals to come back. These round-trip times are converted into distance measurements, which are turned into topographic measurements when they are combined with information about the exact position of the spacecraft in its orbit.
LRO has already mapped more than six-and-a-half billion points on the lunar surface and provided measurements of the steepness of the ground’s slope. The return signals received by the spacecraft also contain information about the roughness of the landscape.
“We know the shape and structure of the lunar surface better than we know any other object in the solar system, including Earth,” said John Keller, LRO project scientist.
LRO tracking pulses also have been used to demonstrate Earth-to-satellite communication using lasers. In 2013, mission scientists reported the successful transmission of an image of the Mona Lisa pixel by pixel by laser pulses sent from Goddard to LRO.
Laser ranging has been tested at even greater distances. In May 2005, Goddard successfully exchanged laser pulses with the MESSENGER spacecraft en route to Mercury. The 15-million-mile (24.3-million-kilometer) distance to the spacecraft was measured with a precision of less than 8 inches (20 centimeters). Later that year, laser pulses transmitted from Earth were successfully detected by a Mars-orbiting spacecraft over a distance of 49.7 million miles (80 million kilometers).
Source : NASA