FEATURE

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Historic Test Brings Laser Spacecraft Ranging to a New Era

An experiment in which an Earth-based observatory successfully exchanged laser pulses with the MESSENGER spacecraft while the two were millions of miles apart has helped to convince mission planners to include the same technology on the Lunar Reconnaissance Orbiter (LRO), which will launch in 2008 to map the lunar surface.

The hope is that the highly precise laser-ranging technology demonstrated last spring with MESSENGER’s Mercury Laser Altimeter (MLA) will allow scientists to measure with centimeter accuracy the disturbances to LRO’s orbit around the moon. With that information, they hope to learn more about the Moon’s well-known gravitational anomalies, which NASA first measured during the early days of the space program.

“We proposed a subsystem for the Lunar Reconnaissance Orbiter and this experiment was the proof that the it would work,” said Xiaoli Sun, MLA Instrument Scientist, referring to the record-breaking test that he and other MLA team members conducted from the Goddard Geophysical and Astronomical Observatory in late May. “Our test with MESSENGER helped to convince us that we could do his with LRO.”

Historic First

The test between MESSENGER, which stands for MErcury Surface, Space ENvironment, GEochemistry and Ranging, and the Goddard observatory made headlines because it represented the first time scientists were able to conduct a two-way exchange of laser signals over vast distances in space. At the time, MESSENGER was 15 million miles from Earth on its 6.6-year journey to Mercury, where it will map the planet’s surface.

In addition to proving that such an exchange could occur at interplanetary distances, the test demonstrated its sub-nanosecond timing accuracy. It also calibrated the laser pointing and receiver bore sight, which was the principal reason for the test, Sun said.

Long Time Coming NASA has long wanted to demonstrate the laser-ranging capability across deep space because of its widespread potential. In addition to determining orbital positioning needed for fundamental geophysical studies, the same laser technology can dramatically improve communications in deep space. The Mars Telecommunications Orbiter, which NASA cancelled last summer due to budget constraints, would have used lasers to transmit data between Earth and Mars at a rate of 1 to 30 million bits per second, depending on how close the two planets were. Currently, the maximum data speed with microwave technology is about 100,000 bits per second.

Goddard technologists used this 1.2-meter Telescope Tracking Facility at the Goddard Geophysical and Astronomical Observatory to fire laser pulses to MESSENGER.

Until last spring, however, a successful demonstration eluded scientists. In 1992, laser pulses were successfully transmitted and detected by the camera onboard the Galileo probe while it was 4 million miles away. However, Galileo carried neither a laser transmitter nor a timing capability. Goddard scientists also attempted similar tests with the Mars Orbiter Laser Altimeter onboard the Mars Global Surveyor and the Laser Ranger on the Near Earth Asteroid Rendezvous mission, but bad weather and spacecraft scheduling problems shut down the experiments.

Lunar Subsystem

Since the successful demonstration, LRO mission planners have decided to install a small optical receiver, which will receive laser pulses from ground stations on Earth. The time that it takes the laser pulses to travel from the ground station to LRO determines with centimeter precision the orbiter’s position. In comparison, conventional radio frequency tracking techniques can only measure spacecraft position to several meters, Sun said.

Knowing highly precise positional data can reveal much about the effect of the lunar gravity on the spacecraft’s orbit. “Such studies are crucial for improving our ability to predict the spacecraft position,” Sun said. “That way we assure safer landings for human and robotic cargo in future exploration.”