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Lunar orbiter set for long-distance laser data linkup
BY STEPHEN CLARK
SPACEFLIGHT NOW

Posted: October 13, 2013


NASA's LADEE spacecraft finished a week-long series of maneuvers to reach an initial operating orbit around the moon Saturday, kicking off a month of commissioning and calibrations while demonstrating a laser communications system that could lead to breakthroughs for future expeditions into deep space.


Artist's concept of LADEE orbiting the moon at sunrise. Credit: NASA
 
Three burns of the probe's main engine over the last week first allowed the moon's gravity to capture LADEE in a high-altitude parking orbit, then lowered the craft's altitude into the first of two operational orbits.

Since LADEE entered lunar orbit Oct. 6, ground controllers at NASA's Ames Research Center in California lowered the spacecraft's altitude in two successive rocket firings. A burn late Saturday put LADEE into a near-circular orbit over the moon's equator at an altitude of approximately 250 kilometers, or 155 miles, mission officials said.

The Lunar Atmosphere and Dust Experiment Explorer will stay in its current orbit for 30 days, allowing engineers to check out the spacecraft's systems, perform an aliveness test on its three science instruments, deploy sensor aperture covers, and begin commissioning activities for the mission's science phase.

The $280 million mission's main purpose is to study the moon's tenuous exosphere, a type of atmosphere so thin that its atoms never collide with each other. Scientists say atmospheres like the moon's are common throughout the solar system on planets like Mercury and among the largest asteroids.

Launched from Wallops Island, Va., on Sept. 6 aboard a Minotaur 5 rocket, LADEE also carries a technology payload to test out a next-generation communications device using laser light to transmit data over long distances at higher speeds than possible with conventional radio systems.

Around Nov. 11, controllers will lower LADEE's altitude to less than 30 miles above the lunar surface to begin a 100-day science mission before the orbiter runs out of propellant and crashes into the moon in early 2014.

The Lunar Laser Communications Demonstration aboard LADEE will begin its experiments Oct. 17, linking up with ground stations in New Mexico, California and the Canary Islands and exchanging data packets at speeds unattainable with radio communications systems.


File photo of an optical laser experiment aimed at the moon. Credit: ESA
 
An infrared laser beam emitted from one of the ground stations will scan the sky around the moon until it intercepts the receiver on LADEE, which will return the signal and acquire a lock. Optical communications systems like LADEE's laser instrument require more precise pointing to obtain a lock than radio transmitters.

"Once the two systems are locked and acquired, then we can send tens of megabits of data per second from the Earth up to the moon, and similarly we can send hundreds of megabits per second from the moon on LADEE down to the Earth," said Don Cornwell, the laser communication demonstration mission manager from NASA's Goddard Space Flight Center in Greenbelt, Md.

NASA says LADEE's laser demo will achieve a downlink speed from the moon to Earth of 622 megabits per second and an uplink speed of about 20 megabits per second.

Built by MIT Lincoln Laboratory, the communications payload will help build confidence for future missions to use laser data links from deep space, including NASA's manned asteroid mission and future rovers on the surface of Mars.

"NASA has a need for faster download speeds for data from space," Cornwell said. "We'd like to be able to send high-resolution images, movies in 3D even, from satellites that not only orbit the Earth but also from probes that will go to the moon and beyond."

"If you're beaming data back over a standard radio link and compare that to what we can do with a laser system, it's about like comparing a modem to fiber optics in terms of your Internet speed," said Greg Delory, LADEE's deputy project scientist at NASA's Ames Research Center. "With a typical radio link, you're talking about tens of kilobits per second on average. With the laser communications system [on LADEE], we're going to get 600 megabits per second equivalent data speed, which is just blindingly fast."


Artist's concept of the Lunar Laser Communications Demonstration mission on LADEE. Credit: NASA
 
Another benefit of laser communications is it requires fewer components and smaller terminals, both on the ground and in space.

"Light waves are shorter in wavelength than radio waves, so we can use smaller transmitters and receivers," Cornwell said.

NASA is also developing a laser terminal to fly as a hosted payload on a commercial communications satellite in geostationary orbit in 2017. The geostationary demonstration mission will test the laser system's ability to relay data from orbiting satellites, such as the International Space Station, to the ground at faster speeds.

"I'm a huge fan of laser communications, and one of the reasons is that as you go farther out into the solar system, it's a much more efficient way to get high bandwidth at low power," said John Grunsfeld, associate administrator of NASA's science mission directorate.

NASA is considering placing a laser communications system on its next Mars rover set for launch in 2020.

"It could be as soon as our Mars 2020 mission," Grunsfeld said. "We've already been having discussions about could you do laser communications on a rover on the surface of Mars."

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