Friday, January 18, 2013

NASA sends the Mona Lisa to the Moon

NASA has turned the Mona Lisa into the first digital image to be transmitted via laser beam from Earth to a spacecraft in lunar orbit, nearly 240,000 miles away, thanks to a technology that may soon become routine.

The experiment took advantage of the laser-tracking system that's in operation aboard NASA's Lunar Reconnaissance Orbiter, which has been circling the moon for the past three and a half years. NASA sends regular laser pulses from the Next Generation Satellite Ranging station at Goddard Space Flight Center in Maryland to the space probe's Lunar Orbiter Laser Altimeter, or LOLA, to measure its precise position in lunar orbit.

For last March's Mona Lisa maneuver, researchers encoded a black-and-white version of Leonardo da Vinci's enigmatic masterpiece as a series of values in a 152-by-200-pixel grid. Each value represented a shade of black to gray to white, ranging from zero to 4,095. The signal for each pixel was then piggybacked on the ranging station's laser-tracking pulses: Each pulse was fired during one of 4,096 super-short designated time slots, at a rate of about 300 bits per second.

As the pulses were received in lunar orbit, LOLA's software used the precise timing of each pulse to figure out the grayscale value for a given pixel — and reassembled the black-and-white image. The process wasn't perfect: Atmospheric turbulence introduced laser transmission errors, even when the sky was clear. To accommodate the 15 percent error rate, the researchers used Reed-Solomon data coding, which is the same method used to smooth out the bumps in the playback of CDs and DVDs.

The picture was reprocessed and sent back to Earth using the orbiter's standard radio communication system, just to make sure that Mona survived the trip intact. Throughout the experiment, Lunar Reconnaissance Orbiter conducted its regular mapping tasks without interruption.

A research report on the experiment, with Goddard's Xiaoli Sun as principal author, was published online by Optics Express on Thursday.

Sun said the Mona Lisa was chosen for the transmission because the painting is so much more visual than strings of random numbers. "It's a familiar image with lots of subtlety," he said. "You can immediately feel whether the image looks right, and how much information got lost."

The feat marked the first time anyone has achieved one-way laser communication at planetary distances, LOLA's principal investigator, David Smith of the Massachusetts Institute of Technology, said in a NASA news release.

"In the near future, this type of simple laser communication might serve as a backup for the radio communication that satellites use," Smith said. "In the more distant future, it may allow communication at higher data rates than present radio links can provide."

A data rate of 300 bits per second may seem achingly slow by today's standards, but NASA is planning a higher-bandwidth laser communication demonstration for its next mission to the moon, known as the Lunar Atmosphere and Dust Environment Explorer. When LADEE is launched in August, it will carry an experimental laser system that's designed to transmit data at a rate exceeding 600 million bits per second.

In 2017, NASA is due to send an experiment called the Laser Communications Radar Demonstration into orbit aboard a commercial satellite to test a full-fledged, beam-based communication system. Studies suggest that laser systems have the potential to transmit data at rates 10 to 100 times faster than traditional radio systems for the same mass and power, or match radio's data rate with a smaller, more efficient package.

Who knows? Mona Lisa may well mark the start of a renaissance in high-speed satellite communications.


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