Before NASA even existed, science-fiction writer Arthur C. Clarke in 1945 imagined spacecraft that could send messages back to Earth using beams of light. After decades of setbacks and dead ends, the technology to do this is finally coming of age.
Two spacecraft set for launch in the coming weeks will carry lasers that allow data to be transferred faster than ever before. One, scheduled for take-off on 5 September, is NASA’s Lunar Atmosphere and Dust Environment Explorer (LADEE), a mission that will beam video and scientific data from the Moon. The other, a European Space Agency (ESA) project called Alphasat, is due to launch on 25 July, and will be the first optical satellite to collect large amounts of scientific data from other satellites.
“This is a big step forward,” says Hamid Hemmati, a specialist in optical communications at NASA’s Jet Propulsion Laboratory in Pasadena, California. “Europe is going beyond demonstrations for the first time and making operational use of the technology.”
These lasers could provide bigger pipes for a coming flood of space information. New Earth-observation satellites promise to deliver petabytes of data every year. Missions such as the Mars Reconnaissance Orbiter (MRO) already have constraints on the volume of data they can send back because of fluctuations in download rates tied to a spacecraft’s varying distance from Earth. “Right now, we’re really far from Earth, so we can’t fit as many images in our downlink,” says Ingrid Daubar, who works on the MRO’s HiRISE camera at the University of Arizona in Tucson. Laser data highways could ultimately allow space agencies to kit their spacecraft with more sophisticated equipment, says John Keller, deputy project scientist for NASA’s Lunar Reconnaissance Orbiter (LRO). That is not yet possible, he says. “We’re limited by the rate at which we can download the data.”
Today’s spacecraft send and receive messages using radio waves. The frequencies used are hundreds of times higher than those put out by music stations on Earth and can cram in more information, allowing orbital broadcasts to transmit hundreds of megabits of information per second. Lasers, which operate at higher frequencies still, can reach gigabits per second (see ‘Tuned in’). And unlike the radio portion of the electromagnetic spectrum, which is crowded and carefully apportioned, optical wavelengths are underused and unregulated.
Efforts to develop laser communication systems struggled for much of the twentieth century: weak lasers and problematic detectors derailed project after project. But recent advances in optics have begun to change the situation. “The technology has matured,” says Frank Heine, chief scientist at Tesat-Spacecom, a company based in Backnang, Germany.
In the 1980s, Europe took advantage of improved lasers and optical detectors to begin work on its first laser communication system, the Semiconductor Laser Intersatellite Link Experiment (SILEX). Equipped with the system, the ESA satellite Artemis received 50 megabits of information per second from a French satellite in 2001and then exchanged messages with a Japanese satellite in 2005. The project taught engineers how to stabilize and point a laser in space. But it was abandoned after its intended application — a constellation of satellites to provide Internet services — was dropped in favor of the network of fiber-optic cables now criss-crossing the globe.
Since then, Heine’s team at Tesat-Spacecom has created a laser terminal for satellite-to-satellite communication, at a cost to the German Aerospace Center of €95 million (US$124 million). The laser, amplified by modern fiber-optic technology, achieves a power of watts — compared with the tens of milliwatts reached by SILEX. In 2008, terminals mounted on two satellites transferred information at gigabits per second over a few thousand kilometers.