LASER pulses have been exchanged between Earth and the Messenger spacecraft, a whopping 24 million kilometres away en route to Mercury. Although no actual information was transmitted, the experiment sets a record and highlights the potential of lasers for interplanetary communications.
Space probes travelling through the solar system usually communicate with Earth using microwaves. But microwaves are not as directional as lasers, so the power of their signals is spread over much larger areas over long distances, limiting the maximum data rate that can be transmitted. NASA’s Mars Odyssey spacecraft, for instance, can send only 128,000 bits per second to Earth. Because laser beams spread out much more slowly, they could deliver more power to ground-based optical receivers and hence allow higher data rates.
This advantage has been clear since Theodore Maiman made the first laser in 1960, prompting him to list space communications as an important potential application. However, because laser beams are so narrow, using them for space communications proved difficult because for decades spacecraft could not be tracked precisely enough to direct the laser. It was only late last year that the first laser link was demonstrated between two satellites in different orbits, the Japanese Kirari satellite in low-Earth orbit and the European Artemis satellite in geosynchronous orbit. And NASA’s Mars Telecommunications Orbiter wold have included a laser link to Earth, but the mission was cancelled.
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The latest exchange was carried out to test the laser altimeter that the Messenger probe will use to map the surface of Mercury from 2011 by firing short laser pulses at the planet and timing the reflections. A team led by David Smith of the NASA Goddard Space Flight Center in Maryland aimed pulses from Messenger towards Earth. Despite travelling 24 million kilometres, the beam’s width had increased to just 1900 kilometres. So a receiver at Goddard picked up the pulses, but only when the laser was pointed directly at it. At the same time, a ground-based laser fired pulses towards Messenger, timed to arrive at the craft’s receiver when it was looking for reflected laser light. “We saw it as the simplest conceivable communications link you could make,” says Smith.
“No information was transmitted, but the experiment highlights the potential of lasers for interplanetary communications”
Although the pulses between the spacecraft and the station on Earth did not carry information, the tests did yield precise measurements of the spacecraft’s distance and speed (Science, vol 311, p 53). The calculated distance from Earth was just 50 metres out from that measured using microwaves. “We would like to get to the stage in interplanetary communications and ranging where we can do with lasers what we routinely do with microwaves but with much greater accuracy,” Smith says.
