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Light show

Lasers will one day improve data transfer from spacecraft, but hurdles must still be overcome.

The first images from another planet trickled to Earth as slowly as an intravenous drip. On 15 July 1965, as the spacecraft Mariner 4 swept past Mars, a small television camera began to roll. It took 22 images of the Martian surface, each 200 pixels square. Mariner 4 swivelled its high-gain, 1-metre-wide parabolic radio antenna towards Earth. It began transmitting the images in the S band, a high-frequency part of the radio spectrum, at a rate of 33.33 bits per second — about a million times slower than broadband Internet connections today.

Mission scientists at NASA’s Jet Propulsion Laboratory in Pasadena, California, were too impatient to wait for the complete first image, and for computers to process it. So as the data arrived, the scientists pinned ticker-tape strips of the greyscale pixel readouts to a wall — each strip a line from the image — and coloured in the rugged scene by hand with pastel crayons. It would take four days for all of the images to be returned to Earth.

Interplanetary communications have since improved considerably. The S band gave way to the X band, then to K and Ka — each at a higher frequency than its predecessor, giving engineers more cycles per second to pack in information. When the Mars Reconnaissance Orbiter (MRO) went into orbit around the red planet in 2006, it was armed with antennas that could transmit information at a rate of six megabits per second. That is a jump in Mars transmission rates of more than five orders of magnitude in the 40 years between Mariner 4 and the MRO, and a doubling of the data-transfer rate every 2 years or so. Moore’s law of computing power seems to have guided planetary data transmissions just as it has the exponential rise in transistor density on a chip.

Yet scientists, ever greedy for more data, are still constricted. The MRO, for instance, is limited in how often it can use some of its data-intensive instruments, such as the high-resolution camera. Such bottlenecks are motivating a push for optical communications (see page 266). In the coming months, both NASA and the European Space Agency will launch spacecraft with communications modules that can use lasers rather than radio transmitters. This has two benefits. First, lasers work at higher frequencies than radio does, so they can transmit more information. Second, a laser stays collimated in a narrow beam, whereas radio transmissions spread out as they travel, meaning that lower-power laser transmitters can be used to convey the same amount of information. Optical communications can also take advantage of the extraordinary investment in lasers, which have shrunk in size and grown in power.

Scientists will surely develop instruments that take advantage of the higher bandwidth. The public could benefit, too. The NASA laser demonstration, which will go to the Moon, claims to be able to return data in high definition. Imagine seeing details of the Moon broadcast live in as much detail as TV viewers spot sweat on a football player’s face.

There are issues to contend with. Lasers have a hard time transmitting through clouds, and so optical communications systems may have to depend on radio relay stations orbiting Earth (or receivers in cloudless deserts) for the final stretch home. But it is not too difficult to envision a day when the Solar System is stocked with spacecraft all networked by pulsed light: an interplanetary Internet.

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Light show. Nature 499, 254 (2013). https://doi.org/10.1038/499254a

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