Render of the LRO, via Wiki Commons
NASA used lasers to beam a picture of Leonardo da Vinci's painting of the Mona Lisa to a spacecraft in lunar orbit. Cool, right? But it’s not the art in question that makes this so interesting, it’s the technology. This transmission marks the first time lasers have passed information across a planetary distance.
Communicating in space is typically done using radiowaves, waves on the longer end of the electromagnetic communications spectrum. The most common type, S-band radiowaves, measure about 4 inches in length. They can’t transmit a lot of data at once, but they’re easy to receive. That’s the benefit of radiowaves: the longer wavelength spreads out quickly and relatively evenly from a point of origin.
Lasers, on the other hand, rely on much shorter wavelengths at the high end of the visible light section of the electromagnetic spectrum. And they’re much smaller – we’re talking nanometer wavelengths instead of inches. This means they can carry loads more information. That’s the real difference. Both radiowaves and laser beams are on the same electromagnetic spectrum and travel through a vacuum at the speed of light. As far as space communications is concerned, neither is faster than the other.
But there’s a trade off to lasers’ high data transfer: receiving lasers is much harder because the data is sent in a very narrowly focused beam of light. Its like pointing a laser pointer at a wall. No matter how far back you stand from that wall, the point of light will always be a point. It might get a little bigger or fuzzier around the edges, but it’s still a definite point. Using lasers to communicate in space is like aiming that laser pointer at your wall from space. The dot will be bigger and fuzzier, but it will still be a dot, which is a lot harder to tune in to.
So getting a laser to transmit data from a point on Earth to a spacecraft in orbit around the Moon is no small feat. In the Mona Lisa’s case, the laser signal was fired from NASA’s Next Generation Satellite Laser Ranging station at the Goddard Space Flight Center in Maryland to its Lunar Reconnaissance Orbiter (LRO) nearly a quarter of a million miles away. LRO was the prime choice for the laser demonstration since it has a laser receiver. NASA tracks most of its spacecraft by radiowaves, but its tracking LRO with radiowaves as well as lasers.
An example of NASA's transmission coding improving resolution.
But having a receiver didn’t ensure success. The transmission had to be executed with perfect precision and timed just right. Before transmission, the Mona Lisa was decoded and divided into sections measuring 150 by 200 pixels. Each section was then transmitted by a laser to the orbiter at a rate of about 300 bits per second. Once LRO received the image it reconstructed the photo, corrected for distortions generated as the laser passed through the Earth's atmosphere. LRO then sent the reconstructed Mona Lisa back to Earth using traditional radiowaves.
In the near future, laser communications will probably serve as a backup method to tried and true radio communications. But in the more distant future, lasers will take over to provide higher data transfer rates than we’ve ever seen.
One mission launching this year is taking the first steps towards making a laser-communications future a reality. Optical Payload for Lasercomm Science – OPALS – will transfer video data from a platform on the International Space Station to a ground station at JPL's Optical Communications Telescope Laboratory in California. The mission, which is scheduled to launch on SpaceX’s September ISS cargo resupply mission, will be another demonstration of the technology but it won’t be a one-off shot like Goddard’s Mona Lisa message to LRO. OPALS will regularly beam lasers from the ISS to Earth for three months. If the mission starts out well, it might be extended for a full year.
Another demonstration of laser communications will come with NASA’s next mission to the Moon. The Lunar Atmosphere and Dust environment Explorer, also set to launch later this year, will feature a high data rate laser-communication-demonstration along with its primary mission of mapping the lunar atmosphere and environment.
If all these laser demonstrations work like they’re supposed to, we may be living in a laser-filled future before long. Forget dotty reconstructions of the Mona Lisa, we’ll have hi-res images from other planets within minutes of spacecraft landing.