杏吧原创

THE interplanetary information superhighway is heading for gridlock. The sort of uncongested communications that we now take for granted on Earth are an unheard-of luxury in space, as NASA鈥檚 decades-old network of dishes and relays struggles with spiralling traffic from dozens of space missions.

The need for an upgrade is becoming urgent with the advent of a new generation of spacecraft carrying powerful cameras. They will soon begin transmitting images of the worlds they explore in unprecedented detail.

While these new probes are great news for planetary scientists, transmitting high-resolution images back to Earth will put increasing strain on NASA鈥檚 40-year-old Deep Space Network (DSN). This global array of listening and transmitting stations is the lifeline for nearly all the agency鈥檚 spacecraft plus a host of international satellites operating at high-altitude Earth orbit and beyond.

The first of the new generation of bandwidth-hungry explorers is the Mars Reconnaissance Orbiter. MRO, which reached the planet in March, is carrying the most powerful camera ever sent into space, the High Resolution Imaging Science Experiment (HiRISE). At present it is dipping into Mars鈥檚 atmosphere to lower its altitude before beginning serious surveys in November, when it will start transmitting image files of up to 28 gigabytes. To save time, images at this high resolution will only be taken of key surface features, but overall MRO should return a whopping 34 terabits of data 鈥� 10 to 20 times more than all previous Mars missions put together.

MRO鈥檚 camera will be used to pinpoint safe, accessible and scientifically interesting landing sites for future robotic probes and human expeditions. It can resolve objects smaller than 1 metre from about 300 kilometres up, using an array of 10 charge-coupled devices (CCDs) 鈥� the kind of detector that earthbound digital cameras use 鈥� that can produce 200-megapixel images. In contrast, cameras aboard other spacecraft, such as the Cassini probe orbiting Saturn, have only one CCD.

Those pictures and the glut of other scientific data to be gathered by the orbiter will be useless, however, unless they can be transmitted back to Earth. The problem facing the DSN is how to share its limited number of antennas between the 35 missions that are currently transmitting data from deep space while retaining the flexibility needed to handle critical events such as launches of new spacecraft, steering manoeuvres, fly-bys and so on, as well as the occasional spacecraft emergency.

鈥淭ypically the network is congested all the time,鈥� says Michael Rodrigues of the deep space mission systems office at NASA鈥檚 Jet Propulsion Laboratory (JPL) in Pasadena, California, where the DSN is based. 鈥淚t鈥檚 like a freeway 鈥� there are only so many cars you can put on it at one time.鈥�

The network comprises three facilities positioned roughly 120 degrees apart around the globe to maintain round-the-clock communications with spacecraft as Earth rotates. The sites 鈥� one in California鈥檚 Mojave desert, another near Madrid, Spain, and the third near Canberra, Australia 鈥� each has one 70-metre diameter parabolic dish antenna, several 34-metre antennas and one 26-metre antenna.

The heavier the demands on this network, the less chance there is of using more than one antenna during transmissions to provide back-up, so the more likely it is that an antenna failure will lead to a loss of data. That happened last September as Cassini made a close approach to Saturn鈥檚 moon Titan. During the fly-by, scientists used radar instruments to search for a suspected methane lake or 鈥渨et鈥� area on the moon鈥檚 surface. However, nearly all the radar data was lost. Although the main cause of the problem turned out to be a software problem on the craft, the DSN had just one antenna targeted at Cassini at the time and failed to receive about half of the data that had been properly recorded and transmitted.

鈥淚t鈥檚 always a challenge to use the Deep Space Network wisely,鈥� says Candice Hansen-Koharcheck, a planetary scientist at JPL who works on both Cassini and MRO. 鈥淭he Cassini team didn鈥檛 want to be a hog about it, and we just thought, 鈥榃ell, if we lose a ground station during transmission that鈥檚 just too bad.鈥� After the fly-by, we thought, 鈥楳aybe that wasn鈥檛 too smart.'鈥� Cassini鈥檚 programme managers have since changed their strategy so that data stored on board during critical activities will not be overwritten immediately, to ensure it will not be lost if the DSN goes down.

Another problem is the great distances that signals must travel from interplanetary spacecraft to reach Earth. Coupled with the expense and difficulty of outfitting probes with high-powered transmitters and heavy antennas, this has previously resulted in excruciatingly slow communication rates. However, spurred by its commitment to support MRO鈥檚 huge transmissions, NASA is now laying the groundwork for the first high-bandwidth deep space network.

The upgrades began with MRO itself, which has a 3-metre antenna dish and a 100-watt transmitter. In comparison, NASA鈥檚 Mars Odyssey spacecraft has just a 1.3-metre antenna transmitting at 15 watts. Data sent by MRO should be able to reach ground stations at a rate of up to 6 megabits per second. The top speed at which interplanetary probes are able to transmit data to Earth at present is about 400 kilobits per second.

MRO is also carrying experimental antennas that operate in a frequency region called the K_a band. At around 32 gigahertz, these frequencies are four times as high as those in the X band, the standard for earlier spacecraft, allowing much higher data rates during transmissions. This portion of the spectrum is also much less crowded, resulting in less interference. MRO will be able to use its K_a-band transmitter alongside its X-band system. 鈥淏ut if all goes as we hope, K_a-band will become the communications band of choice on MRO,鈥� says William Weber, head of JPL鈥檚 interplanetary network directorate.

The experiment will also help NASA engineers design future communications systems. DSN managers intend to have a fully operational K_a-band system ready before the 2008 launch scheduled for NASA鈥檚 Kepler space telescope, which will search for Earth-sized planets around other stars. Kepler will be NASA鈥檚 first spacecraft to rely exclusively on K_a-band, and will also carry a photometer fitted with an array of 42 CCDs, dwarfing MRO鈥檚 camera. 鈥淭hat鈥檚 our next big challenge,鈥� says Rodrigues. 鈥淜epler is going to double how much capacity we need.鈥�

鈥淢RO is a first step in the development of a high-speed internet in space鈥�

Beyond Kepler, the Deep Space Network should get even busier. President Bush鈥檚 vision for a base on the moon by 2020 will require high-bandwidth communications and excellent back-up systems to ensure astronauts can be in constant contact with Earth. MRO is therefore a crucial first step in NASA鈥檚 development of a high-speed 鈥渋nternet鈥� in space, says Chad Edwards, manager of the Mars Network office at JPL.

鈥淚t鈥檚 like your computer,鈥� he says. 鈥淚f you think about that old dial-up modem connection, changing to DSL or cable led to a qualitative change in the way people use and experience the internet. The same can be said of deep-space communications.鈥�