Calling long distance is tough when the telephone lines are busy. For most of us, that simply means waiting a few hours and calling again, but for NASA’s deep space missions, constant and reliable communication is vital.

However, an ever-growing cadre of interplanetary spacecraft aimed at the planets and other bodies in our solar system have swamped the space agency’s Deep Space Network (DSN). The network is a collection of antennas aimed at space and managed by the Jet Propulsion Laboratory (JPL). The network is NASA’s chief phone line to spacecraft like Cassini, on its way to study Saturn, and Stardust, flying toward the comet Wild-2.

It is also overworked, with antenna time for existing missions in short supply.

Airtime will become tighter in the next few years as host of missions - from those to Mars, Stardust’s Wild-2 rendezvous and other spacecraft are set to launch or enter critical phases during the end of 2003 and start of 2004. The Red Planet will play host in 2003-04 to a pair of NASA rovers, the European orbiter Mars Express and its lander Beagle 2, as well as the Japanese orbiter Nozomi – all in addition to NASA’s Mars Odyssey and Mars Global Surveyor already orbiting the planet. Deep Impact will launch as one of the rovers arrives at the Red Planet and Stardust meets Wild-2, Blume told SPACE.com.

Still more missions – Cassini for one –will rely on the DSN to talk to ground control, and JPL scientists have been preparing for this potential signal gridlock by building a new radio antenna, working with mission designers on their communication needs and researching new technologies to streamline the network.

JPL began developing the network in the late 1950s to ensure consistent, two-way communication with future deep space missions without requiring each new flight project to develop its own tracking system. Since then, it has grown into the largest and most sensitive telecommunications systems in the world.

The network is made up of a cornucopia of radio antenna dishes distributed at three sites around the world; Goldstone near Barstow, California, a second site in Spain located near Madrid and a third near Canberra, Australia. The location of each site allows DSN operators to maintain continuous radio contact with a spacecraft as the Earth rotates, and then relay telemetry and vital health information back to its specific mission control.

To do this, each of the three network facilities has its own huge radio antenna – a dish spanning 230 feet (70 meters) in diameter. A cluster of smaller antennas surrounds the main dish to add to each installation’s communication abilities. The amount of time mission scientist get to track and communicate with their spacecraft is determined through a DSN management team, which gives priority to vehicles performing critical maneuvers such as going into orbit around a planet.

"We are concerned that our 70-meter antennas are getting quite old," said Rich Miller, manager of JPL’s Office of Plans and Commitments, part of JPL’s Interplanetary Network Directorate responsible for the DSN. "Late in the next decade, they’ll be 50 years old."

Miller told SPACE.com that the malfunction of a main antenna, or a spacecraft emergency, is one of the biggest concerns plaguing the DSN. Non-network installations, such as radio astronomy antenna or independent deep space tracking systems built by Japan and the European Space Agency could help in a pinch. But finding funds to replace or refurbish the aging main antenna will have to wait until after the communication crunch time next year.

In order to meet the expected communications requirements of 2003-04, NASA is spending $54 million to improve the DSN. The bulk of those funds going to the construction of a new 111-foot (34 meter) radio antenna at the network’s Madrid site where demand for tool is expected to be the greatest out of the three installations. The new antenna should be completed by November 2003 in time for the Mars rover missions, and will add about 70 hours per week of extra spacecraft tracking time onto the facility’s 210 hours to date.

Tracking time for each spacecraft has also been an issue, and DSN operators are working with future and existing mission designers to determine exactly how much time each project needs to track their spacecraft.

"It’s going to be tight, and there are going to be a few compromises," Miller said. "Some missions are going to have to return a little less data here and there, but it looks like we’re going to make it."

In cases where more than one spacecraft must be tracked, negotiations between the two project team’s will determine how to address the overlap. For example, during the Deep Impact mission, the project leader’s will have to give up a couple of hours of tracking time to other missions. Giving up a few hours now and then, however, is pretty easy to do, Blume said.

"Typically, we’re asking for eight hours of tracking time a day," Blume told SPACE.com. "But in reality, there’s often a little cushion built into that number."

Radio traffic from the Mars rovers should ease off the Deep Space Network a bit. Mission planners expect to only get half of their data through the DSN facilities. The rest, Miller said, should be relayed through the Global Surveyor or Odyssey already in orbit. The relay process could also be used to relay data from the European Space Agency’s Beagle 2 lander when it drops to the Martian surface.

Other modifications to the DSN network include hardware upgrades at its three facilities to allow an antenna to listen to more than one spacecraft, as well as cut down the amount of set up time needed before tracking a spacecraft, effectively adding more time to actually receive and send data. Efforts are also underway to use a higher radio frequency, known as the Ka-band, which would increase the data rate between ground controllers and their spacecraft four times over.

In addition to building new dishes to bolster communication efforts, JPL scientists are also looking at the possibility of using devices known as laser light buckets, ground-based telescopes that could listen to future spacecraft. Instead of using radio waves like DSN’s massive antenna dishes, light buckets are optical, collecting information carried on light waves. A test project using laser light buckets is under development at JPL’s Table Mountain facility in Wrightwood, California. Laser light buckets could allow faster data rates than the biggest radio antenna on Earth, Miller said, but it does have its drawbacks.

"The concept looks very promising from a cost standpoint … but it can’t get through clouds at all," he said, adding that to be effective, a number of ground sites would be needed to account for bad weather. "And it would all have to be developed from scratch, but it’s possible that sometime in the next decade we could be using optical instead of radio frequencies."