Engineers and scientists are now pinning their hopes on one or more uplink-downlink sessions over the next two months. This would entail sending commands to the spacecraft using a strong signal via NASAs Deep Space Network and, approximately 22 hours later, looking for a coherent downlink response. With Pioneer 10 now 7.1 billion miles (11.4 billion kilometers) from Earth, it takes 11 hours and 21 minutes for a signal to travel just one way between us and the spacecraft.

Few are holding their breath that the tactic will work. The mission may well be over.

Pioneer 10, managed by the NASA Ames Research Center in Moffett Field, California, was originally intended as a 21-month mission to Jupiter. It quickly earned a place in history as the first spacecraft ever to pass through the Asteroid Belt (July 1972) or to visit an outer planet with its successful flyby of Jupiter (December 1973). Defying all predictions, it continued to be an active NASA mission until March 1997, when funding was all but cut off.

Even then, a variety of creative measures, including volunteer work by the missions original staff, long off to other NASA tasks, kept the telemetry flowing until the spacecraft went silent last year.

The communication troubles began last fall following a successful tracking session (a listen-only session, with no commands being uplinked) on July 8. Pioneer 10 sent back interesting data from the spacecrafts Geiger Tube Telescope instrument indicating the spacecraft was still under the influence of the Sun. That meant that Pioneer 10 was still within the heliosphere, a vast region permeated by charged particles flowing out from the solar wind that extends throughout the solar system.

The next day, the team attempted what is known as a precession maneuver, a firing of small hydrazine thrusters to adjust the high-gain antennas orientation, helping it point more accurately toward Earth. A variety of inconclusive indicators leave the impression that the maneuver was unsuccessful.

In August, there were two tracking sessions during which the signal locked on but no scientific data were obtained.

Subsequent sessions over the next few months, culminating in the most recent attempt on January 16, were unproductive. No one knows if the problem is caused by an unsuccessful precession maneuver, the coldness of outer space impacting on equipment, or a combination of both.

Further tracking sessions from Arecibo in Puerto Rico are scheduled for February and March. But the idea of an uplink-downlink session from the Deep Space Network featuring a strong, stable signal for an exchange of commands seems like the last, best chance to talk to the spacecraft.

Theres just one hitch: the Deep Space Network is oversubscribed, and current active missions have higher priority for antenna time. As Ryan described it, "The Pioneer 10 view period is overlapped with Galileo and Cassini right now; and with our relatively low priority, that makes it difficult to find antenna time. We're a bit on the beggar's side here."

The team remains optimistic that time can indeed be negotiated.

Among those who hope for at least one more successful session is the University of Iowas James Van Allen, after whom the Van Allen Belts (Earth-encircling regions of high-energy particles trapped in the Earths magnetic field) are named. He is the principal investigator on Pioneers Geiger Tube Telescope and is interested in follow-up data to the successful July 8 session.

Van Allen wants to determine whether cosmic ray intensity is decreasing while the Suns influence on Pioneer 10 is increasing, due to the delayed effect of high solar activity reaching the spacecraft.

Said Van Allen, "We await further data from Pioneer 10 with keen interest."