J. Kelly Beatty
Sky & Telescope
 | | Jupiter,
as recorded by Pioneer 10 on December 2, 1972, 44 hours before the spacecraft
passed just 133,000 km from the planet. The Great Red Spot hugs the day-night
terminator at left, and the moon Io (upper right) is casting the round, black
shadow. Pioneer's scanning camera took red and blue images of the scene, then
green was added to produce this color view. Courtesy NASA/Ames Research Center.
|
After
81Ú2 months of silence, Pioneer 10 is once again in touch with its handlers here
on Earth. Ground controllers heard from the spacecraft for about 90 minutes on
April 28th while tracking it with NASA's 70-meter receiving dish in Spain. The
long-distance call came down "sweet as could be Ñ a nice, strong signal," says
Lawrence Lasher, Pioneer project manager at the Ames Research Center in California.
The
breakthrough came after weeks of failing to pick up Pioneer's feeble signal by
merely listening for it. Lasher now believes that the spacecraft can no longer
maintain a stable transmission frequency. To sidestep this malfunction, the Spanish
station beamed a single-frequency carrier signal to the spacecraft, which then
echoed it back to Earth (after a round-trip travel time of 21.8 hours). Ground
controllers used this same two-way communication scheme when they last heard from
Pioneer 10 on August 6, 2000. Keeping in touch with the 29-year-old craft has
become very difficult because it is now 11.7 billion kilometers from Earth and
because onboard power is barely adequate to run the 8-watt transmitter. Although
its mission officially ended in 1997, Pioneer 10 has avoided a complete shutdown
because Ames engineers are using the weakening radio beacon to test a new tracking
method based on chaos theory.
Lasher plans other communication sessions
in the coming weeks to assess Pioneer 10's condition and to beam up some housekeeping
instructions. Commanding the spacecraft came a halt last year when the last of
the project's decades-old PDP computers failed, complicating efforts to keep Pioneer's
antenna pointed toward Earth. But since then critical command sequences have been
transferred to a modern desktop system. "We're in business again," Lasher says.
According to James A. Van Allen, whose Geiger-tube telescope is the sole experiment
still sending back data, Pioneer 10 could reach the boundary marking true interstellar
space within a few years. Even though the odds are long, he and Lasher hope the
spacecraft will still be functioning well enough to announce its arrival there. |
