Fade to Black
Now and then, the faintest whisper returns from NASA's distant space probes.
- By J. Kelly Beatty
- Air & Space magazine, July 2001
(Page 3 of 4)
So could the 29-year-old spacecraft become the first to send a signal from the heliosphere? Unfortunately, Pioneer is last in the queue for Deep Space Network tracking passes. Officially, the project ended on April 1, 1997—a few weeks after a “celebration” of Pioneer 10’s silver anniversary at the Smithsonian’s National Air and Space Museum. “It was a funeral service,” Van Allen snips, “and I gave a eulogy.” But he also worked the hallways, protesting the cutoff to NASA officials. They responded with a reprieve, agreeing to let engineers use the spacecraft’s weakening signal to test a new tracking scheme based on chaos theory.
IMP 8 doesn’t have to compete for time on the Deep Space Network, ironically, because its telemetry system is obsolete. For more than 28 years, this oldster has been continuously transmitting six kilobits of data per second—it has no tape recorder—at the long-abandoned VHF frequency of 137.98 megahertz. The signal is gathered by a trio of Yagi-style receivers (think of rooftop TV antennas on steroids) in Virginia, Belgium, and Australia that are dedicated to the IMP 8. “One of the most challenging aspects of my job,” says project scientist Joseph H. King at NASA’s Goddard Space Flight Center in Maryland, “is cobbling together the VHF ground network.” Once at Goddard, the IMP data is routed to science teams around the country, arriving as an encrypted jumble of timing code, spacecraft positions, and instrument readouts that takes a lot of massaging by decades-old software to be useful.
But space physicists aren’t complaining. The 10th and last of the Interplanetary Monitoring Platform series, IMP 8 circles Earth in an unusually high orbit that extends about halfway to the moon. In its heyday the spacecraft served as something of a sentinel, warning of stormy conditions in the solar wind. Seven of its 11 experiments still work, and their data remains a staple for hundreds of space physicists studying the sun and Earth’s magnetosphere despite the advent of state-of-the-art solar watchdogs like the Advanced Composition Explorer and Wind spacecraft. “It’s so much a part of the culture,” observes MIT’s Paularena. “We accept and use its data without really thinking about it.” For example, on July 14, 2000, the sun uncorked an eruption so powerful that the solar wind’s shock wave disabled some sensors aboard Wind. But IMP 8 took it in stride, sending back readings on the titanic shock that had the MIT team fist-pumping in exultation.
Interest in IMP 8 data soared during the mid-1990s, when there was a hiatus in solar wind coverage by other spacecraft. But the craft’s steeply inclined orbit kept it hidden from the ground stations in Virginia and Belgium for five days out of each 12-day circuit. King had been running the program on a shoestring for years, and his options were limited. But then came a chance conversation with a fellow runner at Goddard. “How’s that old spacecraft doing?” asked Michael Comberiate, who had built some of IMP 8’s electronics early in his career. It turned out that Comberiate would be returning to Antarctica in a few months to service some NASA hardware, and a plan was hatched.
Working with a local Ham wizard named Michael Staal, Comberiate fabricated eight 30-foot VHF antennas, shipped them to McMurdo Station, and mounted them atop a 65-foot-high tower. Throw in some kluged electronics and a desktop computer, and—voilà—IMP 8 data started streaming into Goddard from the South Pole. “Antarctica is a tough place to do anything like that,” Comberiate says, and after two harsh winters he disassembled the antennas and moved them to Australia.
Time is slowly catching up to IMP 8, and its data isn’t as prized as it once was. Last year’s loss of its magnetometer, whose magnetic field readings provide a context for other data sets, didn’t help. “Right now it’s like someone who is red-green colorblind,” Paularena explains. “You can still see the world, but you’re missing something.” The continuing value of IMP 8’s data will be tested later this year, when King will defend his program before a senior review board. Despite its loss of compass, there are good reasons to keep listening to what IMP 8 has to say—if for no other reason than to extend its unbroken 28-year run of solar wind data. “Sometimes I think NASA hasn’t taken sufficient pride in its long-term spacecraft,” King says.
In fact, in a world where a good VCR might last eight or 10 years, NASA’s endurance records seem nothing short of astounding. Budgets permitting, IMP 8 could continue sending its solar weather reports for years to come. The twin Voyagers could prove equally durable (the Voyager Interstellar Mission, as it’s now called, has a timeline that runs at least through 2016). No other nation’s spacefaring efforts come close to these milestones. Giotto, launched in 1985 by the European Space Agency, was tracked after plunging through two comets (Halley and Grigg-Skjellerup) until September 1992. Sakigake, another Halley watcher, remained in contact with Japanese controllers for a decade.
But the Methuselah prize may ultimately go to a spacecraft that will spend the next decade in electronic hibernation. Today NASA calls it the International Cometary Explorer, or ICE, but when launched in 1978 it was christened the International Sun-Earth Explorer 3. Under the direction of trajectory master Robert Farquhar, of the Johns Hopkins University Applied Physics Laboratory, ISEE 3 spent five years flitting here and there around the Earth-moon system. For a while it hovered near the L1 Lagrangian point, a million miles in the sun’s direction; then it crisscrossed Earth’s magnetosphere and lingered downstream for months at a time. Recast and renamed as a comet chaser, ICE dashed off to intercept the ion tail of Comet Giacobini-Zinner in September 1985.