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Mary Hardin, NASA's Jet Propulsion Laboratory
Scientists keep searching for a signal from Mars Polar Lander
Mars Polar Lander, phone home. That was the command researchers issued thrice last week from NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. But scientists at the Earth receiving station in Stanford, Calif., detected no response from the lander, which had fallen silent Dec. 3 just as it was about to enter the Martian atmosphere. This week scientists will continue to study the data collected at the "Dish" radio telescope in hopes that sophisticated computer analysis will locate a signal among all the background noise.
Photos of the Dish
is available at:
Mission officials stress that the recent try is a long shot, and the process of data analysis and confirmation will not yield immediate results. Scientists at Stanford and JPL will continue their sophisticated analysis of the data. In the meantime, commands issued from NASA's Deep Space Network on Feb. 1 and 2 will tell the lander to reset its clock and send a signal to Earth on Friday, Feb. 4. The international scientific community has offered to help confirm any signals. Scientists at radio telescopes in the Netherlands, England and Italy will be listening for a reply, as will scientists at Stanford if they can get time on the Dish, which is booked for another research project.
All this effort was spawned by a faint signal detected by the Dish on Jan. 4. "It was the radio-frequency equivalent of a whistle," says Ivan Linscott, a senior research associate at Stanford's Space, Telecommunications and Radioscience Laboratory. Akin to the single, narrow tone that accompanies television broadcast tests, the whistle was at the ultra-high frequency (UHF) of 401.5 megahertz the right place to indicate a possible communication from the lander.
Just as the pitch of a train whistle drops as the locomotive approaches, the characteristics of the space whistle changed. "The pitch actually had a little curvature to it, and it was that characteristic that got our attention," Linscott says. Some of the changes in the space whistle came from the Doppler Effect created through the rotation of Mars and the Earth. A much larger effect, however, was produced when temperature-sensitive crystals in the lander's transmitter warmed up, creating a frequency profile that fell, then rose, in a characteristic way, Linscott says.
Even though Linscott says scientists are "still hopeful," the search has been frustrating. "It's like having a loved one missing in action," says JPL research scientist John Callas. "You've given up hope, and then there's been a report of a siting and your hopes are raised. Emotionally it's a little bit tough. I think in reality the chances are small, but we want to make sure. We have a responsibility to be sure, and that's why we're here."
Detecting a signal, even if one is there, is no easy feat. Space is noisy. "UHF frequencies are a pretty busy place up here," says Linscott. "But we still manage to find quiet places, or at least moments in those quiet places, to listen."
The transmitter on the lander has a broadcast power of about 14 watts, says Callas. For comparison, the beacon on the Mars Global Surveyor, which is currently in orbit 380 kilometers (228 miles) above the surface of the Red Planet, is weaker only 1 watt. Boding poorly for the mission is the fact that this week the sensitive Dish detected the weaker signal from the surveyor, but not the stronger signal from the lander.
But the main problem is the weakness of the signal. And signals weaken as they traverse the roughly 300 million kilometers (about 180 million miles) from Mars to Earth. "We expect a signal hitting the Dish to be something of the order of one billionth of a billionth of a milliwatt [one-thousandth of a watt] of power," says Callas. "It's extremely tiny. This is equivalent to listening to a cell phone from Mars."
More possible woes
Scientists may no longer know the exact configuration of the lander, since NASA has sent many commands to exercise different aspects after losing contact with the spacecraft. Also, it is possible that the lander may have been "asleep" when some of the commands were sent. If this were the case, it would be responsive only after it completed a complex "waking-up" process.
To help them face these considerable technical challenges, the researchers have a powerful tool on their side: the exquisitely sensitive 150-foot-diameter Dish, which SRI International operates and maintains and the U.S. government owns. From a grassy knoll on the Stanford campus, the parabolic reflector of this radio telescope concentrates electromagnetic waves (radio waves) into an aluminum collecting horn built by SRI's Mike Cousins that Linscott calls "the jewel in the crown."
From there, the signal is fed into a transducer, says Cousins, site manager of the Dish. A transducer changes one type of energy into another. Unlike the pressure waves of audio, electromagnetic waves from space need to be manipulated by transducers and fed into computers before scientists can "see" or "hear" the data.
Using sophisticated software, Linscott and colleagues search among the weak signals for narrow, single tones. "Because of the processing that's involved in trying to extract them out of the background noise, it's not like we put headphones on and hook in like the now-famous image of [Jodie] Foster's search in the movie [Contact]," Linscott says. "We don't have headphones. What we have are the computers that record the signals from the sky, and more computers to try and analyze those."
Even if scientists are able to eventually detect a signal from the lander, will that mean the spacecraft could still serve a useful purpose?
"If the lander is there and it's talking to us, that tells us a tremendous amount of information," Callas says. "It tells us it got to the surface of Mars safely or in a reasonably safe configuration and that it's generating power, that its computers are operating, that it can hear us, and that it can use at least part of its UHF system. That would go a long way to understanding what might have gone wrong to put us where we are today." A functional UHF system might even allow scientists a means of reconfiguring the lander and recovering the mission a possibility that Callas admits is "extremely optimistic."
It will take some time to find out the mission's fate. While strong signals can be displayed in real time as spectra when they arrive, weak signals require lengthier processing. Because of the weakness of the signal, it takes "a fair amount of sophistication in the processing of this data using computer software to try to extract the signal," Callas says. "The human eye is an additional aid to the processing because we can make some displays on the computer screen. By looking at them we can recognize patterns a lot more readily than a computer algorithm can."
Support for signal processing came in part from the Bosack Foundation, established by Stanford alumni Leonard Bosack and Sandy Lerner, founders of Cisco Systems. Mars mission support comes from JPL. Lockheed Martin Astronautics Inc. of Denver is the agency's industrial partner for development and operation of the spacecraft.
Stanford graduate students who helped develop the signal-processing techniques used include Mitch Oslick, who earned his doctorate and now works for Phillips Electronics, and Neza Maslakovic, who will finish her doctoral work this summer.
By Dawn Levy