Scientists Keep Searching For Missing Martian
by Dawn Levy
Stanford - January 31, 2000
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.
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," added Callas.
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."
EARTH INVADES MARS
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