In August 2003, as the twin Mars Exploration Rovers were barreling toward Mars in their flying saucers, scientists and engineers sent a radio signal disguised as the rovers’ “voice” to the Odyssey orbiter at Mars. The call to Odyssey was what Dr. John Callas, Mars Exploration Rover Science Manager, defines as a “can-you-hear-me-now?” test. Scientists and engineers wanted to ensure the UHF (ultra-high frequency) radio system on Odyssey, a primary communications relay between the rovers and Earth, would work. Odyssey responded with a resounding yes, and something else from Mars responded too….
HEARING UNEXPECTED ECHOES IN THE NOISE
When the first, clear “I-can-hear-you” reply beamed back from Odyssey, modest high-fives and conservative cheers were exchanged amongst the small team of PhDs huddled around a computer near a 46-meter (150-foot) antenna at Stanford University known locally as the “Dish.” SRI International manages the radio telescope, the only deep space antenna near the Jet Propulsion Laboratory that can send UHF radio waves from Earth to Mars. As each new line of relay test data streamed down to the computer screen at the Dish, Stanford University’s Dr. Ivan Linscott began to mutter, “Huh…what’s that?…that’s strange.”
A peculiar stripe on the data-return-screen was arcing underneath the straight line that represented the primary communications from Odyssey. The mysterious curve then intersected the primary line, and stopped sending data at the same time the main signal disappeared. The team initially dismissed the strange line as signal noise that engineers term “radio frequency interference” (RFI). But the curved line of data has now earned the title, “Lucky Stripe,” and the so-called static has become the subject of the “Mars Bi-Static UHF Radar Experiment.”
EXPERIMENT IS LIKE A FUN HOUSE MIRROR
After a week of studying the stripe, the team discovered that the extra data was actually a reflection from the surface of Mars. “Anyone who’s used rabbit ears to pick up a television signal has probably seen a ghosting effect – a signal echo,” explains Callas.
The ghosted image arrives off-center and is more transparent because the source signal hit some neighboring wall or structure and then bounced back to the TV receiver a little later and more scattered. The Lucky Stripe is a reflection of the martian surface, and the stripe is curved because the Odyssey spacecraft was traveling in an arcing orbit over the planet as the echo from the surface of Mars reverberated back to Earth.
“Just like a distorted reflection in a fun house mirror tells you something about the mirror’s shape, radar reads an echo of the surface of Mars that tells us about the shape of the surface it’s reflecting,” explains Callas. The team later confirmed multiple reflections, which suggests they are seeing “echoes” of material beneath the surface of Mars.
EXPANDING SCIENCE USING CURRENT RESOURCES
Fast-forward to October, 2005. The bi-static radar experiment team, led by Callas, is now using the Odyssey UHF radio system and SRI’s antenna to hunt for subsurface water ice – a key component to understanding the history or future of life on Mars. Timing is best now as Earth and Mars merge toward a close approach on October 29, 2005 and UHF radio signals become stronger and take less time to travel between the planets. Scientists are hoping to find unexpected treats in the form of “noisy” radar tricks from now until just past Halloween.
This bonus science is possible because the communications team didn’t disregard the “noise” from the original test and later NASA support for the discovery. “This simple, inexpensive technique utilizes resources that are already in place,” explains Callas. The NASA-funded experiment is a collaboration between JPL-Caltech, Stanford University, and SRI, and uses antenna-scheduling resources and communication protocols already in place to support the rover mission.
Rover Mission Planner Byron Jones reflects the mindset of various Mars experts who have helped the experiment succeed. “There’s a real sense of community during our Mars Relay meetings as various missions work to share memory space on Odyssey so the bi-static experiment can operate,” says Jones. When Mars is whispering, “Can you hear me now?”” through cryptic radio echoes, all of the mission teams can’t help but eagerly listen.