Originally, Phoenix was headed elsewhere. Mission scientists had targeted a wide area called ‘Region B’ — B for Best. But then images started coming in from the high-resolution camera (HiRise) on Mars Reconnaissance Orbiter. The region intended landing region soon became B for Bad.
“They get the HiRise data and they see big boulder fields with rocks the size of the lander,” says Tabatha Heet, a 22-year-old graduate student at Washington University in St. Louis, Missouri. “We quickly realized that this is not the place we wanted to go.”
But where would Phoenix go instead? Someone had to scour the new HiRise images for rocks. Washington University’s Ray Arvidson, a mission scientist on Phoenix (as well as for the Viking landers and the Mars Exploration rovers) knew where he could get some cheap, indefatigable labor. He told Heet to start counting.
She went through more than 40 hectares of images, pixel by pixel, looking for the tell-tale shadows from rocks. If the width of the shadow was more than 1.5 meters across, then the rock was probably about that size. And based on previous relationships, martian rocks are roughly half as tall as they are wide. So anything with a shadow that wide posed a problem for Phoenix.
How many rocks did Heet count? “More than I can remember,” she says. But the areas she counted are branded in her brain. Scientists brought her maps, and she quickly told them if the area was good or bad. Tabatha gained a nickname: “The Tabulator.”
“I was counting for about two years,” she says. “I say now that I hate rocks, but that’s not really true.” In fact, she says it was all very much worth it — for the mission, and for her. Scientists at JPL made software to automate the counting, but this had to be calibrated by Heet’s hand counts. In the end, they chose a landing ellipse (pictured below) in which Phoenix will land with 99% confidence. The green areas signfy 0-3 rocks per hectare bigger than 1.5 meters, while the red areas, noticeably absent, contain 20 or more. Based on her work, Heet landed herself a summer gig with the mission: she’ll be one of four documentarians, recording the rationales for the science team’s daily decisions.
After all the counting, Heet had to have Lasik eye surgery to improve her terrible eyesight. But she doesn’t blame Mars. “I was blind before I started counting rocks.”
Image: NASA/JPL-Caltech/University of Arizona