Researchers from NASA and the University of Illinois at Chicago atop the frozen surface of Wisconsin's Lake Mendota this week are preparing for interplanetary exploration. Below them, under a sheet of ice more than a foot (30 centimeters) thick, the space agency's new Environmentally Non-Disturbing Under-Ice Robotic Antarctic Explorer (ENDURANCE) maps the lake's underwater terrain. If this and subsequent voyages are successful, a similar vessel could be sent to navigate the suspected liquid water under the frozen surface of the ocean on Jupiter's moon Europa by the year 2028.
ENDURANCE, a $2.3-million project funded by NASA's Astrobiology Science and Technology for Exploring Planets (ASTEP) program, is an autonomous vehicle designed to operate underwater below the ice. Its mission: to gather environmental data (such as samples of microbial life) and create three-dimensional maps of undersea topography.
The Lake Mendota effort is a practice run for a month-long mission it is set to undertake later this year in Antarctica's permanently frozen Lake Bonney, which is coated with up to 15 feet (4.5 meters) of ice. The lake, which is about 2.5 miles (four kilometers) long, one mile (1.6 kilometers) wide, 130 feet (40 meters) deep and located in the continent's McMurdo Dry Valleys, was chosen because its extreme conditions are about as close as it gets on Earth to those ENDURANCE might encounter on Europa. If this trip to Antarctica is successful, ENDURANCE will do a second mapping next year.
All data gathered from these expeditions will be sent to the University of Illinois's Electronic Visualization Laboratory, where researchers will create 3-D images, maps and data renderings of the lake. ENDURANCE begins mapping from the melt hole through which it enters the water. Using the GPS coordinates of the opening, the vehicle's positions are determined data gleaned from sensors that measure temperature, light and the water's chemistry. During its eight-hour missions, gathered information is stored on board using flash disk memory for later recovery and analysis on the surface.
Workers cut a 10-foot (3.1-meter) by 14-foot (4.3-meter) rectangular dive hole with a chainsaw to prepare an entry point for ENDURANCE in Lake Mendota's icy surface. Because ENDURANCE enters and leaves its underwater habitats via ice holes, it is designed to be a compact vehicle—about 4.7 feet (1.4 meters) long, 3.5 feet (1.1 meters) wide, and 2.6 feet (0.8 meter) high and weighing about 176 pounds (80 kilograms) on land. The propulsion chassis uses commercial components along with custom-designed flight electronics and thrust vector controllers. Maximum vehicle speed is anticipated to be as much as five feet (1.5 meters) per second.
Most of the onboard scientific instrumentation is fiber-optic-based and mounted along the wall of a flow-through tube that runs the entire interior length of the vehicle. Chloride and conductivity probes sense the same flow stream at the rear of the vehicle. A digital camera with lighting sits on the starboard bow (located on the right, if you are facing the sub's front) for capturing visible spectrum images within each volume pixel (voxel) as well as for taking bottom sediment images. Three-dimensional images are composed of voxels in the same way that two-dimensional ones are built from pixels.
ENDURANCE, which runs on two lithium ion batteries, detects and avoids obstacles using sonar arrays mounted on the bow, port (left) and starboard sides that can alert the vessel about an object up to 328 feet (100 meters) away. The sonar's detection range is expected to give ENDURANCE at least a minute before it reaches any obstacle.