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.
An ultrashort baseline transceiver is lowered into the water once ENDURANCE submerges and emits signals that give the autonomous vessel a point of reference. This aids the vessel's mapping mission and also helps the researchers retrieve their robot sub. A malfunction that might cause ENDURANCE to get lost or be unable to make its way back to the entry hole would be disastrous due to the harsh conditions in which it operates. "When you put something in a lake in Antarctica, you don't want to lose it," says John Rummel, a NASA senior scientist for astrobiology.
ENDURANCE is a follow-up to the Deep Phreatic Thermal explorer (DEPTHX) , a NASA-funded project led by StoneAerospace, Inc., a Del Valle, Tex.based maker of technology used to explore the ocean depths as well as deep space. DEPTHX last year underwent extensive underwater field tests in Mexico, and its success has paved the way for ENDURANCE's missions.
Whereas the DEPTHX was tested in waters that were a balmy 86 degrees Fahrenheit (30 degrees Celsius), ENDURANCE will encounter 32-degree F (0-degree C) conditions in Lake Bonney, whose stratified waters contain both fresh- and salt- water. Nearest its surface, Bonney has a layer of about 16 feet (five meters) of freshwater atop about 110 feet (33.5 meters) of saltwater, the latter of which is three times as saline as normal seawater, Rummel says. This increased saltiness will cause ENDURANCE's sonar waves to behave differently, bouncing upward more than they would in fresh or less briny water.
NASA's otherworldly aspirations for ENDURANCE include a trip to Europa, which Rummel says the agency hopes to pull off within in two decades. ENDURANCE has to prove its mettle on Earth, however, before it is ready to dive under Europa's ocean, which features up to 12 miles (20 kilometers) of ice atop water that could be around 62 miles (100 kilometers) deep. Once there, ENDURANCE could play a very special role in space exploration, Rummel says, adding, "It could very well be the first vehicle to find extraterrestrial life."