Deep Mission: Aquatic Robotic Diver Could Aid Europa Exploration
The DEPTHX autonomous underwater vehicule undergoes tank trials.
Credit: Stone Aerospace.

It's robotic, aquatic and ramping up for a long, deep dive, all to test technology that could one day allow its autonomous descendants plunging into alien waters believed to sit beneath the icy crust of Jupiter's moon Europa.

Built by Texas-based Stone Aerospace, the autonomous underwater vehicle Deep Phreatic Thermal eXplorer (DEPTHX) will begin a second round of dives this week to explore the depths of La Pilita, a 377-foot (115-meter) deep geothermal sinkhole in Mexico [image]. They follow a series of successful tests dives to shakedown the vehicle's autonomous navigation and mapping capabilities, researchers said [video].

"The fact that it did it and came home each time, it's like the clouds parted," said Stone Aerospace chief Bill Stone, DEPTHX principal investigator, of the robot's early success. "I've been doing this 27 years and I was blown away."

Stone and his team are priming DEPTHX to take the ultimate dive down El Zacat?n, a water-filled geothermal sinkhole - or cenote - in Mexico known to reach depths beyond 925 feet (282 meters), though its true bottom has not been detected by either human divers, robots or sonar [image]. That expedition is slated for May.

The research is part of a $5 million NASA study to develop technology that could one day allow a waterborne explorer to probe the vast unknown ocean thought to lurk beneath the miles-thick crust of ice covering Europa. The cracked, icy Jovian moon has been billed one of the solar systems' best hunting grounds for extraterrestrial organisms.

Researchers at Stone Aerospace, Carnegie Mellon University, the Colorado School of Mines, University of Colorado and Southwest Research Institute are participating in the DEPTHX project, which is funded by the Astrobiology Science and Technology for Exploring Planets program within NASA's science directorate.

Diving deep

Despite weighing in at a hefty 1.3 tons, DEPTHX's design keeps it neutrally buoyant in water, allowing the 8.2-foot (2.5-meter) wide robot to maneuver in three dimensions (up, down and side-to-side) with the aid of six thrusters [image]. Because no one knows for sure how deep the El Zacat?n cenote goes, the vehicle has been built to dive to a depth of about 3,280 feet (1,000 meters), Stone said.

DEPTHX is armed with 36 onboard computers, 56 sonar sensors and a suite of depth, guidance and velocity sensors to find its way through the murky waters of La Pilita or El Zacat?n.

But the brass ring of DEPTHX's guidance system is its Simultaneous Localization and Mapping (SLAM) system, navigation software designed to allow the robot to autonomous take its sonar and other sensor readings to build reliable and useful three-dimensional maps of unexplored terrain.

"It's kind of the chicken and the egg," said DEPTHX project scientist George Kantor, of Carnegie Mellon University where researchers developed the robot's SLAM software. "As it builds the map, it can actually use that map as it moves around."

Early tests DEPTHX tests 3-D mapping system revealed an underwater tunnel 98 feet (30 meters) below the surface of the 98-foot (30-meter) wide La Pilita cenote, though the system should be put through its paces in full in the upcoming tests, Kantor said. Other autonomous tests using dead reckoning and sonar navigation have proven successful, he added.

"When you're working on Europa....there is not going to be any guidance from Mission Control," Stone said. "They're all going to be sitting back there sweating, and hoping they didn't mess up some line of code."

NASA demonstrated as much last week when its New Horizons probe swung past by Jupiter on Feb. 28, relying on a series of preloaded commands to study the planet and its moons due to the 45 minutes it would take a signal to cross the millions of miles between Earth and the gas giant.  

DEPTHX's navigation system complexity also comes at a cost, researchers said.

"Our cruising speed is about 0.2 meters per second (about half a mile per hour)," Kantor said. "That's not going to work on Europa, where you're going to want to be moving a lot faster."

Science automaton

Finding your way around obstacles or rocky walls in a pitch-black cenote - or the waters of Europa, for that matter - is just the first step for DEPTHX researchers, who are hoping to develop autonomous science capabilities for the aquatic robot - especially those useful for studying microbiology.

"The hat trick beyond what we've already done is to give it the ability to sniff out environmental variables, gradients in the environment's chemistry, that would suggest something is going on," Stone said.

DEPTHX carries a probe-like appendage [image] capable of retrieving solid samples during dives and contains space for up to five two-liter water samples from different locations around cenotes like La Pilita or El Zacat?n. The robot also sports cameras to scan for color changes in rock walls and an onboard microscope to detect moving cells, a hallmark of living microorganisms, researchers said.

"There is absolutely no way of knowing what's in this hole," DEPTHX's chief microbiologist John Spear, of the Colorado School of Mines, told of El Zacat?n.

For Spear, DEPTHX's potential as a pathfinder for a Europa probe is a bit of a bonus. The robot's technology could also be adapted to explore any number of environments on Earth, from the depths of the planet's oceans to the deep lake bottoms below Antarctica.

The key, Spear added, is developing the capability of overlaying science observations - such as changes in water sulfide content - with SLAM location maps to pinpoint their respective positions.

"When you walk up to a hot spring, you see it, you smell it, you can feel it and taste it," Spear said. "We needed to develop a machine that's capable of doing the same thing I can. We had to give the machine senses so to speak."

For Stone, DEPTHX's big test comes in May, when the robot will put its navigation and science capabilities to the test at El Zacat?n.

"The two goals are there," Stone said. "I think we're going to hit them."