Dive for the stars

How an autonomous submersible with a difference could pave the way for a future mission to seek out life in lakes beneath the ice on one of Jupiter’s moons. Niall Firth reports.

The robot was jammed 25m down, just under the roof of one of the the world’s deepest flooded caves at the El Zacaton complex in Mexico.

On its way back from a four-hour mission, the robot — DepthX — had a problem with its sonar processing and drove itself into a wall. The only way for the team on the surface to rescue its £2.5m ($5m) investment was to don scuba equipment and dive down into the inky blackness to help nudge it free.

DepthX (Deep Phreatic Thermal Explorer) is a fully-funded NASA project which hopes to develop a robot that will be capable of searching for life in the lakes hidden beneath the ice of Jupiter’s most Earth-like moon, Europa.

But while the money may be NASA’s, the project itself is undeniably the brainchild of engineer, explorer and prodigious polymath, Bob Stone. Little hiccups along the way, like robots that get stuck, are all part of the development process, according to the man who has not only produced more than 200 peer-reviewed scientific papers but is also arguably the world’s foremost cave-explorer.

‘There are over 100,000 lines of autonomous behaviour software in this thing so there’s going to be a bug somewhere,’ said Stone of the recent minor scrape. ‘But that is why we are doing all of these smaller missions, to iron out all of the little problems.’

In this recent test, DepthX completed the second of three planned missions at El Zacaton. It was the first stage of a long process that will eventually result in an autonomous robot capable of mapping and analysing the water on Europa — the place scientists reckon is the most likely to contain life in our solar system.

Europa is covered by a global ocean of liquid water submerged beneath a layer of ice estimated to be up to 10km (just over six miles) thick. The moon, about the same size as Earth’s Moon, is slightly heated by the friction of tides, meaning its water is in liquid form. And it is this that may well contain our nearest alien neighbours.

The idea for an autonomous under- water robot began with an early prototype Stone and his team had built to map the underwater caves they had explored all over the world. Stone has a track record of developing technologies to fit his expeditionary needs and was the inventor of the closed circuit ‘re-breather’ scuba system that removes carbon dioxide from exhaled air and re-circulates it for re-use. It is now considered standard issue for extreme cave-divers. Stone himself claims to have spent a total of 353 days in caves deeper than 1,500ft (457m).

After developing his human-operated mapping robot, Stone heard that NASA was looking for ideas for its lander for a planned mission to Europa. This was his type of challenge, and together with a team of researchers — including microbiologists and electromechanical engineers — he began work on what was to become DepthX.

The robot’s distinctive mushroom-like shape is designed to maximise its unique geometry-based navigation system. Instead of using radio referencing or even GPS like other robots, the locations in which it will be operating meant that a new mapping engine had to be developed. This simultaneous localisation and mapping (SLAM) algorithm will be the navigation device that DepthX will rely on when operating in unexplored areas. It means it can create 3D maps of its location and then use these stored maps to navigate itself out of any labyrinthine series of underwater caves.

‘It’s like an all-seeing eyeball which gets geometric lock on the features that surround it to work out its position,’ explained Stone. Powered by a lithium ion battery, the vehicle’s six thrusters, two of which are positioned vertically, and four horizontally, enable it to carefully pick its way through the underwater world.

While its 3D mapping and navigation skills are extremely advanced, DepthX has another role, that of autonomous robotic microbiologist — like a floating robotic David Attenborough. As well as its sonar and wide-field camera it is also equipped with a one-metre extension probe fitted with a high-resolution camera, an altimeter and a series of sensors for measuring water samples.

After creating a 3D map of its environment, the robot begins to characterise its surroundings by analysing variables such as pH, salinity, conductivity and dissolved oxygen present. Using complex intelligent algorithms DepthX then divides its vision of the world around it into virtual cubes into which it can split the information it has gathered. Currently, this information is then post-processed back at the base and analysed to see whether the data gives indication of any life in a certain area of the cave. However in future missions, Stone said that all of this analysis will be done on board the robot in real-time.

‘The robot can then follow the gradient of a trace, such as hydrogen sulphide, back to a wall or floor to find its source. Microbiological life likes to be on surfaces rather than in the water column.’


Stone’s team is continuing with ongoing exhaustive tests to develop DepthX’s micro-organism detection and collection ability

After following the signal back to the wall, the robot then has to override the programming instincts which normally keep it clear of obstacles. It pulls in close to the wall — close enough so its probe can be put to work. It then uses machine vision with its wide-field camera to analyse the area for deviations in background colour, a key indication that microbial life may be present.

But DepthX doesn’t stop there. If it finds an area of water that is of sufficient interest, its probe reaches out and collects a sample. This is then processed by the robot’s on-board microscope, a unique design that is rated to survive and operate at up to 1,000m down — the estimated depth of El Zacaton.

It pulls a sample in, lights it from behind, and looks to see if there is any motion involved. if movement is detected that is more than simple Brownian movement of particles, then it can make an assumption that there are microbes present.

The full, deeper mission at El Zacaton next month will test DepthX to its limits. Potentially going 12 times deeper than the first two tests, the final mission will put the robot beyond the reach of any scuba rescue team to map parts of the hitherto completely unexplored caverns. It is a test the robot will have to pass before it can move on to the next stage, which will take place in Antarctica.

There are two follow-on projects which Stone hopes to get funded. The first of these will take a future variant of the robot — known as Endurance — and let it loose beneath the Antarctic ice of Lake Bonnie, a helicopter ride away from McMurdo base. The second is beneath what is believed to be one of the world’s largest lakes, Vostok — also in Antarctica.
However, according to Stone, NASA’s recent budget cuts mean that the Endurance programme is far from being a shoe-in for funding.

‘The NASA science programme has really seen its budget slashed and all of the money has gone to the Crew Exploration Vehicle (CEV) and the manned space missions,’ he complained. ‘All of NASA’s science work, including planetary robotics, is really running at shoestring levels at the moment which in my opinion is a tragedy.’ Indeed, just last week Nasa officials announced that the agency will be winding up its big ideas factory, the Institute for Advanced Concepts(NIAC).

Endurance’s first deployment, if funded, is scheduled for November 2008, and will include many additional technologies beyond DepthX, including a device for melting through ice, and sterilisation equipment so as not to disrupt the eco-balance of any local flora and fauna. This will make it much closer, in terms of technical challenges, to the Europa mission.
‘It’s going to be a lot trickier at Bonnie or Vostok as the lakes are pretty flat underneath, unlike Zacaton which is feature-rich with loads of jutting rocks for the robot to make its spatial calculations from,’ said Stone.

The robot is equipped with an extension probe fitted with a high-resolution camera, an altimeter and a series of water measuring sensors

The ice-covered Lake Vostok is considered to be an even tougher challenge for Endurance. ‘This is about as close to a dress rehearsal as you can get on earth to Europa. There are three and a half kilometres of ice on the surface of the lake and everyone wants to know what’s down there.’

Stone believes that last summer’s historic Cassini-Huygens mission to land on Jupiter’s moon Titan was ‘one the great engineering and intellectual achievements of mankind.’ However, he knows that any future mission to Europa will eclipse the Huygens mission in terms of complexity.

‘Imagine landing on Europa and then getting through 10km of solid ice cap,’ he said. ‘Once you’ve melted through 10km of ice you then have to set in motion the most sophisticated robot we’ve ever built to go looking for life in a world where it must explore on its own, build maps, and then come back and phone home.’

Stone already has a clear idea of what would be required in terms of technology for the eventual Europa mission. On the microbiology side, he envisages that DepthX’s successor will have a far greater degree of microbiological know-how than the current robot. He believes it will need an onboard database of life-forms against which it can check any new findings, a microbiological library which the robot’s team has already started compiling.

‘By the time it’s ready for Europa we could even have an onboard DNA sequencer so we can find out exactly what it is that’s there,’ said Stone. ‘If that was the case you could record the DNA sequence, take pictures, collect samples, and then return to the melthole, uplink to the surface lander, then to the orbiter, then on to the deep-space network and — if we’re lucky — in something like 2019 we will find out if there’s life off earth. That’s the grand game.’

Funding aside, there is one key issue that needs to be resolved before any future mission to Europa. According to Stone, everyone who has been involved with the project has come to the same conclusion as him. The final two stages of the Europa lander will need to be nuclear-powered. Stone’s own rough calculations indicate that an advanced 100KW pebble bed reactor — about the size of a microwave oven — would be needed on board the robot.

The final lander, as envisaged by Stone, would comprise three different sections. First the lander itself which would touch down on to the moon, containing propulsion systems, power and data relay technologies. The second and third stages would be joined together and include a heat exchanger to convert the nuclear power into heat to melt through the ice.
As it moves down through the 10km of ice the robot would leave a network of fibre-optics in its wake. Upon reaching the submerged ocean, the third stage ‘fast-mover’ robot — for which DepthX is the prototype — would separate from the thermal melter, leaving behind a communications system and a navigation beacon and move off into the ocean in search of life. The robot’s onboard nuclear reactor would also provide its thermal pulse propulsion.

Stone has proposed to NASA that this third stage robot would also carry a number of smaller ‘sub-bots’ which would be equipped with sensors, but would be battery powered.
‘What you do then is if you get to an area which is very interesting, like hydrothermal vents at the bottom of the lake, you send one of these sub-bots in first to do some primary mapping and then come back, dock and recharge,’ explained Stone. The nuclear system on board the third-stage robot would also act as a battery recharger for all the electric subsystems, sensors and sub-bots.

Stone hopes that at Lake Vostok a fully-operational version of this Europa mission could be tested, including a thermal melter and a number of the sub-bots, but he is aware that the chances of this happening are slim.

‘At some stage we would like to test the whole thing but for some reason I suspect they will never let us put a nuclear reactor into an Antarctic lake.’
But, he added, at some point in the future the system will need to be tested before a full Europa mission is given the go ahead. ‘It’s going to need to happen if we’re going to Europa. People just need to grow up. Science can be done properly and this is the only power source. If it’s designed correctly it’s entirely safe.’

At present, all attention is on DepthX and the El Zacaton missions. Stone and his team in Mexico are continuing to develop the robot’s ability to autonomously detect and collect micro-organisms in one of the most challenging and fascinating environments on the planet.

For the time being, this is challenge enough. However, for Stone — a man who once only just failed at the final hurdle to qualify as an astronaut — the work they are doing is all focused on seeing if there is life beyond Earth.

‘There are so many steps along the way to reaching Europa but we’ve just taken the first one,’ he said.