Shielded by a thick, brittle ice surface, a vast warm ocean hides beneath. Scientists consider Jupiter’s smallest moon, Europa, to be the most promising for alien life and have set in motion multiple concept ventures to design an autonomous underwater vehicle (AUV) which could successfully explore this remote, off-world environment. Many Europa-bound missions are yet to come to fruition and prove the feasibility of this ambitious and complex task, until now. After four years of research and development, the Europa-Explorer project, engineered by Germany’s Robotics Innovation Center (RIC), have built a working prototype submersible and a mission plan for perhaps one of the most exciting space exploration mission endeavors of our time.
Though smaller than the Earth’s moon, Jupiter’s moon Europa contains more liquid water than Earth. The unusual features of the fractured icy surface and discovery of the water plumes in 2013, add to scientific speculation that tidal heating may warm this voluminous saltwater expanse to temperatures favorable for life. Thought to have a thickness between 3 and 15 kilometers, Europa’s ice surface shields an ocean, an estimated depth of 100km (62 miles) – the Mariana Trench, a shallow 11 kilometers (7 miles) by comparison.
While several spacecraft have already completed a mixture of long-term and flyby missions, 2030 will be the decade both NASA and the European Space Agency (ESA) launch further data-collection missions of Europa’s environment and begin the search for life.
The German Research Center for Artificial Intelligence (DFKI GmbH) Robotics Innovation Center (RIC) launched the Europa-Explorer project in December 2012 – a pilot survey for future missions to Europa. It focuses on the aspect of navigation of robotic systems on and especially under Europa’s surface. After four years of concept development funded by the German Ministry of Economics (BMWI), a possible mission scenario has been drafted which covers all aspects of exploration, from the time of landing until the transmission of data back to Earth.
After arriving on Europa, the mission plan is to have a terrestrial “IceShuttle” melt a narrow passage through the moon’s frozen shell. A swarm of micro gliders will be dispersed into the water, anchor themselves to overhanging ice, and begin transmitting acoustic signals. These signals will allow the AUV “Leng” to orient itself before descending into the depths of Europa’s ocean. After completing fully autonomous exploration of the seafloor, Leng will return to the IceShuttle to dock, upload results, and recharge.
Dr.-Ing Marc Hildebrandt, Project Leader of Europa-Explorer and scientist at DFKI GmbH, explains, “The microgliders are one-way vehicles and will not return to the IceShuttle at the end of a mission. For their deployment, a passive system was devised which makes sure the gliders are deployed flying in separate directions. For the AUV, a lot of energy went into the question on how to deploy, dock and reintegrate a vehicle moving mostly horizontally into a vertically aligned IceShuttle. Our final concept utilizes the AUV’s ability to change its buoyancy and by that its pitch angle. The docking itself is not that easy: unlike most AUV docking-systems it was not possible to deploy large cones for vehicle catching, making it a precision docking approach more akin to spacecraft dockings. We are very proud that this aspect is very robust and reliable.”
The necessity for the development of new systems lies in the level of specialization of the two components required for the mission. The autonomous ice-drilling shuttle with a payload system is a new area of research and not available off the shelf. The small, narrow 200mm diameter AUV is required so that the AUV fits into the payload section of the IceShuttle, adding further to complications in the design, along with highly-specialized sensors needed for the exploration of an under-ice foreign environment. Typical under-ice-exploration AUVs used on Earth are significantly bigger.
“If I had to select one single biggest challenge of this project it would be complexity. When starting the project 3.5 years ago, both the AUV and the IceShuttle were sketched as much simpler devices, which developed tremendously in complexity with each added environmental parameter, security issue or additional requirement. Another more expected challenge is the whole navigation. You can boil it down to this: ‘after an exploration of unknown terrain find your way home to a 100km distant 30cm large hole.’ That’s quite a challenge.”
Into the Unknown
As engineers and computer scientists, the RIC project team did not have a comprehensive knowledge of Europa’s environment, so they asked the experts: the researchers of the MPS, the ‘Max-Planck institute for solar system research.’
Hildebrandt recalls, “They gave us a lot of details on what to expect but also cautioned that most of these facts are inferred and not yet finally proven, since no one, not even a probe or robot, has yet set foot, or wheel, onto Europa. The main facts for us are: there is liquid water with a certain amount of solved salts; there will be shallow currents at the equator, stronger ones at the poles; the ocean could be 100km deep which due to lower gravity conditions, resembles pressure conditions at the Marianna trench; and there will be a magnetic field, but Jupiter’s magnetic field. One hypothesis – in fact one of the reasons this moon is so interesting – states, that there should be hydrothermal vents similar to the ones found on Earth. If the water is liquid, there has to be an energy source – this leads to the assumption of a hot core. If the right conditions could be found in the ocean, life may even exist there, similar to the sun-independent ecosystems around hydrothermal vents in our deep oceans.”
For the exploration of this vast expanse of water, a fully-autonomous system is necessary because of the time it takes to transmit from Earth to Europa (33-53 minutes). While the AUV is submerged and un-docked no external control is possible, and scientists are still not sure what environment to expect. In addition to that, the necessity for the vehicle to dock with the ice-shuttle after each successful dive adds to this complexity. Although there are modern AUVs which can perform long fully autonomous missions, a system which “lives” in an underwater environment and monitored irregularly has yet to be shown in practice.
The primary design goal of the AUV was for it to fit into the IceShuttle: as the energy required to melt a hole through the ice increases quadratically with the diameter of the hole, the size of the IceShuttle should be as compact as possible, limiting the AUV to 190mm in diameter. While AUVs of similar diameters exist, such as Remus100 and Gavia, they do not have the extensive instrumentation required for a prolonged mission to Europa. Getting all these devices (DVL, ADCP, FOG, CTD, cameras, diving-cells and docking capability) into such a small diameter was a challenge for the RIC team.
‘Leng’ Phone Home
“One of the big paradigms we have is ‘always be able to come home.’ While this is evident in a typical mission situation, this ability is also a necessity if something went wrong – if a thruster malfunctions, a sensor does not work anymore or the battery is depleted faster than expected. A ‘typical’ AUV on Earth would surface and start sending out emergency signals using satellite communications. This is not possible on Europa, making the whole autonomy much more comprehensive. The vehicle must always have the possibility to go back to the IceShuttle, because if it can’t then we can’t communicate or update the mission, so the mission is lost. This is a top priority – if it sees the system isn’t doing what it’s supposed to do, then the vehicle will return to ‘base’ and report back to a human on Earth so they can take a look at it,” explains Hildebrandt.
In the first project, CUSLAM, the team at the RIC designed and built an AUV, “Dagon”, for scientific data collection and created a novel vision-based underwater localization system. The Europa-Explorer project, extends this localization system for the special environment of an under-ice long-range exploration as necessary to navigate on Jupiter’s moon, Europa. This navigation system is unique, based on the Dagon AUV which uses stereo camera-based system with a 30cm baseline looking straight down at the ocean floor. The system was modified, taking into account the smaller diameter and, therefore, the reduced baseline of the vision-based localization system. By tracking features in the images, the vision system then can compute the vehicles motion and navigate by recognizing features on the seafloor which may appear uniform to the human eye.
A more extensive sensor package will be included at a later stage such as dissolved gas sensors (O2, CH4), fluorometry sensors, and micro-labs for amino-acid detection or characterization. The AUV will also need to withstand 1,100 bar external pressure, a feature also planned for a later date.
“For us, it was more important to be able to first show our ability to create a functioning, fully-autonomous team of robots which can execute complicated missions without human interaction. A similar reason is the usage of aluminum for the pressure hulls: the real vehicle will likely be manufactured from titanium alloys, but they would have been too expensive and not really necessary at the current project stage. An area where we tried to be as close to the real system was navigation – only sensors that would work on Europa was included in the system. This goes especially for devices such as LBL or compass sensors. An optical avoidance of sorts will also be used: a sonar will be doing avoidance and scanning for an obstacle in front of the vehicle but presently it only stops the vehicle if it becomes too near to the obstacle. We will want the full avoidance capability in the future for a long term mission which allows it to go around the object but at the moment the focus is long-term navigation and autonomy of the submersible.”
Hildebrandt is confident that their concept will work and, after some refinement, succeed in exploring this off-world ocean. The next steps for the team are real-world testing of the instruments here on Earth: first on the 5m thick Arctic ice surface, then on the 100m thick Antarctic ice shelf, or perhaps subglacial lakes. If successful, the RIC team will begin planning for a “real” mission to Europa and seek collaboration with NASA or the ESA.
“As well as space, some people are interested in using this system on Earth. We have a lot of contact with the German Research Center for Arctic exploration, AWI, as they’d like to have a system to deploy in Antarctica for one-year, summer to summer, keeping the ice shuttle and AUV in the water for climate change research. This is not currently possible as AUVs can go down for a few days but always relied on being retrieved for recharging or mission updates which can only be done in the Antarctic summer. So, having something like this for a long term deployment is hugely beneficial to some applications, this just being one example. We are planning on using it on Earth as well as space. If we can prove we have a working system than the ideas of other scientists on how to use it will come quickly.”
“It is a great project, I really enjoyed working on it. Initially, I was a bit reserved at the prospect of developing a mission scenario for a space mission maybe 30-40 years in the future as it sounded a bit far-fetched. Seeing our results after this relatively short time in comparison, I am more confident than ever that such a mission might be possible in the future. And it definitely is a plus that one day I might be able to tell my kids ‘I was part of the project that paved the road to finding extraterrestrial life,’” concluded Hildebrandt.