However, there are already propulsion concepts that might offer alternatives ten or twenty years from now.
A magnetoplasmadynamic drive (MPD) uses very strong magnetic fields to accelerate a reaction mass (like the noble gas argon or the metal lithium) out of the thruster. This makes high exit-velocities of up to 40 km/s possible. A spaceship thus equipped could reach Saturn in one to two years. A current commercial variant is called VASIMR. Its manufacturer, Ad Astra, built a 100 kw prototype that, in a mid-2017 NASA test, worked for 100 continuous hours. One drawback of the MPD, though, is that it only works with an energy input of electricity. As solar energy is rather sparse in the environs of Saturn, one would need a small nuclear reactor, or Radioisotope Thermoelectric Generator (RTG). The latter creates electricity from the heat released by the decay of plutonium.
Another alternative would be a nuclear drive. The concept was already developed in the 1950s, ‘Project Orion.’ Back then, the plan was to ignite a nuclear bomb behind a spaceship (which was shielded by a large mirror) to propel it this way. Using about 1,000 such bomb ignitions in sequence, Saturn could be reached in one or two years. However, these days it would hardly be acceptable to send real atom bombs into space by rocket. The concept was further developed into Mini-Mag Orion. In that project, a small piece of fissile material would be compressed in a magnetic field until it ignited in a miniature explosion. Even that would not be possible without releasing damaging radiation, so the drive would have to be carefully shielded.
The most promising concept these days seems to be the Direct Fusion Drive (DFD). The idea of using nuclear fusion for powering a spaceship has been discussed since the 1990s. In contrast to nuclear fission, nuclear fusion does not create very much radioactive garbage. The only problem might be released neutrons, which then might be captured by stable atoms, turning them into unstable nuclides. Currently, the company Princeton Satellite Systems (PSS) is predominantly active in this field of research, and it also advised me on this book. In the DFD conceptualized by PSS, deuterium (heavy hydrogen) reacts with helium-3 (an isotope of helium) to form helium-4. It achieves an exit velocity of 70 km/s. Under these conditions, a spaceship would reach Mars in a month and Saturn in a year. The fusion reactor generates relatively few neutrons, so the spaceship would not require cumbersome shielding. At the same time, the DFD also generates electricity.
Neither deuterium nor helium-3 is radioactive. However, helium-3 is quite expensive, as it is very rare. There are only about 3,000 tons present in the entire atmosphere of Earth. The annual consumption on Earth is approximately eight kilograms, and one kilogram of the gas costs about 16 million dollars. PSS estimates a round-trip flight to Saturn would require about 20 kilograms of helium-3. Earth’s moon might be a good source. In its upper rock formations, the content of helium-3 is up to 1,000 times higher than on Earth. A DFD could also serve as a source of energy for everything a human crew would want to do on Enceladus.
The final proof of the existence of life on Enceladus could, in all probability, only be found by an expedition directly exploring its ocean. The conditions on Enceladus are actually quite good for this, as at least parts of the ice crust are thinner than on other moons.
Humans have experience drilling through layers of ice. Usually, conventional drill rigs are used for this, but it would be impractical to take them along to Enceladus.
The alternative would be a cryobot, an ice-drilling robot already developed by the German physicist Karl Philberth in the 1960s. In 1968, his ‘Philberth Probe’ reached a depth of 1,000 meters in the Greenland ice sheet.
The current leader in this area is the U.S. company Stone Aerospace with its ‘Valkyrie.’ This ‘Very deep Autonomous Laser-powered Kilowatt-class Yo-yoing Robotic Ice Explorer’ is partially financed by NASA. It does not carry an energy source on board, but is supplied by a laser via a fiber-optic cable. Currently, Stone Aerospace is testing their Valkyrie with a power level of 5-kilowatts on Earth. The version for Enceladus would have to be considerably larger, but it would work according to the same principle. It would need from 250 kilowatts upwards to 1 megawatt of power. The more power, the faster the cryobot can drill, although "drill" is not really the correct term. The laser heats up the water, and the hot water is aimed at the ice, which then melts like butter. This works more quickly and requires less maintenance (which is very important) than a metal drill that would wear out. In addition, the hot water can also be used for generating energy. The fiber-optic cable, in turn, can be used for transmitting information.
However, the Valkyrie concept only works if you bring along a source of energy. One cannot simply generate 5 megawatts with an emergency generator. One would need a small nuclear power plant. Or one could use the dual function of the DFDs, each of which provides 10 megawatts of power. As Enceladus does not have an atmosphere, the energy could be beamed almost without loss via laser from the orbiting vessel to the lander module, which then would feed it to Valkyrie through the fiber-optic cable.
Life on Enceladus?
If there is life on Enceladus, it would be located at the bottom of its ocean. Here, as already described, serpentinization reactions continually occur. These generate heat, hydrogen, and methane—each important for life. At the same time, other minerals that are important as nutrients for life might be dissolved in the water, along with the various salts it contains. The ocean probably has existed longer than there have been conditions on Earth suitable for life as we know it. Thus, there really was plenty of time.
Of course, these organisms would have to survive without photosynthesis, as no light reaches the bottom of the ocean. They also would have to go without oxygen. Yet even here on Earth, researchers have already identified three habitats with similar conditions. One of them was found in the depths of a South African mine. It is based on radioactive decay energy and consists of bacteria-reducing sulfur. However, there is hardly any sulfur on Enceladus. The other two were found by scientists in volcanic rocks near hot springs deep below the ground. They are dominated by archaea. These organisms consume the hydrogen emerging here due to plate movements, and they burn it with carbon dioxide and generate the energy they need for living, as well as traceable byproduct amounts of methane and water. Together with bacteria and eukaryota, the archaea form the three domains of life, and they are the most ancient. Archaea are single-celled, and the DNA containing their genetic information is circular. They possess simple organs of movement (flagella) and sometimes build a kind of skeleton to stabilize their shape. They differ from bacteria in the structure of the ribosomal RNA, which is responsible for translating genetic information into proteins.
On Earth, archaea are often found under extreme conditions. Some varieties only flourish at temperatures above 80 degrees Celsius, while others prefer living in highly concentrated saline solutions, or very acidic or alkaline environments (pH value below 0 or above 10). Even the pressure at the bottom of the ocean, which measures between 2.8 and 4.5 MPa, should pose no problem. After all, there are microorganisms in the Mariana Trench which can withstand a pressure of 50 MPa. Even multi-celled organisms like Pseudoliparis amblystomopsis, a species of snailfish, can survive under these conditions.
Archaea can also perform amazing feats—they are among the fastest creatures on Earth, for instance. In the category ‘body lengths per second’ they achieve a value of 400 to 500. A cheetah only reaches 20, a human 11, and a horse 7. A sports car would have to drive at 6,000 km/h to rival the archaea. The reason archaea are so much faster than bacteria is that they have more flagella (50 versus 5 to 7), and they can also rotate these faster, as they possess a more efficient ‘motor.’ Humans use archaea, among other things, in biogas systems to generate methane.