For a test flight, astronauts steer the vehicle into an orbit that almost always remains above the south pole of the moon. From there they could control a fleet of robotic explorers and investigate the composition of ancient ice deposits in the forever-dark craters of the Aitken basin. Such a mission puts long-duration exploration through its paces with the safety of Earth just a few days away. After the crew returns to Earth, the deep-space vehicle remains in high Earth orbit, awaiting refueling and refurbishment for its first asteroid mission.
We have investigated a wide range of such missions. Some would take astronauts to small objects (less than 100 meters across) just beyond the moon and back to Earth in under six months. Others would venture to large objects (bigger than a kilometer) almost out to Mars and back in two years. Focusing only on an easier mission could stunt exploration by setting a dead end for technological capability. Conversely, striving for a harder mission could perpetually delay any meaningful exploration by setting targets too far out of reach. Our design baseline falls between these two extremes. It is a one-year round-trip that launches in 2024, with 30 days spent exploring asteroid 2008 EV5. This object, about 400 meters across, appears to be a type of asteroid of great interest to many planetary scientists—a type C carbonaceous asteroid, a possible relic from the formation of the solar system and perhaps representative of the original source of Earth’s organic material.
The most efficient way to get there is to use Earth’s gravity for an old trick known as the Oberth effect. It is the reverse of the orbit-insertion maneuvers that robotic space probes routinely undertake. To prepare for it, mission controllers outfit the deep-space vehicle with a high-thrust chemical rocket stage, carried up from Earth by an electrically propelled resupply tug. After the stage is attached and the crew is onboard, the deep-space vehicle free-falls from the vicinity of the moon down to just above Earth’s atmosphere to build up tremendous speed. Then, at just the right moment, the high-thrust stage fires, and the vehicle frees itself from Earth’s grasp in a matter of minutes. This maneuver works best at the moment when the vehicle is traveling at top speed near Earth because the amount of energy the ship gains is proportional to how fast it is already traveling. The Oberth effect is an exception to the rule that ion drives are more efficient than chemical rockets; you need a lot of thrust, quickly, to take full advantage of the gravitational kick start from Earth, and only high-thrust rockets can provide it. Together the ion-propelled spiral and chemical-powered Oberth effect cut the amount of fuel it takes to escape Earth’s gravity by 40 percent compared with an all-chemical system.
Once the astronauts escape Earth, the Hall effect thrusters turn on and steadily push the vehicle toward its destination. Because ion drive provides continuous thrust, it lends itself to flexibility. Mission planners can develop a robust set of abort trajectories should a malfunction occur at any point in the mission. (The Japanese robotic asteroid mission Hayabusa was able to recover from several mishaps because of its ion drive.) If technical or budgetary problems prevent us from getting the deep-space vehicle ready in time to reach the asteroid 2008 EV5, we can choose another target. Likewise, if we encounter technical difficulties, we will improvise. For instance, if high-performance propellants are too hard to store in deep space, we can switch to lower-performing propellants and revise the mission accordingly. Nothing in the mission is locked in.
The Pluses of Pods
In our plan, the astronauts have a month at the asteroid for exploration. Rather than donning space suits, they can take a lesson from deep-sea submersibles and use exploration pods. Space suits are basically big balloons, and an astronaut constantly fights air pressure for every little movement, making space walks hard work and limiting what can be accomplished. A pod with robotic manipulator arms not only alleviates this problem but also provides room to eat and rest. In a pod, an astronaut could zip around for several days at a time. NASA is already developing a Space Exploration Vehicle (SEV) that can be used as a pod at asteroids, and the same design could later be adapted for a surface rover for the moon and Mars.