Project Icarus is an ambitious five-year study into launching an unmanned spacecraft to an interstellar destination. Headed by the Tau Zero Foundation and British Interplanetary Society, a non-profit group of scientists dedicated to interstellar spaceflight, Icarus is working to develop a spacecraft that can travel to a nearby star.
Andreas Tziolas, secondary propulsion lead for Project Icarus, overviews the general Icarus mission timeline, with a focus on the secondary propulsion techniques that will be required to support the fusion-propelled interstellar vehicle.
To appreciate the wide range of systems needed for in interstellar mission, let us consider a nominal profile for Icarus. The Icarus study is still underway, but we will take a bird's eye view of the mission, keeping in mind the importance of achieving its scientific objectives.
The spacecraft construction would necessarily take place in space. If constructed in low-Earth orbit (LEO), we would most likely need a high thrust chemical propulsion system, and ideally a reliable single-stage-to-orbit solution, such as SKYLON or perhaps a revised program for mass production of launch vehicles.
A large scale space-based infrastructure would then be needed, which would most likely use a wide range of solar powered systems.
This orbiting shipyard might look like a fabrication ring capable of moving around the spacecraft (pictured above), with a number of remote manipulators working on sections of it at a time.
The engineers may be operating the manipulators safely from Earth, working in shifts around the clock and from around the world. The sheer size of the orbiting shipyard would make it suitable for visits, effectively making the Icarus construction a tourist attraction:
"Hotel Icarus: All proceeds go towards building mankind's first interstellar starship."
The Daedalus starship required an enormous amount (50,000 tons) of a rare Helium isotope to power its fusion engines: Helium-3 -- extremely rare on Earth, but which can be found in the sun and gas giant planets.
The solar wind that has been splashing against our moon for millions of years, also contains Helium-3, and has been slowly depositing some quantities of it on its surface.
The most abundant source of Helium-3 however is the atmospheres of gas giants. The two best candidates are Jupiter which is the closest to Earth -- but has very powerful and tenuous gravitational field -- and Uranus -- with a slightly higher Helium-3 concentration, but is almost four times further away from Earth.
Team Icarus is actively researching fusion fuels and several interesting fuel acquisition methods. One option, also considered in detail by the Daedalus team, is the placement of atmospheric mining balloons in the atmospheres of gas giants, the designs for which may be informed from research into stratospheric balloons.
Another novel concept system being considered is based on a ramjet type aero-spacecraft, which partially uses the atmospheric gases for propulsion, and includes a forward gas collection scoop.
Yet another concept design involves an orbiting spacecraft with a very long hollow tether which is descended into the gas giant's upper atmosphere and effectively 'sucks' the atmosphere up and distils the Helium-3 in orbit. The tether could be used for power as well as propulsion in this case, because it can generate electrical current by moving through the magnetic field of the gas giant, through induction.
Interplanetary tankers would then carry the fuel back to LEO, perhaps using some of the Helium-3 for their own propulsion. An alternative would be to store the Helium-3 in interplanetary fuel depots orbiting the gas giant, where the Icarus could pick them up on its way out of the solar system.
Navigation and Trajectory Insertion
Attitude control and navigation are of particular importance inside our solar system, where the initial Icarus interstellar trajectory insertion would take place.
An orbital insertion trajectory called the Oberth maneuver, has the spacecraft dip towards the sun and then quickly power outwards, in a move reminiscent to an orbital slingshot. In this case however, the benefit comes from using up as much of the fuel as possible deep inside the gravitational field of the sun, which saves energy because that same amount doesn't need to be carried it up and out of the gravity well.
This move also brings the spacecraft very close to the sun, which gives us an opportunity to use a solar sail to either supplement the main engine, or to assist in trajectory insertion.
The Icarus may suffer malfunctions that would need to be repaired from time to time.
The Daedalus team decided to deal with on-board repairs by using a type of mobile repair robot, they called "Wardens," as they were made to survey and patrol the spacecraft.
If Team Icarus makes the decision to use something similar, these robots will also require a means of propulsion for locomotion. This would most likely be a form of electro-thermal pulsed plasma thruster, which is capable of micro-newton performance, giving the bots the ability to position themselves very accurately.
An alternative is to use a "repair rail" with robotic manipulators, inspired from the Canadarm which has served on board the Space Shuttle (Canadarm 1) and ISS (Canadarm 2) for almost 30 years ( without single event failure ).
While the Icarus is on route to the target star, there are still numerous tasks required of the secondary propulsion systems. The fuel tanks, for instance, would be dropped as they are depleted, after which trajectory corrections are to be expected.
Fuel Tanks as Relays
The use of these dropped fuel tanks as communications relays has been proposed to assist in maintaining a robust communications link with Earth. These parabolic, clam-shelled fuel tanks could split open to reveal two telecommunications dishes, one pointing at the Icarus, while the other back at Earth.
These relays would be, for the most part, free-flying with whatever velocity and direction they had when they were released. To allow them to point towards the Icarus and the Earth, they would require some accurate method of orientation. In this particular case, a Radioisotope Thermal Generator (RTG) powered flywheel fits the bill quite well, as both of these systems can be used for long term operation.
Deceleration Options on Arrival at Target Star
There is a current push in observational astronomy to determine if there are any Earth-like planets in our solar neighborhood.
The Icarus target definition has remained open for precisely this reason, where on discovery of an Earth-like planet within 15 light-years, it will almost certainly become our target star.
In order to carefully study the planets and moons of the target system, the Icarus would need to explore several deceleration options, which may allow the spacecraft to at least spend more time in the target system, if not fully decelerate.
We essentially have two alternatives. We can either decelerate the entire Icarus spacecraft mainly using reverse main engine thrust, or we can decelerate the individual probes using a combination of magsails, solar sails and perhaps nuclear rockets. This task is complicated by the fact that we would need to first survey the target solar system and acquire reliable orbital information for the planets or moons of interest.
What are then the requirements for maximizing the scientific data return from the probes, and what demands does this place on the probe propulsion systems?
Planetary Science Probes
Let's assume full deceleration at the target star has been achieved and planetary orbit information has been processed and assigned to a number of science probes.
By that time, near-Earth telescopes would be sufficiently advanced to verify and inform the Icarus computers on which scientific objectives are most desirable. As might be expected, there will be an incredible amount of "feature creep" at this stage, where scientists will be arguing over which objectives should take priority over others, while the probe power and propulsion systems last.
Essentially this means a solar powered propulsion system would be needed. Solar thermal engines require a reaction mass (usually Hydrogen) for propellant that, however, will have to be carried along and stored safely for the duration of the interstellar journey.
Solar electric engines could be used, especially if combined with a stable and inert ion source like teflon. The best overall option for a reliable and abundant propulsion method would appear to be the use of solar sails, particularly for planetary surveyors and orbiters with prolonged remote sensing agendas.
If we were to take it a step further, we might consider landers with rovers or even submarines. RTG power sources, combined with aerobreaking, parachutes and airbags require the least amount of propellant, but would probably only be chosen for use in relatively dense atmospheres.
Keeping these systems operational after 100 years of storage, sends everyone back to the drawing board, as the entire history of human spaceflight isn't that long. It is however possible with current engineering practices to construct and test many of these systems.
Perhaps testing these advanced systems in dual-role planetary exploration missions, as concurrent precursor and engineering proofing platforms for interstellar missions is the best way to both explore our solar system and prepare for interstellar exploration.