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.
Rob Swinney, member of the British Interplanetary Society and Project Icarus, investigates the realities of navigating through interstellar space with a vehicle traveling at 12 percent the speed of light.
You can imagine the scene: "Helm, come to heading of 250 Mark 3.5," says Captain James T. Kirk, and the USS Enterprise warps-off into the distance on another Star Trek adventure.
Now, what "250 Mark 3.5" actually means is far from certain, but if this fictional future is to be believed, navigating around the Galaxy is simple! But what will it really be like in the future when we finally head off to navigate our way around interstellar space?
That's the question the investigators of Project Daedalus asked themselves in the 1970s when they designed the Daedalus probe to travel to Barnard's Star some six light-years away.
Remember, those were the days before GPS and most of the team members would have been familiar with the old fashioned method of navigating "by the stars." Besides, the effect of the daily rotation of the Earth, the fact that the stars are pretty much fixed in their positions in the sky, and given an accurate time piece, they can provide great positional information.
But with the Daedalus starship heading off toward Barnard's Star, knowing your position as you speed across the interstellar medium could be tricky. Due to the "parallax effect," the nearest stars will appear to move compared to the far distance and can no longer be used as fixed points to reckon your position.
One of the members of Project Daedalus, Geoff Richards, in his paper "Project Daedalus: The Navigation Problem," worked out the equations required to correct for the interstellar probes changing position along with other corrections that would be required due to the high cruise speed (the Daedalus probe design's top speed was 12 percent the speed of light).
These corrections will be incorporated into any future interstellar spacecraft, including Project Icarus.
In his paper, Geoff highlighted another issue with Daedalus being a flyby probe; in the 1970s, there was a serious problem with the known distance of Barnard's Star, and the other local stars too -- distance precision was barely accurate to 10 percent!
Beyond the initial boost phase through interstellar space, the Daedalus starship planned to have a mid-course correction to make sure the probe was on target. As it neared the Barnard's Star system, some final course manoeuvres would be needed to deliver the sub-probes to their final trajectory toward any supposed planets for study. Geoff, even in the 1970s, was convinced that this could be achieved with nothing more than the normal development of technology.
That immediately prompts us to ask what has happened over the last 30 years, and therefore, what are the implications for Project Icarus?
Certainly, the known positions of the local stars, with modern astrometric measurements, are now known to the accuracy and precision that was required by Daedalus, so their planned course corrections would have been achievable.
Nonetheless, the likely mission plan for Icarus (at this point in the project is still open for discussion) is likely to include more regular course corrections as it is hoped the craft, in some way, will be decelerated into stellar orbit at the target system. The modern guidance and navigation equipment of star trackers and attitude sensors etc., flight-proven over the last 40 years or so, is more than up to the task.
But if Icarus is to decelerate, then the demands of navigation will change from simply zeroing in on the target for a flyby like Daedalus to automatically inserting in to orbit. After all, the Icarus probe will be over four light-years away and completely under its own control -- it will be a basic mission requirement that the probe will carry out these tasks automatically.
This may still be challenging in the future, but many commentators suggest that advanced observations will provide the orbital parameters of the target stars' planetary bodies. Whether this will be enough to allow some prior planning to help Icarus is debatable.
If the thought of full autonomy for stellar orbital insertion starts to get the designers a little nervous, then what about what happens next? Presumably the main probe will disperse the sub-probes that will have to navigate their way -- again, automatically -- to their individual targets, which might include planetary orbital insertion or landing in unknown environments.
As a fundamental part of Project Icarus, the basic navigation system may not be a critical system for showing that interstellar travel is possible, whereas the propulsion system will be.
Navigation, Guidance and Communication
But like all the other systems, navigation systems will need to work for up to 100 years of the planned mission, dealing not only with the inherent possibilities of a system or component failure but also the adverse conditions of outer space. Current components of guidance and control systems have typical lifetimes of around 10 to 15 years, so like Daedalus that incorporated repair "wardens" and a truckload of spare parts, Icarus will have to come up with a plan to solve this issue.
A possible mission architecture under discussion is the idea of using multiple probes dispersed in to a constellation for the interstellar transit, rather than cruising to the target as a single stage (as planned with the second stage Daedalus craft).
When arriving at the target system we'd need multiple probes anyway, so why not go prepared? One of the key advantages of this "constellation mode" configuration is that the sub-stages are no longer susceptible to a single catastrophic failure that could have destroyed the entire Daedalus probe. If needed for deceleration at the target system, the sub-stages could reform in to one unit to decelerate perhaps using their combined engines.
If Icarus were to travel in constellation mode then there is also the opportunity (in the timescales of an Icarus mission) of using the platform for an interferometry network (much like the ground-based interferometers in operation today) and enhancing any astronomical observations. Unfortunately, that'll be more sleepless nights for the Icarus designers!
Another complication for the control of the interstellar probe is the pointing accuracy required to send your communications back to Earth. Radio communications by re-using the 40 meter second stage reaction chamber as an antenna was the method of choice for Daedalus but some research suggests that optical laser communications may be the way to go for Icarus to increase the amount of data that can be transmitted.