Project Icarus is an ambitious five-year study into launching an unmanned spacecraft to an interstellar destination. Headed by the Tau Zero Foundation, a non-profit group of scientists dedicated to interstellar spaceflight, Icarus is working to develop a spacecraft that can travel to a nearby star. Dr. Robert Adams, study lead in the Advanced Concepts Office at the NASA Marshall Space Flight Center and Lead Designer for the Mission Analysis and Performance Module for Project Icarus, explains the realities behind getting an interstellar spacecraft to its destination.
One critical feature of any interstellar voyage is the "mission analysis," which organizes science objectives, plots movement through space and determines abort and correction scenarios. Some refer to this effort as "trajectory analysis" or "astrodynamics" but it encompasses more than simply plotting a path through the heavens.
Complex missions, like the one the Icarus team is tackling, involve integration of multiple spacecraft stages, gravity assists around planets and stars, correctional maneuvers, and other tools of the trade.
The mission analyst must juggle all of these options to find the most efficient path to the target, while balancing the cost and risk needs of the mission.
Clearly, the major challenge that one needs to overcome for any interstellar mission is the vast distances that separate the stars. Consider, for example, the nearest star system to the sun: Alpha Centauri. At 4.4 light years away, Alpha Centauri is a binary system, stars A and B revolving about a central point and a third star, Proxima Centauri, which revolves around the other two.
Using current technology, a one-way trip to the Alpha Centauri system would take approximately 75,000 years. To put this in perspective, that's about one hundred and fifty times the amount of time that has passed since Columbus discovered America.
Now let's consider how fast we'd like to get there. Ideally, a very long-duration mission should be a maximum trip time of 50 years. A young man or woman can join the mission team immediately after leaving college and still be alive and nearing the end of their career when the probe reaches its target.
Having someone carrying the torch throughout insures some continuity to the project. Because Icarus is tasked with designing an interstellar mission that would reach the target solar system in under one hundred years, this fact in itself raises yet more remarkable challenges — specifically the creation of an organization that could endure for that length of time.
With a maximum 100 year mission time, our vehicle must achieve a velocity of roughly 0.1 c (10 percent of the speed of light) to reach even the closest stars in this time frame. This is about a thousand times faster than any craft ever built.
For convenience, let's give ourselves a maximum top speed of 0.15 c so that we can put several target star systems in play. Achieving a velocity of 15% the speed of light is daunting, but not impossible using technology we have access to today.
In future Project Icarus articles, the team will discuss various propulsion schemes in more detail. Also to come are the challenges in protecting the vehicle as it continuously slams into interstellar dust at extreme velocities. In this article, we will focus our attention on the most efficient trajectory to reach our target velocity.
Finally, we must strongly consider deceleration methods when we reach our target star, since this is one of the objectives of the Icarus mission. At 15 percent the speed of light, a spacecraft would buzz through a typical solar system in a matter of hours. A 50 year trip that culminates in less than a day's observation and acquisition of data is… unsatisfying. So if possible, we'd like to find a way to slow down the spacecraft and stay a while.
There are a few tools in the bag that we pull out for any difficult space mission. Staging our vehicle allows us to cast off mass like tanks and engines when they're no longer needed. Gravity assists also allow us to gain velocity by making close passes to planets in our solar system. We take a little mission time to fly up on a planet, say Jupiter, and take on it like a NASCAR driver "slingshotting" around a competitor. This method of accelerating space vehicles has been used since 1959 when the Soviet probe Luna 3 photographed the far side of the moon.
However, our usual tricks are useful but are not sufficient to meet the challenges of the Icarus mission because of the incredible velocities we need to achieve. Here we need to bring back a very old and largely forgotten maneuver.
First described by Hermann Oberth in 1927, the two-burn escape maneuver can be very effective for this mission. Consider a spacecraft orbiting a much more massive body like the sun. Oberth described how the spacecraft could reverse its thrust, actually decelerating and slowing down, to drop closer to the massive body.
When the spacecraft reaches the closest point to the body it flips around and makes a "hard burn" to accelerate as much as possible. Calculations by the author show that such a maneuver can achieve 2-3 times the velocity possible without the maneuver. It is important to appreciate that this is very different to the famous 'gravity assist' maneuver.