lunes, 28 de febrero de 2011
What Would an Interstellar Mission Look Like?
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.
Vast Distances, Epic Timescales
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.
Seems like getting something for free, doesn't it? And we all know that the universe is notoriously stingy with freebies. However, this maneuver does not violate any laws of physics.
Note that the fuel in the vehicle is also in orbit around the massive body. If the vehicle burns some fuel while dropping to a lower orbit, then much more propellant can be burned at the lower orbit. The propellant is then left behind in a much lower (less energetic) orbit. Oberth realized that releasing the chemical or nuclear energy of a fuel was not actually using all of the propellant's available energy. The propellant also has mechanical (kinetic and potential) energy that can be released -- thus accelerating the vehicle -- by use of his maneuver.
To Jupiter, the Sun, then the Stars
By using Oberth's maneuver around the sun, we can anticipate that a vehicle that can achieve a velocity of 5 percent the speed of light will actually blast out of the solar system at our target 15 percent the speed of light. However there are still a number of issues to be investigated.
What is the spacecraft's maximum acceleration? How close can the spacecraft actually get to the sun without suffering dire radiation and heating effects? How many stages will the vehicle need to have? And attaining 5 percent the speed of light before Oberth's maneuver is still a very challenging task. Future articles will discuss the propulsion systems that have potential to reach these velocities.
A possible mission profile using the options mentioned above would start in Low Earth Orbit (LEO) where the Icarus probe is built. Using conventional propulsion systems such as liquid (chemical) rockets, the craft begins its journey to Jupiter. The chemical rockets will be jettisoned immediately after this Trans-Jovian Injection (TJI).
Jupiter's gravity will drag the Icarus probe into a new trajectory perpendicular to the plane of the solar system. Additionally this gravity assist will shorten the Icarus orbit so that it starts falling back towards the sun. A short burst from its fusion engines will tighten the probes orbit, so that years later the probe will fly deep inside the sun's atmosphere, the solar corona.
A set of fuel tanks will be jettisoned at Jupiter after the short burst. As the probe nears this closest approach to the sun it will start firing at maximum thrust for weeks, or months. The probe will have to use some of its propulsive force to steer towards the sun as it accelerates. The probe will swing around the sun while continuing to accelerate. Multiple sets of drop tanks will be dropped along the way. The thrusting will not stop until the probe achieves the target velocity.
The probe will continue to thrust in short bursts on its long journey to the target star. These bursts will make up any drag caused by interstellar gases and dust, and to make minor course corrections as needed. As the probe approaches the target star it will conduct the Oberth maneuver in reverse, thrusting at maximum as it swings around the target star. If possible, the probe will slow sufficiently to be captured around the target star, and extend the time the probe has to study the star system.
It is not yet clear if the Icarus team will be able to find the right combination of propulsive technologies and mission options that will enable the full acceleration and deceleration at the target star. And many of the mission elements described above will be traded against numerous other options.
The Icarus team is committed to find the mission profile that will allow humanity to take those first few steps out to the stars.