Sending a probe to another star would be NASA’s greatest adventure, but how do we lay the groundwork for such a mission? The agency likes ‘roadmaps,’ spelling out clear and specific objectives and beginning with missions not so far beyond those we could fly today. NASA’s Interstellar Probe Science and Technology Definition Team (IPSTDT) recently prepared studies on a solar sail mission into nearby interstellar space, reaching approximately 400 AU from the Sun in 20 years of flight time. Think of it as a logical follow-on to the Voyager probes.

But Ralph McNutt and colleagues at Johns Hopkins’ Applied Physics Laboratory have been defining a more ambitious mission. As worked out in several recent papers, McNutt’s probe would approach the Sun to within 4 solar radii before a fifteen minute engine burn would establish its high-speed escape trajectory from the Solar System. At this point all acceleration would end; unlike the IPSTDT design, no sail would be deployed.

The McNutt mission follows the NASA roadmap idea: “If we are to take seriously the notion of interstellar travel toward the middle of the next century, and try to make it happen, then the best we can do is to rely on currently known physics,” McNutt writes, “which can still push the limits of the doable even under the most optimistic of technological advances.”

Getting a probe this far from the Sun is tricky, but the first problem comes in approaching it. Heat constraints are only part of the story. The probe’s angular momentum around the Sun has to be removed to allow the close approach; the team envisions sending it to Jupiter for a gravity ‘slingshot’ maneuver to kill the momentum. It would then fall in toward the solar furnace for the engine burn at perihelion.

Image: Epsilon Eridani. At 10.7 light years, it provides a useful target for early interstellar probe concepts, including tests of optical communications against the background of a stellar spectrum. Credit: European Southern Observatory.

While the IPSTDT design calls for a workable solar sail as its enabling technology, McNutt’s probe relies upon a carbon-carbon thermal shield and a propulsion system capable of high specific impulse (ISP). The baseline mission objective is 1000 AU from the Sun within the lifetime of a researcher, or 50 years. And as McNutt writes, the key is to develop a spacecraft capable of operating in long-term cruise mode for a voyage of this immensity. “With an emphasis on long-lived, self-healing architectures and redundancies that will extend the probe lifetime to well over a century, a long-lived probe could be queried at random over decades of otherwise hands-off operations.”

For propulsion, a scaled-down Orion approach (using nuclear explosions behind the craft) has been considered, as has solar heating of a gas propellant. Intriguingly, the probe would be launched toward Epsilon Eridani, a K-type star some 10.7 light years from Earth. Although not designed as a true star probe (not at these speeds, some 20 AU per year!), the probe would be able to test data downlink capabilities against the background of a stellar spectrum, something that will have to be done to provide communications for the true interstellar probes that will follow. For reasonable bandwidth at distances of 1000 AU or more, an optical communication system would be used.

McNutt sees this probe concept as a 65-year development program costing roughly $1 billion. And it would itself be a precursor mission for a more advanced mission. From a paper in Acta Astronautica:

At 200 AU/yr, such a second generation probe could make the first targeted interstellar crossing in ~3500 years, the approximate duration of the Egyptian Empire. A more robust propulsion system that enabled a similar trajectory toward higher declination stars such as Alpha Centauri could make the corresponding shorter crossing in a correspondingly shorter time of ~1400 years, the time that some buildings have been maintained, e.g. Hagia Sophia in Istanbul (Constantinople) and the Pantheon in Rome. Though far from ideal, the stars would be within our reach.

Centauri Dreams‘ take: the mind-boggling trip times cited for McNutt’s second generation probe are a reminder of something Geoffrey Landis once told me, apropos of science fictional starships that make the journey look easy: “A trip to the stars is going to be hard, and it’s going to take a long time.” As a culture, we have to debate just where mission duration fits into our concept of scientific inquiry. If it is the lifetime of a researcher, then we will need to reach speeds of 30,000 kilometers per second to consider a star mission. But McNutt, in sketching out a hypothetical mission measured in thousands of years, sets out a useful counter-limit. While a thousand year mission may never be launched, the guess here is that if the technology emerges to make a crossing measured in the low hundreds of years, such a probe will be built, its data handed from generation to generation as a gift to the human future.

The paper cited above is R.L. McNutt Jr., G.B. Andrews et al., “Low-cost interstellar probe,” Acta Astronautica 52 (2003), pp. 267-279. See also McNutt’s Phase I and II studies for NIAC, available here. And be aware of McNutt, Andrews, Gold et al., “A realistic interstellar explorer,” Advances in Space Research 34 (1): 192-197 2004. For more on the interstellar roadmap idea, see J.L. Anderson, “Roadmap to a star,” Acta Astronautica 44 (1999) 91-97.