Ralph McNutt’s contributions to interstellar mission studies are long-term and ongoing. We’ve looked at the Innovative Interstellar Explorer concept he has been studying at the Applied Physics Laboratory (Johns Hopkins), but IIE itself rose out of earlier design studies for a spacecraft that would penetrate the heliopause to reach true interstellar space. One possibility for that earlier probe was a ‘Sun-diver’ maneuver, a close pass by the Sun to gain a gravitational slingshot effect, followed by an additional kick from an onboard booster.
The thinking a few years back was to reach 1000 AU in less than fifty years, but Innovative Interstellar Explorer has lost the Sun-diver maneuver and focuses on a more realistic 200 AU, as part of a NASA Vision Mission study that contemplates a gravitational assist at Jupiter and the use of radioisotope electric propulsion. IIE is subject to the same funding constraints as any other mission of this nature but it’s well worth perusing its specs on the site, for McNutt is both scientist and visionary, a man who looks beyond the ‘lifetime of a researcher’ limit for mission duration.
That has taken him into interesting intellectual terrain, writing in a study for NASA’s now defunct Institute for Advanced Concepts of a future technology that could reach speeds of 200 AU per year. That’s fast enough to get you to Epsilon Eridani in 3500 years, approximately the lifetime of the Egyptian empire. Writes McNutt:
“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 Constantinople and the Pantheon in Rome. Though far from ideal, the stars would be within our reach.”
Human Expeditions to the Gas Giants and Beyond
Given these musings, where does McNutt stand on human exploration of the Solar System itself? We learn the answer in an interesting piece that has just appeared in the Johns Hopkins APL Technical Digest where, writing with Jerry Horsewood and Douglas Fiehler, he notes the sharp constraint that radiation exposure places upon mission designers. We know we can reach the outer Solar System — our unmanned probes continue to demonstrate the capability — but humans in deep space have to cope with solar energetic particles from the Sun (SEPs) and galactic cosmic rays (GCRs). That means getting to the destination quickly.
The article looks at optimized trajectories to Callisto, Enceladus, Miranda, Triton and Pluto, five expeditions that each demand one-way flight times of no more than two years, with a total mission time of five years. Solar energetic particles can be shielded against, but running the numbers on galactic cosmic rays shows they would require a huge mass penalty for shielding. To approximate the shielding effect of the Earth’s atmosphere would involve a shield massing thousands of tons. Limiting flight times seems the only solution.
To make this happen, McNutt envisions a nuclear electric propulsion system with an overall power level of 100 MWe, with the electricity generated by the nuclear reactor being used to power up the plasma stream that propels the vehicle. The Neptune mission, targeted for a 2075 launch, would achieve 197.5 kilometers per second with a thrust time of 1.2 years — compare that to the 16.2 kilometers per second New Horizons is currently managing on its trajectory to Pluto/Charon. And the trajectories of these five fast missions are themselves interesting:
The striking point for all of these trajectories, and especially for the three outermost targets, is the lack of curvature. To date, planetary transfer trajectories make use of near-Hohmann-transfer orbits (minimum-energy solutions), albeit sometimes with intermediate planetary gravity assists. Propulsive maneuvers typically are used for gravitational capture at the target, rather than slowing down from faster-than-required transfer orbits. The “straight” trajectories are driven by the requirement of a fixed transit time; without the interplanetary deceleration period before reaching the target planet, the spacecraft in each case would escape from the solar system.
Demands of the Journey
It’s the radiation constraint that pushes our propulsion technologies well past current capabilities, shortening acceptable trip times and demanding speeds that in our current context are almost surreal. Back in 1968, Clarke and Kubrick’s 2001: A Space Odyssey sent the ‘Discovery 1’ mission to Jupiter without evident regard for radiation shielding, and young optimists like me in the audience assumed that the outer planets would be within reach some time in the early 21st Century. Now we’re talking about putting together a set of missions that vaguely resemble Clarke and Kubrick’s a century later than the film had supposed.
Interestingly, by McNutt’s calculations, these expeditions would be mounted in a vehicle offering a habitable volume about twice that of the spaceship in 2001 if we assume a crew of ten (a crew of six is also considered in the paper). And if 2001 didn’t concern itself with enroute radiation, another thing it didn’t dwell on was the method for constructing the interplanetary craft. To build such a vehicle, we’ll need something like the extremely heavy lift launch vehicles (EHLLVs), or ‘Supernovas,’ that were originally studied in the 1960s. McNutt discusses lifting a thousand tons to low-Earth orbit with each launch for assembly of the outer system spacecraft in space. The study envisions 30 Supernova launches for the five expeditions.
Costs of an International Venture
All of this adds up to huge costs, some $4 trillion, which compares to a US GDP of $13 trillion in 2006 and a world GDP in the same year of $48 trillion. The five expeditions to the outer planets would clearly demand an international initiative, one that would cost 1.5 times the U.S. cost of World War II in 2006 dollars. From the study’s summary:
A 5-year round-trip mission will require ~10 t per person of expendable supplies with a likely crew of at least six people and an extremely reliable vehicle with an extremely dedicated and stable crew. Infrastructure capable of putting tens of thousands of metric tons of materials into LEO will be required as well. Such a project is potentially achievable at the cost of at least 10% of the current world GDP. With current investment in human space activity in the United States, even with growth projected on the basis of the growth of the overall U.S. economy, a dedicated, international effort will likely be required if the entire solar system is to have an initial reconnaissance by human crews by the beginning of the 22nd century.
Getting a human presence to the outer planets by the end of the century is going to be tough even if we assume the propulsion advances that can achieve 200 kilometers per second — or in the case of Pluto/Charon, over 300 kilometers per second. But this is exactly the kind of study we need to place our current technology in context. We can’t assume anything about future breakthroughs. We can only define the problems we face so that in that context, future work may produce solutions that can lower travel times and costs to acceptable levels.
The report is McNutt, Horsewood and Fiehler, “Human Missions Throughout the Solar System: Requirements and Implementations,” available online. McNutt’s Phase I and II studies for the NASA Institute for Advanced Concepts are still available on the NIAC site.