VASIMR: hope or hype for Mars exploration?by Jeff Foust
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| Some concepts for VASIMR-powered missions, using 200 megawatts of power, allow transit times from Earth to Mars in as little as 39 days. |
Because of the concerns about the long travel time, there’s been ongoing interest in any system that could potentially allow crews to get to Mars and back more quickly. In recent years one technology in particular has emerged as a favorite: a concept called the Variable Specific Impulse Magnetoplasma Rocket, or VASIMR. The concept, developed by former NASA astronaut Franklin Chang-Diaz, uses a pair of radio wave antennas to ionize and heat a gas such as argon, which is then accelerated away from the engine along the magnetic field lines from superconducting magnets, creating thrust. The concept is not unlike other electric thrusters, although with the ability to vary its performance (hence the “variable specific impulse” in its name) and also operate at much higher power levels.
As with other electric thrusters, and unlike conventional chemical engines, VASIMR does not generate high amounts of thrust. However, it can operate continuously for days or weeks, turning that gentle acceleration into significant velocity. Some concepts for VASIMR-powered missions, using 200 megawatts of power, allow transit times from Earth to Mars in as little as 39 days, nearly five times faster than a conventionally-powered mission.
The VASIMR concept appears to have attracted the attention and interest of NASA’s leadership. On several occasions earlier this year, including speeches and testimony before Congress, administrator Charles Bolden mentioned “gamechanging” technologies that would allow missions to reach Mars “in days, not months”, at one point mentioning the work done by Chang-Diaz. Bolden’s interest appears motivated by concerns about the effects a long transit to and from Mars would have on the human body. “We don’t understand the radiation environment,” he said during a Senate hearing in February. “We don’t understand fully what happens to the human body in transiting for eight months.”
Chang-Diaz (who flew with Bolden on two shuttle missions, STS-61-C in 1986 and STS-60 in 1994) left NASA in 2005 to pursue work on VASIMR after the agency ended its support for VASIMR research. He established a new company, Ad Astra Rocket Company (AARC), with offices in Texas and Chang-Diaz’s native Costa Rica, to continue development of the propulsion concept with a mix of private and government support. Most recently, they’ve tested an experimental 200-kilowatt engine, the VX-200.
Speaking via videoconference at the NewSpace 2010 conference in Sunnyvale, California, in July, Chang-Diaz was focused more on near-term plans for VASIMR than for Mars exploration. The company is developing a flight-qualified version of the VX-200, designated the VF-200, and has a Space Act agreement with NASA to test the VF-200 on the ISS in mid-2014. Beyond that, Chang-Diaz said AARC was interested in developing a solar-powered space tug using a VASIMR engine. In the long run, he said, VASIMR “is something we need very badly in order to really be serious about space exploration with humans.”
One might expect that a technology that could shorten travel times to Mars and mitigate some of the corresponding concerns, and the support for such technology at the highest levels of NASA, would be warmly received by advocates of human Mars exploration. Yet, during a recent panel session on the topic at the Thirteenth Annual International Mars Society Convention in Dayton, Ohio, such advocates were skeptical at best about the potential of VASIMR to revolutionize Mars exploration, and at worst worried a focus on such technologies could actually delay human missions to the Red Planet.
| VASIMR “has become a tollbooth on the way to Mars,” Zubrin said. “This is being used by policymakers as a reason why we can’t go to Mars today.” |
Robert Terry, a physicist who worked for over two decades in the plasma physics division of the Naval Research Laboratory, expressed concern about “leaks and losses” in the VASIMR design that could reduce its effectiveness. He also pointed to a recent analysis that looked at how much usable payload could be sent to Mars as a function of both the specific impulse of the propulsion system and the mass fraction of the aeroshell that would capture the spacecraft in Mars orbit. As it turned out, improvements to the aeroshell’s mass fraction had a much bigger effect than increasing the specific impulse, even to the very high levels (10,000 to 30,000 seconds) of a system like VASIMR. “Even if VASIMR works,” he said, “there may not be a use for it in the context of Mars.”
Another concern is that for a Mars mission, VASIMR would have to use a nuclear power system that doesn’t exist yet. Mars Society president Robert Zubrin warned that mission designs that used VASIMR had unrealistic expectations about the mass of such reactors. The largest space nuclear power systems, the Topaz nuclear reactors developed by the former Soviet Union, generated 10 kilowatts and had a specific power, or alpha, of 100 kilograms per kilowatt. NASA had hoped to get alpha down to 65 kg/kW with its now-cancelled Prometheus program, and Zubrin said that if one is “quite optimistic” an alpha of 20 kg/kW was possible. The VASIMR-based Mars mission concepts, he said, assume an alpha of 1 kg/kW. “That’s like steel with the weight of Styrofoam,” Zubrin said. “It has no relationship with reality.”
Assuming an alpha of 20 kg/kW, Zubrin said, means that a reactor that generates 200 megawatts would weigh 4,000 tons. (By contrast, the VASIMR mission architectures with the 39-day travel times had assumed an overall mission mass of approximately 600 tons.) Moreover, the best travel time you could get with this much more massive system is six to eight months, comparable with conventional chemical propulsion systems, Zubrin claimed. “The numbers don’t add up,” he said.
A separate, and perhaps bigger, concern mentioned by Zubrin and others at the conference is that a system like VASIMR, regardless of its technical capabilities, could be perceived as being required for a human Mars mission in order to reduce the risk to the crew. “Rather than becoming a tool that might be of assistance in the future, a possible addition to our toolbox, it has become a tollbooth on the way to Mars,” Zubrin said. “This is being used by policymakers as a reason why we can’t go to Mars today.”
| “So the real question we have to ask with VASIMR is, where are we going and what do we want to do?” asked Landis. |
Zubrin said that the radiation concerns expressed by some were overblown: the exposure for crews in interplanetary space would be only twice what ISS crews experience, where the Earth serves as a shield blocking cosmic radiation from half of the sky. “This concern that we can’t go to Mars until we have this impossible propulsion system because we have to protect the crew from radiation is belied by the fact that we’re actually going to subject our crews to that much radiation over the coming decade [on the ISS] without going anywhere.”
The Mars Society conference panel was, in general, skeptical about the prospects of VASIMR, with no clear proponents of the system on the panel (AARC was invited to participate, one panelist said afterwards, but declined.) The closest thing to a positive voice on the panel was Geoffrey Landis, a scientist at NASA’s Glenn Research Center and a science fiction author. Even he was skeptical about the utility of VASIMR for human Mars missions. “It’s kind of overkill,” he said, referring to the development of both the propulsion system and its nuclear power source. “You can do it with just chemical propulsion.”
However, for human exploration beyond Mars, it’s a different story. “Maybe we want to colonize the entire solar system,” he offered. “In that case, we probably do want VASIMR, or something like it.”
“So the real question we have to ask with VASIMR is, where are we going and what do we want to do?” he asked. “Is Mars the end or is Mars the beginning of our exploration?”