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Zubrin Claims VASIMR is a Hoax

Artist rendering of the VASIMR powered spacecraft heading to Mars. Credit: Ad Astra

A next-generation plasma rocket being developed by former NASA astronaut Franklin Chang Diaz called the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) has been touted as a way to get astronauts to Mars in weeks rather than months, as well as an innovative, cheap way to re-boost the International Space Station. But in a biting commentary posted on Space News and the Mars Society website, “Mars Direct” advocate Robert Zubrin calls VASIMR a “hoax” saying the engine “is neither revolutionary nor particularly promising. Rather, it is just another addition to the family of electric thrusters, which convert electric power to jet thrust, but are markedly inferior to the ones we already have,” adding, “There is thus no basis whatsoever for believing in the feasibility of Chang Diaz’s fantasy power system.”

The VASIMR uses plasma as a propellant. A gas is ionized using radio waves entering into a plasma state. As ions the plasma can be directed and accelerated by a magnetic field to create specific thrust. The purported advantage of the VASIMR lies in its ability to change from high impulse to low impulse thrust as needed, making it an ideal candidate for a mission beyond low Earth orbit.

Chang Diaz’ company, the Ad Astra Rocket Company successfully tested the VASIMR VX-200 plasma engine in 2009. It ran at 201 kilowatts in a vacuum chamber, passing the 200-kilowatt mark for the first time. “It’s the most powerful plasma rocket in the world right now,” said Chang-Diaz at the time. Ad Astra has signed a Space Act agreement with NASA to test a 200-kilowatt VASIMR engine on the International Space Station, reportedly in 2013.

The tests would provide periodic boosts to the space station, which gradually drops in altitude due to atmospheric drag. ISS boosts are currently provided by spacecraft with conventional thrusters, which consume about 7.5 tons of propellant per year. By cutting this amount down to 0.3 tons, Chang-Diaz estimates that VASIMR could save NASA millions of dollars per year.

For the engine to enable trips to Mars in a reported 39 days, a 10- to 20-megawatt VASIMR engine ion engine would need to be coupled with nuclear power to dramatically shorten human transit times between planets.

Robert Zubrin. Credit: The Mars Society

Zubrin is the president of the Mars Society and author of the book “The Case for Mars: The Plan to Settle the Red Planet and Why We Must.” He has long touted the “Mars Direct” approach of getting humans to Mars to create a sustainable human settlement. The plan includes a series of unmanned and human flights to Mars using existing technology, as well as “living off the land” on Mars by creating rocket fuel to return to Earth, and using underground reservoirs of water on Mars.

In his commentary on VASIMR, Zubrin says, “existing ion thrusters routinely achieve 70 percent efficiency and have operated successfully both on the test stand and in space for thousands of hours. In contrast, after 30 years of research, the VASIMR has only obtained about 50 percent efficiency in test stand burns of a few seconds’ duration.”

On the ‘39 days to Mars’ claim, Zubrin says VASIMR would need to couple with a nuclear reactor system with a power of 200,000 kilowatts and a power-to-mass ratio of 1,000 watts per kilogram, while the largest space nuclear reactor ever built, the Soviet Topaz, had a power of 10 kilowatts and a power-to-mass ratio of 10 watts per kilogram.

Zubrin has invited Chang Diaz to a formal public debate the VASIMR at a Mars Society convention in Dallas next month.

Read Zubrin’s commentary on Space News or the Mars Society website.

More info: Ad Astra Rocket Company


Nancy Atkinson is Universe Today's Senior Editor. She also works with Astronomy Cast, and is a NASA/JPL Solar System Ambassador.

Comments on this entry are closed.

  • Torbjörn Larsson July 15, 2011, 8:16 PM

    Whoa! Where did that come from? I see in the comments that Zubrin’s argument is at least one year old; but the technology and the NASA push for new technology (including nuclear reactors) is older than that.

    Sadly it is fairly useless to try to model an individual’s behavior such as strategy as it rarely can be tested. Instead I can note that I would have been more keen to listen if this had come from someone else than a passionate advocate for the sufficiency of (mostly) traditional technology. (Say a person for which the goal is primary.)

    Is VASIMR a hoax? No, it works demonstratively. Is VASIMR for interplanetary crafts a hoax? No, it is unclear if it can achieve that market, and I believe the project has been upfront with this as it is delivering demonstrators instead of dreams.

    The rest is politics, which can be deferred to the hoaxers … excuse me, politicians, and the vote box. And some pushers and doers. Zubrin is mostly a pusher, Diaz is mostly a doer. Frankly I prefer the later, even if both have their place.

    Also, it is clear that Zubrin is comparing apples with pears:

    – Efficiency should be compared for mature and comparable technology. In VASIMR’s case, chemical rockets (CR).

    I think CR can but dream of achieving ~ 50 % efficiency, isn’t it more like ~ 10 %!?

    – Power source should be compared with similar high mass long duration missions. There NASA has routinely proposed nuclear power.

    – The radiation analysis is interesting if true. However, my impression was that the risk assessment discussed was a newer result of research!?

    I don’t know if Zubrin, an “aerospace engineer and author” has actually been involved in this research. Maybe he is the hoax.

    – Aside from the question of radiation, still unsolved in Zubrin’s approach is:

    1) Landing manned craft on Mars, a problem that VASIMR isn’t set up to solve nor has an intended market that revolves around it as Zubrin’s advoccacy is.

    2) Good enough closure of biosphere environments. Today’s ~ 70 % is claimed to be insufficient for Mars trips, VASIMR or not. IIRC ~ 90 % is the necessary target.

    At the end of the day, manned space is more than “the case for Mars”.

  • Torbjörn Larsson July 15, 2011, 8:43 PM


    “The argument that we must go much faster to avoid cosmic rays is demonstrably false, as proven not only by standard radiation risk analysis — which estimates about a 1 percent cancer risk for the 50 rem dose that astronauts would receive on a Mars round trip — but by the fact that about a dozen astronauts and cosmonauts have already received such a cumulative cosmic ray dose during repeated flights on the international space station or Mir, and, as expected, none of them have evidenced any radiological health effects.”

    This is from the Wikipedia entry on radiation health threats during space missions:

    “Average exposure on the ISS is a rate of 150 mSv per year, though crew rotations are shorter than that.[10] Astronauts on Apollo and Skylab missions received on average 1.2 mSv/day and 1.4 mSv/day respectively.[10]”

    [10 is a NASA ref.]

    A day on ISS averages ~ 0.4 mSv, a day on Moon missions ~ 3 times that. Zubrin isn’t far off, so his space angle model with Earth as shield may be correct.

    Achieving 50 rem = 0.5 Sv, which is subclinical, takes ~ 3.5 years on ISS with the average dose and ~ 1.1 year on a Mars mission.

    Presumably Zubrin’s Mars missions of ~ 2 years do have a substantial radiation problem. If a crew rotation of ~ 0.5 year accumulates 50 rem from episodic events such as CMEs during 1-2 stays, it would mean 300 ~ 600 rem on a Mars mission.

    “Doses of 200 to 1,000 rem will probably cause serious illness with poor outlook at the upper end of the range.”

  • Chris Eiffel July 16, 2011, 12:12 AM

    There are some serious issues with the power required for the VASMIR. It’s worth researching and but it’s technology readiness is too low for anything soon. Elon Musk has my bet for getting there first.