Posted November 13, 2000
A new type of rocket technology being developed at the Advanced Space Propulsion Laboratory has the potential to cut the time needed to take humans to Mars in half! Jennifer Laing looks at this advance in space propulsion and talks to NASA Astronaut Dr. Franklin Chang Díaz, Director of the ASPL, about the benefits of this technology for long duration space flights.
"It represents a quantum leap in space transportation," says NASA astronaut, Dr. Franklin Chang Díaz. He's talking about the plasma rocket, or to be precise the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) currently being developed by the Advanced Space Propulsion Laboratory (ASPL) at Johnson Space Center, Houston. If we are going to send men to Mars, Díaz believes that existing chemical rockets will be inadequate. "It would take far too long to go to Mars. New technologies such as the plasma rocket are the only way to maintain serious exploration of space with humans."
Current rocket technology would result in a journey to Mars of about 8-10 months. In the absence of countermeasures, astronauts would be exposed to microgravity and radiation in space during this long flight, with health risks such as bone loss, muscle atrophy, weakened immune systems and increased cancer risk on the cards. A shorter flight would be less hazardous and easier for the crew to cope with, both from a physical and psychological standpoint. A journey of about four months would be similar to the length of time U.S. astronauts have spent on the Russian Space Station Mir and the International Space Station (ISS). That's where plasma rockets come in. They could potentially shorten a trip to Mars to about 115 days - and also offer the advantage of a variable thrust, which would allow an emergency abort if conditions warrant it.
The VASIMR could also transport a robotic cargo mission to Mars as a precursor to a manned spacecraft mission. This journey could be made at slower speeds and thus would be longer than the 115 day mission desirable for a manned expedition, with the advantage of then being able to transport a larger payload. The manned spacecraft could then follow at a higher velocity and carrying a smaller payload.
Chang Díaz, Director of the ASPL, is a man of many talents. He has a number of challenging missions under his belt since becoming an astronaut, such as taking part in the final shuttle-Mir docking. In a footnote to history, he was spared the fate of the Challenger crew in 1986 as a result of his team being moved up one flight in a last minute re-shuffle. Chang Díaz has a Ph.D. in plasma research and has been working on these rockets since 1979. So how exactly do they work?
Plasma is a state which is achieved when a gas is heated up to extremely high temperatures. Electrons are stripped or lost from the neutral atoms, resulting in an electrically neutral 'soup' of charged particles. Because of the extreme temperatures required by the plasma rocket, no material yet known to man would be able to contain the hot plasma. It can however be partially contained by a magnetic field, and the VASIMR uses three magnetic cells for this purpose. Experiments in the laboratory are done in a vacuum chamber to simulate the vacuum of space.
The neutral gas propellant (normally hydrogen or helium) is injected into the first magnetic cell (forward-end cell) and plasma is created through the action of helicon waves via a radio antenna. The second cell (central cell) increases the energy of the plasma by heating it using electromagnetic waves, and the third (aft-cell) ensures that the plasma is detached from the magnetic field as thrust from the magnetic nozzle.
One of the beauties of VASIMR is its ability to vary the plasma exhaust to maintain optimal propulsive efficiency. Much like the transmission of your car, which uses the power of the engine to optimise performance, VASIMR can adapt to the specific conditions of the flight. Says Chang Díaz, "It's like changing gears on your car. By choking the flow, you can go into a higher gear." Using a Mars mission as an example, the rocket can "climb on first gear for the first 30 days", using maximum thrust, then shift to a "higher gear" 85 days into the flight, before decelerating when reaching Mars.
There are added benefits to using a propellant such as hydrogen. Storage of the liquid hydrogen propellant in locations around the spacecraft will help keep the crew shielded from radiation in space, as well as being able to be used as coolant for high-temperature superconducting magnets. Hydrogen is plentiful throughout the solar system, allowing supplies to be generated on Mars for the return journey.
What about power requirements? Most space architecture is power hungry or as Chang Díaz likes to say, "In space, power is life." The VASIMR rocket will require about 10 megawatts of power for a Mars mission, as well as additional power requirements for the crew in the spacecraft i.e. heating, light, and use of onboard electrical equipment. According to Chang Díaz, solar arrays of the size needed to generate sufficient power so far away from the Sun would not be practical. It is estimated that they would need to be about 24 times the size of the solar panels on the ISS to be up to the task! He argues that the only viable solution is to use nuclear reactors to generate the power needed for both transportation and life support. Chang Díaz calls nuclear power "one of the most controversial aspects we need to resolve before we go to Mars." He proposes the use of uranium reactors, which he feels are "safer and more powerful" than using generators powered by the decay of plutonium. For safety reasons, the reactors and the fuel could be launched in separate missions and assembled "robotically" in "a nuclear safe orbit".
The first test for the VASIMR in space is proposed to take place in 2004. A small solar powered spacecraft will be "lofted" above the Earth using the new rocket technology, and will study the Van Allen radiation belts. A small prototype will also be placed on the ISS to try to eliminate the drag of the atmosphere, "neutralising the only unbalanced force on the Station." "The ISS is a unique laboratory," says Chang Díaz, as there is no need to create an artificial vacuum" for the VASIMR rocket. The unit created will be small enough to pass through the hatch for maintenance. But what about plans for Mars? Well that's the sixty-four million dollar question. Chang Díaz explains that the optimum journey to Mars occurs every 11 years, when the trip is shortest and cheapest due to the alignment of the planets. "This is the one we want to shoot for!" This puts one possible target date for using this technology on a mission to Mars at 2018. At the moment, this timeframe is purely speculative and much will depend on the outcome of the research and testing programs and funding levels.
The new plasma rocket technology could make manned space flight to Mars faster, safer and more cost-efficient. It could even make missions to Europa less than one year in duration! It's an exciting development and one worth watching over the next few years. Zubrin in his book The Case for Mars says that fusion propulsion "...offers totally new and dramatically superior possibilities over any lower technology." Work done by the ASPL could make that first step on Mars closer than you think.
Jennifer Laing is a freelance writer from Melbourne, Australia.