9 December 2003
N.Y. - As the United States considers new goals for NASA after the loss of the Columbia, some space enthusiasts have renewed calls for a mission to Mars.
But a team of physicists and biologists here at a laboratory on Long Island is demonstrating that even if the nation wanted to commit to such a goal, it would be far more complex than the Moon emission that gripped the country in the 60's.
One reason is radiation, in the form of heavy ions from distant stars, zipping through everything in their path. Others include price, estimated at $30 billion to $60 billion, and launching enough food, supplies and fuel for a round trip. Any one of these could make the project impractical.
In a new $34 million NASA laboratory here, part of Brookhaven National Laboratory, scientists are using subatomic particles accelerated to nearly the speed of light to slam into materials that could be used in a spaceship, and tissue samples and small animals. Using tools like PET and M.R.I. scans and DNA sequencing, they hope to shed light on ways that radiation damages biological tissue, and what can be done about it.
On a trip to Mars and back, probably every cell in the body would be hit by an ionized particle or a proton, researchers say, and they have very little idea what that would do. "If every neuron in your brain gets hit, do you come back being a blithering idiot, or not?" asked Dr. Derek I. Lowenstein, the chairman of Brookhaven's collider accelerator department.
A trip to Mars means "trying to live in an environment that human beings were not built to live in," Dr. Lowenstein said. "Space is not `Star Trek,' but the public certainly doesn't understand that."
On earth, radiation shielding is easy; just add concrete or lead. That is not so easy on a spaceship, where weight is of the essence. Nor is there much prospect of significantly reducing the amount of time the astronauts would be exposed, unless NASA develops a much more effective propulsion system.
The NASA administrator, Sean O'Keefe, has identified radiation as one of three problems that will have to be solved before a Mars mission. The others are better propulsion and on-board power generation.
Brookhaven is studying the radiation in a a sprinkling of undistinguished-looking corrugated metal buildings, connected by low earthen berms. "That's where the action is," said Mona Rowe, a spokeswoman. The berms are shields for tracks underneath that carry the accelerated particles that slammed into targets or one another. Above the berms, wild turkeys amble through the woods.
The radiation environment that the accelerator is mimicking is vastly different from the terrestrial one.
The average American receives about 350 millirem of radiation a year: the fraction of solar and cosmic radiation that makes it through Earth's magnetic field and atmosphere; radiation from naturally radioactive rocks and minerals, some incorporated into building materials; higher doses from flying in airplanes; and sources like medical X-rays.
In contrast, the astronauts who went to the Moon on Apollo 14 accumulated about 1,140 millirem, equivalent of about three years on Earth in their nine-day mission. The astronauts on the Skylab 4, who spent 87 days in low Earth orbit, received a dose of about 17,800 millirem (equivalent to a 50-year background dose on Earth).
That dose was near the threshold of radiation exposure that produces clinically measurable symptoms. Longer-term effects like increases in cancer rates have not been observed in adults exposed to doses at that level, but experts presume the effects exist. By comparison, nuclear power plant workers are limited by law to exposures no greater than 5,000 millirem a year; in this country they are generally held below 2,000.
A round trip to Mars would be of a different order of magnitude. Brookhaven puts the exposure at 130,000 millirem over two and a half years. That is equivalent to almost 400 years of natural exposure.
But radiation in space is not like radiation on Earth.
On Earth the dose is mostly made up of gamma rays, which have far less energy than the heavy charged particles of space. But beyond Earth's protective atmosphere and magnetic field, the radiation is mostly ions of every element on the periodic table up to iron (No. 26), moving at a substantial fraction of the speed of light, and approaching from distant stars in all directions. Astronauts in low Earth orbit get some protection from the magnetic field.
Much less is known about the biological effects of this radiation, because very few places can simulate the interplanetary radiation. Brookhaven can do it, but its method, sequentially firing ions of different elements, resembles playing a symphony by mimicking one instrument at a time.
One recent afternoon, scientists were adjusting the flow of iron ions being delivered to the 400-square-foot "target room" of the laboratory here, using a control a bit like a shower head, which could vary the dimensions and density of the spray. The target would eventually be a flask filled with human tissue, but for now was a monitoring instrument that captured an image the way an X-ray film would.
Dr. Adam Rusek, a physicist, shuttled between a control panel and the main room of the Space Radiation Laboratory, where Dr. Betsy Sutherland, a staff biologist and some assistants, were watching instruments that analyzed the beam.
Intermittently, an assistant went into the heavily shielded target room to adjust the target, a procedure that requires a retina scan by a security device and the insertion of special keys to assure that no one unauthorized enters.
Inside the room, the lighting dimmed before each initiation of the beam, so that anyone trapped inside could hit a panic switch to stop it.
At last, the beam assumed the desired size, density and uniformity. "Is that better?" Dr. Rusek asked. "Yes, don't breathe on it," Dr. Sutherland replied.
One persistent question about radiation exposure is the importance of the delivery rate, but Dr. Sutherland is simply trying to hit each cell once. "If a cell is hit once, there is no rate," she said. "Once is once."
After irradiation, the cells are moved to a nutrient medium that is known to support cancer cells but not normal cells.
The experiment is repeated with ions of several elements. Dr. Sutherland also uses protons, which come from the Sun and stars and far outnumber the ions.
One theory holds that cells busy repairing damage from protons will not be able to cope with damage from heavy ions; another says that proton irradiation will prime the cell's repair system to be ready for particle damage.
"It's a reasonable thing to ask, what are these first protons going to do to the later response to iron," said Dr. Sutherland, noting that the theory had not been tested.
Another Brookhaven scientist, Dr. Marcelo Vazquez, a physician, plans to irradiate mice to look for brain damage. Damage from heavy ions, he said, will include a column of cells formed by the track of the ion, and a surrounding halo of cells damaged by electrons.
Dr. Vazquez, who also has a doctorate in neurobiology and radiobiology, said that neither the column nor the penumbra was visible on post-mortem examination. But changes in motor skills are tested by stimulating animals with cocaine and measuring movement with infrared beams, Dr. Vazquez said. Memory can be observed. Mice are put in water and trained to escape to a platform; then they are irradiated and the drill is run again.
NASA's chief scientist, John M. Grunsfeld, who as an astronaut made several spacewalks to maintain the Hubble telescope, said the research would take years. "The current plan is about five years but I suspect we'll extend that," he said in an interview in Washington. He hopes that the research reveals the biological mechanism of radiation damage to cells, he added.
Also, some targets are structural materials. The incoming protons and ions have so much energy that they make neutrons peel off the aluminum or other materials; those neutrons are a potent form of radiation. In addition, irradiating some materials can cause changes that make them radioactive. Such "activation products," commonly produced in nuclear reactors on Earth, give off yet more radiation. Researchers hope they can pick materials that will resist such activation or neutron peeling.
A third area of research is shielding. On Earth, radiation shielding is commonly provided by concrete or lead, but the costs of launching spacecraft are so high that this is not practical. One possible solution is a water tank, with the astronauts' living in a chamber in the middle. "It's just so expensive to put material into orbit that you'd like to use materials you have to bring anyway," Dr. Lowenstein said.
And beyond the spaceship itself, making space safe for extended trips beyond the magnetosphere will probably require a new system to monitor the Sun.
Physicists predict solar storms now by watching the Sun from Earth or from satellites in Earth orbit, but protecting a Mars mission will probably require watching the side of the Sun that faces away from Earth. The job could be done with a small number of satellites launched into orbit around the Sun, somewhere outside the orbit of Mercury, Dr. Lowenstein.