The loss of the shuttle 'Columbia' was a big blow for Dublin Institute for Advanced Studies, which has worked with NASA for more than 30 years, writes Dick Ahlstrom
The break-up of the space shuttle Columbia 12 days ago had special poignancy for two physicists at Dublin Institute for Advanced Studies. They are researching astronaut safety and had experiments on board two recent shuttle missions.
They are assessing the long-term risk faced by astronauts exposed to space radiation. The key to this is getting detailed information about the radiation they absorb on a typical 15-day flight, something that has not been done before, according to Denis O'Sullivan, professor of astrophysics at the institute.
Flight crews on board the shuttles and the International Space Station are daily bombarded by incoming radiation. They absorb highly energetic cosmic rays from deep space, Van Allen belt radiation and particles streaming away from the sun on the solar wind.
O'Sullivan is an expert in these forms of radiation. He leads an EU project assessing aircraft flight crews' exposure to radiation. He also has more than 30 years' involvement with space programmes run by the shuttle's operator, the National Aeronautics and Space Administration. "I worked on lunar samples at [the University of California at\] Berkeley in 1969 from the Apollo 11 and 12 missions," he says.
"We have had a connection [at the institute\] with them since the Apollo 16 mission to the moon, in 1972."
His latest work, also involving postdoctoral researcher Dr Dazhuang Zhou, started almost two years ago. "I was invited by a group from the Johnson Space Center to work on a project to look at the astronauts' exposure to radiation."
Measuring this exposure is extremely complicated, given the range of sources and variety of particles. It ranges from electrons to heavy atomic nuclei moving at near light speeds.
The public hears comparatively little about the risks, but they can be substantial, with the greatest dangers being radiation-induced cancer and fatal radiation poisoning. Unfortunately, it is very difficult to reduce these risks while maintaining a manned space flight programme.
"This is an area that is very controversial," says O'Sullivan. "In general NASA works on the basis that a 3 per cent cancer excess in a lifetime is acceptable."
Yet the crews are willing to fly into space despite the many risks, be it radiation or catastrophic mechanical failure, as with the ill-fated Columbia.
"It brings into question the whole idea of space travel," says O'Sullivan. "Mankind was not built to exist in space for very long."
The institute's team flew instruments on board STS-108, in December 2001, and STS-112, in October last year. Both of these were visits to the International Space Station, says O'Sullivan, and flew at orbits of about 390km above earth.
NASA has done averaged dose studies, but the group from Dublin Institute for Advanced Studies looked in much greater detail, measuring the exposure caused by specific types of particles. "We carried out the first measurement of heavy nuclei at space-station altitudes," says O'Sullivan.
Natural background radiation is measured in millionths of a sievert, or microsieverts; a typical exposure for a person on the ground from land-based and cosmic radiation is about 3,000 microsieverts a year. The astronauts on board STS-108 received that much during a single 15-day flight.
"There is very little you can do about cosmic radiation," says O'Sullivan. These very fast particles easily penetrate the walls of the shuttle and space station.
And the Van Allen belts, energetic particles trapped in the earth's magnetic field, only affect flights when they pass over Brazil, where the belts dip lowest.
Of more concern are the solar flares that blast out vast amounts of high-energy particles, peppering the shuttle with up to 10 billion hits per square centimetre.
This activity waxes and wanes in a predictable way - only recently we passed a solar maximum, when the sun was at its most active. There are enormous flares every few decades; these unpredictable events pose the greatest danger to crews.
One large flare occurred in August 1972, when no Apollo flights were up. "If the astronauts had been out in space there would have been a major problem," says O'Sullivan. There was an estimated one-in-eight chance of dying after being exposed to it. A really big flare could kill four out of five astronauts exposed to it without some level of protection.