A little bit of Ireland was due to go into orbit last night, when the space shuttle Endeavour was to carry aloft a scientific experiment built in Dublin. It will measure radiation and provide information that could help any future manned mission to Mars.
The National Aeronautics & Space Administration, which flies the shuttles, invited researchers from Dublin Institute for Advanced Studies to help measure how much space radiation astronauts are exposed to.
The institute has 30 years of involvement with NASA missions and decades of experience of measuring radiation, according to Prof Denis O'Sullivan of the institute's astrophysics section.
O'Sullivan is involved in an ongoing EU project to measure airline crews' exposure to radiation. His experiments, which have flown on Concorde and on Aer Lingus aircraft, have shown that flight crews are exposed to 10 times more radiation than staff on the ground.
This time, he is going to measure how much radiation astronauts on the shuttle and the International Space Station are exposed to during a 10-day flight. NASA is particularly interested because of proposals for a two-year return trip to Mars, during which radiation exposure could become a problem, according to O'Sullivan.
Until now, he says, astronauts' exposure has been fairly limited, because of the relatively low altitudes involved. The station orbits at about 400 kilometres; the shuttle tends to work in the 200- to 400-kilometre range. This will change with any attempt on Mars, however, which will involve more than two years of space travel, well away from any protective cover provided by earth.
The team's concerns are cosmic and solar radiation; the sun's rays include fast-moving protons, neutrons and heavy atoms stripped clean of their electrons. The particles continuously bombard the earth, but the atmosphere absorbs most of the energy. Space travellers do not benefit from this protective blanket.
Like X-rays and other forms of radiation, cosmic rays can kill cells and cause genetic damage that can lead to cancer; hence NASA's interest.
"The particles come from galactic cosmic rays, trapped particles above the earth, solar energetic particles and also from secondary particles from the station's walls," says O'Sullivan. The neutrons and heavy nuclei in particular can kick up recoil particles if they collide with atomic nuclei in the body or, indeed, in the shielding or other structures in the station or shuttle. "The main thing is to measure the linear energy transfer, or LET. It is basically the energy the particle deposits per unit distance in matter."
Linear energy transfer becomes an issue when the matter being bombarded is human tissue. "As they slow down in human tissue, they cause damage. There are many types of effects that occur. The neutrons do it by interacting with the nuclei of the atoms, causing recoil particles, which go on to do further damage."
Living on the ground gives us about 2.5 millisieverts of natural background radiation, including cosmic rays, in a year.
An astronaut in low earth orbit would get that much in just two or three days, says O'Sullivan; a journey to Mars would deliver 1,000 times as much. A return trip "could easily bring you into that area of one or two sieverts".
O'Sullivan and his colleagues Dr Dazhuang Zhou, a postdoctoral member of the institute, and Eileen Flood, of its technical staff, built the "passive nuclear track detection" measuring device, which, at 15 centimetres square and two centimetres thick, is smaller than a typical hardback book.
It contains a polymer, a form of plastic that behaves like human tissue and records any collisions with cosmic rays. The polymer is laid down in dozens of sheets, each about half a millimetre thick, to form a detector stack, which has been mounted on the mid deck of the shuttle, in the medical-experiment rack.
A particle striking the device leaves a permanent track in the plastic. The size and nature of the tracks give the energy content of the particles - and, therefore, their mass and nature - as well as showing how many particles hit the device. O'Sullivan expects it to record at least 1,000 impacts during the 10-day flight.
When Endeavour returns, the device will be shipped back to Dublin so the team at the institute can calculate how much radiation the shuttle crew were exposed to. The results will be compared with NASA's own on-board measuring devices, to confirm their accuracy.
You can follow the progress of the mission at www.nasa.gov