Radiation levels on a roundtrip mission to Mars would be likely to exceed current US exposure limits for astronauts.
The results, taken during the Nasa's Mars Curiosity rover’s eight-month cruise to Mars, indicate that astronauts would receive a radiation dose of about 660 millisieverts, a measure of radiation exposure, during a 360-day roundtrip flight – the fastest travel possible with today's chemical rockets.
That dosage does not include any time spent on the planet's surface and Nasa limits astronauts' increased cancer risk to 3 per cent, which translates to a cumulative radiation dose of between about 800 millisieverts and 1,200 millisieverts, depending on a person's age, gender and other factors.
"Even for the shortest of (Mars) missions, we are perilously close to the radiation career and health limits that we've established for our astronauts," Nasa's chief medical officer Richard Williams told a National Academy of Sciences' medical committee on Thursday.
Measurements taken by Nasa's Mars Science Laboratory (MSL) mission as it delivered the Curiosity rover to Mars in 2012. The lab’s Radiation Assessment Detector (RAD) is the first instrument to measure the radiation environment during a Mars cruise mission from inside a spacecraft that is similar to potential human exploration spacecraft.
An astronaut living for six months on the International Space Station, which flies about 250 miles above Earth, receives a dosage of about 100 millisieverts. An abdominal X-ray scan generates about 10 millisieverts.
At Nasa's request, the Institute of Medicine panel is looking into ethics and health standards for long-duration spaceflights.
"We're looking at that 3 per cent standard and its applicability for exploration-type missions," added Nasa's Edward Semones, spaceflight radiation health officer at the Johnson Space Center in Houston, during a conference call later with reporters.
"The snapshot today is that we would exceed our limit," Semones said.
The research was published in this week's edition of the journal Science and further information from Curiosity about how much and what type of radiation astronauts can expect on the Martian surface is due to be released later this year.
But Nasa says far from dissuading them from sending a manned mission to Mar’s the results merely serve as a basis for the development of new technologies designed to protect astronauts such as alternative propulsion technologies to speed up the trip to Mars and different types of spacecraft shielding.
"As this nation strives to reach an asteroid and Mars in our lifetimes, we're working to solve every puzzle nature poses to keep astronauts safe so they can explore the unknown and return home," says William Gerstenmaier, Nasa's associate administrator for human exploration and operations in Washington.
Two forms of radiation pose potential health risks to astronauts in deep space: galactic cosmic rays (GCRs), particles caused by supernova explosions and other high-energy events outside the solar system; and solar energetic particles (SEPs) associated with solar flares and coronal mass ejections from the sun.
Current spacecraft shield much more effectively against SEPs than GCRs and to protect against the comparatively low energy of typical SEPs, astronauts might need to move into havens with extra shielding on a spacecraft or on the Martian surface, or employ other countermeasures.
GCRs tend to be highly energetic, highly penetrating particles that are not stopped by the modest shielding provided by a typical spacecraft.
"Scientists need to validate theories and models with actual measurements, which RAD is now providing," said Donald M. Hassler, a program director at the Southwest Research Institute and principal investigator of the RAD investigation.
"These measurements will be used to better understand how radiation travels through deep space and how it is affected and changed by the spacecraft structure itself. The spacecraft protects somewhat against lower energy particles, but others can propagate through the structure unchanged or break down into secondary particles."