Pasadena - Dec 14, 2003
In an article appearing on page 1 of the science section of the New York Times December 9, Times reporter Mathew Wald grossly misrepresented the danger posed by cosmic radiation to astronauts on a human Mars mission.
Wald states: "…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."
The Times report is very misleading, as it compares the Mars mission to Skylab, and then on that basis, claims that to the Mars radiation dose is unprecedented. In reality, astronauts have already spent much longer times aloft than Skylab, and taken doses fully comparable to those of the Mars mission.
According to Wald's data, which appears to have been supplied by Brookhaven radiation researchers, Skylab took 18 Rem in 84 days, which indeed would be markedly less than the 130 Rem cited by the same group for a round trip mission to Mars. However many ISS and Mir astronauts have spent rotations of 180 days on orbit, which, scaling from the Skylab data provided would be 39 Rem.
However ISS and Mir are in a higher inclination orbit than Skylab, and this makes the Earth's magnetic field less effective for shielding. Therefore on the basis of a 39 Rem estimate given for 180 days in Skylab, an estimate of 50 Rem for 180 days on ISS would be reasonable.
But many have spent longer than 180 days on orbit. NASA astronauts Mike Foale and Carl Walz, for example have each accumulated about 230 days on orbit, mostly between Mir and ISS. Based on the above, that represents a dose of 64 Rem, or half the 130 Rem projected for the round trip Mars mission.
Russian cosmonauts, however, have been on orbit even longer. One cosmonaut spent 18 months (~540 days) on Mir, which corresponds 150 Rem, slightly MORE than that projected by Wald's Brookhaven sources for the round trip Mars mission.
No radiation induced health effects have been reported in any of these cases. So Wald's statement that the Mars mission dose represents a new order of magnitude in space radiation exposure is simply false.
In fact, it is not surprising that no radiation-induced health effects have been observed. According to the authoritative (and quite conservative ) Biological Effects of Ionizing Radiation (BEIR) Report, a 130 Rem dose delivered over a long period of time (long with respect to metabolic repair cycles -say > 6 months) would correspond to about a 2% incremental risk of a fatal cancer at some time in the future for a 35 year old male.
This is about the same incremental risk assumed by someone who smokes two packs a day for about 3 years (i.e. the astronauts going to Mars would incur the same cancer risk if they stayed home and smoked for the same duration). No prompt (observable) effects would be expected.
Another comparison of dose rates in interplanetary space with ISS doses is provided by data has been provided by data measured by the Lawrence Berkeley Lab (LBL) MARIE instrument on Mars Odyssey, currently in orbit around Mars.
When normalized for free space (i.e. MARIE actual data doubled to undo the blocking effect of Mars), this data, published recently by MARIE Principal Investigator Dr. Cary Zeitlin, shows that interplanetary dose rates are about a 0.1 Rads per day, which is a factor of two higher than those 0.05 Rads/day encountered on ISS.
Mars surface doses would be somewhat less than ISS- even without sandbag shielding or other recourses. With a modicum of these measures, Mars surface doses could be expected to be less than half ISS levels.
Rads are a measure of the amount of energy radiation deposits in human tissue. 100 Rads = 1 Sievert = 1 J/kg of radiation energy deposition. Rems are a measure of the biological threat represented by such radiation. Rads are converted to Rems by multiplying then by a Quality Factor, which varies with the type of radiation.
The International Commission on Radiation Protection recommends a Quality Factor of 2.0 for galactic cosmic radiation (Reference: ICRP Publ.#60), Ann. Of the ICRP 21, (No. 1-3) (1991)). Thus 1 Rad of cosmic radiation would constitute 2 Rem of dose.
Based on MARIE data, a Mars mission using an UNSHIELDED spacecraft for a mission consisting of a 6 month transfer to Mars, 18 months on the Martian surface, followed by a 6 month transfer from Mars to Earth could expect to receive 18 Rads each way plus 27 Rads on Mars, for a total mission dose of 63 Rads, (0.63 Sieverts) or 120 Rem.
With easily manageable shielding techniques requiring no additional mission mass, such as proper placing of crew supplies within the spacecraft for shielding purposes, and the placement of sand bags on the roof of the hab during the stay on Mars, this dose could be readily cut in half. Such a 63 Rem (0.315 Sievert) dose would represent about a 1 percent incremental risk of future cancer to each member of the crew, roughly half the threat posed by sustaining a smoking habit over the same 2.5 year period.
Additional support to the above analysis is presented in a paper by Dr. Stan Curtis, of the LBL Life Sciences Division. In that paper ("Single Track Effects and New Directions in GCR Risk Assessment," Adv. Space. Res., Vol. 14, No. 10, pp.(10)885-(10)894, 1994 ) Dr. Curtis breaks down the incremental risk to each organ of the body (Table 1, p.(10)888.) due to galactic cosmic radiation, with the total risk to all organs combined reported as 4% per Sievert.
Thus, based on Curtis' findings, the above 0.315 Sievert dose round trip Mars mission would incur a cancer risk of 1.2%, in good agreement with standard BEIR-derived Rem-based methods of radiation risk analysis.
It should be noted that the 6 month outbound, 18 month on Mars, 6 month return Conjunction Class mission plan assumed in the above analysis is entirely feasible using current chemical propulsion technology. In 2001, Mars Odyssey took 6 months to fly from Earth to Mars. We do not need to wait for any futuristic propulsion systems to reduce flight times to make radiation doses acceptable.
Radiation is not a show-stopper for a human Mars mission.
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Japan Poised To Abandon Martian Probe Mission
Tokyo (AFP) - Dec 08, 2003
Japan's trouble-plagued first mission to Mars is set to be abandoned in the latest of a series of costly failures to hit the country's space development programme, news reports and officials said Monday.
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