Friday, June 14, 2013
NASA says we are sending humans to Mars. But there are several problems we need to solve before that can happen. One of the biggest problems is space radiation.
A six-month stay at the International Space Station exposes astronauts to about the same amount of radiation that they would receive in 20 years on the surface of the Earth. This occurs even though both the Earth’s magnetic field and the outer hull of the station provide some protection.
This exposure to radiation increases the risk of cancer-related death. Exposure is measured in units of Sieverts (Sv) and milliSieverts (1/1000 Sv). The dose received depends on the level of radiation and the exposure time, and it accumulates. To determine health risks we look at the lifetime exposure, where 1 Sv of exposure is associated with a 3 percent increase in risk of cancer death.
Two forms of space radiation pose a risk for humans traveling to Mars. One is galactic cosmic rays (GCR), which are very high-energy particles traveling through space from sources like supernova explosions and other high-energy events outside of the solar system.
The other source is the sun, which emits solar energetic particles (SEP) during solar flares and coronal mass ejections. It is believed the exposure rate from GCR is constant, whereas the exposure from SEP depends on sun activity.
NASA’s Curiosity rover now exploring Mars has an instrument to measure both GCR and SEP radiation. The Radiation Assessment Detector (RAD) is in the left-front area inside the rover. It is about the size of a toaster and weighs 3.8 pounds. It has a 60-degree, upward-looking cone that collects the radiation particles and directs them to detectors for measurement.
The radiation instruments on most other spacecraft are mounted on the outside of the spacecraft where they measure the radiation in space, not affected by the spacecraft. The RAD instrument is the first to measure the radiation environment from inside a spacecraft during the cruise phase.
The RAD location inside the spacecraft makes it shielded in much the same way as a crewed spacecraft. Its measurements give us information about the effectiveness of this type of shielding for human exploration spacecraft.
On May 30 NASA released the results of the data collected by RAD during the cruise phase to Mars. The data show that the rover was exposed to an average of 1.8 milliSieverts of GCR per day, with a total exposure of 0.466 Sievert.
It is extremely difficult to develop spacecraft shielding to lower the radiation exposure from these very high energy GCRs. However, with current technologies spacecraft could be designed to protect against the lower-energy SEPs that are randomly produced by the sun. Havens with extra shielding could be used by crew during times of high solar activity.
Based on the RAD results, spacecraft could be built to limit the crew’s exposure to about 0.6 Sievert on a 360-day interplanetary round trip to Mars. In Earth-based terms, that would be like getting a whole-body CT scan every five or six days. That’s a high level of exposure, but within the current NASA lifetime limit of 1 Sievert. New technologies in shielding or faster propulsion systems to shorten travel time could further reduce exposure.
Now that Curiosity is on the surface, RAD is collecting data about Mars radiation. Because Mars has only 1 percent the atmosphere of Earth and no global magnetic field, astronauts will still need protection from radiation after they are on the surface. The RAD information will be used to design shielded shelters for the astronauts to live in during their time on Mars’ surface.
The surface RAD data can also be used to determine the current surface habitability for microbes. Surface radiation levels today are most likely too high to support microbial life, but data from RAD will tell us how far below the surface we would have to dig or drill to reach a microbial safe zone. Combining this data with estimates of past solar activity and an increased Martian atmosphere, we could determine if the surface radiation in the past might have allowed life.
NASA needs to solve the space radiation problems before the first human crew is scheduled to leave for Mars in the mid-2030s. In case you are interested in making the trip, I hear they are looking for volunteers.
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.