Mars magnetic field as obeserved by Mars Global Surveyor
Don Hassler is the principal investigator is Southwest Research Institute, Boulder, Colorado.
The Radiation Assessment Detector, or RAD, investigation on Curiosity will monitor high-energy atomic and subatomic particles reaching Mars from the sun, from distant supernovas and other sources. These particles are naturally occurring radiation that could be harmful to any microbes near the surface of Mars or to astronauts on a future Mars mission.
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RAD’s measurements will help fulfil the Mars Science Laboratory mission’s key goals of assessing whether Curiosity’s landing region has had conditions favourable for life and for preserving evidence about life.
The telescope has detectors for charged particles with masses up to that of an iron ion. RAD can also detect neutrons and gamma rays coming from the Mars atmosphere above or the Mars surface material below the rover.
RAD will also do an additional job. Unlike the rest of the mission, RAD has a special task and funding from the part of NASA that is planning human exploration beyond Earth orbit. It will aid design of human missions by reducing uncertainty about how much shielding from radiation future astronauts will need. RAD will take measurements during the trip from Earth to Mars as well as during Curiosity’s roving on Mars because the radiation levels in interplanetary space are also important in design of human missions.
Measurements of ultraviolet radiation by Curiosity’s Rover Environmental Monitoring Station will supplement RAD’s measurements of other types of radiation.
Galactic cosmic rays make up one type of radiation that RAD will monitor. These are a variable shower of charged particles coming from supernova explosions and other events extremely far from our solar system. The sun is the other main source of energetic particles that this investigation will detect and characterize. The sun spews electrons, protons and heavier ions in “solar particle events” fed by solar flares and ejections of matter from the sun’s corona. Astronauts might need to move into havens with extra shielding on an interplanetary spacecraft or on Mars during solar particle events.
Earth’s magnetic field and atmosphere provide effective shielding, acting like a huge, invisible force field against the possible deadly effects of galactic cosmic rays and solar particle events. Anything that penetrates this barrier is quickly absorbed by Earth’s thick atmosphere. Just to find high enough radiation levels on Earth for checking and calibrating RAD, the instrument team needed to put it inside major particle-accelerator research facilities in the United States, Europe, Japan and South Africa.
Unlike Earth, Mars lacks a thick atmosphere (the Martian atmosphere is only 1% of Earth’s atmosphere) and a magnetic field. This leaves the planet totally vulnerable to radiation from space. The Mars radiation comes from many sources: the Sun’s solar wind, cosmic rays from the Sun, and other stars. Life on the surface of Mars would be exposed to a constantly high dose of radiation, and the occasional lethal blasts that come regularly from strong solar flares.
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The radiation environment at the surface of Mars has never been fully characterized. NASA’s Mars Odyssey orbiter, which reached Mars in October 2001, assessed radiation levels above the Martian atmosphere with an investigation named the Mars Radiation Environment Experiment (MARIE). Since Mars the above mentioned thin atmosphere, radiation detected by Mars Odyssey would be roughly the same as the surface.
Over the course of about 18 months, Mars Odyssey detected ongoing radiation levels which are 2.5 times higher than the astronauts experience on the International Space Station – 22 millirad per day. The spacecraft also detected 2 solar proton events, where radiation levels peaked about 2,000 millirads in a day, and a few other events that got up to about 100 millirads.
Current estimates of the radiation environment at the surface rely on modelling of how the thin atmosphere affects the energetic particles, but uncertainty in the modelling remains large. A single energetic particle hitting the top of the atmosphere can break up into a cascade of lower-energy particles that might be more damaging than a single high-energy particle.
NASA weighs radiation danger in units of cancer risk. A healthy 40-year-old non-smoking American male stands a (whopping) 20% chance of eventually dying from cancer. That’s if he stays on Earth. If he travels to Mars, the risk goes up. The question is, how much? A study done in 2004 calculated that the added risk of a 1000-day Mars mission lies somewhere between 1% and 19%, and the most likely answer is 3.4%.
For these calculations one assumed that the capsule would be build of Aluminum, just like the Apollo era capsules. However if we use polyethylene, the same material garbage bags are made of, we get a different story as polyethylene absorbs 20% more cosmic rays than aluminum. At the Marshall Space Flight Center a form of reinforced polyethylene is developed that is 10 times stronger than aluminum, and lighter, too. This could become a material of choice for spaceship building, if it can be made cheaply enough.
If we are talking shielding capabilities pure hydrogen is even better. Lyguid hudrogen blocks cosmis rays 2.5 times better than aluminium does. Some advanced spacecraft designs call for big tanks of liquid hydrogen fuel and wrapping those fuel tanks around the living space of the astronauts.
In addition to its precursor role for human exploration, RAD will contribute to the mission’s assessment of Mars’ habitability for microbes and search for organics. Radiation levels probably make the surface of modern Mars inhospitable for microbial life. Near-suface Organic compounds would break down due to the radiation. Deeper layers of Mars would be shielded to the radiation by the soil above it. The measurements from RAD will feed calculations of how deeply a possible future robot on a life-detection mission might need to dig or drill to reach a microbial safe zone. For assessing whether the surface radiation environment could have been hospitable for microbes in Mars’ distant past, researchers will combine RAD’s measurements with estimates of how the activity of the sun and the atmosphere of Mars have changed in the past few billion years.
Radiation levels in interplanetary space vary on many time scales, from much longer than a year to shorter than an hour. Assessing the modern radiation environment on the surface will not come from a one-time set of measurements. Operational planning for Curiosity anticipates that RAD will record measurements for 15 minutes of every hour throughout the prime mission, on steady watch so that it can catch any rare but vitally important solar particle events.
The first science data from the mission will come from RAD’s measurements during the trip from Earth to Mars. These en-route measurements will, first, enable correlations with instruments on other spacecraft that monitor solar particle events and galactic cosmic rays in Earth’s neighborhood and then will yield data about the radiation environment farther from Earth. Two months into its journey to Mars the Sun erupted with enough solar particles released to be registered by Curiosity’s Radiation Assessment Detector.
This solar event of 22 January 2012 was followed by some more eruptions which together created the largest radiation storm felt on Earth since 2003. The lucky timing also allowed the detector to measure how much radiation astronauts might be subjected to during a journey to Mars.
The physicist Don Hassler is assisted by an international team of co-investigators including experts in instrument design, astronaut safety, atmospheric science, geology and other fields.
Southwest Research Institute in Boulder and in San Antonio, Texas, together with Christian Albrechts University in Kiel, Germany, built RAD with funding from the NASA Exploration Systems Mission Directorate and Germany’s national aerospace research center, Deutsches Zentrum für Luft- und Raumfahrt.