Measuring the Radiation Environment on Mars
An SwRI-led instrument is determining radiation hazards for future manned missions to Mars
Dr. Donald M. Hassler is a senior program director in the Planetary Science Directorate at Boulder, Colo., part of SwRI’s Space Science and Engineering Division. He has more than 25 years of experience in space physics and the development, characterization and calibration of space instrumentation, and he is principal investigator of the Radiation Assessment Detector.
Image courtesy of NASA/JPL
The car-size Mars Science Laboratory spacecraft has been exploring the surface of the Red Planet since August 2012 to assess past and present habitability of Mars. Positioned near the center of the rover, the SwRI-led Radiation Assessment Detector is about the size of a coffee can and is characterizing the planet’s radiation environment, a key influence on life.
The Radiation Assessment Detector, one of 10 instruments onboard the Mars Science Laboratory, measures fluxes of solar energetic particles and galactic cosmic rays to help assess the radiation environment on Mars, as well as on the journey from Earth to Mars.
The RAD instrument measures charged and neutral particles as they pass through a series of charged-particle detectors (blue surfaces), a gamma ray detector (red) and a neutron detector (brown). Anti-coincidence shielding surrounds the gamma-ray and neutron sections of the instrument.
The round gray cap indicates the location of the RAD on the Mars Science Laboratory
The MSL spacecraft provided some shielding from solar events during cruise, reducing significantly the particle flux observed by RAD during these events. The particle flux observed by RAD inside the MSL spacecraft is shown in the figure to be several orders of magnitude less than that observed by the unshielded SIS instrument on the ACE spacecraft
After the newest Mars rover, Curiosity, landed safely on the Red Planet’s surface on Aug. 6, 2012, scientists began a new round of exploration using the car-size vehicle’s 10 onboard science instruments.
One instrument, however, had already been gathering valuable data during the nine-month journey from Earth to Mars. The Southwest Research Institute (SwRI)-led Radiation Assessment Detector (RAD) was powered up and began collecting data 10 days after the spacecraft was launched from Cape Canaveral on Nov. 26, 2011. Since then, RAD has collected roughly seven months of data during the cruise and now more than five months of data on the Martian surface. These are the first measurements of their kind taken on any other planet’s surface besides Earth.
RAD is a compact but powerful energetic particle analyzer designed to characterize the radiation environment on the surface of Mars and quantify the radiation hazard that astronauts might encounter on future human missions to the Red Planet. RAD’s measurements will help NASA plan future manned missions as well as help validate the radiation transport models that are being used to evaluate spacecraft and spacesuit shielding designs. The radiation environment on Mars is a complex combination of galactic cosmic rays, solar energetic particles, secondary neutrons and other particles created both in the atmosphere and at the Martian surface. One of the unique aspects of RAD is that it is capable of simultaneously measuring and identifying these different particle types, over a wide energy range, using a small (approximately three pounds) package.Dual purpose: astronaut safety and Mars habitability
The primary, overarching scientific objective of the Mars Science Laboratory is to “assess the past and present habitability of Mars,” and to search for the elements needed to support life, such as water and carbon-based materials. RAD plays an essential role in achieving MSL’s prime science objective by helping to characterize and understand “life-limiting” factors, or factors detrimental to habitability, through its measurements of energetic particle fluxes at the surface of Mars. At the same time, RAD’s characterization of the Martian radiation environment is a critical contribution to NASA’s efforts to plan for possible future manned expeditions. By addressing both science and human exploration objectives, RAD has effectively become a “poster child” for cooperation and science exchange between NASA’s Science and Human Exploration Directorates.
In addition to identifying radiation hazards for future human exploration, characterizing the radiation environment on the surface of Mars will also aid understanding the degree to which the radiation environment might put constraints on the existence of microbial life (past or present), or the preservation of signs of such life, since radiation also contributes to the breakdown of near-surface organic compounds. For example, how deep below the regolith, or Martian surface, must microbial life be (or have been) to survive the mutagenic influences of the observed radiation levels? Although current-day radiation levels probably make the surface of modern Mars inhospitable for microbial life as we know it, RAD’s measurements will help determine the depth below the surface that a possible future robot on a life-detection mission might need to dig or drill to reach a “microbial safe zone.”
For the purposes of human exploration, planning for a future manned mission to Mars requires understanding the possible radiation hazards over the course of the entire round-trip mission. In general, a manned mission to Mars can be separated into three phases: the cruise phase to Mars (six to nine months), the time on the Martian surface (more than six months), and the return trip to Earth (six to nine months). Therefore, estimating the total radiation dose that an astronaut will receive on a future Mars mission requires assessing the contributions from all three phases. Consequently, an important secondary objective of RAD is to characterize the radiation environment from inside the MSL spacecraft during its journey through interplanetary space on its way to Mars. Interestingly, the level of shielding provided by the MSL spacecraft is not unlike that of the International Space Station (ISS) or the Crew Exploration Vehicle (CEV) that will be used to take future astronauts into deep space. Thus, during the cruise phase to Mars, radiation levels measured by RAD served as a proxy for the radiation levels that astronauts might experience on a journey to Mars.
The first data packets received from RAD revealed a strong flux in space, even inside the spacecraft, with radiation doses about four times higher than the baseline levels measured on the launch pad from the spacecraft’s own nuclear-powered generator. RAD was measuring the relevant energetic particle species originating from galactic cosmic rays, the Sun and other sources. Of particular interest were particles accelerated by flares and coronal mass ejections (CMEs) originating from the surface of the Sun, which spew fast-moving clouds of radiation across the solar system. Besides being scientifically interesting in terms of their physics, particles from these giant clouds could pose a potentially greater biological hazard as they hit the spacecraft and release an inward cascade of secondary particles inside the capsule. Just as an astronaut would be, RAD was tucked inside the spacecraft for the journey and thus could characterize these secondary particle showers, as well as higher-energy galactic cosmic rays and the secondary particles that they produced inside the spacecraft as well. Thus, measurements taken by RAD are providing insight into the shielding required for spacecraft to be used in future manned missions to deep space.Weathering a solar storm
The decision to power RAD on during the journey to Mars was validated when the spacecraft was exposed to the largest solar particle event since 2003. The flood of energetic particles unleashed by a solar flare and a fast-moving CME swept over not only the spacecraft, but also Earth and Mars. Although solar storms create the Earth’s aurorae and can affect Earth satellites, air travel and GPS systems, this one did no damage to the Mars Science Laboratory. However, its effects could be seen clearly in data downloaded from RAD.
The event was particularly exciting because of the alignment of Earth, the MSL and Mars at the time, and also because of the opportunity it afforded to compare data from RAD with data from other spacecraft that also observed the storm. The solar particle event was observed by the Solar Dynamics Observatory (SDO), Geostationary Operational Environment Satellites (GOES), the Advanced Composition Explorer (ACE) and the twin Solar Terrestrial Relations Observatory (STEREO) spacecraft in Earth orbit, as well as by the Solar Heliospheric Observatory (SOHO) flying at the Lagrangian Point L1 between Earth and the Sun.
During the seven months of cruise observations, as the Sun’s activity was increasing, RAD observed several large X-class and M-class solar flares. Data from RAD, taken from inside the MSL, are now being compared with data from the other satellites to better understand and predict the dose rate that future astronauts will experience.First measurements from the surface of another planet
Curiosity landed on Mars with a flawless, “picture-perfect” landing, on Aug. 6, 2012. The next day, or “sol” (the term for a Martian day), RAD, which had been switched off during the final approach to Mars, was turned back on (the first scientific instrument to be turned on after landing, other than the cameras). Serendipitously, the day RAD made its first measurements of cosmic rays on the surface of Mars, Aug. 7, 2012, was the 100th year anniversary of the discovery of cosmic rays on Earth by Victor Hess (Aug. 7, 1912), using measurements from a balloon flight in Austria. Since Aug. 7, the team has collected more than 150 sols (five Earth months) of data, and continues to operate RAD around the clock. As Curiosity begins its traverse of the Red Planet, sampling the soil and sniffing the air, RAD quietly collects data in the background, accumulating statistics and keeping a watchful eye for any signs of the type of flares or solar storms seen during cruise. So far, the space weather on Mars has been quiet. But as the 11-year solar maximum approaches, many more large solar particle events or solar storms are expected over the course of the mission. And given that the rover and all 10 instruments are working perfectly, it is hoped that the mission will be long-lived, perhaps lasting until the next solar maximum — the period when the Sun is most active — 10 to 12 years from now.Operating on “Mars time”
Although the operation of RAD on Mars is relatively simple, the operation of the rover, with all 10 scientific instruments, is quite complex. Not only does the science operations team need to coordinate the daily observing programs of each of the 10 instruments, the team also needs to assess the results of the previous sol’s activities and plan the activities for the next sol, including coordinating the daily drives or traverses, selecting, handling and processing soil samples, as well as the daily commanding and telemetry schedules. Complicating these daily activities is the fact that a “sol” is 39 minutes longer than an Earth day. So, for the past four months the entire science operations team has been operating on “Mars time,” meaning that the start of each workday begins about 39 minutes later than the previous one, making it difficult to establish a daily routine. An operations day that begins at 8 a.m. one day would start at 8 p.m. two weeks later. Because the operations team includes engineers, operations specialists and scientists to perform all of the tasks associated with operating the rover and science instruments, as well as assessing the science and engineering results from each previous day’s activities, more than 200 people have been adjusting to Mars time. One of the more interesting aspects of operating on Mars time was to deliver a science lecture with 200 MSL scientists in the audience, at 3 a.m. Earth time.Radiation environment sensitive to Mars weather and climate
One of the first exciting results from RAD is that the radiation environment on Mars is very sensitive to daily changes in weather, primarily atmospheric pressure. The Martian atmosphere provides some level of shielding from the harsh galactic cosmic rays coming from space, and the RAD team is finding that the thickness of the atmosphere as a function of daily heating and cooling varies by about 10 percent, which causes a few percentage points variation in the radiation dose observed at the surface. As longer time series of data are accumulated, seasonal variations may appear as well.
Not only are there diurnal variations caused by thermal tides in the atmosphere, but also the team is observing longer-term variations associated with changes in the magnetic structure in the heliosphere or interplanetary space surrounding Mars. This heliospheric structure is magnetically tied to the Sun, and it rotates with the Sun with a 27- to 28-day period. Although many new discoveries are being made about the Mars environment with RAD, the team is still waiting for solar activity to pick up and the first large solar storm to be observed from the Martian surface.The RAD instrument’s makeup
Positioned near the left-front corner of the rover, the three-pound RAD is only about the size of a coffee can, but performs the same functions as Earth-bound devices 10 times its size. RAD consists of a charged particle telescope comprising three solid-state silicon detectors and a cesium iodide (CsI) calorimeter. An additional plastic scintillator is used together with the CsI calorimeter, surrounded by an anti-coincidence shield, to detect and characterize neutral particles, such as neutrons and gamma rays. The outputs of the various solid-state detectors, and photodiodes used with the CsI and plastic scintillators, are converted to digital signals for further processing. The digital logic includes an embedded microcontroller to bin and format the data.
The RAD instrument is mounted just below the top deck of the rover, with the charged particle telescope pointed in the zenith direction. In this way, RAD detects charged particles arriving from space as well as neutrons and gamma rays coming from Mars’ atmosphere above, as well as from the surface below.Conclusion
RAD continues to operate flawlessly on the surface of Mars, and is expected to do so throughout the nominal two-year mission, as well as for any extended mission, which it is hoped will last 10 years or more, providing an unprecedented, entire solar cycle of radiation data from the surface of another planet. The importance of characterizing the radiation environment wherever humans go in space with an instrument such as RAD has been recognized by NASA’s Human Exploration and Operations Directorate. SwRI scientists are building a next-generation RAD for Johnson Space Center to go on the International Space Station in 2014.
Questions about this article? Contact Hassler at (303) 546-9670 or firstname.lastname@example.org.
The RAD project is a team effort, with many individuals and organizations providing significant contributions. SwRI, together with Christian Albrechts University in Kiel, Germany, built RAD. The dedicated efforts of the many scientists, engineers, technicians and support staff, at both SwRI and CAU, are gratefully acknowledged. In particular, the efforts of Dr. Cary Zeitlin, John Andrews, Dr. Bent Ehresmann, Kerry Neal, Joe Peterson, Dr. Scot Rafkin, Kelly Smith, Yvette Tyler and Eddie Weigle at SwRI, Robert Wimmer-Schweingruber, Eckart Boehm, Stephan Boettcher, Soenke Burmeister, Jan Kohler, Jingnan Guo, Cesar Martin and Lars Seimetz at CAU, Guenther Reitz at Germany’s national aerospace research center, Deutsches Zentrum für Luft- und Raumfahrt, Dave Brinza at Jet Propulsion Laboratory, Arik Posner at NASA HQ and Frank Cucinotta at Johnson Space Center, have been fundamental to RAD’s success. RAD is supported by funding from the NASA Human Exploration and Operations Mission Directorate and DLR. Early development for RAD was supported by SwRI’s internal research and development program.