Measurements taken by NASA's Mars Science Laboratory (MSL) mission as it
delivered the Curiosity rover to Mars in 2012 are providing NASA the
information it needs to design systems to protect human explorers from
radiation exposure on deep-space expeditions in the future. MSL'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. The findings will reduce uncertainty about the effectiveness of
radiation shielding and provide vital information to space mission designers who
will need to build in protection for spacecraft occupants in the future. The
findings, which are published in the May 31 edition of the journal Science,
indicate radiation exposure for human explorers could exceed NASA's career
limit for astronauts if current propulsion systems are used.
Two forms of radiation pose potential health risks to astronauts in deep
space. One is galactic cosmic rays (GCRs), particles caused by supernova
explosions and other high-energy events outside the solar system. The other is
solar energetic particles (SEPs) associated with solar flares and coronal mass
ejections from the sun.
Galactic cosmic rays are one of the most important barriers standing in the way of plans for interplanetary travel by crewed spacecraft.
Galactic cosmic rays are one of the most important barriers standing in the way of plans for interplanetary travel by crewed spacecraft.
Cosmic rays are energetic particles originating from outer space
that travel towards every direction and also impinge on Earth's atmosphere.
Almost 90% of all the incoming cosmic ray particles are simple protons, with
nearly 10% being helium nuclei (alpha particles), and slightly less than 1% is
other heavier elements. Electrons (beta particles) or gamma ray photons with
similar origin are also included under that denomination. The term ray
is a historical accident, as cosmic rays were at first, and wrongly, thought to
be mostly electromagnetic radiation. In modern common usage high-energy
particles with intrinsic mass are known as "cosmic" rays, and photons,
which are quanta of electromagnetic radiation (and so have no intrinsic mass)
are known by their common names, such as “gamma rays” or “X-rays”, depending on
their frequencies. Cosmic rays attract great interest practically, due to the
damage they inflict on microelectronics and life outside the protection of an
atmosphere and magnetic field, and scientifically, because the energies of the
most energetic ultra-high-energy cosmic rays (UHECRs) have been observed to
approach 3 × 1020 eV, about 40 million times the energy of particles
accelerated by the Large Hadron Collider (LHC). At 50 Joules, the
highest-energy ultra-high-energy cosmic rays have energies comparable to the kinetic
energy of a 90-kilometre-per-hour (56 mph) baseball.
Galactic cosmic rays (GCRs) are one of the most
important barriers standing in the way of plans for interplanetary travel by
crewed spacecraft. Galactic cosmic rays are the high-energy particles
that flow into our solar system from far away in the Galaxy. GCRs are mostly
pieces of atoms: protons, electrons, and atomic nuclei which have had all of
the surrounding electrons stripped during their high-speed (almost the speed of
light, 299 792 458 m/s) passage through the Galaxy. Cosmic rays provide one of
our few direct samples of matter from outside the solar system. The magnetic
fields of the Galaxy, the solar system, and the Earth have scrambled the flight
paths of these particles so much that we can no longer point back to their
sources in the Galaxy. Cosmic rays also pose a threat to electronics placed
aboard outgoing probes. In 2010, a malfunction aboard the Voyager 2 space probe
was credited to a single flipped bit, probably caused by a cosmic ray.
Strategies such as physical or magnetic shielding for spacecraft have been
considered in order to minimize the damage to electronics and human beings
caused by cosmic rays.
The second form of “radiation”
to boost the levels of risk towards a Mars missions is represented by the Solar
energetic particles (SEP).
These are high-energy particles coming from the Sun. They consist of protons, electrons,
and alpha particles helium ions, and HZE ions (which have an electric charge
greater than +2) that are highly energetic. They are of particular interest and
importance because they can endanger life in outer space. Solar energetic
particles can originate from two processes: Particles accelerated at a solar-flare
site or by shock waves associated with coronal mass ejections (CMEs). However,
only about 1% of the CMEs produce strong SEP events.
Radiation exposure is measured in units of Sievert (Sv) or milisievert
(one one-thousandth Sv). Long-term population studies have shown exposure to
radiation increases a person's lifetime cancer risk. Exposure to a dose of 1
Sv, accumulated over time, is associated with a 5 percent increase in risk for
developing fatal cancer.
NASA has established a 3 percent increased risk of fatal cancer as an
acceptable career limit for its astronauts currently operating in low-Earth
orbit. The RAD data showed the Curiosity rover was exposed to an average of 1.8
milisievert of GCR per day on its journey to Mars. Only about 5 percent of the
radiation dose was associated with solar particles because of a relatively
quiet solar cycle and the shielding provided by the spacecraft. “In terms of
accumulated dose, it's like getting a whole-body CT scan once every five or six
days," said Cary Zeitlin, a principal scientist at the Southwest Research
Institute (SwRI) in San Antonio and lead author of the paper on the findings.
"Understanding the radiation environment inside a spacecraft carrying
humans to Mars or other deep space destinations is critical for planning future
crewed missions.”
The RAD data will help inform current discussions in the United States
medical community, which is working to establish exposure, limits for
deep-space explorers in the future.
References:
1.
Radiation
Measured by NASA's Curiosity on Voyage to Mars has Implications for Future
Human Missions; Access: (accessed on: May 30, 2013);
2.
Cosmic ray; http://en.wikipedia.org/wiki/Cosmic_ray
Access: (accessed on: May 31, 2013);
3.
Cosmic
rays: What are cosmic rays?; Access: http://imagine.gsfc.nasa.gov/docs/science/know_l1/cosmic_rays.html
(accessed on: May 31, 2013);
5.
Image:
Access: http://www.popsci.com/science/article/2013-04/apply-now-one-way-trip-mars
(accessed on: May 31, 2013)
