Update on the Worsening Particle Radiation Environment Observed by CRaTER and Implications for Future Human Deep‐Space Exploration

Abstract

Over the last decade, the solar wind has exhibited low densities and magnetic field strengths, representing anomalous states that have never been observed during the space age. As discussed by Schwadron, Blake, et al. (2014, https://doi.org/10.1002/2014SW001084), the cycle 23–24 solar activity led to the longest solar minimum in more than 80 years and continued into the “mini” solar maximum of cycle 24. During this weak activity, we observed galactic cosmic ray fluxes that exceeded theERobserved small solar energetic particle events. Here we provide an update to the Schwadron, Blake, et al. (2014, https://doi.org/10.1002/2014SW001084) observations from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter. The Schwadron, Blake, et al. (2014, https://doi.org/10.1002/2014SW001084) study examined the evolution of the interplanetary magnetic field and utilized a previously published study by Goelzer et al. (2013, https://doi.org/10.1002/2013JA019404) projecting out the interplanetary magnetic field strength based on the evolution of sunspots as a proxy for the rate that the Sun releases coronal mass ejections. This led to a projection of dose rates from galactic cosmic rays on the lunar surface, which suggested a ∼20% increase of dose rates from one solar minimum to the next and indicated that the radiation environment in space may be a worsening factor important for consideration in future planning of human space exploration. We compare the predictions of Schwadron, Blake, et al. (2014, https://doi.org/10.1002/2014SW001084) with the actual dose rates observed by CRaTER in the last 4 years. The observed dose rates exceed the predictions by ∼10%, showing that the radiation environment is worsening more rapidly than previously estimated. Much of this increase is attributable to relatively low‐energy ions, which can be effectively shielded. Despite the continued paucity of solar activity, one of the hardest solar events in almost a decade occurred in September 2017 after more than a year of all‐clear periods. These particle radiation conditions present important issues that must be carefully studied and accounted for in the planning and design of future missions (to the Moon, Mars, asteroids, and beyond).

September 2017's Geoeffective Space Weather and Impacts to Caribbean Radio Communications During Hurricane Response

“…The measurements from the westward‐viewing telescopes for EPEAD are shown here because they observe larger solar proton fluxes than the eastward view due to the former seeing particles whose gyro centers lie outside geosynchronous orbit and are hence less filtered by the geomagnetic field (e.g., Rodriguez et al, ). Several SEP enhancements are annotated by their cause, solar eruption (4, 6, and 10 September), or ICME1 or ICME2 energized (7 and 8, and 12 September), in agreement with the findings of Schwadron et al (, this special collection) through their analysis of the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) detector. The period 5–15 September elevated the risks of astronaut radiation, space hardware single‐event upsets, and high‐latitude trans‐ionospheric radio absorption.…”

The Solar Particle Event on 10 September 2017 as observed onboard the International Space Station (ISS)

“…This was the first SPE observed inside the ISS since May 2012, and in addition, it was an event which has been clearly seen by the CRaTER instrument orbiting Moon (Schwadron et al, ) and by the MSL‐RAD instrument on the surface of Mars where it contributed around 418 μGy in Si of additional dose. With all these instruments, we are now generating a unique data set for comparison of the (a) timeline of the event and (b) the influences of the event in terms of radiation limits for crews in LEO and for exploration missions.…”

“…Schwadron et al () concluded that the lens and skin dose on the surface of the Moon would approach the NASA 30‐day limits (see Table ); however, even moderate shielding would reduce the radiation dose below the limits.…”

Update on Galactic Cosmic Ray Integral Flux Measurements in Lunar Orbit With CRaTER

“…In the latter case, we expect to provide a further update, provided the LRO mission and CRaTER instrument continue nominal operations. Dosimetric results that include most of the same period reported here have been published elsewhere (Schwadron et al, ). CRaTER dose rate and LET data are also available online (see http://crater-web.sr.unh.edu/products.php).…”