Influences on Space and Exploration Target Bodies

How do variations in the Sun’s activity, interplanetary plasmas, and inputs from the interstellar medium influence the energetic particle and dust environment of space and exploration target bodies?

All objects within the heliosphere, out to about 150 AU, are embedded in, interact with, and are influenced by the extended magnetized atmosphere of our Sun. This key project will advance our understanding of the near space energetic particle and nanometer-to-micron scale dust environments of target bodies for exploration. Understanding the production and evolution of energetic particles and nanometer dust in the heliosphere are high priority science goals in heliophysics, as identified in the Solar & Space Physics Decadal Survey, but they are also critical to enabling robotic and human exploration beyond low Earth orbit. Absent prudent shielding and a realistic assessment of relativistic particle variability, radiation may sharply constrain future human exploration. Nanometer to micron-scale dust is charged and accelerated to the solar wind speed, potentially producing orders of magnitude more damage upon impact than heavier dust orbiting at Keplerian speeds, thus contributing to the weathering of spacecraft and airless bodies alike. Charged nanoscale dust presents a hazard for both human and robotic missions on the surface of targets such as moons and asteroids.

Maximization of Solar and Radio Bursts

What observational capabilities are needed to maximize the application of solar and other radio bursts to facilitate improved monitoring of solar particle events?

Radio Observatory on the Lunar Surface for Solar Studies (ROLSS), is a concept for a near-side, low frequency, interferometric radio imaging array. It would be deployed during a lunar sortie or by a robotic rover as part of an unmanned landing. The prime science mission is to image intense type II and type III solar radio bursts to determine where and how the radiating particles are accelerated. Secondary science goals include constraining the density of the lunar ionosphere, based on the lower cutoff frequencies of the radio bursts, detecting the flux and arrival direction of interplanetary dust striking the antenna, imaging the terrestrial electron radiation belts, and detecting bright astrophysical sources. ROLSS complements the upcoming NASA Parker Solar Probe and European Space Agency (ESA) Solar Orbiter missions to the inner heliosphere, as it would image emission from electrons and shocks as they pass by and are measured directly by the spacecraft. Furthermore, ROLSS serves as a pathfinder for aspects of larger, farside radio arrays, like the Lunar Radio Telescope Array (LRTA).

Operational and Design Requirements

What are the operational and design requirements to provide new dust measurements from antennas deployed on the surface of the Moon, and how do these measurements aid in our understanding of exploration requirements for the various target bodies?

Measurements of dust properties have been performed with instruments specifically designed to characterize dust particles. It has been shown that space-based radio receivers can also be used to measure dust. Our goal is to further develop the techniques for extracting dust properties from the electric field signals. Radio instruments detect dust by measuring the electrical signals produced when grains impact objects at high speed and create expanding clouds of plasma. Work on the use of radio receivers and lunar arrays for studying dust have shown that radio observations have two particular strengths when it comes to surveying dust. First, radio arrays are very sensitive to nanodust, a significant fraction of the dust population in the solar system that produces weak signals in standard instruments. Second, a radio array is also ideal for searching for the highest mass, most rare dust particles, because the entire surface area of the array, which exceeds hundreds of square meters, becomes a single sensitive dust detector. When a grain impacts an object at extremely high speeds (a hyperkinetic impact), it generates a cloud of high temperature plasma of total charge dependent upon the mass of the grain and its speed (∝ mv3.5). Since ions and electrons in the cloud have the same thermal energy, the electrons expand much more quickly. The passage of the cloud disrupts the photoelectrons from the antenna, which then cannot return because their new angular momentum does not allow it. The resulting net photoelectron current is strong enough to allow for a fast positive charging of the antenna, which is compatible with the measured field intensities on the STEREO/WAVES antennas. Our analysis suggests that this technique works very well for measurements that cover the mass intervals 1022 − 1020 kg and 1017 − 5×1016 kg.

The flux of the larger dust agrees with measurements of other instruments on different spacecraft, and the flux of the smaller dust grains agrees with theoretical predictions. For this task, we will gather dust observations from the STEREO and Wind spacecraft to produce a database of expected distributions at the Moon. This database will be used to extend models of electric field signal from nanodust impact on a spacecraft to the geometry and electrical grounding of a radio array on the lunar surface.  

Initial results on the mass distribution of dust detected through low frequency radio emission suggest an exciting additional science topic for low frequency lunar radio arrays, including sensitive characterization of interplanetary and interstellar dust. For a lunar radio array with 3 arms of 500 m length each and average antenna width on the arms of 1 m, the surface area would be 1500 m2. Given the nanodust flux distributions previously reported this would correspond to approximately 103 dust impacts per second for nanodust, and detections of the heavy ~10 micron dust several times a minute. Thus, a lunar radio array would be capable of measuring nanodust variation at a sufficiently high cadence to identify correlation of dust fluxes with solar wind conditions. A radio array would also have the collecting area needed to detect much less frequent massive dust impacts. It is relevant to note that the time interval for the dust impacts to significantly damage the metal antennas and leads on the polyimide film will be tens of years, substantially longer than the mission lifetime.