Leveraging the Civil Investment in Space Science

J. H. Clemmons, J. B. Blake, and J. F. Fennell

Understanding the space environment and its effects on space systems has been an important endeavor for The Aerospace Corporation throughout its history. Leveraging the civil investment in space science has become a vital strategy for continued progress.

Space assets need protection from the space environment, which can present numerous hazards. Best known is the degraded performance of electronics caused by the effects of long-term exposure to radiation—but lesser-known effects can also cause profound consequences. Some of these effects are caused by particles energetic enough to penetrate vehicle shielding, while others are rooted in the presence of less-energetic charged particles. For example, electrostatic charging and discharge phenomena cause problems on spacecraft, and ionospheric variability causes problems with the propagation of radio signals in space. In low Earth orbit, further difficulties stem from the neutral gas environment, including variable drag and materials degradation caused by the presence of reactive gas species.

Current methods to mitigate space environment effects, however, generally do not lead to optimal spacecraft designs. These methods encompass a variety of approaches, including specialized design requirements and practices, testing regimens, and monitoring and modeling the environment itself. Central to all of these approaches is the need for understanding the space environment and the conditions it is capable of presenting.

Research and other activity at The Aerospace Corporation directly addresses this need. Space is vast, and the environment is both highly structured and extremely variable on a wide range of time and distance scales. Its full range of behavior has not been explored with any degree of rigor. The work at Aerospace plays an integral role in the effort to understand this environment and its effects on space systems and apply that knowledge to important civil and national security space programs.

Early Space Environment Research

Aerospace first began conducting space environment satellite investigations in 1961. The research programs at this time were broadly focused on three areas: energetic space radiation, the upper atmosphere, and the active sun. The goal was to understand the little-known environment in which space systems must operate, so research centered on making and understanding appropriate measurements of the space environment.

The high-altitude U.S. nuclear test Starfish in 1962 dramatically demonstrated that not only was it important to understand the natural environment in which satellites were deployed, but that artificial changes in this environment could also affect space assets. During the tests, high-energy electrons became trapped and formed new radiation belts around Earth. The failures of several satellites were attributed to the products of the Starfish test.

The Orbiting Vehicle (OV) series of spacecraft were launched by the Air Force beginning in 1965 to provide information on a variety of space phenomena and spaceflight issues. These satellites were approximately 81 centimeters long and 69 centimeters in diameter. Many of these experiments focused on understanding the space environment, and Aerospace provided instrumentation for several of them. For example OV1-14 (pictured), launched in 1968, included seven Aerospace experiments.

During the 1960s Aerospace provided scientific investigations for 20 space missions, but launch vehicles failed on approximately half of them. Nevertheless, Aerospace scientists and engineers were eager to fly new experiments. Space science, scientific satellite instrumentation, and spacecraft engineering were new disciplines that held exciting challenges and an air of discovery. For example, although the instrumentation had antecedents in laboratory hardware, transformation to spaceflight hardware often presented significant challenges.

In general, each scientific mission included several instruments—often both charged-particle and plasma-wave instruments and optical instruments for observing Earth's upper atmosphere and the sun. Various sensors measured protons from 100 to a few hundred electron volts and electrons from 100 to 5 million electron volts. Heavy ions also were measured from a few hundred thousand electron volts to tens of millions. Optical instrumentation included instruments to measure all-sky Lyman-alpha, ultraviolet dayglow, multicolor nightglow; a crystal spectrometer to measure solar x rays in the 0.5-12 angstrom range; and broadband solar x-ray monitors. New Aerospace radiation models from this era were used to estimate the radiation environment for a variety of orbits. Their descendants, AE8 and AP8 ("A" stands for Aerospace), are the industry-standard models used today.

During the 1960s, the U.S. Air Force supported almost all Aerospace missions and instruments. A significant exception was an energetic particle instrument included on ATS-1, a NASA spacecraft flown in geosynchronous orbit. During the 1970s, three factors accounted for a change in focus for Aerospace research: electronic components based on new technologies were found to be much more susceptible to radiation than their predecessors; effects due to single particles penetrating electronic components (single-event effects) were discovered; and deleterious effects from spacecraft charging to kilovolt potentials were found to be commonplace. One new major Aerospace investigation at this time was testing devices on the ground for single-event effects. Today, these investigations have grown increasingly important as new technologies have been introduced into spaceflight circuitry.

Air Force and NASA Partnership

SCATHA (Spacecraft Charging at High Altitudes) and CRRES (Combined Release and Radiation Effects Satellite) were two investigations sponsored jointly by the Air Force and NASA. Aerospace played an integral role for these missions, which also marked a time of growing support from the civil space program to Aerospace's work and objectives in the space environment.

SCATHA—also designated P78-2 within the DOD Space Test Program—was designed to measure the radiation environment and its effects on space systems in geosynchronous orbit. The goals were to characterize the conditions that caused electrostatic charging of satellite surfaces, measure the levels of charging that could occur on different satellite materials, and detect and characterize charging-induced electrostatic discharges. The investigation offered insight into how to control charging of spacecraft surfaces, which in turn affected future design criteria, materials specification, construction techniques, and testing procedures. The overall program included the SCATHA flight mission and a laboratory-based technology program.

Aerospace constructed science payloads of plasma, energetic-particle, and plasma-wave sensors designed to measure the elements of the space environment that cause surface charging for SCATHA. Aerospace also developed a suite of engineering instruments that measured the level of charging on thermal blanket and thermal radiator materials, the occurrence of electrostatic discharges, the amplitude and frequency content in the electrostatic discharges, the contamination of sample surfaces, and the degradation of the thermal properties of several thermal control materials. In all, Aerospace delivered and integrated 19 sensors and electronics boxes into the SCATHA spacecraft (see sidebar, Spacecraft Charging). Aerospace also performed systems engineering functions and oversaw the performance of the spacecraft fabricator and integrator for the Air Force.

SCATHA was a jointly sponsored investigation because the charging phenomena it was designed to understand had been implicated in numerous anomalies experienced by several DOD, commercial, and NASA spacecraft. The SCATHA satellite was launched in early 1979 and operated for nearly 10 years. It provided the measurements that form the basis for current spacecraft charging characterizations and the charging-related specifications for design and construction of spacecraft that must survive in the near geosynchronous radiation environment. The data continue to be actively used to gain new understanding of the space environment and its interactions with, and effects on, spacecraft.

A spectrogram showing the evolution of relativistic electron fluxes, a dominant component of the space radiation environment, as seen by the NASA/SAMPEX satellite throughout more than an eleven-year solar cycle. The vertical axis is magnetic latitude and is expressed in terms of L value, L being the equatorial crossing of a magnetic field line measured in Earth radii. The color scale, covering more than four orders of magnitude, indicates the intensity of the relativistic electrons. The temporal and spatial variability of the relativistic electrons is highly variable on several timescales. Although not visible in this figure, this variability extends to much shorter timescales.

The CRRES project goals were to measure the near-Earth radiation environment and its effects on state-of-the-art microelectronics and other spacecraft components, as well as to perform chemical releases in the near-Earth environment. Aerospace was in charge of systems engineering and oversight of the program for the Air Force.

CRRES carried two suites of science and engineering payloads and 24 chemical canisters. The science payload suites were instruments that investigated the radiation belts and radiation effects, called SPACERAD, and those that supported the chemical release program and the study of ionospheric irregularities, called LASSII. Aerospace served as principal investigator on three of the SPACERAD payloads and one of the LASSII payloads and as coinvestigator on four other science payloads with supporting hardware responsibility. Those experiments measured the radiation-belt electrons, relativistic protons, magnetospheric ion composition, heavy ions, and plasma waves.

CRRES was launched into a geosynchronous transfer orbit in 1990. It was the first major science mission to the outer Van Allen radiation belt in a decade. The CRRES mission completed the chemical release program and garnered an unparalleled set of measurements in Earth's radiation belts. The observations and measurements gleaned from this mission are the cornerstone of current understanding of the radiation belts and their impact on space systems. The data are still being mined today to develop next-generation radiation-belt models (see sidebar, Radiation Models).

The Growing Role of Civil Space

The SCATHA and CRRES experiments demonstrated the benefits of participating in civil space missions. Pooling resources proved beneficial to increasing the quantity, breadth, and frequency of measurements. The scientific and technical findings were valuable to both national security and civil space programs, as they were facing similar problems in the space environment.

The number of DOD flights carrying space environment sensors declined in the 1980s, but NASA continued to be interested in space science and pursued progress in the field. Although the number of NASA missions flown was not large, they were well planned and incorporated comprehensive sensor suites designed to make large strides in scientific understanding. Top researchers from a variety of institutions (including Aerospace) were involved in these missions, as they are today. The results elevated this work from gathering data and developing simple models to establishing the field as a true science discipline in which understanding begins at the sun and stretches to Earth. Many cause-and-effect relationships in the space environment have now been quantified, and physical models have become ever more quantitative and insightful over the years.

During the 1980s and 1990s, Aerospace increasingly looked to the civil space program to improve its understanding of the space environment. It was clear that a sustained course with multiple investigations was necessary. The space environment had exhibited a wide range of behavior as a function of many parameters, including solar and geomagnetic activity, time, and position in space. There was also growing recognition that several components of the space environment posed significant risks to space systems. The sum of these realizations was that understanding the space environment was a large task, and that significant research efforts were still required. The data at hand were inadequate, and more measurements and analyses were needed. However, the resources needed to make progress were not available within the DOD, so Aerospace pursued opportunities within the civil space program with vigor.

Polar and SAMPEX Missions

Aerospace participated in NASA missions during the 1980s and 1990s, and two notable projects were the Polar and SAMPEX missions. Polar was part of a larger effort to gain understanding of the solar-terrestrial environment as a connected physical system through which energy and matter flow. Polar focused primarily on the high-latitude magnetosphere, which is a crossroads through which much of the variability of the space environment acts. It was thus natural for Aerospace to participate, so the corporation constructed instrumentation and analyzed the returned data. Polar was launched in 1996 and returned useful data for more than 12 years. Much has been learned about the space environment as a result, and Aerospace hardware produced key insights about how charged particles are energized and transported throughout the magnetosphere.

SAMPEX was the first of NASA's reinvigorated line of Small Explorers, designed as a new way to conduct space investigations with greater emphasis on focused science objectives and an expanded role for non-NASA project management. SAMPEX was designed to measure fluxes of energetic charged particles in the magnetosphere, with an emphasis on the populations that cause single-event effects. The University of Maryland led the mission, and Aerospace built some of the hardware, assisted with engineering the instrumentation, and analyzed the measurements. The mission was launched in 1992 and is still providing useful measurements. The SAMPEX team made many discoveries, and its long history of measurements provides a vivid illustration of extreme variability in the space environment.

Evolution of the Business Model for Space Environment Work

The Aerospace business model for understanding the space environment has evolved since the 1960s. The goal is still to provide the understanding necessary to conduct the national security space mission effectively and reliably. Ongoing research and measurements of the space environment are necessary to grapple with its complexities. Changes in technology and design practice have produced new vulnerabilities that require new knowledge of the space environment. Aerospace has developed a diversified approach to acquiring this knowledge, and the civil space program plays an important role.

Much of the needed progress requires new observations of the space environment, so a large amount of activity focuses on obtaining appropriate measurements. Researchers specify which measurements are needed and devise systems to obtain them. This work involves design and construction of instruments and finding ways to fly them in appropriate places. Once the measurements are obtained, the data are analyzed, cast into physical models, and new theory is developed. These activities mesh well with those of the civil space program, and Aerospace is able to apply these investments so that all of its customers benefit from new gains in understanding the space environment.

For example, measurements from Polar and other spacecraft have been incorporated into a model of the charged particle environment tailored for GPS orbits. Measurements from SCATHA have been reanalyzed to provide a better understanding of electrostatic charging in space. Theory developed under NASA funding has been used to understand the evolution of the radiation belts. Research into the dynamics of the magnetosphere funded by the National Science Foundation has been used to understand the behavior of some portions of the energetic particle environment.

Current Projects

New missions to gather needed data are the keystones for progress in the space environment, and Aerospace actively pursues opportunities for flights. This pursuit has led to involvement in several current NASA missions.

An especially well-placed mission is the Radiation Belt Storm Probes (RBSP), which will use two spacecraft with identical instruments to explore the dynamics of the radiation belts under extreme conditions. Aerospace participates in a consortium of research institutions that will provide a suite of instruments to measure the energetic particle environment (see sidebar, Space Science Experiments Under Development). This contract was awarded through NASA's competitive acquisition process, and Aerospace has been funded to provide several instruments and data analysis support for the flights. In addition, NASA was asked by the National Reconnaissance Office (NRO) to host a pair of sensors to measure relativistic protons. NASA agreed, and the NRO has tasked Aerospace to provide the hardware for this project. Thus, the RBSP mission is another partnership between NASA and national security space, and Aerospace is involved on both sides of the partnership.

Another current project is NASA's Magnetospheric Multiscale mission, which will send four spacecraft to regions in the magnetosphere where a phenomenon called magnetic reconnection occurs. This phenomenon is responsible for energizing charged particles, both directly and indirectly, and the spacecraft will probe its fundamental physics. Aerospace will produce one of the components of the energetic charged particles instrumentation. This basic research will help improve understanding of how charged particles are energized and transported in the space environment.

Aerospace also participates in smaller endeavors through the civil space program. Suborbital missions carried by sounding rockets allow detailed probes of precisely targeted phenomena, such as the aurora. These missions enable rapid turnaround and testing of new technologies and instruments. Other civil projects include ground-based monitors, advanced data analysis, and development of models and theory.

Conclusion

Aerospace's scientific participation in civil space activities leverages efforts to better understand the space environment and protect national space assets. The goal is to apply knowledge of the environment to national security and civil space missions to improve their reliability and effectiveness. New space environment measurements and their scientific interpretation are used to advance this goal, and participation in the civil space program is an essential component of Aerospace's strategy.

One of the drawbacks of reliance on the civil space program is that the targets of interest are not always well aligned with the needs of national security space. An example is the ionosphere-thermosphere (I-T) system, which is the region below about 2000 km altitude. The environment in this region exhibits a range of behavior that impacts national security space systems by affecting radiowave propagation and drag. However, I-T research has received low priority within NASA, so it has progressed slowly.

One way to address this problem is the way it was done in the early days—fly a new mission. Aerospace has conceived a mission involving two satellites called the Paired Ionosphere-Thermosphere Orbiters (PITO). PITO was designed to resolve important outstanding issues in I-T research, and a recent study concluded that it is quite feasible and can be performed with modest resources (see sidebar, PITO). In the past, Aerospace has taken the lead in uncovering many aspects of the space environment and its effects on space systems. The examples cited above, especially SCATHA and CRRES, show that Aerospace has the capability to handle and manage mission-sized efforts in this realm. Marshalling these strengths to lead a space environment mission like PITO provides an opportunity for Aerospace to serve its customers through leadership.