Exploring the Galactic Frontier
The Interstellar Boundary Explorer is set to discover the global interaction in the region separating our solar system from interstellar space
By David J. McComas, Ph.D.
Decades after successful flybys to study the giant planets, Voyagers 1 and 2 continue their history-making flights by being the first spacecraft to cross the closest boundary of our solar system. They are now in the sheath region of our heliosphere, the region of space dominated by the Sun. This “heliosheath” is a no man’s land between our solar system and interstellar space, the tenuous region within galaxies not occupied by planetary systems. As the spacecraft move toward interstellar space, they continue to gather valuable data at points along the paths they travel, raising numerous questions about the edge of the solar system and the interactions that occur at its boundaries.
This fall, the Interstellar Boundary Explorer (IBEX) spacecraft will explore far beyond Voyagers’ point measurements, opening a new realm of discovery, by making the first global observations of the Sun’s interaction with the interstellar medium. IBEX will make these pioneering observations from a highly elliptical orbit that reaches outside Earth’s magnetosphere, the dynamic region around our planet controlled by its magnetic field. From this vantage far above Earth’s relatively bright magnetosphere, the spacecraft’s energetic neutral atom (ENA) sensors will assemble global images that reveal the broad heliospheric interactions for the first time. These ENAs are produced by a process called “charge exchange,” which neutralizes previously charged particles in the interactive region within the interstellar medium. Once neutralized, some ENAs travel inward toward the Sun where they can be observed by the Earth-orbiting IBEX spacecraft.
Southwest Research Institute (SwRI) leads the mission and consortium of numerous U.S. and international partners for the National Aeronautics and Space Administration (NASA) as part of the Small Explorer Program of highly focused, relatively inexpensive space science missions.
IBEX science mission
At the center of the solar system is the Sun, whose outer atmosphere is constantly evaporating into space and forming a million-mile-per-hour stream of charged particles called the solar wind. This supersonic solar wind expands radially in all directions, inflating the giant bubble that makes up the heliosphere, the region of space influenced by our Sun. The heliosphere also plows through the galaxy at about 50,000 miles per hour. The innermost boundary of this giant bubble is called the termination shock, at a distance more than three times farther than the furthest planets.
The global interactions in this region have never before been observed and are important because they shield us from cosmic radiation. This dangerous radiation poses significant hazards to astronauts on space missions, including future manned missions to the moon and Mars. Some researchers theorize that galactic cosmic ray radiation levels could have been many times higher at various times since the formation of Earth, and might have influenced the formation and evolution of life itself.
IBEX will examine the structures, dynamics, energetic particle acceleration and charged particle propagation in the very complex and important regions of the outer heliosphere. By providing the first global observations of this interstellar interaction, IBEX will reveal the heliosphere’s fundamental nature and provide images for detailed modeling and improved understanding.
The spacecraft has a simple science payload with flight-proven sensor technologies for very high sensitivity and low background ENA observations. Two large high-sensitivity, single-pixel ENA cameras and a combined electronics unit (CEU) make up the payload. The IBEX-Lo sensor, built by Lockheed Martin, the University of New Hampshire, NASA Goddard Space Flight Center, and the University of Bern in Switzerland, measures ENAs from about 10 electron volts (eV) to 2 kilo-electron volts (keV). IBEX-Hi, built by SwRI and Los Alamos National Laboratory, measures particles from about 300 eV to 6 keV. SwRI engineers also built the CEU, which contains all but one of the high-voltage power supplies, along with support electronics for the sensors, a digital data processing unit and data storage. The spacecraft will begin returning scientific data approximately a month after launch, at the end of spacecraft and payload commissioning.
The sensors view perpendicular to the spacecraft’s sun-pointed axis as it rotates at 4 revolutions per minute. Just as the charge exchange process produces ENAs in the inner heliosheath, the two IBEX sensors use charge exchange to convert ENAs back into ions so they can be analyzed and detected.
As the spacecraft spins, the measured ENAs fill in the pixels to build a crescent-shaped portion of the all-sky map. As the spacecraft’s spin axis tracks the Sun, these crescents move across the sky, completing one full-sky map every six months.
Because of the relatively low number of heliospheric ENAs, the IBEX team put much effort into quantifying and minimizing noise and background sources, both of which interfere with target measurements.
Maximizing the spacecraft’s apogee altitude, the furthest point in its elliptical orbit, is important because of the relatively bright ENA emissions from the Earth’s magnetosphere and the background contamination that could be generated in the magnetosphere and its surrounding regions. While the IBEX mission can achieve its science from a lower apogee, observations become increasingly better as the apogee is raised, so the team developed a novel, relatively inexpensive method for launching IBEX into a highly elliptical orbit.
IBEX launch and mission design
During the mission development phase, the IBEX team designed a groundbreaking concept for placing a spacecraft into a high-altitude orbit using the least expensive launch vehicle in NASA’s arsenal Ñ a Pegasus rocket Ñ coupled with an additional IBEX-supplied solid rocket motor (SRM) and the spacecraft’s hydrazine propulsion system.
Pegasus will deliver IBEX to an altitude of about 120 miles, point it in the desired direction, spin it up to 60 revolutions per minute and release it. After discarding a lightweight adapter cone, the additional IBEX STAR-27 SRM will carry IBEX into a medium-altitude parking orbit. After discarding the spent SRM casing, IBEX will use its internal hydrazine system over several orbits to raise its apogee to approximately 200,000 miles and its perigee to about 4,400 miles above the Earth. This orbit, which evolves over time because of the effects of solar and lunar gravitation, is ideal for IBEX’s heliospheric ENA imaging.
The team’s invention of this new launch and orbit-raising approach has produced a new and robust method for launching spacecraft into high-altitude orbits using a standard Pegasus rocket. This process could be used to fly a new range of small, relatively inexpensive missions for NASA and other government and commercial sponsors. At IBEX’s highest altitude, it is very nearly at full escape energy from Earth orbit, enabling the efficient launch of not just future high-altitude Earth-orbiting missions, but also low-cost missions to the moon, interplanetary space, and potentially even to other planets.
The IBEX spacecraft will launch from Kwajalein Atoll, near the equator in the Pacific, at about 11 degrees N latitude. While more remote and expensive, launching from this site takes advantage of the increased rotational energy of the Earth, making a small but important improvement in the total mass that Pegasus can carry to orbit.
As a simple, nearly Sun-pointed spinner, the IBEX design allows for straightforward and repetitive mission operations. The spacecraft makes observations while at high altitude and completes one orbit about every eight days. As IBEX approaches Earth, the sensors are put into a low-power safe mode. During the low-altitude segment of each orbit, the data from the previous orbit are downlinked and new commands for the following two orbits are uploaded. Then as IBEX rises back toward high altitudes, the sensors are re-energized and resume their heliospheric ENA imaging. Essentially all orbits follow the same repetitive process.
The IBEX approach to mission operations builds on Orbital Sciences Corporation’s Mission Control Center, which has been used to control more than 30 spacecraft, and on SwRI’s extensive experience in managing science operations and data analysis. IBEX will use the Tracking and Data Relay Satellite System for gathering real-time data during the SRM burn and the existing Universal Space Network ground stations once it is in orbit. The IBEX Science Operations Center, located at Boston University, with an identical backup system at SwRI, will evaluate mission data, monitor payload performance, and deliver and archive data.
While the spacecraft’s primary mission is to explore the interactions of the heliosphere with interstellar space, IBEX also provides valuable information for other areas of space science. Its very high-sensitivity ENA observations of the Earth’s magnetosphere will provide invaluable new data for this field. In addition, the astrophysical and heliospheric communities will be able to explore synergies in heliospheres and astrospheres throughout the galaxy.
IBEX also has a comprehensive education and public outreach program, which is overseen and implemented by the Adler Planetarium and Astronomy Museum in Chicago. Adler leads 12 partners in promoting the mission’s exploration and discoveries. A full-length planetarium show will premiere in fall 2008, highlighting the solar system’s interaction with the galaxy and development of the IBEX mission.
At press time, the IBEX spacecraft is on track for an October 2008 launch, kicking off the first global ENA measurements and images of the interactions between the heliosphere and interstellar medium. Building on the pioneering Voyagers, IBEX is poised to blaze new trails that will ultimately reveal much more about our home in the galaxy and how it protects life from deadly galactic radiation.
Published in the Summer 2008 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.