Satellite’s final days improve predictions of orbital decay
Researchers are gaining new insights into how the upper atmosphere and ionosphere affect satellites in space, as well as communications and navigation on Earth, because of new data from a U.S. Air Force satellite that recently finished its mission after more than seven years in orbit.
While the Communication/Navigation Outage Forecasting System (C/NOFS) satellite burned up in Earth’s atmosphere during a planned re-entry on Nov. 28, it left behind a valuable archive of data about a part of the space environment that has been difficult to study. Data gathered by C/NOFS will be used to improve models that predict satellite trajectories, orbital drag, and uncontrolled re-entries. Researchers from the U.S. Air Force and the University of Texas presented their findings last week at the American Geophysical Union Fall Meeting in San Francisco, California.
The C/NOFS satellite, which launched on April 26, 2008, studied a region above the upper atmosphere known as the ionosphere, a layer of electrically charged particles created by ultraviolet radiation from the Sun that lies some 40 to 60 miles (64 to 96 kilometers) above the Earth’s surface where it interacts and mixes with the neutral particles of the thin upper atmosphere.
The upper atmosphere and the ionosphere both constantly change in response to forces from above and below, such as explosions on the Sun, intense upper atmospheric winds, and dynamic electrical field charges. Such changes can affect satellite orbits and also produce turbulence in the atmosphere that cause what are known as scintillations, which interfere with radio wave communication and navigation systems – especially at low latitudes near the equator.
During most of its time in orbit, C/NOFS never came closer to the Earth’s surface than about 250 miles (402 kilometers). However, the spacecraft began to orbit at lower and lower altitudes as solar activity increased. Eventually, it descended to an altitude lower than 160 miles (257 kilometers) above Earth.
During the satellite’s last 13 months of operation, it recorded a unique set of observations as it traveled through the same space environment that can directly cause premature orbital decay. Data from this region of the atmosphere is rare because orbits cannot be sustained long-term without onboard propulsion.
“One thing we learned clearly from C/NOFS is just how hard it is to predict the precise time and location of re-entry,” said Cassandra Fesen, principal investigator for C/NOFS at the Air Force Research Laboratory at the Kirtland Air Force Base in Albuquerque, New Mexico.
Data collected by C/NOFS show that the upper atmosphere and ionosphere react strongly to even small changes in near-Earth space.
“The neutral atmosphere responds very dramatically to quite small energy inputs,” said Rod Heelis, principal investigator at the UT-Dallas for NASA’s Coupled Ion-Neutral Dynamics Investigation (CINDI) instrument suite on board the satellite. “Even though the energy is put in at high latitudes – closer to the poles – the reaction at lower latitudes, near the equator, is significant.”
Rob Pfaff, project scientist for CINDI at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and principal investigator for another C/NOFS instrument, the Vector Electric Field Investigation (VEFI), is studying observations related to one of the original goals of the C/NOFS mission: Why does the low latitude ionosphere become so turbulent, it can wreak havoc on navigation and communication signals?
The C/NOFS low-altitude observations were vital to forming a complete picture of these disturbances. The observations revealed the presences of strong shears in the horizontal ionosphere motions at the base of the ionosphere, areas where charged particles flow by each other in opposite directions. The shears and undulations, which are observed throughout the nighttime, equatorial ionosphere, are believed by researchers to be the source of large-scale instabilities that drive the detrimental scintillations.
Jim Sharkey is a lab assistant, writer and general science enthusiast who grew up in Enid, Oklahoma, the hometown of Skylab and Shuttle astronaut Owen K. Garriott. As a young Star Trek fan he participated in the letter-writing campaign which resulted in the space shuttle prototype being named Enterprise. While his academic studies have ranged from psychology and archaeology to biology, he has never lost his passion for space exploration. Jim began blogging about science, science fiction and futurism in 2004. Jim resides in the San Francisco Bay area and has attended NASA Socials for the Mars Science Laboratory Curiosity rover landing and the NASA LADEE lunar orbiter launch.