Joint Air Force-NASA Mission To Study High-radiation Orbits

WASHINGTON — The U.S. Air Force and NASA are on track to launch a joint satellite mission next year to study how radiation affects space hardware in some of the most rarely used and unpredictable Earth orbits.

The results of the Demonstration and Science Experiments (DSX) mission could open up potentially valuable medium Earth orbits for communications and surveillance satellites and enable the development of satellite components that are less prone to damage from radiation, program officials said.

The $155 million DSX mission, like NASA’s pioneering Lunar Crater Observation and Sensing Satellite mission that deliberately slammed into the Moon in 2009, will use a standardized Atlas 5 payload adapter ring as a free-flying spacecraft.

The DSX spacecraft, outfitted with three main scientific payloads, will be placed into a highly elliptical retrograde orbit that takes it 12,000 kilometers from Earth at apogee and 6,000 kilometers at perigee.

The DSX satellite was at one time planned for launch in 2008, but it has had a hard time finding a ride to space because of its relatively low priority as a research and development mission and the fact that it is headed for an uncommon orbit, said Mark Scherbarth, program manager at the Space Vehicles Directorate at Kirtland Air Force Base, N.M. The satellite finally found a launch opportunity as a secondary payload on the Air Force’s Defense Meteorological Satellite Program-19 mission scheduled for launch in October 2012. An Atlas 5 rocket will lift off from Vandenberg Air Force Base, Calif., and drop off the operational weather satellite in low Earth orbit before carrying DSX to a higher altitude.

The launch should have DSX on orbit just in time to study the space environment during solar maximum, when the sun is most active and spacecraft are most vulnerable, Scherbarth said in a March 14 interview. The sun’s current 11-year cycle is predicted to peak between November 2012 and May 2013.

The satellite’s unique orbit is designed to allow it to study the inner and outer Van Allen radiation belts, which can wreak havoc on spacecraft, Scherbarth said. The Van Allen belts are areas where high concentrations of ionized particles exist due to the Earth’s magnetic field. The inner belt extends from roughly 6,700 kilometers to 9,600 kilometers above the equator, and the outer belt is roughly 19,100 kilometers to 63,700 kilometers above the equator; both belts curve closer to the Earth at the poles.

“It’s a very high-radiation environment, and radiation has a lot of detrimental effects not only on structural materials and thermal materials, but probably most importantly on the electronics in spacecraft, which have only increased over time,” Scherbarth said.

The two Air Force payloads, the Wave Particle Interaction Experiment and Space Weather Experiment, will collect data on the electron and proton populations in the regions. This information will be used to create better analytical models for designing spacecraft to operate in medium Earth orbits, Scherbarth said.

“The analytical models that we have don’t predict it very well,” he said. “So you don’t know what to expect up there. If you design your spacecraft a certain way and you guess wrong, you have either under-shielded and you die very quickly, or you’ve put way too much shielding, which is very expensive.”

Of the nearly 1,000 operating satellites currently orbiting the planet, 90 percent are either in low Earth orbits or 36,000 kilometers above the equator in the geosynchronous orbits favored by commercial communications satellites, according to the Union of Concerned Scientists’ satellite database. Medium Earth orbit — the sparsely populated region above 2,000 kilometers but below 36,000 — is home to only about 6 percent of operating satellites, including the U.S. GPS and Russian Glonass navigation satellites (20,200 and 19,100 kilometers, respectively).

The Air Force believes the region between the inner and outer Van Allen belts at around 10,000 kilometers has great promise. Satellites orbiting in the so-called slot region could enable communications about eight times faster than geosynchronous satellites and view the entire Earth with fewer satellites than needed in low Earth orbits, according to an Air Force fact sheet.

NASA’s contribution to the DSX mission is the Space Environment Testbeds (SET) payload designed to study space weather effects on spacecraft.

Satellites rely on microelectronics that have become smaller and smaller over the years. While this has enabled them to be faster and more efficient and to process more data, it also has made them more vulnerable to radiation effects, said Dana Brewer, program executive for NASA’s Living With a Star program.

“The microelectronics change every two years,” Brewer said in a March 16 interview. “As we go to smaller feature size, which is the gap between two junctions in a device, the smaller we go, the more damage we can produce from either an electron or a heavy ion in space.”

This problem is compounded by the fact that U.S. spacecraft are increasingly reliant on components derived from commercial hardware, Brewer said.

“The government used to drive the electronics market, and now it’s driven by commercial requirements,” she said. “So we used to have radiation-hardened manufacturing sites, and with the big gaps in the devices, we didn’t have as much of a problem.

“We used to keep track of exactly how things were manufactured every step of the way, and a manufacturer couldn’t change anything unless we knew about it. Now because we’re doing more commercial, changes can occur in the manufacturing … and that can affect their performance in space.”

The SET payload will use multiple dosimeters to detect radiation at different parts of the spacecraft. It also features experiments that will study the physics of how certain types of components degrade due to different types of radiation.