SAN FRANCISCO — A team of researchers is eager to use a constellation of miniature satellites to delve into one of the most vexing questions facing scientists studying Earth’s climate: Is the planet producing more radiation than it absorbs from sunlight, or do the incoming and outgoing radiation levels balance each other out?
An imbalance between incoming and outgoing radiation levels would be the single best predictor of future climate change because if energy from the sun is not being reflected and emitted back into space it can lead to climate instability, said Lars Dyrud, director of Cambridge, Mass.-based Draper Laboratory’s Earth and Space Science programs.
With the backing of NASA’s Earth Science Technology Office at the Goddard Spaceflight Center in Greenbelt, Md., researchers from the Johns Hopkins University’s Applied Physics Laboratory of Laurel, Md., and Draper Laboratory are developing the Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) mission. The Applied Physics Laboratory and Draper plan to launch a single cubesat equipped with a radiometer developed by L-1 Standards and Technology of New Windsor, Md., that is roughly the size of a deck of cards to measure Earth’s outgoing radiation. The mission is designed to demonstrate technology that could be deployed in a cubesat constellation to provide a comprehensive look at Earth’s reflected and emitted radiation, said Bill Swartz, the Applied Physics Laboratory’s RAVAN principal investigator.
L-1 President Steven Lorentz said the idea for the RAVAN instrument stems from the National Institute of Standards and Technology Advanced Radiometer (NISTAR), built between 1999 and 2001 for the Deep Space Climate Observatory, a NASA mission proposed in 1998, then stalled until the U.S. National Oceanic and Atmospheric Administration revived the initiative in 2012. NISTAR is a three-channel cavity radiometer that weighs approximately 25.5 kilograms and uses 43 watts of power. After producing the radiometer, L-1 began exploring other missions that would benefit from similar technology, Lorentz said.
The RAVAN instrument weighs about 1.5 kilograms and uses less than 1 watt of power, said Lorentz, who also serves as NISTAR principle investigator. L-1 was able to produce RAVAN’s tiny radiometer by employing passive temperature control techniques instead of the active systems on NISTAR and equipping the new instrument with sophisticated analog-to-digital converters. Although the two instruments employ different technology and measurement techniques, both are designed to offer detailed data on irradiance reflected and emitted from the sunlit face of Earth, Lorentz said.
The RAVAN spacecraft, which measures 10 centimeters by 10 centimeters by 34 centimeters, includes its own black body calibration source and vertically aligned carbon nanotubes to absorb incoming light. Neither the vertically aligned carbon nanotubes nor the gallium black body has been used before in an orbiting instrument, according to a NASA statement released in May 2013, when the RAVAN cubesat won funding as part of the space agency’s In-Space Validation of Earth Science Technologies program.
NASA’s Earth Science Technology Office is expected to provide roughly $4 million over three years to support development and construction of the first RAVAN cubesat. A portion of that funding also will be used to enable the RAVAN team to develop manufacturing processes and engineering principles to build a constellation of RAVAN cubesats as efficiently as possible. “Standardization allows you to build a lot of satellites inexpensively,” Dyrud said.
To provide a comprehensive measurement of Earth’s outgoing solar radiation, researchers would need to obtain data from between 30 and 40 cubesats similar to RAVAN.
“We did analysis showing that 30 cubesats is the minimum we can fly to reach this holy grail of measuring outgoing radiation with sufficient accuracy to verify climate change models,” Dyrud said.
The first RAVAN cubesat is scheduled for launch in late 2015 through NASA’s Cubesat Launch Initiative, which finds rideshare opportunities for small satellites on space agency missions. The RAVAN mission is designed to operate in any low Earth orbit.
“As long as we can remain in space for a couple of years, we hardly care where we go,” Dyrud said.
Currently, NASA obtains data on solar radiation reaching Earth’s atmosphere with a variety of instruments, including the Total Irradiance Monitor on NASA’s 2003 Solar Radiation and Climate Experiment and the Total Solar Irradiance Calibration Transfer Experiment launched in November on the U.S. Air Force Space Test Program Satellite-3. The Clouds and Earth’s Radiant Energy System sensors flying on NASA’s Aqua and Terra Earth observation satellites and the NOAA-NASA Suomi National Polar-orbiting Partnership spacecraft provide data on solar energy reflected by Earth and Earth’s emitted thermal energy.
While the Clouds and Earth’s Radiant Energy System has proved adept at highlighting long-term trends in outgoing radiation, a RAVAN constellation would be designed to reveal both long-term and short-term variations, Swartz said. For example, a RAVAN constellation would be able to detect changes in the levels of energy radiated into space from any location on Earth and during any time of the day or night. Researchers could use that information to improve climate models, Swartz said.
RAVAN is being built using the same satellite bus the Applied Physics Laboratory designed for two cubesats launched in November on a Minotaur 1 rocket from Wallops Flight Facility in Virginia as part of the Defense Department’s third Operationally Responsive Space mission. The laboratory designed the new cubesat bus to meet the requirements of future national security and space science customers. The bus is designed to mimic the command and control, communications, navigation and power capabilities of larger spacecraft. The Applied Physics Laboratory cubesats, which remain in orbit, continue to demonstrate excellent performance, Swartz said Jan. 15.