A fogbank is the least useful location for a telescope, yet today's space observatories effectively operate inside one. That's because Venus, Earth and Mars orbit within a vast dust cloud produced by comets and occasional collisions among asteroids. After the sun, this so-called zodiacal cloud is the solar system's most luminous feature, and its light has interfered with infrared, optical and ultraviolet observations made by every astronomical space mission to date.
"To put it simply, it has never been night for space astronomers," said Matthew Greenhouse, an astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Md. Light from zodiacal dust can be a thousand times brighter than the sources astronomers actually target, limiting sensitivity in much the same way that bright moonlight hampers ground-based observatories. The dust and its unwanted illumination are greatest in the plane of Earth's orbit, the same plane in which every space telescope operates.
Placing future astronomy missions on more tilted orbits would let spacecraft spend significant amounts of time above and below the thickest dust and thereby reduce its impact on observations. So Greenhouse teamed with Scott Benson at NASA's Glenn Research Center in Cleveland, Ohio, to investigate how these "dark sky" or extra-zodiacal orbits might improve mission science and to develop a means of cost-effectively reaching them.
"Just by placing a space telescope on these inclined orbits, we can improve its sensitivity by a factor of two in the near-ultraviolet and by 13 times in the infrared," Greenhouse explained. "That's a breakthrough in science capability with absolutely no increase in the size of the telescope's mirror."
Greenhouse, Benson and the COllaborative Modeling and Parametric Assessment of Space Systems (COMPASS) study team at NASA Glenn designed a mission that utilizes new developments in solar arrays, electric propulsion and lower-cost expendable launch vehicles. Their proof-of-concept mission is the Extra-Zodiacal Explorer (EZE), a 1,500-pound EX-class observatory that could accommodate a telescope in the size range of the recently completed WISE mission — all within the cost and schedule constraints of NASA’s Explorer Program.
Launched on a SpaceX Falcon 9 rocket, EZE would use a powerful new solar-electric drive as an upper stage to direct the spacecraft on a gravity-assist maneuver past Earth or Mars. This flyby would redirect the mission into an orbit inclined by as much as 30 degrees to Earth's.
The result, the scientists say, will be the highest-performance observatory ever achieved in the decades-long history of NASA's Explorer program.
"We see EZE as a game-changer, the first step on a new path for NASA Explorers that will yield major science goals despite limited resources," said Benson, who previously managed the new electric propulsion technology project.