SAGE III-ISS Picks Up Where Others Left Off
By: Denise Lineberry
NASA's Stratospheric Aerosol and Gas Experiment (SAGE) III has reached its third generation in a lineage of remote sensing instruments that study the Earth's atmosphere and protective ozone layer.
Most recently, it lived in orbit during the SAGE III Meteor-3M satellite mission, which was a joint partnership between NASA and the former Russian Aviation and Space Agency (RASA), now known as Роскосмос. That satellite lost power more than five years ago.
The next, and possibly most challenging destination for a sibling of SAGE-III will be on the International Space Station (ISS), where it will serve as a pathfinder for NASA's Earth Observing System.
Engineers at NASA's Langley Research Center have been running functional tests on the instrument as they prepare SAGE-III ISS for Systems Requirements and Mission Design Reviews, which are scheduled to take place on December 13 and 14.
In late August, SAGE III-ISS took cover at Langley when Hurricane Irene threatened the East Coast. It recently emerged from its storage container in a Langley clean room facility to prepare for the light of the sun and the moon.
From within a clean tent at the edge of an opened storage bay at NASA Langley in February, the instrument completed preliminary sun and moon look tests, scanning along their vertical centerlines.
According to Charles Hill, a NASA postdoctoral fellow from Langley’s Chemistry and Dynamics Branch, that first look after a decade in storage under a dry nitrogen purge, gave the SAGE III-ISS team the confidence for it to hitch a ride into space.
"The tests ensured that the performance of the SAGE III instrument itself has not degraded since its last sun and moon look test in December of 2000," said Hill to a class of Hampton University's Atmospheric and Planet Science graduate program students.
The instrument moon look test occurred during the wee hours of the morning. "It was a long night with no sleep, but we did get some great data," said Hill, who graduated from the program almost two years ago. He wanted to update the group on the future of SAGE III, because Hampton University is one of many partners for the experiment.
The day- and night-long tests measured the spectrum of sunlight and moonlight arriving at the surface of the instrument, which is a scanning, CCD (charge coupled device) -based grating spectrograph. As the ISS orbits Earth, SAGE III will observe sunrises and sunsets while passing between regions within direct sunlight and regions within the solar cone of umbra, the darkest part of the shadow where the light source is completely blocked by an obstructing body.
Similarly, the instrument will observe moonrises and moonsets. During such events, opportunities will exist to measure light from either the sun or moon after it passes through Earth’s atmosphere. Those measurements will be used to retrieve vertical profiles of atmospheric chemical and aerosol components. The technique is referred to as atmospheric "occultation measurements."
Before SAGE-III ISS launches from NASA’s Kennedy Space Center in late 2014 or early 2015 in the unpressurized trunk of a SpaceX Falcon 9 Dragon module, the team hopes to conduct one or, ideally, two more sun and moon look tests at Langley to calibrate the instrument boresight alignment and wavelength registration, and to test a new solar attenuator that might be installed.
The instrument will need to be qualified and a nadir-viewing platform, which is in its design phase, will be added so that the instrument scan head will be aligned within one degree of its nadir, or point position toward the Earth's center of mass.
Hurdles lie ahead for the mission, and the ISS environment can be challenging at times. The instrument optics and thermal radiators will have to be protected from ammonia and silicon gas coming out from the space station.
Also, the orbital motion of the ISS admits a slow wobble. To maintain the instrument scan head’s orientation with respect to the nadir direction, a hexapod, which will move SAGE III with six degrees of freedom, will be installed to compensate for the wobble. The hexapod was manufactured by Thales Alenia in Italy under contract to the European Space Agency (ESA) and is scheduled for delivery to NASA Langley 18 months prior to launch.
Once complete, the instrument payload will include five pieces of hardware built at NASA Langley, two from the European Space Agency and one from Johnson Space Center's ISS program. The nadir-viewing platform includes three new pieces of hardware from Langley, and two existing pieces from JSC’s ISS program. The payload and the platform are still in the design phase.
Looking past the launch and assembly of SAGE III-ISS, there are proposals being structured to reassemble the Flight of Opportunity (FOO) Instrument, a possible third mission of SAGE III.
Originally, the three individual SAGE III instruments were given the three-letter designations M3M, ISS and FOO, indicating that the first mission had been slated to operate aboard Meteor-3M, the second aboard the International Space Station and the third is awaiting an undetermined Flight of Opportunity.
With each new generation, comes new opportunity.