On July 24, 2012, NASA successfully launched a pair of newly developed spectrometers aboard a sounding rocket from the White Sands Missile Range, New Mexico to an altitude of 323.8 km (201.2 mi). This may not seem to have much to do with extending the life of a satellite floating between the Sun and Earth about 1.5 million kilometers (932,000 mi) away, but it does. That’s because the tests' purpose was both to test new instruments for a potential future replacement of the SOHO solar observatory satellite and to recalibrate SOHO’s existing instruments.
It’s great when a space mission lasts longer than expected. Though the history of space exploration has been punctuated by failure and even tragedy, some missions shine out, such as the Viking and Opportunity Mars Landers, which operated years beyond their very short mission objectives and, of course, Voyager, a craft that is still working a generation after its launch. However, success can bring its own problems. One of these is that a still-functioning craft may have to work with instruments never meant to last so long and are now showing their age.
A case in point is the Solar and Heliospheric Observatory (SOHO). This joint project between the European Space Agency (ESA) and NASA was launched on December 2, 1995 and is currently parked at the Lagrange point between the Earth and the Sun where gravitational forces balance, leaving it forever in the one spot. Since its launch, it’s been studying the Sun and has discovered over 2,200 comets. Originally planned as a two-year mission, SOHO continues to send back data. It’s done a great job and, more importantly, is the main source of near-real time data that helps look out for solar flares. Trouble is, the instruments weren’t designed to run for 18 years and they show it. Filters degrade, surfaces become contaminated, telescope mirrors dim... In other words, it’s going slowly blind.
There isn’t much that can be done to repair SOHO, but future missions will benefit from more durable instruments. That is the purpose of the sounding rocket test. Among its payload were two Degradation Free Spectrometers (DFS). These are similar to the spectrometers used by SOHO, but where the satellite’s are gradually failing, these are designed to avoid that fate on a future mission. Instead of conventional optics, they use a rare gas photoionization-based Optics-Free Spectrometer (OFS) and a Dual Grating Spectrometer (DGS). These are made filter-free and optics-free by using rare-gas chambers, photoelectron focusing techniques, gratings and light baffles to exclude unwanted light without filters.
The mission was mainly to test the spectrometers, which are capable of, in the words of NASA’s press release, “high cadence measurements of the highly variable Extreme Ultraviolet (EUV) solar flux and have minimal degradation over multi-year time scales while observing the sun 24/7." What that means is that the spectrometers can make precise observations of the Sun at the extreme end of the ultraviolet spectrum for years on end without the mechanism wearing out.
The other purpose was to help calibrate SOHO. In addition to the new spectrometers, the sounding rocket also carried a clone of SOHO’s Solar Extreme Ultraviolet Monitor (SEM). This was calibrated at the National Institute of Standards and Technology both before and after flight to provide a calibration check for SOHO, so observations from the satellite can be corrected. If all goes well, it may give SOHO a little more life and its successor a lot more time.
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