The GOLD mission will launch later this month on a commercial satellite.
The last time scientists got a good look at half the Earth in far ultraviolet light came in 1972, when astronaut John Young snapped some photos with a special camera during a spare moment of the Apollo 16 mission to the Moon. Since then, heliophysicists and other scientists interested in the complicated interplay between Earth’s upper atmosphere and outer space have suffered from a paucity of data.
That should finally change later this year, after two NASA-sponsored satellites begin to collect data about the composition and temperature of the ionosphere, ranging from an altitude of about 60km above Earth to more than 1,000km. Scientists used to think the Sun’s radiation dominated the Earth’s extremely tenuous atmosphere at this altitude, but in the last decade they have begun to understand that weather at the planet’s surface also can change conditions far above.
“We’d really like to be able tease out the effects from the Sun above and the Earth below,” said Sarah Jones, a mission scientist with the GOLD (Global-Scale Observations of the Limb and Disk) mission that is scheduled to launch later this month. She and other mission scientists spoke this week during a NASA briefing. “The ionosphere is a really dynamic place.”
A public-private mission
The microwave-sized GOLD mission will launch as a component on a much larger communications satellite, SES-14. This marks the first time NASA has used this commercial pathway, known as a hosted payload, for one of its science missions. It did so to help keep costs down. According to NASA, the cost for this mission is capped at $55 million.
The satellite has already been delivered to Kourou, in French Guiana, for a scheduled launch on Jan. 25th aboard an Ariane 5 rocket. After reaching a geostationary transfer orbit, the spacecraft will take a few months to reach a geostationary orbit at 35,000km above the Earth. From there it will have a grand view of Earth’s western hemisphere.
GOLD’s primary scientific instrument is an ultraviolet-imaging spectrograph, which will take images of the entire disk of Earth in ultraviolet light and then break it down by wavelength. Analysis of each wavelength will provide information about the temperature, density, and composition of the ionosphere at that moment. Earth’s entire disk can be imaged every 30 minutes, providing ongoing views of how the upper atmosphere changes over time.
Presently, models that seek to describe the behavior of the ionosphere break down after less than 24 hours, said Richard Eastes, the mission’s principal investigator. Previous missions have observed a small location of the ionosphere, or only been able to observe a larger area for a short period of time. GOLD will provide a continuous stream of data across an entire hemisphere of the planet.
A second satellite
The behavior of the ionosphere isn’t just an academic question. Its charged particles, such as ions and electrons, can garble radio wave signals propagating through the upper atmosphere from one part of the Earth to another. In particular, dense bubbles of this charged gas rise over the equator and tropics at unpredictable intervals. Better understanding the ionosphere and modeling its behavior should improve communication systems used by the aviation and maritime industries, as well as the military.
After its cruise to geostationary orbit, and subsequent checkouts, scientists anticipate beginning to take data from GOLD by around October. At that time it should be paired with another mission, ICON, also dedicated to studying the ionosphere. This second NASA mission, a small, dedicated satellite, will travel through the ionosphere itself in low-Earth orbit making in-situ observations.
Carrying an interferometer and other instruments, ICON will complement the GOLD mission by providing fine-scale detail measurements of the larger phenomena observed by the satellite in geostationary orbit. ICON should launch later this year with a two-year primary mission. By early in the 2020s, then, space scientists should have a much better grasp on Earth’s final frontier—its interface with the rest of the universe.
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