Measuring Helium in the Exosphere

When solar weather reaches Earth, it heats the gas in the outer atmosphere and increases drag against satellites and other spacecraft in orbit. To better predict these effects of space weather, scientists need more information on what exactly happens up there.

Goddard Heliophysicist Hyunju Connor plans to modify an instrument developed for the Mars Atmosphere and Volatile Evolution (MAVEN) mission to study the extreme upper reaches of Earth’s atmosphere – called the exosphere. This thin realm starts above 310 miles (500 km), and Connor specifically wants to measure helium in the area from 430 to 930 miles (approximately 700 to 1500 km).

The Neutral Gas and Ion Mass Spectrometer (NGIMS) instrument built by Goddard planetary scientists Paul Mahaffy and Mehdi Benna can accurately measure as little as 1,000 helium molecules

per cubic centimeter, Connor said. She is using Internal Research and Development (IRAD) funding to adapt NGIMS to a small satellite – something the size of a medium to large kitchen appliance.

“The technology itself is already flight proven,” Connor said. “Now I want to bring that instrument to Earth to understand how our outer atmosphere behaves.”

The NGIMS instrument was tested in Earth’s upper atmosphere before being sent to Mars, Benna said. “We honed our techniques on planetary missions,” he said. “Now we are bringing all those measurement improvements back to Earth science.”

Solar weather stirs the diffuse gasses of the upper atmosphere and exosphere, causing unpredictable amounts of drag on spacecraft. That subtle pressure of atoms striking a moving object affects satellites’ orbits as well as how accurately mission controllers can point spacecraft communications arrays at ground stations to transmit data back to Earth.

Currently, mission controllers infer density changes in the outer atmosphere by measuring changes in spacecraft orbit-keeping maneuvers and pointing tasks, then calculating the additional fuel or spin from reaction wheels those tasks require. In-situ measurements like those Connor’s ISEND mission could provide would improve our understanding of the weather up there and help predict changes in drag on spacecraft for more accurate maneuvering operations.

Connor said her ISEND concept would complement NASA’s Geospace Dynamics Constellation (GDC) mission currently under development. GDC would use a constellation of six identical satellites to measure helium and heavier atoms at 200 miles (350 km) – where the International Space Station and many satellites orbit.

“If you could fly several small satellites above the GDC constellation,” Connor said, “it would provide a third dimension to better understand how solar weather affects our spacecraft.”

She became interested in Earth’s exosphere after studying soft X-rays emitted when solar winds interact with hydrogen. These gases fill an increasingly thin, hazy bubble that can extend beyond the orbit of the Moon, or more than 240,000 miles from Earth.

Connor said she is focusing on helium, a noble gas that is easier to measure, but behaves similarly to hydrogen in response to space weather. The data ISEND returns will help fill gaps in our understanding of these thin exosphere environments, she said.

Studying the dynamic space environment is critical to understanding how Earth’s atmosphere evolves, our planet’s relationship with its star, and how to improve remote sensing for future terrestrial and planetary missions, Connor said.