The scientists of the BARREL team sent their last balloon soaring toward the stratosphere over Sweden on Aug. 30, 2016, capping off four years of launches to study how electrons precipitate into Earth's atmosphere.

Throughout August 2016, the BARREL team was at Esrange Space Center near Sweden's northernmost town of Kiruna, launching a series of instrument payloads on scientific balloons. BARREL, short for Balloon Array for Radiation-belt Relativistic Electron Losses, is funded by NASA to study X-rays in Earth's atmosphere near the North and South poles.

These X-rays are caused by electrons that rain down into Earth's atmosphere from the giant swaths of radiation that surround Earth, called the Van Allen belts. Understanding this radiation and its interaction with Earth's atmosphere helps us to better protect satellites that orbit near Earth.

In addition to X-ray instruments, several of the BARREL balloons on this campaign also carried one of two student-built instruments. One of the instruments was designed to study the total electron content of Earth's ionosphere - a layer of charged particles overlapping with Earth's neutral atmosphere.

The other measured low-frequency electromagnetic waves that help to scatter electrons from space down into Earth's atmosphere.

This was the fourth series of balloon launches for BARREL, but this year's campaign provided unique scientific opportunities.

"We were able to launch eight payloads," said Robyn Millan, principal investigator of BARREL and a space scientist at Dartmouth College. The team was able to observe parts of near-Earth space at the same time as other key NASA missions. "And we obtained multiple conjunctions with both the Van Allen Probes and MMS spacecraft."

NASA's four MMS spacecraft began the science phase of their mission in September 2015, midway through BARREL's third balloon campaign - making this the first opportunity for the BARREL team to coordinate with MMS throughout a complete series of launches. The conjunctions with MMS, the Van Allen Probes, and other missions mean that the team has a more comprehensive data set than ever before.

"BARREL, like any science mission, is a stand-alone campaign - we are able to do great science with just our payloads and science instruments," said Alexa Halford, a BARREL team member based at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

"But by collaborating with such a large group, we're able to see connections between events that we would have perhaps never even thought to look for if we were only working by ourselves."

This was the final campaign of the BARREL mission, said Millan. But the team has come far in the decade since BARREL was first proposed.

"We designed and built 45 balloon payloads, carried out eight test flights and 55 science flights, managed the logistics of campaigns from four different locations, and obtained an incredible amount of data that will continue to be analyzed for years to come," said Millan.

"I am so proud of our team for all of their hard work, dedication and accomplishments."

BARREL is led by Dartmouth College in Hanover, New Hampshire. The undergraduate student instrument team is led by the University of Houston and funded by the Undergraduate Student Instrument Project out of NASA's Wallops Flight Facility at Wallops Island, Virginia.