The hole in Earth’s ozone layer that forms over Antarctica each September was the smallest observed since 1988, NASA satellite measurements from this year have revealed.

The ozone hole reached its peak extent on September 11, covering an area about two and a half times the size of the U.S. — 7.6 million square miles in extent — and then declined through the remainder of September and into October, according to scientists from NASA. Ground- and balloon-based measurements from the National Oceanic and Atmospheric Administration (NOAA) also showed the least amount of ozone depletion above the continent during the peak of the ozone depletion cycle since 1988. NOAA and NASA collaborate to monitor the growth and recovery of the ozone hole every year.

“The Antarctic ozone hole was exceptionally weak this year,” said Paul A Newman from NASA.

The two missions — GOLD and ICON will team up to explore the ionosphere.