Look for lightning, find fireballs

The new year has started with a bang in Pittsburgh, Pennsylvania. Early on New Year’s Day, many local residents heard a loud boom and felt the ground shake, prompting calls to 911. Allegheny County quickly acknowledged the event, noting that it wasn’t an earthquake or a thunder and admitting “we have no explanation for the reports”.

The culprit was later confirmed by NASA meteor watch: it was a racing car, a very large bright ball of fire (a meteor brighter than Venus). The meteor was estimated to be half a ton, one meter wide and moving at around 45,000 miles per hour. When it exploded in the atmosphere it released the energy equivalent of a 30 ton TNT explosion which was recorded by detectors at a an infrasound station near Pittsburgh.

Although there are a few space bolide detection programs around the world, the majority are ground-based, including the NASA Meteorite Tracking and Recovery Network and the NASA All-Sky Fireball Network. However, most fireballs enter the atmosphere on the 70% of the Earth that is covered by ocean.

“Bolids are rare, and due to the limited observation areas of ground-based systems, very few bolides are detected from the ground, perhaps only a few per year,” said Jeffrey C. Smith, data scientist at the SETI Institute and principal investigator of a cooperative project with the Asteroid Threat Assessment Project at NASA Ames Research Center. “Bolide explosions are also very fast, usually only lasting a fraction of a second, so you need very fast detectors.”

Recently, scientists discovered that they have such a detector, although it was not designed to detect space rocks passing through the atmosphere. In 2018, astronomer Peter Jenniskens (also from SETI and NASA Ames) and his colleagues show that the geostationary Lightning Mapper (GLM) aboard NOAA’s GOES-16 weather satellite could be used to observe fleeting lightning from fireballs. The GLM samples transient light at a rate of 500 frames per second. It can detect fireballs from about 4 inches (1 decimeter) up to about 9 feet (3 meters) wide.

Two years ago, Smith and his colleagues began developing and training a machine learning algorithm for computers to automatically detect fireballs in GLM data. Their goal was to create a publicly accessible database of racing events and their light curves— the trajectories and the intensity of the luminous trails they left in the sky. Smith and his team described their work in the newspaper Icarus in November 2021.

The map above shows the distribution of more than 3,000 bolides detected by GLMs on board GOES-16 and GOES-17 between July 2017 and January 2022. The blue dots are the bolides detected by GOES-16; pink dots were detected by GOES-17. The lone pink spot over the Atlantic Ocean was detected by GOES-17 during its commissioning phase before being moved into its operational orbit over the west coast.

Bolides observed by both GOES-16 and -17 are recorded in stereo. On the map, the slight shift between the stereo detections is due to the different perspectives from which they were seen by each satellite. Stereo detection allows researchers to reconstruct the trajectories of fireballs in the atmosphere. These data, along with the light curves, are useful for modeling how asteroids enter the atmosphere, break up and impact the Earth. These data can also inform models that assess the risk of larger meteor impacts, while helping asteroid population studies that improve our understanding of solar system evolution.

No humans observed the New Year’s fireball over overcast Pittsburgh, but the GLM detected four light flashes. It wasn’t a particularly bright car or even the brightest on record that day, Smith said. The others were just over the ocean or in rural areas, where they were less likely to be seen.

“That’s one of the big advantages of using a geostationary satellite — we can detect events in very distant areas that are missed by ground-based observers,” Smith said. The geostationary orbits of the GOES satellites allow them to monitor the Western Hemisphere from 55 degrees north latitude to 55 degrees south. Although the coverage is not global, it allows scientists to capture unprecedented numbers of meteors in publicly available data. “At present, GLM is the only accessible tool available to obtain hemispherical coverage at the scale of bolide search.”

Currently, events identified by the computer algorithm are reviewed by humans before being added to the database. After several iterations of the program, the computer becomes quite good at correctly identifying fireballs. “Four out of five detections we make are legitimate,” Smith said. “A very small amount of manual checking is now required to eliminate false positives.”

The team’s goal is to improve detection accuracy enough that humans aren’t needed in the process, Smith said. “Then we can automatically post our bolide detections very soon after events occur, perhaps within a minute.”

NASA Earth Observatory image by Joshua Stevens, using NASA Bolides data courtesy of Jeffrey Smith/SETI. Story by Sara E. Pratt.

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