Two years ago, US government sensors detected an extremely bright light over the western Pacific. Fortunately, it came from the passage of a large meteorite through the atmosphere, rather than a potential military threat, but it was so strong astronomers thought it deserved further investigation. Scanning through archival images, a team from the University of Western Ontario found what appears to be the asteroid’s trail 10 minutes before it hit Earth’s atmosphere.
Meteorites are extremely important sources of information about the formation of the Solar System. Those belonging to rarer categories are especially valuable to astronomers. Recently, the increasing availability of video cameras has made it possible to track the path of the incoming object through the atmosphere. From this, it is sometimes possible to calculate the trajectory of the object before it fell, determining the sources of different types of meteorites.
After the detection known as CNEOS 20200918, astronomers tried to do better, figuring out whatever made a flare so bright should have been detectable in astronomical photographs. In a paper submitted to the Planetary Science Journal they report that their suspicion was correct, with the help of additional detections from the ATLAS Haleakalā telescope in Hawaii and the Geostationary Lightning Mapper, an infrared detector carried on a satellite.
Before its encounter with Earth’s atmosphere, asteroid CNEOS 20200918 was about 3 meters (10 feet) across and weighed about 23 tons, the authors estimate, although they acknowledge that this is based on some assumptions about its density. Ten minutes before it hit the atmosphere, when the image was taken, it was 11,900 kilometers (7,500 miles) from Earth, which believe it or not is pretty slow for a space rock. Its orbit was slightly longer than Earth’s but more elongated, so it crossed our path twice a year. The variation in its brightness suggested that it was spinning unusually, but not exceptionally, fast.
It’s not the first time an object has been photographed both in space and burning up in the atmosphere. However, the previous five cases all involved calculating the trajectory of an asteroid before impact, so we were on the lookout, most recently in the case of the object that hit off the coast of Iceland in March. This particular event gained internet fame when it was described in the highly remarkable “half giraffe” measurement.
The authors, led by Dr David Clark, have made similar efforts to find the asteroids responsible for the fireballs (very bright meteors) in archival photographs, but so far without success.
Because the meteorite sank in the western Pacific, the chances of recovering any pieces are negligible. The precision with which the orbit has been calculated increases the scientific value of matching it to the composition of any fragments. However, it is unlikely to be considered so valuable that there will be moves to retrieve it from the ocean floor, as happened with a 2014 meteorite believed to have come from outside the Solar System. It takes a really special space rock to warrant that kind of effort.
On the other hand, the authors hope that if we can repeat this success with future bright meteorites, we will one day succeed with one that lands somewhere more accessible.
A preprint of the paper is available at ArXiv.org.
[H/T: New Scientist]