Last month, at least two major, but unrelated, meteor events occurred in the skies over highly populated areas of the U.S. Both fireballs, often referred to as bolides, were seen — and heard — during daylight hours, suggesting they were unusually large. University of Illinois Urbana-Champaign aerospace engineering communications coordinator Debra Levey Larson spoke with aerospace engineering professor Siegfried Eggl about the recent bolide events and how scientists track and predict their movement in the event that any sort of planetary defense is required.
How common is it for space debris of this size to enter Earth’s atmosphere?
Objects in the 1–2-meter size category, such as the recent March 17 event over Ohio, estimated at about 1.8 meters in diameter, hit the Earth’s atmosphere roughly once per year.
When objects larger than a meter in size enter the atmosphere, they almost always produce noticeable shockwaves. How powerful those are and how much damage they can do depends on the size of the objects and entry conditions, such as how fast and how steeply they dive into the atmosphere. All those objects enter at hypersonic speeds, and when they disintegrate, they convert a significant fraction of their kinetic energy into heat and a shockwave.
In 2013, a roughly 18-meter-sized object entered the atmosphere over the Russian city of Chelyabinsk. Its trajectory was quite flat, and the resulting shockwave was powerful enough to destroy windows, sending more than 1,000 people to seek medical care afterward.
The meteorite that lit up the skies over Ohio wasn’t detected until it entered Earth’s atmosphere. Was it too small to be seen earlier?
Yes, such small objects are very difficult to spot. We basically have no idea how many of those objects are out there, and that will likely not change much at the current level of investment. They are very, very faint and light pollution caused by satellites doesn’t help astronomers in that regard.
On the other hand, most of those objects don’t reach the ground, so they are only dangerous to air traffic and satellites. Even then, the probability of collision is relatively low because the objects move so fast. The more airplanes are up and the more satellites we have, the higher the eventual risk of collisions.
How do you and your colleagues use information about events like these to better understand and predict orbital debris entering our atmosphere?
My research group at U. of I. is more concerned with “city-killers” and larger asteroids — like the asteroid 2024YR4 that briefly had a non-negligible chance to hit the Earth or the moon in 2032 — and leave the study of smaller meteorites to specialists.
We do, however, use the information smaller bolides provide to estimate the orbital distribution on the lower end of the size scale. A very recent near-Earth object model explicitly accounts for those.
What are the biggest sources of uncertainty when modeling the path of a fast-moving object entering the Earth’s atmosphere?
Often we detect such objects fairly late, which means we only have a tiny fraction of the orbit and very little time to predict where the objects end up. Other big sources of uncertainty, especially during daytime, are the lack of background stars for proper astrometry. Background stars can be used as navigation beacons that can tell astronomers very accurately where objects are. During daylight, it is very difficult to determine the actual path of the bolide. In the best-case scenario, observers have detected the same event from multiple stations. Then the trajectory can be triangulated relatively well.
Another challenge is that these objects become very bright very quickly, which sometimes also causes issues with locating the center in detectors. Finally, once they are slowing down, atmospheric uncertainties, such as local wind speeds, can also play a role in determining the exact trajectory.
How does an event like the Ohio meteor fit into the broader context of planetary defense planning?
Since we know so little about objects 50 meters in size and smaller, the plan is not to deflect them but to provide as much warning time as possible. If needed, FEMA would evacuate regions in the U.S. that would be affected by an impact.
Are there any efforts being made to help with detection?
The Vera C. Rubin Observatory, located on Cerro Pachón in Chile, is coming online this year and will significantly boost our ability to detect asteroids between 50 and 160 meters in size. After 10 years, the aim is to know where roughly 90% of all potentially hazardous asteroids, 140 meters and larger, are located and whether they pose a risk of impact in the next century. In 2027 NASA is planning to launch the Near-Earth Object Surveillance Mission, which could help us spot asteroids that orbit interior to the Earth.
Editor’s note: To contact Siegfried Eggl, email eggl@illinois.edu. Edited by News Bureau physical science editor Lois Yoksoulian. Aerospace engineering is part of The Grainger College of Engineering.
