On the morning of Aug. 13, a mysterious BOOM rattled windows and Utahns’ nerves across the Wasatch Front. Immediately, people speculated on social media: Was it thunder? Construction? Jets from Hill Air Force Base? The answer came via grainy video feeds from residential security cameras—it appeared that a meteor had crashed into Earth’s atmosphere to produce what has been called a “sonic boom.”
Ben Bromley, a University of Utah professor of astronomy who researches planetary formation, spoke with @theU about the meteor.
When you heard the sound, did you know immediately that it was a meteor?
No. I was getting ready to run some Saturday morning errands when I heard what I thought was a rocket boom. I went through a list of possibilities—loud neighborhood kids, my own kids, road construction, thunder—I guess I should have thought of a meteor impact, given that I study these impacts on other planets, but I didn’t.
Are we sure that a meteor caused the noise?
I’m certain. People’s security cameras caught a visible trace in the sky that was the meteor coming through the atmosphere. We have weather satellites that caught light flashes that are consistent with a meteor hitting the atmosphere, and that boom noise is an expected outcome of something like this.
Why did it make that boom noise?
The noise comes from the meteor. This thing is cruising in, going tens of thousands of miles per hour, until it crashes into our atmosphere. The extreme friction between the meteor and the air rapidly heats up the material in the meteor until explodes in a fiery air burst.
Is this type of impact rare?
We get inundated with tiny micro-meteors and meteorites constantly. Tons of them fall on the Earth every year. Something that could make the noise we heard had to have been the size of a mini fridge or so. We can guess that this meteor was around 3-feet-long by the amount of energy required to hit the Earth’s atmosphere, creating an air burst as loud as the one we heard.
We know it wasn’t the size of a bus, as was the case in the Chelyabinsk meteor that exploded over the Russian region in 2013. Saturday’s event didn’t have the signature characteristics of a meteor crash that big—the Chelyabinsk meteor flashed across the sky as bright as the sun before plunging through the atmosphere with such power that windows in local buildings shattered.
Was the sound a ‘sonic boom’?
A sonic boom occurs when an object is moving faster than the speed of sound, displacing the air as it tears through the atmosphere. The ‘boom’ happens because all sound make by the supersonic object is bunched into a single pressure wave when it reaches us. I’d feel better calling the noise we heard a blast wave rather than a sonic boom. The meteor was certainly moving faster than the speed of sound, but I think the air burst itself likely contributed to boom.
How often do big meteors hit the Earth?
There is lots of debris floating around in the solar system that hits our atmosphere constantly. Most are too small to make a noise – instead, they burn upon entering the atmosphere and produce the falling stars that you may see in Utah’s dark skies. Collisions that are big enough to produce a boom like this happen about once per year—they’re not too destructive because they blow up tens of miles in the upper atmosphere. The bus-sized objects like the Chelyabinsk meteor, which was about 65 feet across, happen once every 50 years. When these hit the atmosphere, they produce energy equivalent to 20 to 30 times the energy of an atomic bomb. Even rarer are those like the Tunguska event in 1908 which happen roughly once every 500 years. This meteor is estimated to have been about 200 feet across. When it exploded over the sparsely populated Eastern Siberian region, the blast leveled around 80 million trees in 800 square miles of forest.
Luckily, the miles-in-diameter-meteor that caused the dinosaur extinction was even rarer, an event that happens once in tens of millions of years.
How does this meteor event coincide with your research?
One of the things that I liked about his event is that I study planet formation in the solar system. Planets form when stuff hits other stuff in space—it’s all about collisions. My students and I work on techniques, data science strategies and code that have applicability to problems that are important here on Earth. But it doesn’t really feel on a day-to-day level that we’re studying something that’s relevant to us humans. Then on Saturday, it happened. Yes.
For that brief window-shaking moment, all the relevance became clear.