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World Asteroid Day started with a real bang.

On June 30, 1908, an asteroid about 65 meters wide collided with Earth’s atmosphere and exploded several miles above Siberia; the force of the blast flattened and burned millions of trees over an area of more than 2,000 square kilometers. Today, the anniversary of the Tunguska blast has become World Asteroid Day: a science holiday co-founded by a rock music legend and an Apollo astronaut.

In 2015, Apollo 9 lunar module pilot Rusty Schweickart helped launch World Asteroid Day with astrophysicist and Queen guitarist Brian May. The United Nations officially recognized the event a year later in 2016. Earlier this month, Arizona senator Mark Kelly – also a former astronaut – introduced a Senate resolution that, if passed, would officially recognize June 30 as World Asteroid Day in the U.S.

I spoke with Kevin Schindler, resident historian at Lowell Observatory in Arizona, about the origins of World Asteroid Day, the history of planetary defense, and what asteroids can reveal about the history of our Solar System.

Discovering the Danger from Outer Space

Around 200 years ago, in the 1830s, geologists began to study fossils and figure out that several mass extinctions had wiped out whole ecosystems of species on Earth in the distant past.

“In recent decades, they realized that those weren’t necessarily caused by something on Earth, but by something impacting from space – like the Cretaceous Tertiary boundary,” says Schindler.

In the 1960s, geologist Walter Alvarez discovered a thin layer of black clay in rocks around the world. Below the black line, the rocks were rich in fossils; above it, they were nearly barren. The same layer of black clay showed up all around in the world: in rock outcroppings in Italy and New Zealand, and in samples from the floor of the Pacific Ocean. And it clearly marked a deadly before-and-after moment in Earth’s history – one that happened around 66 million years ago.

Alvarez suspected that the black clay was something alien; it contained bizarrely large amounts of an element called iridium, which is vanishingly rare here on Earth but more common in asteroids. He began to realize that an asteroid or comet may have slammed into our planet 66 million years ago, kicking off a mass extinction and scattering iridium-rich black dust over the planet like a burial shroud.

The pieces came together in 1978 when geophysicists Glen Penfield and Antonio Camargo discovered the outline of a crater hundreds of kilometers wide at the edge of Mexico’s Yucatan Peninsula. Its center lies at the bottom of the Gulf of Mexico. Penfield and Camargo named the crater for one of the communities that now lies within its boundaries: Chicxulub Pueblo.

Other craters – smaller but still impressive – also make it obvious that our planet has had more than a few run-ins with meteors during its long history.

“And while there’s not as much debris floating around in our Solar System as when it was newly-formed, there’s still stuff out there,” says Schindler. “And it’s inevitable that at some point that stuff will come back and get us again.”

From Deep Impact to DART

So we’ve known almost 60 years that asteroids and comets could threaten life on Earth.

“In the 1980s and 1990s, there was a search to look for bodies that specifically could impact Earth,” says Schindler. “Phase one of all this started with, ‘okay, let’s look for these bodies that could hit us,’ and then a couple decades later is when we got to phase two, ‘what can we do about it if we do find these things?’”

Strangely enough, it was a pair of high-budget, low-scientific-accuracy Hollywood blockbusters that really brought planetary defense to public attention, according to Schindler. The summer of 1998 featured not just one but two movies about humanity trying to save itself from extinction by blowing up an incoming chunk of space rock. In Armageddon, a wildly-improbable effort by a team of offshore drillers saves Earth from an asteroid impact; in Deep Impact, a similarly-improbable effort fails to save Earth from a comet (so the summer ends in a cinematic tie).

“The good thing about those movies is that, even though they’re not scientifically accurate in every way, they certainly built awareness enough to where lawmakers said, you know, we should put some money aside to study this stuff more,” says Schindler. “Hollywood, in some ways, has helped the cause to learn more.”

And, as science fiction often does, Deep Impact and Armageddon provided thought experiments (albeit not super-accurate ones, to put it mildly) for the ideas that would eventually become actual efforts at planetary defense. According to Schindler, theoretical ideas about whether we could destroy an incoming meteor eventually shifted to ideas about just nudging the deadly object off-course.

“This is just something that’s really been developed in the last decade or so and – I wouldn’t say culminated, but really became well-known with the mission that went up to deflect the moon of an asteroid to see if it was possible,” says Schindler.

That mission was NASA’s Double Asteroid Redirection Test, or DART, in which an intrepid little spacecraft flew 7 million miles to crash into the asteroid Dimorphos and knock it off-course. Dimorphos is actually a mini-moon that orbits another, larger asteroid called Didymos. Astronomers at Lowell carefully measured Dimorphos’s orbital path around its parent asteroid before and after the impact – and they saw evidence that DART had succeeded in knocking Dimorphos into a different orbit.

It’s a long, long way from deflecting one tiny asteroid moonlet onto a different path around its parent asteroid to deflecting something the size of the Chicxulub impactor – or even Tunguska – as it’s barreling toward Earth. But the consensus seems to be that DART was a good start.

“The biggest thing, I think, was that it is possible. This was a very controlled initial step,” says Schindler. “This was certainly promising enough that we should keep doing these tests in different sizes of body and different compositions, because depending on what it’s made of, a body might react differently to something impacting it.”

Fossils of the Early Solar System

Meanwhile, Schindler and World Asteroid Day also want the public to know that asteroids are more than potential threats: they’re an orbiting treasure trove of information about the history of our Solar System and even the origins of life.

Most asteroids are chunks of rock that coalesced early in our Solar System’s history but never grew massive enough to become planets; they’re like the seeds of planets that might have been. Others are the debris left behind by collisions between objects in those chaotic early days of the Solar System, when planets were forming and gas giants migrated, scattering lesser objects in their wake.

“They tell us what the early composition was and what a chaotic time it was in the early part of our Solar System,” says Schindler.

Those clues are written not just in the chemical and physical makeup of asteroids, but in their orbital paths around the Sun. By studying and modelling how those paths have changed over the years, scientists can reconstruct how asteroids and planets may have interacted. The orbits of modern asteroids are like the “footprints” of planet formation, migrating gas giants, and long-ago collisions.

Today, NASA’s Lucy mission is exploring the asteroid belt, getting up close and personal with several of these objects. Meanwhile, NASA’s OSIRIS-APEX mission is on its way to study the asteroid Apophis, which will pass close (but not too close!) to Earth in 2029.

“And now we are studying planetary systems around other stars. Better understanding our Solar System, we can now look at others and see how typical we are,” says Schindler. “You don’t know that without knowing your own Solar System pretty well, so it really has helped us to learn about, sort of, our heritage, I guess.”

World Asteroid Day

World Asteroid Day aims to tie all of those things together, promoting awareness of planetary defense but also of the immense scientific value – and maybe monetary value, eventually – of asteroids.

At Lowell Observatory, that awareness is hard to escape; the observatory stands just an hour’s drive from Meteor Crater – which is exactly what the name suggests, a 213-meter-deep, 1200-meter-wide crater where an object about the size of a Boeing 747 slammed into the desert floor around 50,000 years ago.

“The proximity of Lowell Observatory, where we’re studying bodies in space, and Meteor crater, where we’ve seen the result of one of those bodies hitting Earth – how convenient is that? We’re looking at both ends of it, from when it’s still up in space to the final product if something like this hits.”