Picture of the Asteroid Ida and its Moon

By David Dannecker
Senior Editor

On Monday, January 26 this week, Earth had a close call with the asteroid 2004 BL86. Late Monday morning, the asteroid in question passed by our planet at a distance of just 1.2 million kilometers. That’s only three times further from the Earth than the Moon’s orbit, and plenty close enough for the asteroid to be seen through an average telescope. This particular asteroid is about 300 meters in diameter and has a small moon of its own, judged to be between 50 and 100 meters in diameter. No asteroid this big is expected to pass as close to Earth for the next 12 years, so Monday’s event is notable.

Luckily, 2004 BL86 was only passing through the neighborhood, and we avoided an actual impact. But what would the consequences have been if there had been a collision on Monday? And how does this asteroid compare to the other asteroids that populate our solar system?

At 300 meters in diameter, 2004 BL86 pales in comparison to the asteroid that is thought to have been responsible for the K-T mass extinction event that wiped out the dinosaurs. That asteroid is estimated to have been 10 to 12 kilometers in diameter, or at least 33 times further across than Monday’s interloper (and significantly more massive, as volume scales faster than length). Collisions with asteroids on that scale are far more likely to cause mass extinctions, since the impacts are so powerful as to have global consequences. The roughly 10 kilometer asteroid left behind a crater 100 kilometers in diameter and scattered iridium-rich sediment around the globe. Earth would not have been at risk of repercussions on that scale if 2004 BL86 had collided on Monday. Asteroids larger than 10 kilometers are not actually that common in the solar system; out of the tens of millions of asteroids that orbit the Sun, only about 12,000 are larger than 10 kilometers in diameter, and astronomers believe we have identified 94 percent of the asteroids at that scale.

2004 BL86 falls into a category of more average asteroids that are a few hundred meters in diameter. This category includes many of the millions of asteroids that have been discovered, and likely many more that have yet to be noticed. What effects might a collision with an asteroid of this scale produce? While not causing the widespread devastation of larger asteroids, according to National Geographic, asteroids of this middle size would still be fairly catastrophic, devastating whatever region they impacted and potentially destroying entire nations. Given that assessment, we should consider 2004 BL86’s flyby extremely fortunate. Even more fortunate, collisions of that scale are only believed to occur once every half a million years. In addition to these average-sized asteroids, there are millions and millions of smaller, harder-to-detect asteroids that are in the range a few meters to a few dozen meters, only the largest of which would avoid burning up in the atmosphere. Many readers might recall the meteor that exploded over Russia in 2013; that object was about 18 meters in diameter, and while it didn’t impact the Earth’s surface, the explosion was enough to cause over a thousand injuries.

But what are asteroids doing in Earth’s neighborhood anyway? It is true that the highest concentration of asteroids is found in the main asteroid belt that lies between the orbits of Mars and Jupiter. These asteroids would likely have coalesced into a planet in that orbital region long ago, were it not for Jupiter’s immense gravitational influence. Besides the belt asteroids however, there are other asteroids whose regular orbits are dispersed throughout the solar system. A belt of asteroids known as Near Earth Asteroids (NEAs) exists in the vicinity of Earth’s orbit. This belt consists of approximately 12,000 asteroids one meter or larger, of which fewer than 1,000 are larger than one kilometer in diameter. These asteroids tend to orbit for a few million years before colliding with the Sun, colliding with a planet, or being slingshotted out of the solar system altogether. Given that NEAs have such a short lifespan, it’s generally believed that a gradual, but constant, supply of asteroids is nudged out of the main asteroid belt by way of gravitational interactions with Jupiter. So as it turns out, Jupiter’s orbital dynamics are responsible for both the main asteroid belt, and our own smaller belt of NEAs.

Despite the apparently constant stream of NEAs being thrown Earth’s way, astronomers are fairly sure of the low probability of a significant impact. Large extraterrestrial objects are searched for, identified and tracked long before they cross paths with Earth, so the general wisdom suggests we would know a collision was coming long before its arrival; “Armageddon”-style scenarios, where Texas-sized asteroids are noticed with only 18 days to prepare, are incredibly unlikely (especially considering there is literally only one asteroid that large in the entire solar system, and we know where it is). As mentioned, 2004 BL86’s passage is the closest call we’ll have until 2027, when the asteroid 1999 AN10 will pass by Earth on schedule. So humanity can rest easy this week, and probably every week, since astronomers are constantly scanning to find asteroids before they find us. Newer and better telescopes are being constructed. Events like 2004 BL86’s passing give astronomers the chance to observe asteroids up close to learn more about their behavior, and new technologies may aid in the search even further. But with millions of asteroids out there, thank goodness humans have a fascination with watching the skies.


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