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Well, damn it: on Friday (November 13, 2020), a small asteroid passed so close to Earth that it was just above our atmosphere – it came within 400 kilometers (240 miles) of the Earth’s surface!
It is inferior to most satellites, even those in low orbit. Yegads!
The good news is that it’s so small that even if it hit us it wouldn’t hurt, and it would almost certainly burn in our atmosphere.
The asteroid, initially called A10sHcN and then officially renamed 2020 VT4, was discovered by the Asteroid Terrestrial-impact Last Alert System (ATLAS) observatory in Hawaii. It was first seen around 15 hours after has passed the Earth, which is common. Many times, the geometry of the pitch makes it difficult to see before it gets closer.
The asteroid arrived from the east and crossed a staggering 6,772 km from the center of the Earth, above the Pacific Ocean. Considering the Earth’s radius of 6,371 km, that means it only passed about 400 km above the Earth’s surface.
!!!
Surprisingly, this is not a record. Many meteors have been seen to pass through Earth’s upper atmosphere and return to space, such as the 1972 Great Daylight Fireball which lowered to 35 miles above the ground (and lost a significant amount of mass as it did, leaving behind it a vapor trail easily visible during the day). Others like this have also been seen.
But in recent years, with modern equipment, more near misses are being recorded. In August the asteroid 2020 QC passed 3,000 km from the Earth’s surface and in May 2020 JJ moved to 7,000 km.
So if you don’t count asteroids that temporarily (or permanently) become meteors – or actually hit the ground – 2020 VT4 is a record.
From its brightness the size is about 5-10 meters. At the upper end, this is about half the diameter of the 2013 Chelyabinsk impactor. For a given composition (say rock), the mass of an asteroid depends on its volume, which in turn depends on the cube of the radius. Since 2020 VT4 was at most half the diameter of Chelyabinsk, it would have had about 1/8 the mass. At the low end it would have had only 1/50 the mass.
The Chelyabinsk impact had an energy yield of half a megaton of TNT. If 2020’s VT4 had been reached, it would have been 2-10% higher. Easily visible even during the day, but most likely it would have burned about 50 kilometers above the ground. It’s big enough that small pieces could have been hit like meteorites, which is great. But remember that he didn’t hit.
The shape and geometry of its orbit have changed a lot due to Earth’s gravity from such a close pass. It used to have a period of 1.5 years, but is now just under 0.9 years. The semi-major axis of its orbit (half the long diameter of an ellipse; think of it as a kind of mean distance from the Sun) has changed from 195 million kilometers to 136 million. The average distance of the Earth from the Sun is 150 million km, for comparison.
The orbit extended from within Earth’s orbit to beyond Mars. Now, it goes from the orbit of Venus just outside of Earth’s orbit. It is an extraordinary change!
I will notice, as I always do, that asteroids of this size overtake us quite frequently, but only affect us in the decade or so. The small ones, about one meter in diameter, hit us once a month! But they obviously disintegrate in the atmosphere. Also, the Earth is large and mostly water, which is why we don’t hear much about them.
So this whole thing is good news for me. Locate such a small asteroid hard – they are faint and move fast in the sky – and every time we see another one means we are getting better at detecting them. They are out there whether we watch or not, so I prefer us to watch! Soon, as bigger and better telescopes go online, we’ll get more warnings, better orbital predictions, and perhaps, if we see one big enough to do real damage, we can actually do something about it.
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