Well, on Friday (November 13, 2020), a small asteroid passed so close to Earth that it was just above our atmosphere: It came about 400 kilometers (240 miles) from the Earth’s surface!
This is lower than most satellites, even those in low Earth orbit. Yegadi!
The good news is that it is so small that even if it had hit us, it would not be bad and would almost certainly burn in our atmosphere.
The asteroid, originally named A10sHcN and then officially renamed 2020 VT4, was discovered by the ATLAS (Asteroid Terrestrial-impact Last Alert System) observatory in Hawaii. It was first seen around 3 p.m. after passed by the Earth, which is common. Many times the geometry of the passage makes it difficult to see before reaching the nearest approach.
The asteroid came from the east and passed an incredible 6772 km from the center of the Earth, over the Pacific Ocean. Given the radius of the Earth of 6,371
Amazingly, this is not a record. Many meteors can be seen passing through the Earth’s upper layers and returning into space, such as the 1972 Great Daily Fireball, which reaches 57 km above the earth (and which loses a significant amount of mass, leaving a trail of steam, easily visible during the day). Others like this have been seen.
But in recent years, with modern equipment, almost more gaps have been recorded. In August, asteroid 2020 QC moved 3,000 km from the Earth’s surface, and in May 2020, JJ moved 7,000 km.
So, if you don’t count the asteroids that temporarily (or forever) turn into meteors – or actually hit the ground – the 2020 VT4 is a record.
From its brightness, the size is approximately 5-10 meters. At the upper end, this is about half the diameter of the impact element from Chelyabinsk since 2013. For a given composition (say, a rock), the mass of an asteroid depends on its volume, which itself depends on the cube of the radius. Since the 2020 VT4 was at best half the diameter of Chelyabinsk, it would have about 1/8 mass. There would be only 1/50 of the table at the bottom.
The strike from Chelyabinsk had an energy yield of half a megaton of TNT. If 2020 VT4 had hit, it would have been 2-10% more. It is easy to see even during the day, but it would most likely burn about 50 kilometers above the ground. It’s big enough that small pieces could hit like meteorites, which is great. But remember no hits.
The shape and geometry of its orbit have changed a lot due to the Earth’s gravity from such a close pass. There used to be a 1.5-year period, but now it’s a little less than 0.9 years. The semi-major axis of its orbit (half the long diameter of the ellipse; think of it as the average distance from the Sun) has changed from 195 million kilometers to 136 million. For comparison, the average distance of the Earth from the Sun is 150 million km.
The orbit stretched from Earth’s orbit to Mars. It now passes from the orbit of Venus to just outside the orbit of Earth. This is an amazing change!
I will note, as always, that asteroids of this size take us quite often, but only hit us in a decade or so. Small, with a meter or so, hit us once a month! But, of course, they disintegrate upwards in the atmosphere. In addition, the Earth is large and mostly water, so not much is heard about them.
So for me, this whole thing is good news. Detection of such a small asteroid hard – they faint and move quickly in the sky – and every time we see another, it means that we are getting better and better to notice them. They are there, whether we watch or not, so I prefer to look! Soon, when bigger and better telescopes go online, we will get more warnings, better orbital predictions, and maybe if we see one big enough to do real damage, we can actually do something about it.