Earth’s Planetary Protection in Action

Asteroid 2020 HS7, as seen by Tautenburg Observatory in Germany on April 28, 2020, shortly after the space rock was first spotted.
(Image: © ESA/Tautenburg Observatory, S. Melnikov, C. Hoegner, B. Stecklum)

On April 27, Pan-STARRS Observatory in Hawaii identified a previously unknown space rock that came remarkably close to Earth.

The asteroid, now named 2020 HS7 had such a close shave that first-hour observation suggested a 10% chance of colliding with Earth. This sent planetary defense experts around the world busy studying the trajectory.

This NASA graphic shows the path of the newfound asteroid 2020 HS7, which passed safely by Earth on April 28, 2020 at a distance of 23,000 miles (36,400 km). (Image credit: NASA/JPL-Caltech)

But as more scientists studied the nature of its trajectory over the next hour, their concerns dissipated. 2020 HS7 was on a safe path and would flyby without any trouble

It passed Earth at a distance of 36,400 kilometers when it made its closest approach at 12:21 am IST. That range is close to the orbits of some geosynchronous satellites 36,000 km above Earth.

In fact, it was just 1,200 km from the nearest satellite in geostationary orbit. The space rock passed below the satellite and left it undamaged

These observations also showed that the object was just 4 to 8 meters wide, suggesting that even if it had collided with Earth, it would simply burn up in the planet’s thick atmosphere.

“Small asteroids like 2020 HS7 safely pass by Earth a few times per month,” NASA’s Planetary Defense Officer Lindley Johnson said in a statement released on April 28. “It poses no threat to our planet, and even if it were on a collision path with Earth it is small enough that it would be disintegrated by our Earth’s atmosphere.”

According to ESA, the flyby is one of the 50 closest on record.

A larger space rock’s flyby

An animated orbital diagram shows the four inner planets and asteroid 2020 HS7, including the space rock’s close pass by Earth in April. (Image credit: ESA)

Coincidentally, the alert came as a rather large asteroid called 1998 OR2 was making its closest approach to Earth.

With a diameter of about 2 km, 1998 OR2 is much larger than 2020 HS7. But it’s also passing Earth at a much greater distance – about 6.9 million km. That’s about 16 times the distance between the Earth and the moon (about 385,000 km).

Despite its size, asteroid 1998 OR2 is too small and dim to be seen with the unaided eye.

Both these instances show how planetary defense systems work. After identifying as many objects as possible, they track them long enough to plot their orbits.

If observations show the impact probability increasing, alert systems are enacted to prepare areas at risk and evaluate potential mitigation approaches. But, those systems weren’t necessary for 2020 HS7.

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