747-size asteroid grazed by Earth, and scientists didn’t see it coming

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A large asteroid as large as the wingspan of a Boeing 747 passed close to Earth recently, and scientists didn’t even see it coming.

Dubbed 2021 SG, according to the Minor Planet Center of the International Astronomical Union backed by NASA, the asteroid has a diameter of between 42 and 94 meters, with an average diameter of 68 meters. This size is about the wingspan of a 747, and is also about the height of Cinderella’s Castle at Disney World and about half the size of the Great Pyramid of Giza.

The asteroid flew over Earth on September 16, as noted by asteroid tracker EarthSky. However, scientists did not detect it until the next day.

This may seem surprising, given the number of asteroids regularly detected by astronomers and space agencies around the world. However, this asteroid came from a relative blind spot: the direction of the Sun.

Artist's impression of the Parker probe as it approaches the Sun. (credit: NASA / John Hopkins / APL / Steve Gribben) Artist’s impression of the Parker probe as it approaches the Sun. (credit: NASA / John Hopkins / APL / Steve Gribben)

Most asteroids detected by agencies like NASA arrive on Earth from “the front”, which means that they are coming from the direction facing the interior of the solar system, towards the Earth and the Sun.

But there are asteroids coming from “behind”, heading towards Earth from the direction of the Sun and heading outward.

For this reason, it is very difficult to see these objects as they approach Earth, especially since they often tend to approach during the day when visibility is poor due to glare. of the Sun.

Usually the best time to spot these objects is at dusk. This is the case for all objects in the space between the Earth and the Sun, such as the planets Mercury and Venus.

According to EarthSky, the asteroid passed a very close distance, about half the distance between Earth and the Moon. It was closest to the planet at around 4:28 p.m. EST, passing through Greenland and Canada. This is a very close call, as most asteroids that pass near the planet do so much further.

And while it may seem like an isolated incident, it may not be.

On September 7, astronomers in Arizona detected an asteroid, now designated 2021 RS2, coming from the direction of the Sun just hours before it passed near the planet, as EarthSky noted. Although it is small, around 3.5 meters, it has only traveled 15,340 km. far from the surface of the planet.

It’s the closest an asteroid has hit the planet in 2021, although if it had hit it it probably wouldn’t have done much damage, if any.

We see an asteroid fall to Earth, disintegrate in the atmosphere (illustrative). (credit: Wikimedia Commons)

But the same cannot be said for 2021 SG. With its large size and top speed of 85,748 km / h (approximately 23.8 km / s), the asteroid certainly could have made an impact if it had struck.

For comparison, the last known significant asteroid impact occurred on February 15, 2013, when an asteroid exploded in the air over Chelyabinsk, Russia. The asteroid pointed at 17 meters, and although it claimed no casualties, the shock wave from the explosion shattered the windows of six different Russian cities and forced 1,500 people to receive medical treatment.

Chelyabinsk Meteor, February 15, 2013 (credit: Wikimedia Commons)

With its larger size, the possible destruction that 2021 SG could have caused is difficult to estimate, but would certainly be much worse.

Asteroids closer to the Sun than to Earth show a noticeable hole in most near-Earth asteroid readings because they are so difficult to detect. Therefore, understanding them is of vital importance.

“One of the reasons it’s difficult to meet the goal of finding 100% of all Near-Earth Objects (NEOs) over a kilometer long is that some have orbits that help ‘hide’ them from observers. terrestrial, ”said University of Hawaii astronomer Dave Tholen, who was part of a 2021 study that discovered PH27, an asteroid closer to the Sun that orbits faster than any object in the system. solar except for Mercury.

The impact of an asteroid remains one of the most dangerous natural disasters possible, although they are unlikely. It is for this reason that astronomers around the world, including at NASA’s Planetary Defense Coordination Office (PDCO), are working to monitor all nearby asteroids and calculate their trajectory to see if the one of them poses a threat to the planet.

This is done through the use of special “asteroid chaser” telescopes, but some projects, such as the Double Asteroid Redirection Test (DART) mission launched by PDCO and Johns Hopkins University, are looking to find ways of doing so. defend against the asteroids themselves.

But currently, NASA has no way of accurately detecting asteroids closer to the Sun.

However, that may soon change. Currently, NASA is building a new space telescope that could help. Called the Near-Earth Object (NEO) Surveyor space telescope, it is expected to launch in 2026 and will orbit between the Earth and the Sun, allowing it to better detect these objects. It is hoped that NEO Surveyor will be able to help find around 90% of near-Earth asteroids 140 meters or more wide – a size that could see them capable of destroying a city on impact.

In March, NASA announced that the planet was at little or no risk of an asteroid impacting the planet over the next century, following calculations by astronomers that 9942 Apophis – a massive asteroid of 340 meters – will pass safely through the planet at a distance of less than 32,000 km. April 13, 2029.

However, as the 2021 SG discovery shows, the risk of unexpected asteroids closer to the Sun remains a possible threat.

“Objects like this … are hard to find and track,” Tholen said. “There are probably more such objects that have yet to be discovered, and we need a better census to estimate what the threat of impact on Earth might be.”

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