747-sized asteroid skimmed by Earth, and scientists didn't see it coming

Dubbed 2021 SG, the asteroid flew close to the planet on September 16, but because it came from the direction of the Sun, scientists didn't see it coming.

An asteroid is seen approaching Earth (illustrative). (photo credit: PIXABAY)
An asteroid is seen approaching Earth (illustrative).
(photo credit: PIXABAY)

A large asteroid as wide as the wingspan of a Boeing 747 passed by the Earth recently – and scientists didn't even see it coming.

Dubbed 2021 SG, according to the NASA-backed International Astronomical Union's Minor Planet Center, the asteroid has a diameter of anywhere between 42-94 meters, with an average diameter of 68 meters, which is around the wingspan of a 747,  the height of the Cinderella Castle in Disney World and around half the size of the Great Pyramid of Giza. 

The asteroid flew past the Earth on September 16, as noted by asteroid tracker EarthSky. However, scientists only detected it the next day.

This may seem surprising, given how many asteroids are routinely detected by astronomers and space agencies all over the world. However, this asteroid came from a relative blind spot: the direction of the Sun.

 An artist's rendition of the Parker Probe as it approaches the Sun. (credit: NASA/John Hopkins/APL/Steve Gribben) An artist's rendition of the Parker Probe as it approaches the Sun. (credit: NASA/John Hopkins/APL/Steve Gribben)

Most asteroids detected by agencies like NASA come at Earth from the "front," meaning they come from the direction facing into the interior of the solar system, coming towards the Earth and the Sun.

But there are asteroids that come from the "back," heading towards Earth from the direction of the Sun and heading outwards.

It is therefore very difficult to see these objects as they approach Earth, especially as they often tend to approach during the daytime when visibility is low due to the Sun's glare.

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

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

And while it may seem like a freak, isolated incident, that might not be the case.

On September 7, astronomers in Arizona detected an asteroid, now designated 2021 RS2, coming from the direction of the Sun just hours before it was set to pass by the planet, as noted by EarthSky. Though it was small, around 3.5 meters, it came just 15,340 km. away from the planet's surface.

This is the closest an asteroid has come to hitting the planet in all of 2021, though if it did hit, it likely wouldn't have done a lot of damage, if any.

An asteroid is seen falling to Earth, breaking apart in the atmosphere (illustrative). (credit: Wikimedia Commons)An asteroid is seen falling to Earth, breaking apart in the atmosphere (illustrative). (credit: Wikimedia Commons)

But the same cannot be said for 2021 SG. With its large size and clocking in at 85,748 km/h (around 23.8 km. per second), the asteroid could certainly have made an impact if it hit. 

For comparison, the last known significant asteroid impact was on February 15, 2013, when an asteroid exploded in the air above Chelyabinsk, Russia. This asteroid was 17 meters wide, and while it didn't result in any casualties, the shock wave from the explosion shattered windows in six different Russian cities and caused 1,500 people to need medical attention.

 Chelyabinsk meteor, February 15, 2013. (credit: Wikimedia Commons) Chelyabinsk meteor, February 15, 2013. (credit: Wikimedia Commons)

With its larger size, the possible destruction 2021 SG could have caused is hard to estimate, but would certainly have been far worse.

Asteroids closer to the Sun than the Earth present a notable hole in most near-Earth surveys because they are so difficult to detect. As a result, understanding them is of vital importance.

“One of the reasons why it is hard to achieve the goal of finding 100% of all Near-Earth objects (NEO)... is because some have orbits that help ‘hide’ them from Earth-based observers,” said University of Hawaii astronomer Dave Tholen, who had been part of a study that discovered 2021 PH27, an asteroid closer to the Sun that orbits faster than any object in the solar system except for Mercury. 

An asteroid impact remains one of the most dangerous possible natural disasters that could occur, however unlikely. It is for this reason that astronomers around the world, including at NASA's Planetary Defense Coordination Office (PDCO), work to monitor all nearby asteroids and calculate their trajectory to see if any of them pose a threat to the planet.

This is done through the use of special "asteroid hunter" telescopes, but some projects, like the Double Asteroid Redirection Test (DART) Mission launched by the PDCO and John Hopkins University, seek to find ways of defending against asteroids themselves.

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

However, this may soon change. NASA is in the middle of constructing a new space telescope that would help with this effort. Called Near-Earth Object (NEO) Surveyor space telescope, it is set to be launched in 2026 and will be in 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 with a width of 140 meters or more – a size that could destroy a city if they impacted.

Back in March, NASA had announced that the planet was at little to no risk of an asteroid impact for the next century, following calculations by astronomers that 9942 Apophis – a massive 340-meter asteroid – will safely pass by the planet at a distance of under 32,000 km. on April 13, 2029.

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

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