An asteroid skimmed past the Earth last week at just 3,000 kilometers away from the planet's surface, but no one noticed until after the fact, as noted by NASA data.
Dubbed 2021 UA1, the asteroid, which skimmed by Antarctica last Sunday, was very small — Only around 2 meters in diameter, which is roughly the size of a golf cart. As such, it is unlikely it would have actually done any damage if it impacted the planet, as it would likely have burnt up in the atmosphere.
But while the damage it could have caused should it have impacted was minimal, the true danger is that an asteroid came so close to the planet and nobody noticed until after the fact.
2021 UA1 flew very close to the planet, and is estimated to have been the third closest asteroid flyby ever recorded without impacting, following 2020 QG in August 2020 and 2020 VT4 in November 2020.
With a distance of just 3,000 kilometers, 2021 UA1 was far closer to the Earth than the Moon, which orbits at a distance of 384,400 kilometers from the planet. A simulation shows how close it was.
Newly-discovered #asteroid 2021 UA1 missed Antarctica by only 3000 km Sunday evening.It came from the daytime sky, so it was undiscoverable prior to closest approach.https://t.co/Y0zY7mAYue pic.twitter.com/R9VpMo2X9G
— Tony Dunn (@tony873004) October 27, 2021
2021 UA1 is not as close to the Earth as the International Space Station, which has an average altitude of 408 kilometers. However, it is far closer to the planet than many of Earth's communications satellites, most of which are in orbit at a distance of around 35,785 kilometers.
Asteroid impacts are one of the greatest possible disasters that could affect the planet, which is why space agencies around the world monitor many of these asteroids, calculating their sizes, distance, orbits and whether they could potentially strike the planet.
So why didn't scientists detect 2021 UA1 before it passed by the planet?
This is because it came from a blind spot.
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.
This is not the first time such an asteroid passed the planet without anyone noticing: On September 16, 2021 SG, an asteroid with a diameter between 42 and 94 meters, flew past the planet at around half the distance between Earth and the Moon, and nobody noticed until a day later.
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.
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 require medical attention.
This asteroid, too, came from the "back."
The destructive nature of asteroids, even small ones, is something well-known to experts, with space agencies around the world monitoring for potential catastrophic impacts, as well as researching potential means of stopping them.
One method for possibly stopping the impact of an asteroid is through the use of deflection, which would mean launching something to slightly alter an asteroid's path. The most prominent of these efforts is the Double Asteroid Redirection Test (DART) Mission, set to be launched in November, the result of efforts by NASA and the Applied Physics Laboratory.
In layman's terms, it means punching an asteroid with a rocket with enough speed to change its direction by a fraction of a percent.
However, this method does have its flaws, most notably timing. The spacecraft used in the DART mission has taken a considerably long amount of time and resources to develop and launch. In case of an asteroid impact that seems so sudden, that kind of time could be a luxury the planet can't afford.
This is especially true with asteroids coming from the "back," as they are far more difficult to track.
In fact, 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 incidents like the close flyby of 2021 UA1 show, the risk of unexpected asteroids closer to the Sun remains a possible threat.