Asteroid 2021 SG came from the sun’s direction

Landscape with brilliant sun in the sky and tick marks near it showing location of asteroid.
This illustration shows asteroid 2021 SG’s location just 24 hours before its closest approach to Earth on September 16, 2021. It was close to the sun’s direction in our sky. So astronomers didn’t spot it until September 17, the day after its closest approach. Illustration by Eddie Irizarry /Stellarium.

Asteroid 2021 SG: Bigger than Chelyabinsk

New-found asteroid 2021 SG is some four times larger than the 17-meter (18-yard) space rock that disintegrated over Chelyabinsk, Russia, on February 15, 2013. The Chelyabinsk meteor created a shock wave that broke windows in six Russian cities. It caused some 1,500 people to seek medical attention, mostly from flying glass. But newly found asteroid 2021 SG didn’t hit. It just passed close, at only about half the distance from Earth to the moon, last week. Astronomers finally picked up the asteroid – discovering it for the first time – a day later on September 17, 2021. They were using a large telescope, the 48-inch (1.2 meter) telescope at Mount Palomar in California. Why didn’t they spot it sooner? Because it came from the direction of the sun.

An analysis of its orbit indicates that asteroid 2021 SG was closest to Earth on September 16 at 20:28 UTC (4:28 p.m. ET).

Asteroid 2021 SG has an estimated diameter of between 42 – 94 meters (138-308 feet). Its average diameter is 68 meters (223 feet). That’s in contrast to 17 meters for the Chelyabinsk meteor before it entered Earth’s atmosphere.

It came from the direction of the sun. Those words might sound chilling to you. And they do, too – perhaps more so – to scientists who work to detect near-Earth asteroids, in an effort to keep our planet safe. The Chelyabinsk meteor that did so much damage and caused so much consternation in 2013 also came, unexpectedly, from the sun’s direction. The fact is that astronomers have become very good at detecting near-Earth asteroids. And there are programs in place to watch for them. Some observatories constantly take images of the night skies in search for new asteroids. And astronomers feel they have a good handle on all the potentially damaging asteroids out there … except those that might come to us from the sun’s direction.

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If it entered our atmosphere, an asteroid as big as 2021 SG would produce a huge, very impressive meteor. Asteroid 2021 SG isn’t just big. It’s also a fast-moving asteroid, traveling through space at the amazing speed of 53,281 miles per hour (85,748 km/h or 23.8 km per second), relative to Earth. At closest approach on September 16, 2021, asteroid 2021 SG came closest to Canada and Greenland.

The orbit of 2021 SG shows it’s an Apollo type asteroid that completes a revolution or orbit around the sun every 27 months (2.24 years). This time, it passed Earth just after just passing Mercury’s orbit.

Top view of solar system with orbits of inner planets and long oval orbit of asteroid 2021 SG.
The orbit of asteroid 2021 SG shows it comes as close to the sun as Mercury, our sun’s innermost planet. And then it goes as far out as between the orbits of Mars and Jupiter. In 2021, it swept past Earth not long after passing close to Mercury’s orbit. Image via NASA/JPL.

Asteroids from the sun’s direction

Can astronomers detect asteroids coming from the sun’s direction?

Right now, no, they can’t. But astronomers will soon have a new tool to detect many space rocks, including those hiding in the sun’s glare. NASA is developing a new Infrared Space Telescope called the Near-Earth Object Surveyor space telescope, or NEO Surveyor. NASA expects this telescope to find 90% of near-Earth objects with diameters of at least 140 meters. An impact from an object that large could level a city. This telescope – expected to launch in 2026 – would have spotted both the Chelyabinsk space rock and 2021 SG. It should improve our planetary defense.

Meanwhile, scientists now think that a Chelyabinsk-type event might occur more frequently than previously thought. For example, another good-sized space rock passed by Earth in 2015. 2015 TB45 was about the same size as 2021 SG. Its diameter was about 2,000 feet (610 meters). It passed a bit farther, just outside the moon’s orbit. Still, in the vastness of our solar system … that’s pretty close. Astronomers spotted it three weeks before the closest approach on October 31, 2015. Some radar images coincidently showed a skull-shaped space rock. And so some dubbed it the Halloween Asteroid.

What does it all mean? Perhaps that – even with astronomers watching – an impressive asteroid event is currently possible without previous warning, at any time. That would be true if the object came from the sun’s direction. But in a few years we should all have extra protection from the new NEO Surveyor. And that fact ought to help all of us – astronomers included – feel safer!

Bottom line: Asteroid 2021 SG – about four times the diameter of the Chelyabinsk meteor in 2013 – made a close pass by Earth on September 16, 2021, without being previously detected.

September 20, 2021

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