Asteroid passing so close to Earth its trajectory will change

Small asteroid 2020 SW will pass so close to Earth on September 24, that our planet’s gravity will alter the space rock’s speed and bend its path through space. The green line indicates the asteroid’s trajectory, while the yellow line points to the sun. Illustration by the Minor Planet Center, with modifications by Eddie Irizarry.

By Eddie Irizarry, EarthSky.org

A small asteroid discovered on September 18 from Mt. Lemmon Observatory in Arizona will pass a lot closer than the moon’s distance this week, passing even closer than geostationary satellites. This approach is so close, and the asteroid is so small, that Earth’s gravity will bend the space rock’s trajectory, as shown in the illustration above.

Location of Asteroid 2020 SW on the night of Wednesday, September 23, at around 10:30 pm CDT (03:30 UTC on September 24). Facing east, as seen from U.S. The space rock will be located not too far from where we see the star Algenib (Gamma Pegasi), in the Great Square of Pegasus. Illustration by Eddie Irizarry using Stellarium.

Closest approach of this asteroid – labeled 2020 SW – should occur on September 24 at around 11:18 UTC (7:18 am ET; translate UTC to your time). At closest approach, asteroid 2020 SW should pass at an estimated distance of 17,556 miles (28,254 km) from Earth, which is just 7% of the moon’s distance. For comparison, television and meteorological satellites orbit at some 22,300 miles (35,888 km) from our planet’s surface.

Although there’s still some uncertainty in the space rock’s orbit, calculations indicate there is no risk of impact. As a result of the asteroid’s orbit’s uncertainty, closest approach might occur up to six hours earlier or later than expected.

Asteroid 2020 SW is estimated to be about 14 to 32 feet (about 4.5 to 10 meters) in diameter. It’ll brighten as it draws near, but won’t ever get bright enough to see with the eye alone, but you can watch it online.

Preliminary calculations indicate that asteroid 2020 SW orbits the sun every 372 days. Its orbit is just seven days longer than Earth’s. However – despite the similarities our orbits – calculations indicate the small asteroid will not impact Earth, at least not for the next approximately 50 years for which its orbit has been calculated.

The space rock is currently traveling at a speed of 17,336 miles per hour (27,900 km/h) or 7.75 km/s relative to Earth.

Asteroid 2020 SW will pass at a safe distance, probably over Australia or New Zealand, during its closest approach, as shown on the illustration below:

Asteroid 2020 SW might reach a visual magnitude of 13.0 to 13.5, too faint to be seen by the unaided eye, but within the reach of observers using a 6- or 8-inch diameter telescope – that is, a very small telescope – and also larger telescopes. The space rock is currently in the constellation of Pegasus the Flying Horse, and will then move to the constellation of Pisces the Fish.

The asteroid might still have a magnitude close to 15 on the night of Wednesday, September 23, but will gradually increase its brightness as it approaches, reaching a magnitude of around 13 to 13.5 during the first hours of Thursday, September 24, its brightest being just before dawn.

Observers using a telescope might detect the asteroid, which will look like a “star” moving very slowly because of the distance. If you are pointing the telescope at the correct position and time, carefully observe the stars’ pattern and compare it just five minutes later. If you are able to see a “star” that changed position, then it’s not a star, you have found the asteroid!

Asteroid 2020 SW will pass at a safe distance, probably over Australia or New Zealand, during its closest approach. Green line indicates the asteroid’s trajectory, with blue marks indicating position every 30 minutes. Yellow line points toward the sun. Illustration by the Minor Planet Center, with modifications by Eddie Irizarry.
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