Asteroid Larger Than Washington Monument Heading Past Earth Before Halloween

An asteroid with a diameter greater than the Washington Monument will pass Earth in the lead-up to Halloween 2021. According to NASA's Centre for Near-Earth Objects (CNEOS) on Monday, October 25, asteroid 2017 SJ20 makes its nearest approach to our planet for at least the next 100 years. The asteroid, which could have a diameter as large as around 200 meters, wider than the Washington Monument is tall, will pass within 5 million miles of Earth. To put this into perspective, the asteroid will come much closer to Earth than Mars, which on average is around 140 million miles from Earth but won't come anywhere near as close as the moon, which is only around 240,000 miles from the surface of our planet. The asteroid 2017 SJ20 is traveling at around 10 miles per second as it orbits the sun on a path that crosses the orbit of our planet. After it passes Earth it will swing past the sun and head out to Jupiter.

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Using NASA's Small-Body Database Lookup, skywatchers can track 2017 SJ20's progress through the solar system. This image from the application shows where, in their respective paths around the sun, both Earth (blue path) and 2017 SJ20 (white path) were on October 10.

A second snapshot from the Small-Body Database Lookup taken from a different angle shows Earth and asteroid 2017 SJ20's relative positions on Monday October 25.

By Halloween evening, October 31, the asteroid and Earth will have separated continuing their respective paths around the sun. While Earth has an orbit that takes it around the sun roughly every 365 days, like most asteroids 2017 SJ20 has an orbit that is much wider, taking it out past Mars and towards Jupiter. This screengrab from the Small-Body Database shows Earth in the same position around the sun next Halloween. In October 2022, asteroid 2017 SJ20 is way out past the orbit of Mars.

When asteroid 2017 SJ20 is out past Mars and towards Jupiter, the fact that it is far from the sun means its speed has reduced to around 3 miles per second. Its speed builds as it swings around close to the sun and as it passes close to Earth. The asteroid will closely approach Earth's orbit in November 2025 but will be ahead of Earth by over 57 million miles. The asteroid won't come anywhere as close to Earth as it does on October 25, 2021, until November 30, 2105, when it will pass within around 8 million miles of our planet. Asteroid 2017 SJ20 is classified by NASA's Jet Propulsion Laboratory (JPL) as a near-Earth object or NEO, as is any asteroid or comet that comes within a distance of around 120 million miles of Earth. The asteroid isn't classed as a Potentially Hazardous Asteroid (PHA). PHAs are asteroids over 140 meters in diameter that are considered to be capable of making a threatening approach to Earth. This is generally considered to mean an asteroid that passes within 4.6 million miles of Earth. The asteroid 2017 SJ20 meets both of these criteria, but researchers at CNEOS have ruled out the possibility that it will hit Earth within the next 100 years. In fact, this October marks the closest the asteroid will come to that dubious accolade.