Astro Bob: Astronomers spot potential 'planet killer' asteroid hiding near sun

A hunky asteroid that could threaten the planet in the distant future orbits between Earth and Venus. Closer to home, be on the lookout for the Taurid meteors this week.

Asteroid illustration
Recently, an international team of astronomers spotted three near-Earth asteroids (NEAs) hiding in the glare of the Sun using a special wide-field camera. One of the asteroids is the largest object potentially hazardous to Earth to be discovered in the last eight years. Above is an artist’s impression of an asteroid that orbits closer to the Sun than Earth’s orbit.
Contributed / National Science Foundation's NOIR Lab
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We currently know of about 2,300 potentially hazardous asteroids or PHAs (and a few comets) that could one day wreak havoc on Earth. These are objects that pass close to Earth's orbit and have diameters of 460 feet (140 meters) or larger. At the lower limit, that's big enough to cause regional devastation or a major tsunami in the event of an impact. Fortunately, such encounters are infrequent, occurring about once every 10,000 years.

Larger PHAs — from 1 kilometer (0.6 mile) and up — number around 153 and strike about once every 500,000 years. Astronomers keep track of them all and confirm that no thwackings are expected in the foreseeable future.

NEA totals
This recent chart shows the discovery statistics for near-Earth asteroids (NEAs), potentially hazardous asteroids (PHAs) and near-Earth comets (NEC) as of Oct. 30, 2022. It was compiled by the Center for Near-Earth Object Studies (CNEOS).
Contributed / JPL-CNEOS

There's just one problem. We haven't found all of the potential killers yet. NASA satellite data indicates there are from 4,000 to 6,000 potentially hazardous asteroids larger than 328 feet (100 meters) across, so we've still got about half to go. That's why the recent discovery of three "twilight" asteroids is good news and an omen for the future.

Using the Dark Energy Camera at Cerro Tololo Inter-American Observatory in Chile, astronomers found three near-Earth asteroids (NEAs) in the twilight sky nearly hidden in the solar glow. They're part of an elusive group of asteroids that lurks inside the orbits of Earth and Venus. Like Venus, they're only visible during twilight because they never appear far from the sun. One of them, called 2022 AP7, is the largest PHA to be discovered in the last eight years.

PHA 2022 RM4
PHAs regularly pass safely near Earth. For example, the asteroid 2022 RM4 (pictured here) buzzes the planet on Tuesday afternoon, Nov. 1 at a distance of 1.4 million miles (2.3 million km). It's estimated to be between 1,100 and 2,400 feet (330-740 meters) across.
Contributed / Gianluca Masi, Virtual Telescope Project

The bugger's 0.9 miles (1.5 km) across and has an orbit that may someday place it in Earth's path. The other asteroids — 2021 LJ4 and 2021 PH27— have orbits that safely remain interior to Earth’s orbit. The latter is of special interest to astronomers and astrophysicists because it's the closest known asteroid to the Sun and has a surface temperature hot enough to melt lead.


“Our twilight survey is scouring the area within the orbits of Earth and Venus for asteroids,” said Scott S. Sheppard, an astronomer at the Earth and Planets Laboratory of the Carnegie Institution for Science and the lead author of the paper describing this work . “So far we have found two large near-Earth asteroids that are about 1 kilometer across, a size that we call planet killers.”

Sensitive eye

Finding asteroids near the sun is no easy feat for obvious reasons. To date only about 25 of them with orbits completely within Earth's orbit have been plucked from the glare.

Twilight with crescent moon
Twilight is a tricky time to look for asteroids that orbit closer to the sun than the Earth, but it's the only time. Further searches with the Dark Energy Camera and its ilk will hopefully give us a more realistic idea of how many asteroids populate the space close to the sun.
Contributed / Bob King

The team had only two brief 10-minute windows each night when the sky was dark enough to survey the area. Moreover, they had to make those observations near to the horizon, where the atmosphere blurred and distorted their images. That's where the Dark Energy Camera proved its mettle. It can capture large areas of the sky with great sensitivity — exactly what you need when you're on a deadline.

“Large areas of sky are required because the inner asteroids are rare, and deep images are needed because asteroids are faint and you are fighting the bright twilight sky near the Sun as well as the distorting effect of Earth’s atmosphere,” said Sheppard.

It's relatively easy to detect asteroids that are farther from the sun, since we can spot them in a dark sky. This biases our ideas of how these bodies are distributed across the solar system. Ones that approach the Earth from the direction of the sun are much more difficult to discover, making it more important than ever to get a handle on them before it's— dare I say — too late.

Meteors from Taurus

Northern and Southern Taurids
The Northern and Southern Taurid meteor showers will be active from now until mid-November. Taurid rates are typically low and meteors move relatively slowly across the sky, but fireballs are common. Meteors will appear to stream from two locations in western Taurus below the Pleiades.
Contributed / Stellarium

While we're on the topic of space rocks, fragments of Comet 2P/Encke will light up the skies through about mid-month in a pair of meteor showers, the Southern and Northern Taurids. At best we'll see only about 10 meteors an hour. On the bright side they're known for their fireballs.

Both displays will radiate from points in the constellation Taurus not far from the Pleiades star cluster. Meteor shower experts also expect a contribution from a related shower called the Taurid Swarm , so keep your eyes peeled for sparks overhead.

Read more from Astro Bob
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"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune. You can reach him at
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