Twilight Telescopes find city-killing asteroids in an uncharted region of the solar system

When it comes to finding asteroids, we have a blind spot. It may seem counterintuitive, but the most important asteroid discoveries are now being made at twilight, when astronomers can look close to the horizon — and close to the Sun — for obscure asteroids orbiting the orbits of Earth, Venus, and the Sun. even Mercury.

In a review published today in the journal Science, asteroid hunter Scott Sheppard of the Carnegie Institute of Science talks about new twilight telescope research and the riches it is beginning to uncover. This includes the first asteroid with an inner orbit around Venus and one with the shortest known orbital period around the Sun, both of which were discovered in the last two years. It also includes “city killers,” asteroids large enough that if they collided with Earth, the damage would be severe.

“We’re doing a full-scale survey looking for anything that orbits Venus, and that’s somewhere that we haven’t really explored very deeply in the past with anything other than small one-meter telescopes,” Sheppard, who leads the survey twilight using the Dark Energy Camera (DECam) on the 4-meter Victor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile, reported. “That’s pretty hard to do, and generally big telescopes don’t have a very large field of view, so you can’t cover much of the sky.”

However, DECam and another telescope make it much easier to explore the previously hidden world of asteroids, which until now has been obscured by sunlight.

Related: How many threatening asteroids are there? It’s Complicated.

Why look for asteroids at dusk

About 30 years of methodical sky surveys have led to the discovery of most asteroids with a diameter of 3 miles (5 kilometers). Models and studies show that more than 90% of “planet killer” near-Earth objects (NEOs) (larger than 0.6 miles or 1 km in size) have been detected, but only about half of the “city killer” NEOs (those larger than 460 feet or 140 meters).

So where are the rest? “There will be other objects either close to the Sun, so they will be difficult to observe, or in orbits that coincide with the orbits of the Earth, which makes it difficult to detect them with conventional research,” Sheppard said. Their eccentric orbits make them visible only at dusk.

Sheppard’s team has already identified a medium-sized asteroid called 2022 AP7, whose orbit intersects Earth’s orbit, meeting the criteria for a “potentially hazardous asteroid.” But others appear to be yet to be found. “The main reason we haven’t found all the ‘city killers’ is simply because we haven’t looked at the sky at the same depth for years to find them,” Sheppard said.

The 4-meter Victor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory, Chile. (Image credit: CTIO/NOIRLab/NSF/AURA/P. Marenfeld)

Asteroid language.

Near-Earth asteroids come in many varieties, all determined by the characteristics of the space rock’s orbit. For example, amors approach the Earth, but never cross its orbital path around the Sun, so they do not pose a danger to us.

The situation is different with the Apollo asteroids, which cross the Earth’s orbit, but are mostly outside it. This category includes objects such as Apophis and Bennu, and these space rocks typically orbit the Sun just behind the Earth’s orbital path, meaning that wide-field telescopes operating at night are best suited to detect these asteroids.

Other categories of near-Earth asteroids are much harder to find, such as Athenae (which cross the Earth’s orbit and stay mostly inside it), Athyras (also called Apoheles, which orbit inside the Earth’s orbit), and Vathira (which orbit inside the Earth’s orbit). planet Venus). However, Sheppard’s survey, which uses just 10 minutes of telescope time just after sunset and before sunrise to search close to the sun, does offer some surprises.

Orbits of Earth, Venus and Mercury and Ailo’chaknim. The dots show the exact position of the planets at the time they were discovered on January 1st. February 4, 2020, when both Ailokaxnim and Venus were in the evening sky over Mount Palomar. (Image credit: Caltech-IPAC/R. Hurt)

The only real “Venus Girl”

So far, astronomers know only one space rock, Vathiras.

Asteroid 2020 AV2 was discovered in January. 4 using the Zwicky Transient Facility (ZTF) telescope at the Palomar Observatory near San Diego, California. The object is located in the ancestral lands of the Pauma indigenous group, who were asked to give it a name. They chose “Ayló’chaxnim”, which means “Venus girl” in their Luiseño language.

The asteroid is 0.6 to 1.9 miles (1 to 3 km) across, orbits on a 15-degree inclined path with respect to the plane of the solar system, and takes 151 days to orbit the sun. Scientists suspect that the asteroid was likely thrown into the orbit of Venus after a close collision with another planet.

Sun’s nearest neighbor

At dusk Aug. On February 13, 2021, Sheppard discovered the asteroid with the shortest orbital period. According to DECam, asteroid 2021 PH27 is about 0.6 miles in diameter, and its surface likely heats up to about 930 degrees Fahrenheit (500 degrees Celsius) — hot enough to melt lead — because its 113-day orbit brings it to 12 million miles (20 million km) from the Sun. Only Mercury has a shorter orbit around the Sun – 88 days. However, because its orbit intersects both the orbits of Mercury and Venus, this asteroid is classified as Atira.

2021 PH27 may be an extinct comet, scientists say, given that its orbit is tilted 32 degrees from the main plane of the solar system. This tilt suggests that the object may be from the outer solar system, sent into a closer orbit around the Sun after passing near one of the terrestrial planets.

Dark Energy Survey Camera (DECam) (Image credit: DOE/FNAL/DECam/R.Hahn/CTIO/NOIRLab/NSF/AURA)

The best “twilight telescopes”

ZTF and DECam are what you need to find asteroids orbiting inside Venus.

You might think that a larger telescope is better for hunting asteroids, but larger telescopes have a smaller field of view. ZTF, which scans the skies quickly, has already detected one Vathira asteroid and several Athir asteroids. DECam, a 570-megapixel CCD scanner designed for dark energy exploration (DES), has detected several Atira asteroids, including 2021 PH27. ZTF has a larger field of view, but DECam can detect objects that are significantly reduced in brightness in magnitude.

“DECam changes everything,” Sheppard said. “Now we’re going deeper than humans have gotten before — we’re opening up a whole new area of ​​space that we can constantly monitor that hasn’t been tracked very well in the past. ”

Expect to hear much more about new asteroids discovered in an uncharted region of our solar system.

Jamie Carter is the author of The Beginner’s Stargazing Program. (will open in a new tab)(Springer, 2015) and edits Follow him on Twitter @jamieacarter. Follow us on Twitter @Spacedotcom or on Facebook.

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