When it comes to looking for asteroids, we have a blind spot. It may seem counterintuitive, but the most important asteroid discoveries are now made at twilight, when astronomers are able to look near the horizon, and near the sun, for the uninitiated. asteroids which orbit within the orbits of Earth, Venus and even Mercury.
In perspective published in Science today, asteroid hunter Scott Sheppard of the Carnegie Institution of Science highlights the new “twilight telescope” surveys and the riches they are 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 discovered in the past two years. It also includes “city killers,” asteroids large enough to hit landthe damage would be severe.
“We’re doing a comprehensive survey looking for anything moving around the orbit of Venus, which is a place we haven’t studied very deeply in the past with anything other than small one-meter telescopes,” Sheppard, who leads A twilight survey using the Dark Energy Camera (DECam) on the 4-meter Víctor M. Blanco telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile, he told Space.com. “It’s quite difficult to do, and generally the biggest telescopes don’t have a very large field of view, so you can’t cover much of the sky.”
However, DECam and another telescope are making it much easier to investigate a previously hidden asteroid world that has until now been obscured by groundthe shining.
Related: How many threatening asteroids are there? It is complicated.
Why look for asteroids at twilight?
Some 30 years of methodical searching of the skies have resulted in the finding of most asteroids 5 kilometers in diameter. Models and surveys suggest that more than 90% of near-Earth objects (NEOs) “planet killers” (those larger than 0.6 miles or 1 km) have been found, but only about half of NEOs “city killers” (the largest of 460 feet, or 140 meters) are known.
So where are the rest? “There will be others close to the sun, so hard to observe, or in aliased orbits with Earth that make them hard to find by normal surveying,” Sheppard said. Their eccentric orbits make them visible only in twilight skies.
Sheppard’s team has already identified a medium-sized asteroid, called 2022 AP7, whose orbit crosses that of Earth, matching the criteria for a “potentially hazardous asteroid.” But others, in all probability, remain to be found. “The main reason we haven’t found all the ‘city killers’ is simply because we haven’t been looking at the sky at the same depth for years and years to find them,” Sheppard said.
The 4-meter Victor M. Blanco Telescope of the Cerro Tololo Inter-American Observatory, Chile. (Image credit: CTIO/NOIRLab/NSF/AURA/P. Marenfeld)
The language of asteroids
Near-Earth asteroids come in a variety of flavors, all designated by characteristics of the space rock’s orbit. For example, the Loves come close to Earth, but never cross their orbital path around the sun, so they pose no danger to us.
Not so with the Apollo asteroids, which cross Earth’s orbit but are mostly beyond. This category includes the likes of Apophis i to determineand these space rocks generally orbit the sun from beyond Earth’s orbital path, meaning wide-field telescope surveys operating at night are best positioned to detect these asteroids.
Other categories of near-Earth asteroids are much harder to find, such as Atens (which cross Earth’s orbit and remain mostly within it), Atiras (also called Apohele, which orbit inside the ‘Earth’s orbit) and Vatiras (which orbit within the orbital path). of the planet Venus). However, Sheppard’s survey, which uses just 10 minutes of telescope time just after sunset and before sunrise to search near the sun, is turning up some surprises.
The orbits of the Earth, Venus and Mercury, and ‘Ayló’chaxnim. The dots show the exact positions of the planets at the time of discovery on January 4, 2020, when both ‘Aylóchaxnim and Venus were in the evening sky over Mount Palomar. (Image credit: Caltech-IPAC/R. Hurt)
The One True “Venus Girl”
So far, astronomers know only one Vatiras space rock.
Asteroid 2020 AV2 was discovered on January 4 by the Zwicky Transient Facility (ZTF) telescope at the Palomar Observatory near San Diego, California. The facility is located on ancestral lands of the Pauma indigenous group, who were asked to name it. they chose ‘Ayló’chaxnim, meaning “girl Venus” in their Luiseño language.
The asteroid is between 0.6 miles and 1.9 miles wide (between 1 and 3 km) in diameter, orbiting on a path that is tilted 15 degrees relative to the plane of the solar system, and it takes 151 days to go around the sun. Scientists suspect that the asteroid was probably thrown into the orbit of Venus after a close encounter with another planet.
The sun’s nearest neighbor
In the twilight hours of August 13, 2021, Sheppard discovered an asteroid with the shortest orbital period to date. Captured in the DECam data, asteroid 2021 PH27 is about 0.6 miles across and its surface is likely heated to about 930 degrees Fahrenheit (500 degrees Celsius), hot enough to melt lead, because its 113-day orbit brings it within 12. million miles (20 million km) of the sun. Not more Mercury it has a shorter solar orbit, at 88 days. However, because its orbit crosses both the orbits of Mercury and Venus, this asteroid was classified as an Atira.
2021 PH27 could be an extinct comet, scientists think, given that its orbit is tilted 32 degrees to the main plane of the solar system. This tilt suggests the object may be from the outer solar system, sent into a closer orbit around the sun after passing close to one of the terrestrial planets.
The Dark Energy Survey Camera (DECam) (Image credit: DOE/FNAL/DECam/R. Hahn/CTIO/NOIRLab/NSF/AURA)
The main ‘twilight telescopes’
ZTF and DECam are where they are when looking for asteroids orbiting the interior of Venus.
You might think that the bigger the telescope, the better for asteroid hunting, but larger telescopes have smaller fields of view. ZTF, which scans the sky rapidly, has so far detected one Vatira and several Atira asteroids. DECam, a 570-megapixel CCD imager designed for the Dark Energy Survey (DES) has found a few Atira asteroids, including 2021 PH27. ZTF has a larger field of view, but DECam can detect objects much fainter in brightness as measured by magnitude.
“DECam changes everything,” said Sheppard. “Now we’re going an order of magnitude deeper than people have gone before: we’re opening up a whole new area of space that we can constantly monitor that hasn’t been well monitored in the past.”
Expect to hear a lot more about new asteroids being discovered in an unexplored region of our solar system.
Jamie Carter is the author of “A stargazing program for beginners (opens in a new tab)” (Springer, 2015) and edit WhenIsTheNextEclipse.com. Follow him on Twitter @jamieacarter. follow us on twitter @Spacedotcom or on Facebook.