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 dusk, when astronomers can look close to the horizon – and close to the sun – for little-known asteroids that orbit within the orbits of Earth, Venus and even Mercury.
In a perspective published in Science Today, asteroid hunter Scott Sheppard of the Carnegie Institution of Science highlights the new “twilight telescope” investigations and the riches they are beginning to discover. That includes the first asteroid to orbit Venus and one with the shortest known orbit around the Sun, both of which have been unearthed in the past two years. It also includes ‘city killers’, asteroids big enough that, if they hit, Soilthe damage would be great.
“We’re doing a full-fledged survey looking for anything that moves around the orbit of Venus, a place that we haven’t really explored deeply in the past with anything but small one-meter telescopes,” Sheppard, who uses a Twilight survey. from the Dark Energy Camera (DECam) on the Víctor M. Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory (CTIO) in Chile, Space.com told Space.com. “It’s quite difficult to do and generally the larger telescopes don’t have a very large field of view, so you can’t cover a lot of sky.”
DECam and another telescope, however, are making it much easier to survey a previously hidden world of asteroids that have so far been obscured by the Sun‘s shine.
Related: How many threatening asteroids are there? It’s complicated.
Why search for asteroids at dusk?
About 30 years of methodical searching in the sky has resulted in finding most asteroids with a diameter of 3 miles (5 kilometers). Models and studies suggest that over 90% of “planet-killer” Near-Earth Objects (NEOs) (those larger than 0.6 miles or 1 km) have been found, but only about half of the “city-killer NEOs (that are taller than 460 feet or 140 meters) are well known.
So where’s the rest? “There will be others, either close to the sun, so hard to observe, or on alias orbits with Earth that make them hard to find by normal research,” Sheppard said. Due to their eccentric orbits, they are only visible at dusk.
Sheppard’s team has already identified a medium-sized asteroid, called 2022 AP7, whose orbit intersects Earth’s and meets the criteria of a “potentially dangerous asteroid.” But others are probably yet to be found. “The main reason we haven’t found all the ‘city killers’ is simply because we haven’t observed the sky at the same depth for years to find them,” Sheppard said.
The language of asteroids
Near-Earth asteroids come in a variety of flavors, all indicated by features of the space rock’s orbit. For example, Amors come close to Earth, but never cross its orbit around the Sun, so pose no danger to us.
Not so for the Apollo asteroids, which traverse Earth’s orbit, but are usually outside of it. This category includes the likes of apophis and Am nowand these space rocks generally orbit the sun from just outside Earth’s orbital path, meaning wide-field telescope surveys conducted at night are best for spotting these asteroids.
Other categories of asteroids near Earth are much harder to find, such as Atens (which traverse Earth’s orbit and stay in it for the most part), Atiras (also called Apohele, which orbits Earth) and Vatiras (circling within the track path). of the planet Venus). However, Sheppard’s research — which uses just 10 minutes of telescope time to search close to the sun just after sunset and before dawn — yields some surprises.
The one and only ‘Venus Girl’
Until now, astronomers know only one space rock from Vatiras.
Asteroid 2020 AV2 was discovered on January 4 using the Zwicky Transient Facility (ZTF) telescope at the Palomar Observatory near San Diego, California. The facility is located on the ancestral land of the indigenous Pauma group, who were asked to name it. They chose ‘Ayló’chaxnim, which means ‘Venus Girl’ in their Luiseño language.
The asteroid is between 0.6 miles to 1.9 miles (1 to 3 km) wide, orbits on a path tilted 15 degrees from the plane of the solar system, and takes 151 days to circle the sun. Scientists suspect the asteroid was likely thrown into Venus’ orbit after a close encounter with another planet.
The closest neighbor to the sun
In the twilight of August 13, 2021, Sheppard discovered an asteroid with the shortest orbital period to date. Captured in data from the DECam, asteroid 2021 PH27 is about 0.6 miles across and its surface is likely warming to about 930 degrees Fahrenheit (500 degrees Celsius) — hot enough to melt lead — because its 113-day orbit makes it so as close as 12 million miles (20 million km) from the sun. Nothing but Mercury has a shorter orbit from the sun, at 88 days. However, because its orbit intersects both Mercury and Venus, this asteroid is classified as an Atira.
2021 PH27 could be an extinct comet, scientists think, as its orbit is inclined 32 degrees from the main plane of the solar system. That tilt suggests the object may have come from the outer solar system and been sent into a closer orbit around the sun after passing near one of the terrestrial planets.
The top ‘twilight telescopes’
ZTF and DECam are where it is when looking for asteroids orbiting Venus.
You might think that the bigger the telescope, the better for asteroid hunting, but larger telescopes have smaller fields of view. ZTF, which is rapidly scanning the sky, has seen one Vatira and several Atira asteroids so far. 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 see objects of much weaker brightness as measured by magnitude.
“DECam changes everything,” Sheppard said. “We’re going much deeper now than humans have gone before — we’re opening up a whole new area of space that we can constantly monitor that wasn’t really well controlled in the past.”
Expect to hear much 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 new tab)(Springer, 2015) and he edited WhenIsTheNextEclipse.com. Follow him on Twitter @jamieacarter. follow us on twitter @Spacedotcom or on facebook.