A potentially dangerous “Planet Killer” asteroid hidden in the Inner Solar System has been discovered.

A potentially dangerous "Planet Killer" asteroid hidden in the Inner Solar System has been discovered.
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The asteroid orbits closer to the Sun than Earth does

Twilight observations with the US Department of Energy’s Dark Energy Camera at NOIRLab’s Cerro Tololo Inter-American Observatory in Chile allowed astronomers to spot three near-Earth asteroids (NEAs) hiding in the Sun’s glare. These NEAs are part of an elusive population lurking in the orbits of Earth and Venus. One of the asteroids is the largest object potentially dangerous to Earth discovered in the last eight years. Credit: DOE/FNAL/DECam/CTIO/NOIRLab/NSF/AURA/J. da Silva / Space Engine

Twilight observations reveal three massive near-Earth objects lurking in the inner Solar System.

Astronomers have discovered three near-Earth asteroids (NEAs) hiding in the Sun’s glare thanks to twilight observations with the US Department of Energy’s Dark Energy Camera at the Cerro Tololo Inter-American Observatory in Chile.. These NEAs are part of the elusive population lurking in Earth’s orbits and[{” attribute=””>Venus. One of the asteroids is the largest object that is potentially hazardous to Earth to be discovered in the last eight years.

An international team of astronomers has discovered three new near-Earth asteroids (NEAs) hiding in the inner Solar System, the region interior to the orbits of Earth and Venus. Due to the intense glare of the sun, this is a notoriously challenging region for asteroid hunters to make observations. The detections were possible due to observations using the Dark Energy Camera (DECam) mounted on the Víctor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory in Chile, a Program of NSF’s NOIRLab.

However, the astronomers uncovered an elusive trio of NEAs by taking advantage of the brief yet favorable observing conditions during twilight. One of the NEAs is a 1.5-kilometer-wide (0.9-mile-wide) asteroid called 2022 AP7. It has an orbit that may place it in Earth’s path at some point in the future. Fortunately, the other asteroids, designated 2021 LJ4 and 2021 PH27, have orbits that safely remain completely interior to Earth’s orbit. Also of special interest to astronomers and astrophysicists, 2021 PH27 is the closest known asteroid to the Sun. Subsequently, it has the largest general-relativity effects[1] of any object in the solar system. Its surface becomes hot enough to melt lead during its orbit.

“Our twilight survey examines the area for asteroids within the orbits of Earth and Venus,” said astronomer Scott S. Sheppard of the Carnegie Institution for Science’s Earth and Planetary Laboratory. Journal of Astronomy describes this work. “So far we have found two large near-Earth asteroids that are about 1 kilometer across, which we call planet killers.”

“Only a few NEAs of similar sizes will be found, and these large undiscovered asteroids often have orbits that keep them close to the orbits of Earth and Venus,” Sheppard said. “Due to the difficulty of observing near the Sun’s glow, about 25 asteroids with orbits completely around the Earth have been discovered to date.”

Locating asteroids in the Inner Solar System is a formidable observational challenge. Each night, astronomers have two short 10-minute windows to survey this area and have to contend with a bright background sky caused by the Sun’s glare. In addition, such observations are very close to the horizon. This means that by observing through the thick layer of Earth’s atmosphere, astronomers can blur and distort their observations.[2]

Despite these major challenges, DECam’s unique observational capabilities enabled the discovery of these three new asteroids. One of the world’s highest-performance, wide-field CCD imagers, this state-of-the-art instrument allows astronomers to image large areas of the sky with great sensitivity. If observations capture faint objects, astronomers call them “deep”. The ability to capture both deep and wide-field observations is invaluable when hunting asteroids in Earth orbit. DECam was built and tested at DOE’s Fermilab and funded by the US Department of Energy (DOE).

“Large areas of the sky are required because internal asteroids are rare, and deep imaging is necessary because asteroids are faint, and you’re dealing with bright twilight skies near the Sun, as well as the distorting effects of Earth’s atmosphere,” Sheppard said. “DECam can cover large areas of the sky to depths unreachable with smaller telescopes, allowing us to go deeper, cover more of the sky, and explore the inner Solar System like never before.”

This research is an important step towards understanding the distribution of small bodies in our Solar System, in addition to detecting potentially dangerous asteroids for Earth. Asteroids farther from the Sun than Earth are easiest to detect. Therefore, these more distant asteroids dominate current theoretical models of the asteroid population.[3]

Detecting these objects also allows astronomers to understand how asteroids are transported throughout the inner Solar System and how gravitational interactions and the Sun’s heat can help break them up.

“Our DECam survey is one of the largest and most sensitive searches ever conducted for objects in Earth orbit and near the orbit of Venus,” said Sheppard. “This is a unique chance to understand what kinds of objects are hidden in the inner Solar System.”

“After ten years of remarkable service, DECam continues to make important scientific discoveries while contributing to planetary defense, an important service that benefits all of humanity,” said Chris Davis, NSF Program Director for NOIRLab.

DECam was originally established by DOE and the US National Science Foundation to carry out the Dark Energy Study, which was conducted between 2013 and 2019.


  1. Einstein’s theory of general relativity explains how massive objects bend the structure of spacetime and how this affects the motion of objects in the Universe. In our solar system, for example, this effect can be measured directly Precession of the orbit of the planet Mercurycannot be accurately explained by Newtonian physics alone.
  2. Observing the inner Solar System is difficult for ground-based telescopes and impossible for space-based optical/infrared telescopes.[{” attribute=””>NASA’s Hubble and JWST telescopes. The intense light and heat of the Sun would fry the sensitive electronics. For this reason, both Hubble and JSWT are always pointed away from the Sun.
  3. Atria asteroids — also known by the Hawaiian term Apohele asteroids — are the smallest group of near-Earth asteroids. Their orbits have an aphelion (farthest point from the Sun) smaller than Earth’s perihelion (nearest point to the Sun).

Reference: “A deep and wide twilight survey for asteroids interior to Earth and Venus” by Scott S. Sheppard, David J. Tholen, Petr Pokorný, Marco Micheli, Ian Dell’Antonio, Shenming Fu, Chadwick A. Trujillo, Rachael Beaton, Scott Carlsten, Alex Drlica-Wagner, Clara Martínez-Vázquez, Sidney Mau, Toni Santana-Ros, Luidhy Santana-Silva, Cristóbal Sifón, Sunil Simha, Audrey Thirouin, David Trilling, A. Katherina Vivas and Alfredo Zenteno, 29 September 2022, The Astronomical Journal.
DOI: 10.3847/1538-3881/ac8cff

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