Free Astronomy Magazine January-February 2023

24 JANUARY-FEBRUARY 2023 ASTRO PUBLISHING ble. DECamwas funded by the US De- partment of Energy (DOE) and was built and tested at DOE’s Fermilab. “Large areas of sky are required be- cause the inner asteroids are rare, and deep images are needed because as- teroids 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. “DECam can cover large areas of sky to depths not achievable on smaller telescopes, allowing us to go deeper, cover more sky, and probe the inner Solar System in ways never done be- fore.” As well as detecting asteroids that could potentially pose a threat to Earth, this research is an important step toward understanding the distri- bution of small bodies in our Solar System. Asteroids that are further from the Sun than Earth are easiest to detect. Because of that these more- distant asteroids tend to dominate current theoretical models of the as- teroid population. Detecting these objects also allows as- tronomers to understand how aster- oids are transported throughout the inner Solar System and how gravita- tional interactions and the heat of the Sun can contribute to their frag- mentation. “Our DECam survey is one of the largest and most sensitive searches ever performed for objects within Earth’s orbit and near to Venus’s orbit,” said Sheppard. “This is a unique chance to understand what types of objects are lurking in the inner Solar System.” “After ten years of remarkable serv- ice, DECam continues to yield impor- tant scientific discoveries while at the same time contributing to planetary defense, a crucial service that benefits all humanity,” said Chris Davis, NSF Program Director for NOIRLab. DECam was originally built to carry out the Dark Energy Survey, which was conducted by the DOE and the US National Science Foundation be- tween 2013 and 2019. T wilight observations with the US Department of Energy-fabricated Dark En- ergy Camera at Cerro Tololo Inter-American Observatory in Chile, a Program of NSF’s NOIRLab, have enabled astronomers to spot three near-Earth asteroids (NEA) hiding in the glare of the Sun. These NEAs are part of an elusive population that lurks inside the orbits of Earth and Venus. One of the asteroids is the largest object that is potentially hazardous to Earth to be discovered in the last eight years. [DOE/FNAL/DECam/CTIO/NOIRLab/NSF/AURA/J. da Silva/Spaceengine, D. Mu- nizaga, T. Puzia (PUC), NASA, ESA/Hubble/L. Calçada/M. Kornmesser, N. Bartmann] known asteroid to the Sun. As such, it has the largest general-relativity ef- fects of any object in our Solar System and during its orbit its surface gets 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 describ- ing 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.” “There are likely only a few NEAs with similar sizes left to find, and these large undiscovered asteroids likely have orbits that keep them in- terior to the orbits of Earth and Venus most of the time,” said Shep- pard. “Only about 25 asteroids with orbits completely within Earth’s orbit have been discovered to date because of the difficulty of observing near the glare of the Sun.” Finding asteroids in the inner Solar System is a daunting observational challenge. Astronomers have only two brief 10-minute windows each night to survey this area and have to contend with a bright background sky resulting from the Sun’s glare. Ad- ditionally, such observations are very near to the horizon, meaning that as- tronomers have to observe through a thick layer of Earth’s atmosphere, which can blur and distort their ob- servations. Discovering these three new asteroids despite these challenges was possible thanks to the unique observing capa- bilities of DECam. The state-of-the-art instrument is one of the highest-per- formance, wide-field CCD imagers in the world, giving astronomers the ability to capture large areas of sky with great sensitivity. Astronomers refer to observations as ‘deep’ if they capture faint objects. When hunting for asteroids inside Earth’s orbit, the capability to capture both deep and wide-field observations is indispensa- !