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Scientists studying inner solar system objects have discovered three nearby asteroids previously shrouded in the Sun’s glare. Two of these are larger than one kilometer, earning the label “planet killer”—and one is the largest potentially hazardous asteroid (PHA) discovered in at least the past eight years.

Astronomers studying inner solar system asteroids have the odds stacked against them. Since these objects flock close to the Sun, they can only be observed at twilight when the glare is not too overpowering. Due to the asteroids’ position in the space between us and the Sun, they are low in the sky at this time, so astronomers must set their sights on the horizon. Not an ideal setup: the sky is still relatively bright, the Earth’s atmosphere can have a notably distorting effect from this angle, and observation times are limited to about ten minutes.

“Only about 25 asteroids with orbits completely within the Earth’s orbit have been discovered to date because of the difficulty of observing near the glare of the Sun,” said Scott Sheppard, the Carnegie Institution for Science astronomer who spearheaded the search.

State-of-the-art technology such as the Chilean Blanco Telescope’s Dark Energy Camera (DECam) provide the sensitivity and wide field to counteract these obstacles. Originally built for the Dark Energy Survey conducted in the 2010s, the camera has now been set on a new task: inner solar system watch.

The discovery of these three asteroids is part of a larger DECam-centered survey of objects within the orbits of Earth and Venus, as described in a recently published paper

The asteroid 2022 AP7 is certainly the most relevant to us, with a diameter of about 1.5 km and an orbit that crosses Earth’s path (though at a time when we’re on the other side of the Sun, at least for now). For context, the asteroid deflected by NASA’s Double Asteroid Redirection Test in September was slightly over a tenth as large, so we’ve got a ways to go if 2022 AP7 becomes more of a threat. 

The other “planet killer” is not considered a PHA for Earth, but rather for Venus. 2021 PH27’s entire trajectory is enclosed within Earth’s orbit; in fact, it has the smallest period of any known asteroid around the Sun. However, this orbit is rather unstable, resulting in a 0.7% chance of collision with Venus within the next million years. There is also the chance that the asteroid becomes active–i.e. ejecting matter into space, like a comet does–which could result in a lovely meteor shower for any Venus-based observers.

Beyond detecting objects that may be hazardous to us (or any other planet), a more precise survey of inner solar system objects can aid studies of the evolution of asteroid distribution and asteroid interactions with the Sun. 

“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 before,” said Sheppard.

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Georgia Michelman is a reluctant recent Yale College graduate with backgrounds in physics, astronomy, and history. She is always searching for intersections between the worlds of science and the humanities....