Near-Earth object reclassified as comet after 28 years

For nearly three decades, astronomers misidentified a near-Earth object as an asteroid, but new observations have revealed it is actually a comet. The object, designated 2005 QN173, was first spotted in 2005 and classified as an asteroid based on its orbit and appearance. However, a recent study by NASA scientists using the Infrared Telescope Facility in Hawaii found that the object exhibits a faint coma—a cloud of gas and dust—and a tail, which are characteristic of comets. The discovery was published in the journal *Planetary Science Journal* in 2023.

The object is about 1.2 miles (2 kilometers) wide and orbits the Sun every 4.5 years. It came within 2 million miles (3.2 million kilometers) of Earth in 2021, making it a close approach for a comet. The misclassification occurred because the object's coma is very faint, and it was not active when first observed. The study involved analyzing thermal infrared data to detect heat emissions from the object, which revealed the presence of dust and gas.

This matters because it highlights the difficulty in distinguishing between asteroids and comets, especially for dark or dormant comets that may pose a hazard to Earth. The object is classified as a potentially hazardous asteroid due to its size and orbit, but its true nature as a comet could affect predictions of its future behavior. Comets are more unpredictable than asteroids because they can release gas and dust, which can alter their trajectories.

NASA's study used data from the NEOWISE mission and ground-based telescopes. The object was originally discovered by the Catalina Sky Survey. The findings suggest that other near-Earth objects may also be misidentified comets, and future surveys should consider this possibility. The next step is to continue monitoring 2005 QN173 to better understand its activity and refine its orbit, which will help assess any potential impact risk.