Astronomers have discovered an object in the Solar System that resembles both an asteroid and a comet, with a tail that is twice the distance between the Earth and the Moon.
2005 QN173 flying through the Solar System with an extremely long tail. Photo: NASA
The asteroid 2005 QN173 orbits the Sun and is unlike any other asteroid, located in an area known as the main asteroid belt in the Solar System. Unlike the rocky asteroids in the same region between Mars and Jupiter, 2005 QN173 appears to be changing as it moves through the Solar System, developing a tail long enough to sweep from Earth to the Moon and back.
Observations of the asteroid 2005 QN173, discovered in 2005, indicate that the object is actively emitting a thin tail of dust and gas, a characteristic typically associated with comets. The tail suggests that material containing ice is transitioning from a solid state to a gaseous state in a process known as sublimation.
When examining an asteroid like 2005 QN173, the tail reveals sublimated ice at its surface. What makes 2005 QN173 even more unique is that this asteroid has shown active behavior more than once. Among the half a million known asteroids in the main belt, 2005 QN173 is the eighth asteroid recorded to have exhibited strong activity twice in history.
According to scientist Henry Hsieh at the Planetary Science Institute, 2005 QN173 is both an asteroid and a comet. More specifically, it is an asteroid in the main belt that has recently been recognized as a comet. It fits the definition of a comet, being composed of ice and releasing dust into space.
Researchers discovered that 2005 QN173 is actively emitting material through observations from the Asteroid Terrestrial-Impact Last Alert System (ATLAS) in mid-July and August of this year. Hsieh, the lead researcher, and his team measured that 2005 QN173 has a solid nucleus with a diameter of 3.2 km. They also calculated that the comet’s tail extends over 724,000 km, twice the distance between the Earth and the Moon. Despite its extreme length, the tail of 2005 QN173 is so thin that if the entire asteroid were scaled down to the length of a football field, its tail would only be 18 cm wide, with the nucleus measuring just about one millimeter.
Comets develop tails containing ice because they spend less time near the Sun compared to asteroids. Most comets originate from the cold outer edges of the Solar System, around Neptune. Their elliptical orbits mean they only occasionally pass close to the Sun. In contrast, most asteroids have spent about 4.6 billion years in the warm inner belt of the Solar System, near Jupiter, which means they are relatively close to the Sun.
In 2006, Hsieh and colleague David Jewitt classified a newly discovered group of celestial objects called main-belt comets or active asteroids. These objects possess asteroid-like orbits and comet-like physical properties. The study of 2005 QN173 could help scientists explore the roles of comets and asteroids in the evolution of Earth and life.
