2 new comets and a rare trojan asteroid orbiting Earth discovered by astronomers within 3 weeks

First it was C/2021 A1 (Leonard), then C/2021 B3 (NEOWISE) and finally asteroid 2020 XL5.

C/2021 A1 (Leonard)

MPEC 2021-A99 announced the discovery of a comet (magnitude ~19.5) by Gregory J. Leonard on CCD images taken on Jan. 3.54-3.56 UT with the Mount Lemmon Survey’s 1.5-m reflector. Follow-up measurements were performed by professional astronomers, who acknowledged the discovery.

C/2021 A1 (Leonard) has a diffuse coma about 7″ in diameter. This comet has excellent brightness prospects for December 2021.

In fact, before the perihelion on January 3, 2022, at a distance of 0.6 AU, comet C/2021 A1 (Leonard) will pass just 0.233 AU from Earth on December 12, 2021 and it will have an exceptionally close pass of Venus at 0.028 AU on December 18, 2021.

Comet C/2021 A1 (Leonard) could potentially become a naked-eye object.

C/2021 B3 (NEOWISE)

MPEC 2021-C16 announce the discovery of a new comet (magnitude ~19) in infrared images obtained during Jan. 22 UT with the Near-Earth Object Wide-field Infrared Survey Explorer (or NEOWISE). The new comet has been designated C/2021 B3 (NEOWISE).

Follow-up measurements show that this object is a comet with a compact coma about 10″ arcsecond in diameter.

Earth Trojan Asteroid 2020 XL5

Earth has a second Trojan asteroid sharing its orbit, reports amateur Tony Dunn on the Minor Planet Mailing List.

The asteroid, dubbed 2020 XL5, is a few hundred meters across and its orbit is tied to a gravitationally stable ahead of the Earth in its orbit.

What are Trojan Asteroids?

Trojans are asteroids gravitationally locked to stable Lagrange points either 60° ahead (L4) or behind (L5) the planets in their orbits around the Sun. 2020 XL5 was found around the L4 point. Massive Jupiter has more than 9,000 Trojans.

So far, Trojans have been found sharing orbits — at least temporarily — with Neptune, Uranus, Mars, Venus, and Earth.

Wha are Earth Trojan Asteroids so hard to find?

Earth Trojans are hard to find because during most of their orbits, they appear close to the Sun in the sky. Not only that, but the gravitational resonance does not hold them in lockstep at 60° ahead and behind of the Earth, explains Dunn.

Instead, the objects trace paths around the L4 and L5 points, which are themselves moving as Earth orbits the Sun.

The first Earth trojan, 2010 TK7, comes within 20 million kilometers (12 million miles) to Earth every few hundred years; it is currently drifting away. Models show its orbit is stable enough to stay in a one-to-one resonance with Earth for about a quarter million years. While there are Earth Trojan orbits that are stable for the life of the solar system, no objects have been found occupying them.

Two spacecraft on their way to visit near-Earth objects searched Trojan regions in 2017, but NASA’s Osiris-Rex found nothing at L4 and the Japanese Hayabusa 2 found nothing at L5.

However, the observations were not definitive, and in 2019 Renu Malhotra (University of Arizona) wrote that the Earth could still have up to several hundred Trojans at least a few hundred meters in diameter, amounting to several percent of the some 10,000 near-Earth objects of that size.

So are we going to find some more of these celestial troyan horses? Are they inhabited by deadly viruses? Next generations will know!

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