Comet 21P: Everything You Need to Know About the Amazing Green Astronomical Object

The comet known as 21P/Giacobini-Zinner (21P for short) is currently flying towards the inner Solar System at speeds of more than 51,000 miles per hour. On September 10, it is set to make its closest approach in 72 years to both the Sun and our planet.

But what exactly is a comet and what do we know about 21P?

Comets are celestial objects—thought to be remnants of planet formation in the early Solar System—which circle the Sun in elliptical or oval-shaped orbits. This means that they can spend hundreds of thousands of years out in the depths of the Solar System before returning to pass relatively close to our star. Like all orbiting bodies, the closer they are to the Sun, the faster they move.

The orbits of comets are not limited to the plane in which the planets of the Solar System lie. 21P, for example, orbits at a tilt in a path that takes it above and below this plane.

Comets consist of a solid nucleus that usually measures between 1 and 10 kilometers (0.62 to 6.2 miles) across and is mostly made up of dust and frozen materials—such as carbon dioxide, methane and water—although it may contain a rocky core. They are sometimes referred to as "dirty snowballs" due to their composition.

As comets move closer to the Sun, the star's heat warms them, causing the ice to sublimate (or turn directly from a solid to a gas), while releasing debris ranging in size from dust to small boulders. This produces both a thin atmosphere, or coma, and a characteristic tail that points in the opposite direction to the Sun, rather than behind its direction of travel.

The coma can range in size from hundreds to millions of miles wide, while the tail can extend for tens of millions of miles and can sometimes even surpass the distance between the Earth and the Sun.

As comets travel close to the Sun—and the Earth—they are often visible from our planet, as is the case with 21P. Sometimes the objects are viewable in the night sky with the naked eye if they are particularly bright. But to see 21P, you will need a modest set of binoculars or a telescope.

Comet 21P/Giacobini-Zinner was discovered on December 20, 1900 by French astronomer Michel Giacobini, at the Nice Observatory in France, according to NASA. In October 1913, it was "rediscovered" by German astronomer Ernst Zinner, hence why the object is named after both of them.

21P is a fairly small comet—its nucleus is estimated to measure around 1.24 miles in diameter—which takes about 6.6 years to orbit the Sun. The "P" in the title indicates that it is a periodic comet, meaning it has an orbital period of less than 200 years.

The comet is responsible for the annual Draconids meteor shower on Earth, which usually peaks in early October. Whenever the Earth crosses through the debris field of a comet, the material enters the atmosphere and rapidly burns up, resulting in a spike in meteors or "shooting stars."

21P is notable for being the target of a joint NASA-European Space Agency mission, known as International Cometary Explorer, which in 1985 achieved the first flyby of a comet.

On September 10, 21P will make its closest approach to Earth, coming within around 36 million miles of our planet. This may seem like a huge distance but in space terms it is relatively paltry.

The best viewing time for stargazers will probably be between midnight and dawn on the 10th when the comet will be moving through the constellations Auriga and Taurus in the northeastern sky, according to Northolt Branch Observatories.

Comet Giacobini-Zinner captured by the Kitt Peak 0.9-m telescope on October 31, 1998. N.A.Sharp/NOAO/AURA/NSF

The comet is also visible now and will continue to be in the days leading up to the 10th, although it will appear slightly less bright. In fact, tonight presents a prime opportunity to view the comet as it will appear just over one degree away from the bright star Capella in Auriga. This makes it easier to spot, according to Comet Watch.

If you're blessed with clear skies in your area, head to places with minimal light pollution to have the best chance of seeing 21P. Sunlight reflecting off the comet will give it a slightly bluish-green appearance.

The online observatory Slooh will also be broadcasting a livestream for those interested in catching a glimpse of the comet from the comfort of their own home. The broadcast, which can be can be viewed here from 10 p.m. EDT, will use images from Slooh's flagship observatory at the Canary Islands Institute of Astrophysics, Spain.