The Tale of the Comet

Recently the passing of the comet Neowise across the starry skies of the Earth caused a lot a buzz across the globe. It was discovered by a team using the WISE space telescope under the NEOWISE program on March 27, 2020. What was worth mentioning about this is the fact that the comet Neowise, once it disappears from view, it will not be seen across the Earth's skies for another 6800 years. It was visible across the Earth skies all through July 2020 and 22 July being the day it was closest to the Earth, just 64 million miles away. For the space novices, let's start with what these celestial bodies are.

The comet Neowise as photographed from the ISS

What are comets?

A comet is a very small solar system body made mostly of ices mixed with smaller amounts of dust and rock. Most comets are no larger than a few kilometers across. The main body of the comet is called the nucleus, and it can contain water, methane, nitrogen, and other ices.

When a comet is heated by the Sun, its ices begin to sublimate. The mixture of ice crystals and dust blows away from the comet nucleus in the solar wind, creating a pair of tails. The dust tail is what we normally see when we view comets from Earth.

A plasma tail also forms when molecules of the gas are “excited” by interaction with the solar wind. The plasma tail is not normally seen with the naked eye but can be imaged. Comets normally orbit the Sun and have their origins in the Oort Cloud and Kuiper Belt regions of the outer solar system.

Observatory image of the comet ISON

Classification of comets

Comets come in several categories. The most common are periodic and non-periodic.

In the past, comets were named for their discoverers, such as Comet Halley for Sir Edmond Halley. In modern times, comet names are governed by rules set forth by the International Astronomical Union (IAU). A comet is given an official designation, and can also be identified by the last names of up to three independent discoverers.

Here’s how it works. Once a comet has been confirmed, the following naming rules are followed. 

First, if the comet is a periodic comet, then it is indicated with a P/ followed by the year of its discovery, a letter indicating the half-month in which it was discovered, followed by a number indicating its order of discovery. So, for example, the second periodic comet found in the first half of January 2015 would be called P/2015 A2. The periodic comet Halley (1P/Halley) is the most famous in history. It returns to the inner solar system once every 76 years.

A non-periodic comet would be indicated with a C/ followed by the year of its discovery, a letter indicating the half-month in which it was discovered, followed by a number indicating its order of discovery. A famous non-periodic comet Hale-Bopp(C/1995 O1) was last seen across the Earth skies in the year 1997.

If a comet is independently discovered by three people named Smith, Jones, and Petersen, it could also be called Comet Smith-Jones-Petersen, in addition to its formal designation. Today, many comets are found through automated instrument searches, and so the formal designations are more commonly used.

The mysterious relation between Earth's water & comet

Earth first formed about 4.5 billion years ago. At first, it was too hot to have an ocean. Any water would have boiled right off. After Earth cooled down, the water would stop boiling off into space. Any water would actually stick around. The rocky material that formed Earth in the first place contained some water. But that probably doesn't account for all the water. 

                                       

Suspected water-vapor in the comet Hartley

       

Comets are mostly water ice. It could be that comets made regular water deliveries to Earth. It would take a lot of comets to fill the ocean! But comets could well have made a big contribution. Asteroids contain some water too, so they may have contributed. comets typically have a greater concentration of ingredients that vaporize when heated, accounting for their iconic gaseous tails. Both comets and asteroids can contain ice. And if by colliding with Earth, they added the amount of material some scientists suspect, such bodies could easily have delivered oceans’ worth of water.

I hope it was a worthwhile read and you all got to learn more about comets.

That's it for now. Stay Safe. Stay healthy.