Comets: Frozen Messengers of the Cosmos

For many amateur astronomers, few sights in the night sky rival the thrill of spotting a comet. These mysterious visitors from the outer reaches of the Solar System often appear unexpectedly, their glowing tails stretching across the heavens. But what exactly are comets, and why do they capture our imaginations? Let’s explore the nature of these celestial objects and their fascinating journey through space.

Weekly Information about Bright Comets: Updated 9/25/24 

First off, click the link below from from the world famous comet observer, Seiichi Yoshida, to review the bright comets that are visible now, including Comet C/2023 A3 (Tsuchinshan-ATLAS) that the media is focused on at the moment. You will notice that there are also other bright comets that can be accessed with amateur telescopes and binoculars. Take note of each comet that you wish to observe and then come back to this page to learn more about comets. 

How to Observe Comets with Gear from Explore Scientific

What is a Comet and Why Are They Important?

Comet Illustration Courtesy of NASA, and ESA

Approximate true color image of comet 67P taken by the Rosetta spacecraft's OSIRIS Narrow Angle Camera on March 17, 2015. Credit: ESA / Rosetta / MPS for OSIRIS Team (MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA) / Justin Cowart

Comets are ancient remnants from the formation of our solar system, dating back about 4.6 billion years. Coined by Dr. Fred Whipple as "dirty snowballs," they are made primarily of ice, dust, and dark organic material. These icy bodies hold valuable clues to understanding how the solar system formed and evolved. Some scientists believe that comets may have played a crucial role in delivering water and organic compounds—the essential building blocks of life—to early Earth and other planets within our solar system.

Electron microscopic image of a comet dust grain. Source: NASA 

At its core, a comet is an icy body that originates from the distant regions of the Solar System. As a comet travels closer to the Sun, solar radiation heats the surface, causing its ices to vaporize in a process known as *outgassing*. This releases gas and dust, creating a glowing atmosphere called the *coma*. At the same time, the solar wind pushes these materials away, forming a brilliant *tail* that can extend for millions of kilometers. Despite its size, the tail always points away from the Sun, blown by the pressure of sunlight and solar particles. 

The ion tail of a comet is composed of ions such as CO+, N2+, and CO2+. As the comet enters the inner Solar System, the Sun’s radiation heats its nucleus, causing these gases to "boil off" and become ionized. These ions, now electrically charged, interact with the solar wind—streams of charged particles emitted by the Sun. This interaction distorts the magnetic field lines, creating a magnetotail that always points away from the Sun. The charged ions follow the magnetic field lines within the tail, which is why the ion tail consistently points in the opposite direction of the Sun.

One of these ions, CO+, absorbs sunlight and fluoresces, emitting blue light at a wavelength of 4200 Angstroms.

The Composition of a Comet: A “Dirty Snowball”

In the early 1980s, as the world anticipated the return of Halley’s Comet, a young Scott Roberts was working at a telescope shop, eagerly absorbing all he could about astronomy. It was during this time that Scott first learned of Dr. Fred Whipple’s groundbreaking "Dirty Snowball" theory, which described comets as being composed of a mixture of ice, dust, and rock. Dr. Whipple theorized that comets were remnants from the formation of the Solar System, preserved for billions of years in the deep freeze of space.

Intrigued, Scott decided to take a bold step. He contacted his local library, hoping to find a way to reach Dr. Whipple. To his astonishment, the librarian provided a phone number. Hesitant but curious, Scott called—and to his amazement, Dr. Fred Whipple himself answered.

What followed was a remarkable conversation in which Dr. Whipple personally explained his theory. He described how the icy core, or nucleus, of a comet behaves as it nears the Sun, creating the glowing comas and the difference between the sweeping ion and dust tails visible from Earth. For Scott, this experience was more than just a lesson in comet science—it was an unforgettable reminder that reaching out to those we admire can open doors we never imagined.

The Size and Scale of a Comet

Comets can vary greatly in size, but even the smallest are impressive. The nucleus of a comet can range from just a few hundred meters to tens of kilometers across. But it’s the coma—the cloud of gas and dust surrounding the nucleus—that can grow to enormous proportions, sometimes expanding to be as wide as 15 times the diameter of Earth. The tail of a comet, meanwhile, can stretch for millions of kilometers, often extending well beyond the orbit of our planet

While most comets are too faint to be seen without the help of a telescope, a few—known as “Great Comets”—become bright enough to be visible to the naked eye. These rare events are a spectacular treat for stargazers.

Where Do Comets Come From?

While comets can be found all over the solar system, they are concentrated in and emerge from two primary regions in the outer Solar System: the Kuiper Belt and the distant Oort Cloud. The Kuiper Belt, which lies beyond Neptune, is home to short-period comets that take only a few decades to complete an orbit around the Sun. The Oort Cloud, a vast shell of icy bodies at the edge of the Solar System, sends long-period comets on orbits that can span thousands or even millions of years.

However, some comets are of interstellar origin, perhaps from other solar systems. The first confirmed interstellar object was ʻOumuamua (1I/ʻOumuamua), discovered in 2017. While initially thought to be a comet, it lacked the characteristic coma and tail. Its unusual trajectory and speed suggested it originated from outside our Solar System.

The second confirmed interstellar object, and the first confirmed interstellar comet, is 2I/Borisov, discovered in 2019. Unlike ʻOumuamua, 2I/Borisov exhibited the typical characteristics of a comet, including a visible coma and tail. Its hyperbolic orbit and high velocity indicated that it came from interstellar space and would not return.

These objects are believed to have been ejected from their home star systems due to gravitational interactions and sent on a journey through interstellar space. Their discovery has opened a new chapter in astronomy, offering scientists an opportunity to study materials from other star systems.

Gravitational interactions with planets or nearby stars can alter a comet’s path, sending it spiraling into the inner Solar System where we can observe it. Some comets make only one appearance before being flung back into the depths of space, while others return periodically.

How are Comets Named?

Comets are named and designated according to specific guidelines set by the International Astronomical Union (IAU). The naming system provides both a unique name for each comet and a designation that conveys important information about its discovery. Here’s a breakdown of how comets are named and designated:

Provisional Designation

When a comet is first discovered, it is given a provisional designation based on the year, half-month of discovery, and the order of its discovery within that half-month. This designation follows a specific format:

• First letter: Indicates the half-month of discovery (e.g., "A" for the first half of January, "B" for the second half, etc.).
• Second letter: Indicates the order of discovery within that half-month (e.g., "C/2023 A1" means the first comet discovered in the first half of January 2023).

For example:

• C/2023 A1: The first comet discovered in the first half of January 2023.

Comet Type Prefix

Before the provisional designation, a letter prefix indicates the type of comet:

• C/: A long-period comet (with an orbital period longer than 200 years).
• P/: A periodic (short-period) comet with an orbital period less than 200 years.
• D/: A comet that has been lost or has disintegrated.
• X/: A comet for which an orbit has not been determined.
• A/: An object originally thought to be a comet but later identified as an asteroid.

Example:

• P/2023 A1: A periodic comet discovered in the first half of January 2023.

Naming After Discoverers

Once a comet’s discovery is confirmed, it is typically named after its discoverer(s). The name can honor an individual astronomer or an automated survey program that discovered the comet. A comet may bear the names of up to three discoverers.

For example:

• Comet P/Shoemaker-Levy 9: Named after three astronomers, Gene and Carolyn Shoemaker, and David H. Levy, who discovered it as a team.
• Comet Hale-Bopp: Named after the two astronomers, Alan Hale and Thomas Bopp, who independently discovered it.
• Comet 2I/Borisov: Named after Gennadiy Borisov, who discovered it, with the "2I" indicating it was the second known interstellar comet.

Numbering of Periodic Comets

Once a periodic comet is observed on multiple returns (usually at least two), it is assigned a number in the order of its discovery. This number is placed before the name, indicating that it has a confirmed, repeatable orbit.

Example:

- 1P/Halley: The first periodic comet to be identified, named after Edmond Halley.

- 67P/Churyumov–Gerasimenko: The 67th periodic comet discovered, named after its discoverers, Klim Churyumov and Svetlana Gerasimenko.

Example of Full Designation:

• C/2023 A1 (Smith): A long-period comet, the first discovered in the first half of January 2023, named after the discoverer Smith.
• P/1996 R2 (SOHO): A periodic comet discovered in the second half of September 1996 by the SOHO (Solar and Heliospheric Observatory) spacecraft.

This structured system of naming and designation ensures clarity in identifying and tracking comets, making it easier for astronomers to catalog and study these fascinating celestial objects.

How to Search for and Spot a Comet

David H. Levy, one of the most successful comet discoverers of our time, has spent countless nights scanning the skies in search of comets. His approach to comet hunting is both meticulous and methodical, combining patience, observational skill, and a deep passion for the night sky.

Levy's search typically begins with one of his trusted telescopes, often a modest instrument, such as an 8-inch or 16-inch reflector. He spends hours systematically sweeping the sky, one section at a time. His goal is to identify objects that look out of place against the familiar background of stars. Unlike stars, which remain fixed in their positions relative to one another, comets appear as fuzzy or diffuse objects with a slight motion over time. This motion, combined with their characteristic coma or tail, sets them apart from the point-like stars.

As Levy sweeps the sky, he is keenly attuned to spotting anything that deviates from the starry backdrop. The process involves examining the same patch of sky over time, allowing him to detect the subtle movement of potential comets. Objects that seem blurry or slightly out of focus compared to the sharp points of starlight are often the first clue. If an object appears and seems to move over successive observations, it could be a comet—an icy wanderer from the outer Solar System making its way inward.

Levy’s skill comes from years of practice, learning to differentiate between stars, galaxies, nebulae, and other celestial objects, and recognizing that slight fuzziness that signals a potential new comet. When he identifies something suspicious, he rechecks the area repeatedly to confirm its movement and appearance, then takes careful notes and measurements to track the object.

This method of "visual sweeping" is an old-school, hands-on approach that Levy has mastered over decades. His persistence and dedication have led to the discovery of more than 20 comets, including the famous Comet Shoemaker-Levy 9, which collided with Jupiter in 1994.

Levy's comet-hunting journey is a testament to the rewards of patience, careful observation, and the joy of finding something new in the vastness of space.

How to Report a Comet Discovery

Reporting a comet discovery requires following a specific procedure to ensure the discovery is verified and officially recognized. It is crucial to confirm that the object is genuinely new before reporting it. As a beginning comet hunter, it’s important to avoid making false reports, as this could lead to having a reputation for making poor observations. Here’s how to properly report a comet discovery:

Confirm the Object is New

Before reporting, you need to make sure that the object you’ve discovered isn’t already known or cataloged. Check against existing databases like the Minor Planet Center (MPC) and comet catalogs. If it seems to be a new object, proceed with the report.

Gather Accurate Observations

Make sure to collect as much accurate data about the object as possible. Essential information includes:

• Date and time of discovery (in Universal Time, UT).
• Position of the object (Right Ascension and Declination, preferably from several observations).
• Brightness of the object (magnitude).
• Movement of the object across the sky (rate and direction).
• Description of the object (appearance, coma size, tail, etc.).
• Telescope and equipment used to observe the object.

Make multiple observations over time to confirm the object is a comet, especially noting if it has a diffuse appearance, a tail, or a coma.

Document the Discovery

Keep a detailed log of all observations, including exact times, equipment used, and any environmental conditions that might affect the observation.

Report the Discovery to the Minor Planet Center (MPC)

The Minor Planet Center (MPC) is the official clearinghouse for new comet discoveries. You can submit your report to the MPC, which will evaluate the discovery. 

 How to Submit

• Email the MPC at cbat@cfa.harvard.edu with the subject line "Comet Discovery."
• Include all relevant information (position, motion, brightness, and any other observations) in your report.
• Add your contact information and any collaborators’ names if multiple people were involved in the discovery.
• The MPC’s online submission tool also allows amateur astronomers to submit comet observations.

Provide Images (If Possible)

If you have images or sketches of the object, include them with your report. Highquality images can help confirm the comet’s characteristics, such as its coma or tail.

Wait for Verification

After you submit your report, the Minor Planet Center and other professional astronomers will attempt to verify the discovery. They will:

• Crossreference existing objects to ensure it’s not previously cataloged.
• Collect independent observations from other astronomers.
• Analyze the comet’s orbit to confirm its nature.

Receive Official Recognition

If your discovery is confirmed to be a new comet, it will be officially recognized. The comet will receive a provisional designation (e.g., C/2024 A1), and if the discovery is confirmed, your name (or your team’s) may be attached to the comet (e.g., Comet Smith).

Continue Observing the Comet

Once your discovery has been verified, continue making observations. These followup observations help refine the comet's orbit and are important for tracking its path through the Solar System.

Tips for Discovery Success:

• Observe regularly: Comet discovery often comes from regular sky surveys and systematic observations of areas near the horizon or far from the ecliptic, where comets often appear.
• Join comet discovery groups: Many comets today are discovered by amateur astronomers participating in online networks and collaborations like the Comet Section of the British Astronomical Association or the International Astronomical Union (IAU).
• Use automated software: Many amateur astronomers use imaging software that automatically scans for moving objects.

To report a comet discovery, you need to confirm the object is new, gather accurate observations, and submit your discovery to the Minor Planet Center. If verified, the comet will be given a designation, and the discoverer’s name will be associated with it. Patience and persistence are key, as verification can take some time!

A Final Thought for Aspiring Comet Hunters

Comets have fascinated humans for millennia, and they continue to inspire awe and wonder. Whether you’re a seasoned observer or a novice astronomer, spotting a comet is a deeply rewarding experience that connects us to the history of the universe.

So the next time you hear about a comet making its way through the night sky, don’t hesitate—grab your telescope or binoculars and take a look. You never know when you might witness one of these icy visitors, linking you to the ancient past of our Solar System. 

And remember, as Scott Roberts learned in his conversation with Dr. Whipple, sometimes all it takes is a little curiosity and the courage to ask a question—doors to understanding may open in unexpected ways.