Comets, Meteor Showers, and other Astronomical (or astrophotography) Events

Comet Leonard (C2021A1) with the American flag; Captured 8 December 2021 in Colorado Springs, Colorado

What is a Comet?

A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or coma surrounding the nucleus, and sometimes a tail of gas and dust gas blown out from the coma. These phenomena are due to the effects of solar radiation and the outstreaming solar wind plasma acting upon the nucleus of the comet. Comet nuclei range from a few hundred meters to tens of kilometers across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times Earth’s diameter, while the tail may stretch beyond one astronomical unit. If sufficiently close and bright, a comet may be seen from Earth without the aid of a telescope and can subtend an arc of up to 30° (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures and religions.

Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Long-period comets are set in motion towards the Sun by gravitational perturbations from passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition.

As of November 2021, there are 4,584 known comets. However, this represents a very small fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud) is about one trillion. Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular. Particularly bright examples are called “great comets”. Comets have been visited by uncrewed probes such as NASA’s Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior, and the European Space Agency’s Rosetta, which became the first to land a robotic spacecraft on a comet.

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What is a Meteor Shower?

A meteor shower is a celestial event in which a number of meteors are observed to radiate, or originate, from one point in the night sky. These meteors are caused by streams of cosmic debris called meteoroids entering Earth’s atmosphere at extremely high speeds on parallel trajectories. Most meteors are smaller than a grain of sand, so almost all of them disintegrate and never hit the Earth’s surface. Very intense or unusual meteor showers are known as meteor outbursts and meteor storms, which produce at least 1,000 meteors an hour, most notably from the Leonids. The Meteor Data Centre lists over 900 suspected meteor showers of which about 100 are well established. Several organizations point to viewing opportunities on the Internet. NASA maintains a daily map of active meteor showers.

A meteor shower results from an interaction between a planet, such as Earth, and streams of debris from a comet. Comets can produce debris by water vapor drag, as demonstrated by Fred Whipple in 1951, and by breakup. Whipple envisioned comets as “dirty snowballs,” made up of rock embedded in ice, orbiting the Sun. The “ice” may be water, methane, ammonia, or other volatiles, alone or in combination. The “rock” may vary in size from a dust mote to a small boulder. Dust mote sized solids are orders of magnitude more common than those the size of sand grains, which, in turn, are similarly more common than those the size of pebbles, and so on. When the ice warms and sublimates, the vapor can drag along dust, sand, and pebbles.

Each time a comet swings by the Sun in its orbit, some of its ice vaporizes, and a certain number of meteoroids will be shed. The meteoroids spread out along the entire trajectory of the comet to form a meteoroid stream, also known as a “dust trail” (as opposed to a comet’s “gas tail” caused by the tiny particles that are quickly blown away by solar radiation pressure).

Recently, Peter Jenniskens has argued that most of our short-period meteor showers are not from the normal water vapor drag of active comets, but the product of infrequent disintegrations, when large chunks break off a mostly dormant comet. Examples are the Quadrantids and Geminids, which originated from a breakup of asteroid-looking objects, (196256) 2003 EH1 and 3200 Phaethon, respectively, about 500 and 1000 years ago. The fragments tend to fall apart quickly into dust, sand, and pebbles and spread out along the comet’s orbit to form a dense meteoroid stream, which subsequently evolves into Earth’s path.

Meteor showers are named after the nearest constellation, or bright star with a Greek or Roman letter assigned that is close to the radiant position at the peak of the shower, whereby the grammatical declension of the Latin possessive form is replaced by “id” or “ids.” Hence, meteors radiating from near the star Delta Aquarii (declension “-i”) are called the Delta Aquariids. The International Astronomical Union’s Task Group on Meteor Shower Nomenclature and the IAU’s Meteor Data Center keep track of meteor shower nomenclature and which showers are established.

What is the Radiant Point? Because meteor shower particles are all traveling in parallel paths and at the same velocity, they will appear to an observer below to radiate away from a single point in the sky. This radiant point is caused by the effect of perspective, similar to parallel railroad tracks converging at a single vanishing point on the horizon. Meteor showers are normally named after the constellation from which the meteors appear to originate. This “fixed point” slowly moves across the sky during the night due to the Earth turning on its axis, the same reason the stars appear to slowly march across the sky. The radiant also moves slightly from night to night against the background stars (radiant drift) due to the Earth moving in its orbit around the Sun. See IMO Meteor Shower Calendar 2017 (International Meteor Organization) for maps of drifting “fixed points.”

When the moving radiant is at the highest point, it will reach the observer’s sky that night. The Sun will be just clearing the eastern horizon. For this reason, the best viewing time for a meteor shower is generally slightly before dawn — a compromise between the maximum number of meteors available for viewing and the brightening sky, which makes them harder to see.

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What is a Star Trail?

A star trail isn’t really an astronomical event, but a photograph using long exposure times to capture diurnal circles, the apparent motion of stars in the night sky due to Earth’s rotation. A star-trail photograph shows individual stars as streaks across the image, with longer exposures yielding longer arcs. The term is used for similar photos captured elsewhere, such as on board the International Space Station and on Mars.

Typical shutter speeds for a star trail range from 15 minutes to several hours, requiring a “Bulb” setting on the camera to open the shutter for a period longer than usual. However, a more practiced technique (and the one that I prefer to use) is to blend a number of frames together to create the final star trail image.

Star trail photographs are possible because of the rotation of Earth about its axis. The apparent motion of the stars is recorded as mostly curved streaks on the film or detector. For observers in the Northern Hemisphere, aiming the camera northward creates an image with concentric circular arcs centered on the north celestial pole (very near Polaris). For those in the Southern Hemisphere, this same effect is achieved by aiming the camera southward. In this case, the arc streaks are centered on the south celestial pole (near Sigma Octantis). Aiming the camera eastward or westward shows straight streaks on the celestial equator, which is tilted at angle with respect to the horizon. The angular measure of this tilt depends on the photographer’s latitude, and is equal to 90° − L.

Star trail photographs can be used by astronomers to determine the quality of a location for telescope observations. Star trail observations of Polaris have been used to measure the quality of seeing in the atmosphere, and the vibrations in telescope mounting systems. The first recorded suggestion of this technique is from E.S. Skinner’s 1931 book A Manual of Celestial Photography.

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What is the Aurora Borealis?

At any given moment, the sun is ejecting charged particles from its corona, or upper atmosphere, creating the solar wind. When that wind slams into Earth’s ionosphere, or upper atmosphere, the aurora is born. In the Northern Hemisphere, the phenomenon is called the northern lights (aurora borealis), while in the Southern Hemisphere, it’s called the southern lights (aurora australis).

What causes the aurora? The particles are deflected towards the poles of Earth by our planet’s magnetic field and interact with our atmosphere, depositing energy and causing the atmosphere to fluoresce. The bright colors of the northern lights are dictated by the chemical composition of Earth’s atmosphere. Every type of atom or molecule, whether it’s atomic hydrogen or a molecule like carbon dioxide, absorbs and radiates its own unique set of colors. Some of the dominant colors seen in aurorae are red, a hue produced by the nitrogen molecules, and green, which is produced by oxygen molecules.

What causes the movement and shape of auroras? Constantly changing input from the sun, varying responses from the Earth’s upper atmosphere, and the motion of the planet and particles in near-Earth space all conspire to cause different auroral motions and shapes. From these motions and shapes, we can learn about the physics happening further out in space along the Earth’s magnetic field lines.

What do auroras tell us about Earth’s atmosphere? Auroras tell us many things about Earth’s upper atmosphere, including its density, composition, flow speeds, and the strength of electrical currents flowing in the upper atmosphere. These in turn tell us about the Earth’s magnetic field, how it extends into space, and how it changes dynamically. All of this is important for protecting Earth and space-borne technologies from hazards of “space weather” of which aurora is one part.

While solar wind is constant, the sun’s emissions go through a roughly 11-year cycle of activity. Sometimes there’s a lull, but other times, there are vast storms that bombard Earth with extreme amounts of energy. This is when the northern lights are at their brightest and most frequent. We are currently approaching solar maximum which is predicted to peak between early 2024 to late 2025. Scientists cannot pinpoint exactly when solar maximum will occur but we do know it’s on its way. “Currently for solar cycle 25, by synthesizing all published predictions, the time interval for the cycle maximum ranges from late 2023 to early 2025” according to Frédéric Clette, solar physicist, World Data Center Sunspot Index and Long-term Solar Observations (SILSO) and Solar Influence Data analysis Center (SIDC). NOAA’s Space Weather Prediction Center (SWPC) recently issued a revised prediction that suggests solar maximum may occur between January and October 2024.