2016 Eta Aquarid Meteor Shower Peaks May 5-6

The Eta Aquarid meteor shower peaks shortly before dawn on Friday and Saturday mornings. The radiant lies in Aquarius near the star Eta. Diagram: Bob King, source: Stellarium
The Eta Aquarid meteor shower peaks shortly before dawn on Thursday and Friday mornings. The radiant lies in Aquarius near the star Eta. Diagram: Bob King, source: Stellarium

Itching to watch a meteor shower and don’t mind getting up at an early hour? Good because this should be a great year for the annual Eta Aquarid (AY-tuh ah-QWAR-ids) shower which peaks on Thursday and Friday mornings May 5-6. While the shower is best viewed from tropical and southern latitudes, where a single observer might see between 25-40 meteors an hour, northern views won’t be too shabby. Expect to see between 10-15 per hour in the hours before dawn.

Most showers trace their parentage to a particular comet. The Perseids of August originate from dust strewn along the orbit of comet 109P/Swift-Tuttle, which drops by the inner solar system every 133 years after “wintering” for decades just beyond the orbit of Pluto.

Photo of Haley's Comet crossing the Milky Way, taken by the Kuiper Airborne Observatory in New Zealand on April 8th/9th, 1986. Credit: NASA
Halley’s Comet crossing the Milky Way, taken by the Kuiper Airborne Observatory in New Zealand on April 8-9, 1986. Credit: NASA

The upcoming Eta Aquarids  have the best known and arguably most famous parent of all: Halley’s Comet. Twice each year, Earth’s orbital path intersects dust and minute rock particles strewn by Halley during its cyclic 76-year journey from just beyond Uranus to within the orbit of Venus.

Our first pass through Halley’s remains happens this week, the second in late October during the Orionid meteor shower. Like bugs hitting a windshield, the grains meet their demise when they smash into the atmosphere at 147,000 mph (237,000 km/hr) and fire up for a brief moment as meteors. Most comet grains are only crumb-sized and don’t have a chance of reaching the ground as meteorites. To date, not a single meteorite has ever been positively associated with a particular shower.

A bright, earthgrazer Eta Aquarids streaks across Perseus May 6, 2013. Because the radiant is low for northern hemisphere observers, earthgrazers - long, bright meteors that come up from near the horizon and have long-lasting trails. Credit: Bob King
A bright, earthgrazing Eta Aquarid streaks across Perseus and through the aurora on May 6, 2013. Because the radiant is low for northern hemisphere observers, earthgrazers – long, bright meteors that come up from near the horizon and have long-lasting trails. Credit: Bob King

The farther south you live, the higher the shower radiant will appear in the sky and the more meteors you’ll spot.  A low radiant means less sky where meteors might be seen. But it also means visits from “earthgrazers”. These are meteors that skim or graze the atmosphere at a shallow angle and take many seconds to cross the sky. Several years back, I saw a couple Eta Aquarid earthgrazers during a very active shower. One other plus this year — no moon to trouble the view, making for ideal conditions especially if you can observe from a dark sky.

From mid-northern latitudes the radiant or point in the sky from which the meteors will appear to originate is low in the southeast before dawn. At latitude 50° north the viewing window lasts about 1 1/2 hours before the light of dawn encroaches; at 40° north, it’s a little more than 2 hours. If you live in the southern U.S. you’ll have nearly 3 hours of viewing time with the radiant 35° high.

At some personal peril, I grabbed a photo of snow in the headlights while driving home in a recent storm. Meteors in a meteor shower appear to radiate from a point in the distance in identical fashion. Photo: Bob King
Meteors in a meteor shower appear to radiate from a point in the distance in identical fashion to the way snow or rain radiates from a point in front of your car when you’re driving. Credit: Bob King

Grab a reclining chair, face east and kick back for an hour or so between 3 and 4:30 a.m. An added bonus this spring season will be hearing the first birdsong as the sky brightens toward the end of your viewing session. And don’t forget the sights above: a spectacular Milky Way arching across the southern sky and the planets of Mars and Saturn paired up in the southwestern sky.

Meteor shower members can appear in any part of the sky, but if you trace their paths in reverse, they’ll all point back to the radiant. Other random meteors you might see are called sporadics and not related to the Eta Aquarids. Meteor showers take on the name of the constellation from which they originate.

Aquarius is home to at least two showers. This one’s called the Eta Aquarids because it emanates from near the star Eta Aquarii. An unrelated shower, the Delta Aquarids, is active in July and early August. Don’t sweat it if weather doesn’t cooperate the next couple mornings. The shower will be active throughout the weekend, too.

Happy viewing and clear skies!

What is the Oort Cloud?

The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA

For thousands of years, astronomers have watched comets travel close to Earth and light up the night sky. In time, these observations led to a number of paradoxes. For instance, where were these comets all coming from? And if their surface material vaporizes as they approach the Sun (thus forming their famous halos), they must formed farther away, where they would have existed there for most of their lifespans.

In time, these observations led to the theory that far beyond the Sun and planets, there exists a large cloud of icy material and rock where most of these comets come from. This existence of this cloud, which is known as the Oort Cloud (after its principal theoretical founder), remains unproven. But from the many short and long-period comets that are believed to have come from there, astronomers have learned a great deal about it structure and composition.

Definition:

The Oort Cloud is a theoretical spherical cloud of predominantly icy planetesimals that is believed to surround the Sun at a distance of up to around 100,000 AU (2 ly). This places it in interstellar space, beyond the Sun’s Heliosphere where it defines the cosmological boundary between the Solar System and the region of the Sun’s gravitational dominance.

Like the Kuiper Belt and the Scattered Disc, the Oort Cloud is a reservoir of trans-Neptunian objects, though it is over a thousands times more distant from our Sun as these other two. The idea of a cloud of icy infinitesimals was first proposed in 1932 by Estonian astronomer Ernst Öpik, who postulated that long-period comets originated in an orbiting cloud at the outermost edge of the Solar System.

In 1950, the concept was resurrected by Jan Oort, who independently hypothesized its existence to explain the behavior of long-term comets. Although it has not yet been proven through direct observation, the existence of the Oort Cloud is widely accepted in the scientific community.

Structure and Composition:

The Oort Cloud is thought to extend from between 2,000 and 5,000 AU (0.03 and 0.08 ly) to as far as 50,000 AU (0.79 ly) from the Sun, though some estimates place the outer edge as far as 100,000 and 200,000 AU (1.58 and 3.16 ly). The Cloud is thought to be comprised of two regions – a spherical outer Oort Cloud of 20,000 – 50,000 AU (0.32 – 0.79 ly), and disc-shaped inner Oort (or Hills) Cloud of 2,000 – 20,000 AU (0.03 – 0.32 ly).

The outer Oort cloud may have trillions of objects larger than 1 km (0.62 mi), and billions that measure 20 kilometers (12 mi) in diameter. Its total mass is not known, but – assuming that Halley’s Comet is a typical representation of outer Oort Cloud objects – it has the combined mass of roughly 3×1025 kilograms (6.6×1025 pounds), or five Earths.

Based on the analyses of past comets, the vast majority of Oort Cloud objects are composed of icy volatiles – such as water, methane, ethane, carbon monoxide, hydrogen cyanide, and ammonia. The appearance of asteroids thought to be originating from the Oort Cloud has also prompted theoretical research that suggests that the population consists of 1-2% asteroids.

Earlier estimates placed its mass up to 380 Earth masses, but improved knowledge of the size distribution of long-period comets has led to lower estimates. The mass of the inner Oort Cloud, meanwhile, has yet to be characterized. The contents of both Kuiper Belt and the Oort Cloud are known as Trans-Neptunian Objects (TNOs), because the objects of both regions have orbits that that are further from the Sun than Neptune’s orbit.

A belt of comets called the Oort Cloud is theorized to encircle the Solar system (image credit: NASA/JPL).
A belt of comets called the Oort Cloud is theorized to encircle the Solar system (image credit: NASA/JPL).

Origin:

The Oort cloud is thought to be a remnant of the original protoplanetary disc that formed around the Sun approximately 4.6 billion years ago. The most widely accepted hypothesis is that the Oort cloud’s objects initially coalesced much closer to the Sun as part of the same process that formed the planets and minor planets, but that gravitational interaction with young gas giants such as Jupiter ejected them into extremely long elliptic or parabolic orbits.

Recent research by NASA suggests that a large number of Oort cloud objects are the product of an exchange of materials between the Sun and its sibling stars as they formed and drifted apart. It is also suggested that many – possibly the majority – of Oort cloud objects were not formed in close proximity to the Sun.

Alessandro Morbidelli of the Observatoire de la Cote d’Azur has conducted simulations on the evolution of the Oort cloud from the beginnings of the Solar System to the present. These simulations indicate that gravitational interaction with nearby stars and galactic tides modified cometary orbits to make them more circular. This is offered as an explanation for why the outer Oort Cloud is nearly spherical in shape while the Hills cloud, which is bound more strongly to the Sun, has not acquired a spherical shape.

A comparison of the Solar System and its Oort Cloud. 70,000 years ago, Scholz's Star and companion passed along the outer boundaries of our Solar System (Credit: NASA, Michael Osadciw/University of Rochester)
A comparison of the Solar System and its Oort Cloud. 70,000 years ago, Scholz’s Star and companion passed along the outer boundaries of our Solar System. Credit: NASA, Michael Osadciw/University of Rochester

Recent studies have shown that the formation of the Oort cloud is broadly compatible with the hypothesis that the Solar System formed as part of an embedded cluster of 200–400 stars. These early stars likely played a role in the cloud’s formation, since the number of close stellar passages within the cluster was much higher than today, leading to far more frequent perturbations.

Comets:

Comets are thought to have two points of origin within the Solar System. They start as infinitesimals in the Oort Cloud and then become comets when passing stars knock some of them out of their orbits, sending into a long-term orbit that take them into the inner solar system and out again.

Short-period comets have orbits that last up to two hundred years while the orbits of long-period comets can last for thousands of years. Whereas short-period comets are believed to have emerged from either the Kuiper Belt or the scattered disc, the accepted hypothesis is that long-period comets originate in the Oort Cloud. However, there are some exceptions to this rule.

For example, there are two main varieties of short-period comet: Jupiter-family comets and Halley-family comets. Halley-family comets, named for their prototype (Halley’s Comet) are unusual in that although they are short in period, they are believed to have originated from the Oort cloud. Based on their orbits, it is suggested they were once long-period comets that were captured by the gravity of a gas giant and sent into the inner Solar System.

Evolution of a comet as it orbits the sun. Credit: Laboratory for Atmospheric and Space Sciences/ NASA
Evolution of a comet as it orbits the sun. Credit: Laboratory for Atmospheric and Space Sciences/ NASA

Exploration:

Because the Oort Cloud is so much farther out than the Kuiper Belt, the region remained unexplored and largely undocumented. Space probes have yet to reach the area of the Oort cloud, and Voyager 1 – the fastest and farthest of the interplanetary space probes currently exiting the Solar System – is not likely to provide any information on it.

At its current speed, Voyager 1 will reach the Oort cloud in about 300 years, and will will take about 30,000 years to pass through it. However, by around 2025, the probe’s radioisotope thermoelectric generators will no longer supply enough power to operate any of its scientific instruments. The other four probes currently escaping the Solar System – Voyager 2, Pioneer 10 and 11, and New Horizons – will also be non-functional when they reach the Oort cloud.

Exploring the Oort Cloud presents numerous difficulties, most of which arise from the fact that it is incredible distant from Earth. By the time a robotic probe could actually reach it and begin exploring the area in earnest, centuries will have passed here on Earth. Not only would those who had sent it out in the first place be long dead, but humanity will have most likely invented far more sophisticated probes or even manned craft in the meantime.

Still, studies can be (and are) conducted by examining the comets that it periodically spits out, and long-range observatories are likely to make some interesting discoveries from this region of space in the coming years. It’s a big cloud. Who knows what we might find lurking in there?

We have many interesting articles about the Oort Cloud and Solar System for Universe Today. Here’s an article about how big the Solar System is, and one on the diameter of the Solar System. And here’s all you need to know about Halley’s Comet and Beyond Pluto.

You might also want to check out this article from NASA on the Oort Cloud and one from the University of Michigan on the origin of comets.

Do not forget to take a look at the podcast from Astronomy Cast. Episode 64: Pluto and the Icy Outer Solar System and Episode 292: The Oort Cloud.

Reference:
NASA Solar System Exploration: Kuiper Belt & Oort Cloud

What is Halley’s Comet?

Halley’s Comet, also known as 1P/Halley, is the most well known comet in the Solar System. As a periodic (or short-term comet) it has orbital period that is less than 200 years, and has therefore been observed more than once by people here on Earth over the centuries.

It’s appearance in the skies above Earth has been noted since ancient times, and was associated with both bad and good omens by many cultures. But in truth, its behavior is no different than any short-term visitor that swings by from time to time. And its visits have become entirely predictable!

Discovery:
Halley’s Comet has been observed and recorded by astronomers since at least 240 BCE, with clear references to the comet being made by Chinese, Babylonian, and medieval European chroniclers. However, these records did not recognize that the comet was the same object reappearing over time. It was not until 1705 that English astronomer Edmond Halley, who used Newton’s Three Laws of Motion to determine that it was periodic.

Until the Renaissance, astronomers’ believed that comets – consistent with Aristotle’s views – were merely disturbances in the Earth’s atmosphere. This idea was disproved in 1577 by Tycho Brahe, who used parallax measurements to show that comets must lie beyond the Moon. However, for another century, astronomers would continue to believe that comets traveled in a straight line through the Solar System rather than orbiting the Sun.

In 1687, in his Philosophiæ Naturalis Principia Mathematica, Isaac Newton theorized that comets could travel in an orbit of some sort. Unfortunately, he was unable to develop a coherent model for explaining this at the time. As such, it was Edmond Halley – Newton’s friend and editor –  who showed how Newton’s theories on motion and gravity could be applied to comets.

In his 1705 publication, Synopsis of the Astronomy of Comets, Halley calculated the effect that Jupiter and Saturn’s gravitational fields would have on the path of comets. Using these calculations and recorded observations made of comets, he was able to determine that a comet observed in 1682 followed the same path as a comet observed in 1607.

Pairing this with another observation made in 1531, he concluded that these observations were all of the same comet, and predicted that it would return in another 76 years. His prediction proved to be correct, as it was seen on Christmas Day, 1758, by a German farmer and amateur astronomer named Johann Georg Palitzsch.

His predictions not only constituted the first successful test of Newtonian physics, it was also the first time that an object besides the planets was shown to be orbiting the Sun. Unfortunately for Halley, he did not live to see the comet’s return (having died in 1742). But thanks to French astronomer Nicolas Louis de Lacaille, the comet was named in Halley’s honor in 1759.

The illustration shows a view of Augsburg, Germany with the comets of 1680, 1682, and 1683 in the sky. Click on image for larger view. Image credit: NASA/JPL
The illustration shows a view of Augsburg, Germany with the comets of 1680, 1682 (Halley’s Comet), and 1683 in the sky. Credit: NASA/JPL

Origin and Orbit:
Like all comets that take less than about 200 years to orbit the Sun, Halley’s Comet is believed to have originated from the Kuiper Belt. Periodically, some of these blocks of rock and ice – which are essentially leftover matter from the formation of the Solar System some 4.6 billion years ago – are pulled deeper into the Solar System and becomes active comets.

In 2008, another point of origin for the Halley-type comets had been proposed when a trans-Neptunian object with a retrograde orbit similar to Halley’s was discovered. Known as 2008 KV42, this comet’s orbit takes it from just outside the orbit of Uranus to twice the distance of Pluto. This suggests that Halley ‘s Comet could in fact be member of a new population of small Solar System bodies that is unrelated to the Kuiper Belt.

Halley is classified as a periodic or short-period comet, one with an orbit lasting 200 years or less. This contrasts with long-period comets, whose orbits last for thousands of years and which originate from the Oort Cloud – the sphere of cometary bodies that is 20,000 – 50,000 AU from the Sun at its inner edge. Other comets that resemble Halley’s orbit, with periods of between 20 to 200 years, are called Halley-type comets. To date, only 54 have been observed, compared with nearly 400 identified Jupiter-family comets.

Artists' impression of the Kuiper belt and Oort cloud, showing both the origin and path of Halley's Comet. Image credit: NASA/JPL.
Artists’ impression of the Kuiper belt and Oort cloud, showing both the origin and path of Halley’s Comet. Credit: NASA/JPL

Halley’s orbital period over the last 3 centuries has been between 75–76 years, although it has varied between 74–79 years since 240 BC. Its orbit around the Sun is highly elliptical. It has a perihelion (i.e. the point where it is nearest the Sun) of just 0.6 AU, which places it between the orbits of Mercury and Venus. Meanwhile, it’s aphelion – the farthest distance from the Sun – is 35 AU, the same distance as Pluto.

Unusual for an object in the Solar System, Halley’s orbit is retrograde – which means that it orbits the Sun in the opposite direction to the planets (or clockwise from above the Sun’s north pole). Due to the retrograde orbit, it has one of the highest velocities relative to the Earth of any object in the Solar System.

The orbits of the Halley-type comets suggest that they were originally long-period comets whose orbits were perturbed by the gravity of the gas giants and directed into the inner Solar System. If Halley was once a long-period comet, it is likely to have originated in the Oort Cloud. However, Halley is believed to have been a short-term comet for the past 16,000–200,000 years.

Because its orbit comes close to Earth’s in two places, Halley is the parent body of two meteor showers: the Eta Aquariids in early May, and the Orionids in late October. Observations conducted around the time of Halley’s appearance in 1986, however, suggest that the Eta Aquarid meteor shower might not originate from Halley’s Comet, although it might be perturbed by it.

Photo of Haley's Comet crossing the Milky Way, taken by the Kuiper Airborne Observatory in New Zealand on April 8th/9th, 1986. Credit: NASA
Photo of Haley’s Comet crossing the Milky Way, taken by the Kuiper Airborne Observatory in New Zealand on April 8th/9th, 1986. Credit: NASA

Structure and Composition:
As Halley approaches the Sun, it expels jets of sublimating gas from its surface, which knock it very slightly off its orbital path. This process causes the comet to form a bright tail of ionized gas (ion tail), and a faint one made up of dust particles. The ion tail is also known as a coma (a small atmosphere) which spans up to 100,000 km across and consists of violatiles such as water, methane, ammonia and carbon dioxide.

Despite the vast size of its coma, Halley’s nucleus is relatively small – barely 15 kilometers long, 8 kilometers wide and roughly 8 kilometers thick. Its mass is also relatively low (an estimated 2.2 × 1014 kg, or 242.5 billion tons) and its average density is about 0.6 g/cm3, indicating that it is made of a large number of small pieces held loosely together.

Spacecraft observations have shown that the gases ejected from the nucleus were 80% water vapor, 17% carbon monoxide and 3–4% carbon dioxide, with traces of hydrocarbons (although more-recent sources give a value of 10% for carbon monoxide and also include traces of methane and ammonia).

The dust particles have been found to be primarily a mixture of carbon–hydrogen–oxygen–nitrogen (CHON) compounds – which are common in the outer Solar System – and silicates, like those found in terrestrial rocks. At one time, it was thought that Halley could have delivered water to Earth in the distant past – based on the ratio of deuterium to hydrogen found in the comet’s water that showed it to be chemically similar to the Earth’s oceans. However, subsequent observations have indicated that this is unlikely.

This view of comet Halley's nucleus was obtained by the Halley Multicolour Camera (HMC) on board the Giotto spacecraft, as it passed within 600 km of the comet nucleus on March 13, 1986. Credit: ESA
The nuclear of Halley’s Comet, obtained by the Halley Multicolour Camera (HMC) on board the Giotto spacecraft during its flyby on March 13, 1986. Credit: ESA

The ESA’s Giotto (1985-1992) and Russia’s Vega missions (1986) gave planetary scientists their first view of Halley’s surface and structure. The images could only capture roughly 25% of the comet’s surface, but nevertheless revealed an extremely varied topography – with hills, mountains, ridges, depressions, and at least one crater.

Role in Myths and Superstitions:
As already noted, Halley’s Comet has a long and rich history when it comes to being observed by humans. Including its most recent visits, Halley’s Comet has been visible from Earth on 30 separate occasions. The earliest record of which were the Shih Chi and Wen Hsien Thung Khao chronicles, written in China ca. 240 BCE.

While it is believed that Babylonian scribes recorded the appearance of Halley’s Comet when it returned in 164 and 87 BCE, it’s most famous appearance occurred shortly before the 1066 invasion of England by William the Conqueror. Whereas King Harold of England saw the comet as a bad omen, William and his forces interpreted it as a sign of their impending victory (at least according to legend).

Throughout the Middle Ages, the appearances of comets in the night sky were seen as heralds of bad news, indicating that either a person of royal standing had died, or that dark days lay ahead. This is perhaps owing to what was seen as the erratic and unpredictable behavior of comets, when compared to the Sun, the Moon and the stars.

The Bayeux Tapestry, showing the appearance of Halley's Comet in the sky prior to William the Conqueror's invasion of England. Credit: Wikipedia Commons/Myrabella
The Bayeux Tapestry, showing the appearance of Halley’s Comet in the sky in 1066. Credit: Wikipedia Commons/Myrabella

With the development of modern astronomy, this view of comets has been largely dispelled. However, there are many who still hold to the “doom and gloom” view of Halley’s Comet, believing that it will strike the Earth at some point and trigger an Extinction Level Event, the likes of which has not been seen since the Dinosaurs.

Disappearance:
Halley’s overall lifespan is difficult to predict, and opinions do vary. In 1989, Russian astronomers Boris Chirikov and Vitaly Vecheslavov performed an analysis of 46 apparitions of Halley’s Comet taken from historical records and computer simulations. Their study showed that the comet’s dynamics were chaotic and unpredictable over long timescales, and indicated that its lifetime could be as long as 10 million years.

In 2002, David C. Jewitt conducted a study that indicated that Halley will likely evaporate, or split in two, within the next few tens of thousands of years. Alternately, Jewitt predicted that it could survive long enough to be ejected from the Solar System entirely within a few hundred thousand years.

Meanwhile, observations conducted by D.W. Hughes et al. suggests that Halley’s nucleus has been reduced in mass by 80–90% over the last 2000–3000 revolutions (i.e. 150,000 – 230,000 years). By their estimations, it would not be surprising at all if the comet evaporated entirely within the next 300 revolutions or so (approx. 25,000 years).

The last time Halley’s Comet was seen was in 1986, which means it will not reappear until 2061. As always, some are choosing to prepare for the worst – believing its next pass will signal the end of life as we know it – while others are contemplating if they will live long enough to witness it.

Universe Today has articles on famous comets and distant Halley’s Comet.

For more information, take a look at Comet Halley and Halley’s Comet.

Astronomy Cast has an episode on comets.

Sources: Wikipedia, NASA

What Are Comet Tails?

Comets are renowned for their big beautiful tails that stretch across the sky. But what’s in those things, anyway? And how can comets get multiple tails?

In the past, humans generally used one of two greetings for comets:
1. Dear God, what is that thing? Terrible omens! Surely we will all die in fire.
2. Dear God, what is that thing? Great omens! Surely we will all have a big party… where we all die in fire?

For example, the appearance of what came to be known as Halley’s comet in 1066 was seen as a bad omen for King Harold II. Conversely, it was a good omen for William the Conqueror.

Because of their tails and transitory nature, comets were long thought to be products of the Earth’s atmosphere. It wasn’t until the 1500s, when Tycho Brahe used parallax to determine a comet’s distance. He realized that they were Solar System objects, like planets.

So, good news, we no longer regard them as omens and everyone stopped panicking. Right? Wrong. When Comet Halley approached Earth in 1910, astronomers detected cyanide gas in its tail. French astronomer Camille Flammarion was quoted as saying the gas could “impregnate the atmosphere and possibly snuff out all life on the planet.” This caused a great deal of hysteria. Many bought gas masks and “comet pills” to protect themselves.

With the rise of photographic astronomy, it was found that comets often have two types of tails. A bright tail composed of ionized gas, and a dimmer one composed of dust particles. The ion tail always points away from the Sun. It’s actually being pushed away from the comet by the solar wind.

Comets often develop two tails as they near the sun - a curved dust tail and straight, ion tail. Credit: NASA
Comets often develop two tails as they near the sun – a curved dust tail and straight, ion tail. Credit: NASA

We now know that a comet’s ion tail contains “volatiles” such as water, methane, ammonia and carbon dioxide. These volatiles are frozen near the comet’s surface, and as they approach the Sun, they warm and become gaseous. This also causes dust on the comet’s surface to stream away. The heating of a comet by the Sun is not uniform.

Because of a comet’s irregular shape and rotation, some parts of the surface can be heated by sunlight, while other parts remain cold. In some cases this can mean that comets can have multiple tails, which creates amazing effects where different regions of a comet stream off volatiles.

Comet Lovejoy passing behind green oxygen and sodium airglow layers on December 22, 2011 seen from the space station. Credit: NASA/Dan Burbank
Comet Lovejoy passing behind green oxygen and sodium airglow layers on December 22, 2011 seen from the space station. Credit: NASA/Dan Burbank

These ion tails can be quite large, and some have been observed to be nearly 4 times the distance of the Earth from the Sun. And even though they fill a great volume, they are also pretty diffuse. If you condensed a comet’s tail down to the density of water, it wouldn’t even fill a swimming pool.

We also now know that there isn’t a clear dividing line between comets and asteroids. It’s not the case the comets are dirty snowballs and asteroids are dry rocks. There is a range of variation, and asteroids can gain dusty or gaseous tails and take on a comet-like appearance. In addition, we’ve also found comets orbiting other stars, known as exocomets.

And finally one last fact, the term comet comes from the Latin cometa, which indicated a hairy star.

So, what’s your favorite comet? Tell us in the comets below. And if you like what you see, come check out our Patreon page and find out how you can get these videos early while helping us bring you more great content!

Rosetta’s Comet Looks Like A Kidney Flying Through Space

Up for a little abstract art, anyone? The latest images of the nucleus of Rosetta’s comet makes it look like the celestial object is a kidney. Or perhaps a bean. But regardless of what you “see” in the shape, scientists agree that the comet’s heart certainly isn’t round.

It’s a tantalizing view as the spacecraft speeds towards Comet 67P/Churyumov-Gerasimenko for an August rendezvous. These pictures were taken just a few days ago from 23,000 miles (37,000 kilometers) away, and the spacecraft is drawing noticeably nearer every week. What will a closer view reveal?

“Irregular, elongated, and structured shapes are not uncommon for small bodies such as asteroids and comets,” stated the Max Planck Institute for Solar System Research in a release. “Of the five cometary nuclei that have been visited by spacecraft in close flybys so far, all are far from spherical.”

To illustrate, we’ve put some examples below of the other comets that have had close-up views:

Jets can be seen streaming out of the nucleus, or main body, of comet Hartley 2 in this image from NASA's EPOXI mission. The nucleus is approximately 2 kilometers (1.2 miles) long and .4 kilometers (.25 miles) across at the narrow "neck."  Credit: NASA/JPL-Caltech/UMD
Jets can be seen streaming out of the nucleus, or main body, of comet Hartley 2 in this image from NASA’s EPOXI mission. The nucleus is approximately 2 kilometers (1.2 miles) long and .4 kilometers (.25 miles) across at the narrow “neck.” Credit: NASA/JPL-Caltech/UMD
Halley's Comet, as seen by the European Giotto probe. Credit: Halley Multicolor Camera Team, Giotto Project, ESA
Halley’s Comet, as seen by the European Giotto probe. Credit: Halley Multicolor Camera Team, Giotto Project, ESA
NASA's Stardust-NExT mission took this image of comet Tempel 1 at 8:39 p.m. PST (11:39 p.m. EST) on Feb 14, 2011. The comet was first visited by NASA's Deep Impact mission in 2005. Credit: NASA/JPL-Caltech/Cornell. Image brightened and enhanced to show additional detail.
NASA’s Stardust-NExT mission took this image of comet Tempel 1 at 8:39 p.m. PST (11:39 p.m. EST) on Feb 14, 2011. The comet was first visited by NASA’s Deep Impact mission in 2005. Credit: NASA/JPL-Caltech/Cornell. Image brightened and enhanced to show additional detail.
comet Borrelly's 5-mile (8-kilometer) long nucleus taken from more than 2,000 miles (3,400 kilometers) away. Picture from NASA's Deep Space 1 probe. Credit: NASA/JPL
Comet Borrelly’s 5-mile (8-kilometer) long nucleus taken from more than 2,000 miles (3,400 kilometers) away. Picture from NASA’s Deep Space 1 probe. Credit: NASA/JPL
The nucleus of Comet 81P/Wild taken by NASA's Stardust probe in 2004. Credit: NASA
The nucleus of Comet 81P/Wild taken by NASA’s Stardust probe in 2004. Credit: NASA

The new pictures from Rosetta come shortly after the spacecraft caught its comet tumbling through space. It’s not really known for sure what the nucleus will look like, although several artists have lent their ideas over the years. Luckily, the European Space Agency probe will give us a very close-up view of the comet, as it plans to deploy a lander called Philae to land on the comet’s surface in November.

Both Rosetta and Philae successfully awoke from hibernation earlier this year and all systems appear to be working well so far as they get ready for the close-up encounter with the comet. The spacecraft have been flying through space for about a decade, and will remain with Comet 67P/Churyumov-Gerasimenko as it sweeps to its closest approach to the sun in 2015, between the orbits of Earth and Mars.

Revisit Halley’s Comet – Stay Up Late for This Week’s Eta Aquarid Meteor Shower

UPDATE: Watch a live webcast of the meteor shower, below, from NASA’s Marshall Space Flight Center during the night of Monday, May 5 to the early morning of May 6.

Halley’s Comet won’t be back in Earth’s vicinity until the summer of 2061, but that doesn’t mean you have to wait 47 years to see it. The comet’s offspring return this week as the annual Eta Aquarid meteor shower. Most meteor showers trace their parentage to a particular comet. The Perseids of August originate from dust strewn along the orbit of comet 109P/Swift-Tuttle, which drops by the inner solar system every 133 years after “wintering” for decades just beyond the orbit of Pluto, but the Eta Aquarids (AY-tuh ah-QWAR-ids) have the best known and arguably most famous parent of all – Halley’s Comet. Twice each year, Earth’s orbital path intersects dust and rock particles strewn by Halley during its cyclic 76-year journey from just beyond Uranus to within the orbit of Venus. When we do, the grit meets its demise in spectacular fashion as wow-inducing meteors.



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Composite of Aquarid meteors from the 2012 shower. Credit: John Chumack
Composite of Aquarid meteors from the 2012 shower. Credit: John Chumack

Meteoroids enter the atmosphere and begin to glow some 70 miles high. The majority of them range from sand to pebble sized but most no more than a gram or two.  Speeds range from 25,000-160,000 mph (11-72 km/sec) with the Eta Aquarids right down the middle at 42 miles per second (68 km/sec). Most burn white though ‘burn’ doesn’t quite hit the nail on the head. While friction with the air heats the entering meteoroid, the actual meteor or bright streak is created by the speedy rock exciting atoms along its path. As the atoms return to their neutral state, they emit light. That’s what we see as meteors. Picture them as tubes of glowing gas.

The farther south you live, the higher the shower radiant will appear in the sky and the more meteors you’ll see. For southern hemisphere observers this is one of the better showers of the year with rates around 30-40 meteors per hour. With no moon to brighten the sky, viewing conditions are ideal. Except for maybe the early hour. The shower is best seen in the hour or two before the start of dawn.

The Eta Aquarid shower originates with material left behind by Halley's Comet when the sun boils dust and ice from its nucleus around the time of perihelion. This photo from May 1986 during its last pass by Earth. Credit: Bob King
The Eta Aquarid shower originates with material left behind by Halley’s Comet when the sun boils dust and ice off its nucleus around the time of perihelion. This photo from May 1986 during Halley’s last visit. Credit: Bob King

From mid-northern latitudes the radiant or point in the sky from which the meteors will appear to originate is low in the southeast before dawn. At latitude 50 degrees north the viewing window lasts about 1 1/2 hours; at 40 degrees north, it’s a little more than 2 hours. If you live in the southern U.S. you’ll have nearly 3 hours of viewing time with the radiant 35 degrees high.

A bright, earthgrazer Eta Aquarids streaks across Perseus May 6, 2013. Because the radiant is low for northern hemisphere observers, earthgrazers - long, bright meteors that come up from near the horizon and have long-lasting trails. Credit: Bob King
A bright, earthgrazing Eta Aquarid meteor streaks across Perseus May 6, 2013. Because the radiant is low for northern hemisphere observers, watch for earthgrazers – long, bright meteors that come up from near the horizon and have long-lasting trails. Credit: Bob King

Northerners might spy 5-10 meteors per hour over the next few mornings. Face east for the best view and relax in a reclining chair. One good thing about this event – it won’t be anywhere near as cold as watching the December Geminids or January’s Quadrantids. We must be grateful whenever we can.

Meteor shower members can appear in any part of the sky, but if you trace their paths in reverse, they’ll all point back to the radiant. Other random meteors you might see are called sporadics and not related to the Eta Aquarids. Because Aquarius is home to at least two radiants, we distinguish the Etas, which radiate from near Eta Aquarii, from the Delta Aquarids, an unrelated shower active in July and August.

Wishing you clear skies and plenty of  hot coffee at the ready.

Debunking Comet ISON Conspiracy Theories (No, ISON is Not Nibiru)

Comets always seem to bring ‘em out of the wood work.

There’s a scene from the 1998 movie Deep Impact where the president, played by Morgan Freeman, reveals a terrible truth… the U.S. government has known for over a year that a doomsday comet is headed straight towards Earth, with Hollywood CGI destruction sure to follow.

While dramatic, the scenario is also extremely implausible. On any given evening, amateur astronomers are sweeping the skies using telescopes mounted in backyard observatories that are the envy of many major universities. This effort to discover comets is collaborative and worldwide. If the “Big One” were headed our way, even the likes of Morgan Freeman couldn’t keep it secret.

Trouble is, many unfounded claims are already making their way around the internet about this years’ much anticipated “Comet of the Century,” C/2012 S1 ISON.

Many of these conspiracy theories seem to be a recycling of last years’ Nibiru nonsense. The train of thought runs something like this: Does NASA know something that they’re not telling us? Why are they so interested in this comet? We’ve even had folks ask us why certain patches of Google Earth are “blacked out!”

What ARE they hiding, man?

It’s funny how pseudoscience seems to bubble to the top on YouTube, but I won’t give these conspiracy videos the exposure of the Universe Today platform. With hundreds of thousands of hits, they certainly don’t seem to need it. A simple YouTube search of “ISON” will scare up many wacky ideas about the comet.

In any event, we’ve already fielded several questions from friends and the public on the “dangers” posed by this comet, so we can only imagine that these will grow in intensity as the comet approaches the inner solar system, especially if it performs up to expectations.

What are some of the conspiracy theories out there about Comet ISON?

One currently circulating claim states that Comet ISON has “companions” that have been imaged trailing it. While comets do indeed fragment on occasion, the culprits that can be seen in the .gif animation circulating the internet are easily identified by photography experts as hot pixels in the camera.

Another even more extravagant claim is that Comet ISON will somehow appear “as bright as the Sun.” Even if Comet ISON reaches an expected magnitude equal to that of the full Moon at -13, it will do so when it is less than a degree  from the Sun. Our Sun shines at magnitude -26.74, or over 158,000 times brighter, so it would be very difficult for this comet to compete with the Sun’s brightness in the daytime!

Others seem to worry that this comet — or particles from ISON — could impact Earth. Comet ISON will be making its inner solar system passage safely 0.426 A.U., or a little over 63 million kilometers from Earth even on its closest approach on December 26th. Scientists have defined this comet’s orbit very precisely, and it won’t hit Earth. So, no Comet ISON is not Nibiru — that ‘tenth planet’ destined to destroy Earth that conspiracy lovers can’t seem to let go of.

The debris — which might create a very nice meteor shower — is made up of extremely tiny grains of dust, no more than a few microns wide. Since they will be hitting Earth’s atmosphere at speeds up to 200,000 km/hr (125,000 miles per hour), the particles will burn up.

Here’s a video NASA released about the potential meteor shower from ISON:

Other claims focus on how this comet may cause earthquakes or wreak other untold havoc on Earth. This type of comet hysteria is nothing new. Name a bright comet in history, and you can find a historical event for a convenient tie-in. When haven’t there been earthquakes, pandemics, and wars in history? Plus, according to the US Geological Survey, on any given day there will be an average of 2,750 earthquakes around the world of which 275 are large enough to be felt by humans. But only about 100 earthquakes a year are large enough to cause any damage.

And so, its too easy to tie the “causes” of earthquakes and other events to comets in the sky. Comets have been seen before and during the Norman invasion of England in 1066, an outbreak of the Black Plague in London in 1665, and much more. Gary Kronk maintains a wacky and wonderful list of historical (and sometimes comical) comet “signs and omens” on his Cometography site.

Comet Lovejoy as seen from the International Space Station.
Another brilliant sungrazer, Comet Lovejoy as seen from the International Space Station on December, 2011. (Credit: NASA).

Halley’s Comet produced one of the first great comet hypes of the 20th century with its 1910 passage. Ironically, another comet made a brilliant passage just a few months prior, which became known as the Great Comet of 1910. In fact, many viewers in the general public actually saw this comet and confused it with Halley’s! The recent discovery of cyanogen in the comet’s spectra sparked a panic in the public as hucksters made a small fortune hawking “comet pills” and gas masks to panicked buyers. Never mind that folks ingest more toxic carcinogens from their daily environment than are ever seeded by the tenuous tails of comets.

Another curious bit of hype sprung up in 2011 around Comet Elenin, which promptly broke up and dissipated without even putting on a show. And the supposed earthquakes that conspiracy theorists predicted? Well, the evidence speaks loudly: nothing happened. And the same will be true of Comet ISON. It won’t cause any earthquakes or other disasters. As Don Yeomans from NASA said about Comet Elenin, “It will have an immeasurably miniscule influence on our planet. By comparison, my subcompact automobile exerts a greater influence on the ocean’s tides than comet Elenin ever will.”

So, what’s the harm in all the comet hysteria? Well, one only has to look at the mass suicide of the Heaven’s Gate cult in 1997 to realize that it can be no laughing matter. The suicide was sparked by the idea popularized on the late night Coast to Coast with Art Bell radio show that a spacecraft had been spotted following Comet Hale-Bopp.

Dozens of comets are discovered every year. A great majority are tiny iceballs in unfavorable orbits that never rise above magnitude +10 and are thus of little interest to backyard observers. A couple of times a year, a comet might reach magnitude +6 to +10 and become a fine binocular object.

When a discovery is made — be it by amateur or professional — the first task is to gain enough observations of the object to ascertain its orbit. Once again, we see the international collaborative methods employed by modern science. Already, the cosmic cat’s out of the bag as observatories worldwide make follow up measurements. There are no secrets about Comet ISON that hundreds of astronomers could keep quiet.

You get the idea... a 1687 leaflet depicting the havoc that a comet is sure to bring. (Wikimedia Commons image in the Public Domain).
You get the idea… a 1687 leaflet depicting the havoc that a comet is sure to bring. (Wikimedia Commons image in the Public Domain).

But here are some facts about Comet C/2012 S1 ISON. It was discovered last September by Russian amateurs Vitali Nevski and Artyom Novichonok while making observations for the International Scientific Optical Network (ISON), hence the comet’s name. At the time, it was farther than Jupiter and impossibly faint, but once ISON’s orbit was determined, astronomers realized the comet would pass only 1.1 million miles from center of the Sun (680,000 miles above its surface) on November 28, 2013.

Comet ISON belongs to a special category of comets called sungrazers. As the comet performs a hairpin turn around the Sun on that date, its ices will vaporize furiously in the intense solar heat. Assuming it defies death by evaporation, ISON is expected to become a brilliant object perhaps 10 times brighter than Venus, or maybe even brighter. But that would only occur for a brief time around at perihelion (closest approach to the Sun).

In the end, Comet ISON may put on a good show, but don’t believe the hype.

Comets are notoriously unpredictable when it comes to brightness estimations. To quote comet-hunter David Levy, “Comets are like cats… they have tails, and they do exactly what they want.” But they cannot, however, violate the laws of orbital mechanics!

The orbit and orientation of Comet ISON the day after Christmas 2013 on closest approach to the Earth. (Credit: NASA/JPL's Small-Body Database Browser).
The orbit and orientation of Comet ISON the day after Christmas 2013 on it closest approach to the Earth. (Credit: NASA/JPL’s Small-Body Database Browser).

Was the Repeating Passage of Halley’s Comet Known of in Ancient Times?

An interesting and largely unknown tale of ancient astronomy recently came our way while reading author and astrophysicist Mario Livio’s blog. The story involves the passage of the most famous of all comets.  

It’s fascinating to consider ancient knowledge of the skies. While our knowledge of ancient astronomy is often sparse, we know that cultures lived and perished by carefully monitoring the passage of the heavens.  A heliacal rising of Sirius might coincide with the impending flooding of the life-giving waters of the Nile, or the tracking of the solstices and equinoxes might mark the start of the seasons.

To the ancients, comets were “hairy stars” which appeared unpredictably in the sky. We generally attribute the first realization that comets are periodic to Sir Edmond Halley, who successfully utilized Newton’s laws of gravity and Kepler’s laws of planetary motion to predict the return of Halley’s Comet in 1758. Such a prediction was a vindication of science.

But an interesting tale comes to us from the 1st century CE that Rabbi & Jewish Scholar Yehoshua Ben Hananiah may have known something of “a star that appears every 70 years.” The tale, as told in the Horayoth (rulings) of the Talmud and described in Mr. Livio’s blog is intriguing:

Rabbi Gamliel and Rabbi Yehoshua went together on a voyage at sea. Rabbi Gamliel carried a supply of bread. Rabbi Yehoshua carried a similar amount of bread and in addition a reserve of flour. At sea, they used up the entire supply of bread and had to utilize Rabbi Yehoshua’s flour reserve. Rabbi Gamliel then asked Rabbi Yehoshua: “Did you know that this trip would be longer than usual, when you decided to carry this flour reserve?” Rabbi Yehoshua answered: “There is a star that appears every 70 years and induces navigation errors. I thought it might appear and cause us to go astray.”   

The Rabbi’s assertion is a fascinating one. There aren’t a whole lot of astronomical phenomena on 70 cycles that would have been noticeable to ancient astronomers. With an orbital period of 75.3 years, Halley’s Comet seems to fit the bill the best. The earliest confirmed description of Halley’s comes from Chinese astronomers during its 240 BCE passage. Later subsequent passages of the comet through the inner solar system were noted by the Babylonians in 164 & 87 BCE.

Of course, there’s no further evidence that ancient scholars identified those passages as the same comet. Some great comets such as Hale-Bopp seen in 1997 and this year’s anticipated Comet C/2012 S1 ISON are on orbits spanning thousands of years that outlast most Earthly civilizations.

Mr. Livio also notes that historical knowledge of ancient apparitions of Halley’s may have been accessible to the Great Knesset scholars during the Babylonian exile of the 6th century BCE.

One of the chief objections raised to the Halley hypothesis is the circumstances of the appearance of Halley’s Comet in the Rabbi’s lifetime. Remember, most folks didn’t live for 70 years in the 1st century. Any tales of a periodic comet would have been handed down by generations. You would be lucky to see Halley’s Comet once in your lifetime. Plus, not all apparitions of Halley’s Comet are favorable. For example, Halley’s was bright enough to induce “comet hysteria” with the public in 1910. In contrast, few northern hemisphere members of the general public got a good view of it during its 1986 passage.

Medieval woodcut depicting the supposed destructive influence of a 4th century comet. (Credit: Stanilaus Lubienietski's Theatrum Cometicum, Amsterdam 1668).
Medieval woodcut depicting the supposed destructive influence of a 4th century comet. (Credit: Stanilaus Lubienietski’s Theatrum Cometicum, Amsterdam 1668).

Halley’s Comet was visible on and around January 25th, 66 CE during the Rabbi’s lifetime. However, the Rabbi would have been in his 20’s and have been a student (and not yet a Rabbi) himself. One can imagine that if he was fearful of a “false star” leading them astray, he must’ve known that the 70 year period was just about neigh.

The 66 CE apparition of Halley’s Comet would have appeared around the time of the Jewish Rebellion and just four years before the destruction of the Second Temple in Jerusalem by the Romans in 70 CE.

One other possible astronomical culprit has been cited over the years. The classic variable star Mira (Omicron Ceti) currently has a 332 day cycle which ranges from magnitude +3.5 to below naked eye visibility at +8.6 to +10.1. The variability of Mira was first discovered by astronomer David Fabricius on August 3rd 1596. There are suggestions that ancient Chinese and Babylonian astronomers may have known of this “vanishing star”.

The variable star Mira as imaged by the Hubble Space Telescope. (Credit: NASA/STScl/Margarita Karovska at the Harvard-Smithsonian Center for Astrophysics).
The variable star Mira as imaged by the Hubble Space Telescope. (Credit: NASA/STScl/Margarita Karovska at the Harvard-Smithsonian Center for Astrophysics).

Mira is expected to reach maximum for 2013 from July 21st to 31st.

Not all maxima for Mira are of equal brightness. Mira can peak anywhere from magnitude +2.0 to +4.9 (a 15-fold difference) and there’s evidence to suggest it may have been brighter in the past. Astronomer Philippe Veron noted in 1982 that a larger oscillation period of 60 years for the peak maxima of Mira falls just a decade short of Rabbi Yehoshua’s mention of an errant star.

Whatever the case, its fascinating to consider what celestial object might’ve been referred to, and how many other astronomical tales might be awaiting discovery in ancient texts. We’ve got lots of comets to ponder this year as Comet PanSTARRS, Lemmon, and ISON grace our skies in 2013. Halley’s will make its next visit to the inner solar system in 2061. I’ll open it up to you, the astute Universe Today reading public; was the Rabbi’s Star a comet, a variable star, a meteor storm, or none of the above?

Halley's Comet as seen from latitude 30 north on the morning of July 31st, 2061. (Created by the author using Starry Night software).
Halley’s Comet as seen from latitude 30 north on the morning of July 31st, 2061. (Created by the author using Starry Night software).

-Dr. Mario Livio blogs at A Curious Mind. Be sure to check out his new book Brilliant Blunders: From Darwin to Einstein – Colossal Mistakes by Great Scientists That Changed Our Understanding of Life in the Universe out on May 14th!

 

How the Space Shuttle Killed an American Halley’s Comet Mission

Halley's Comet, as seen by the European Giotto probe. Credit: Halley Multicolor Camera Team, Giotto Project, ESA

NASA missed the chance to visit Halley’s Comet in 1986 when the famed sentinel swung close to Earth, as it does every 76 years. Luckily for history, the Europeans flew Giotto past it on this day (March 13) in 1986, and some other nations sent their own probes.

The full story of NASA’s withdrawal is in Bruce Murray’s Journey Into Space: The First Three Decades of Space Exploration. Murray, the former director of the Jet Propulsion Laboratory, has chapters upon chapters on Halley, but here are some notable highlights.

First of all, there were at least three initiatives for NASA to send a mission to the famed comet. The missions below are in chronological order, and it appears it was only when the preceding one was killed that the next was envisioned:

– Solar sail. This mission would use the power of the solar wind — bits streaming from the sun — to bring a spacecraft within Halley’s gravitational influence. In fact, the spacecraft would stay with Halley as it whisked out of the solar system and would return (long dead) when Halley came back in 2061.

A rendezvous with Comet Tempel 2. Another idea would see a spacecraft swing close to Comet Tempel 2 but also have a probe that would take a picture of Halley from a distance. NASA also considered splitting the mission in two to meet annual budgetary requirements, but the Comet Science Working Group was cool to the idea. There also was some thought about bringing the Europeans into this mission, but that never worked out.

Galileo-type hardware. A third initiative had the Jet Propulsion Laboratory envisioning a distant flyby of Halley, basically using similar types of parts that flew in a spacecraft (called Galileo) to Jupiter.

All three of these initiatives fell to budget cuts during the 1970s and 1980s. What caused the budget cuts? In large part, the space shuttle program. To be sure, the shuttle was an impressive piece of hardware, and we are not doubting what it contributed to the construction of the International Space Station and to human spaceflight in general. But it was a large project and in those tight times, something had to give.

Perhaps the most interesting cancellation came in 1979, when NASA administrator Robert Frosch and his deputy went to President Jimmy Carter’s office to plead for the case of two projects: a solar electric propulsion system that would eventually power the Halley-Tempel 2 mission, and the Compton Gamma Ray Observatory (which flew into space, after many delays, in 1991).

Carter, according to Murray, was reading a book on black holes penned by Walter Sullivan of the New York Times. (We’re assuming it’s the 1979 book Black Holes: The Edge of Space, the End of Time.) When presented with the options, Carter said he was “partial to the gamma-ray thing because of this connection with the black-hole problem.”

That signaled the beginning of the end for NASA’s Halley-Tempel 2 mission.

3 Comets That Fizzled

Take a dirty snowball in space and hurl it towards the Sun. I dare you… and then make a prediction as to how that will look.

This is the problem comet scientists face when talking about how bright a comet will appear from Earth. They’re imaging a conglomerate of dust, ice and other materials millions of miles away. After figuring out where the comet will go, then they have to predict how it will behave.

It’s a science, to be sure, but an unpredictable one. That’s why it’s so hard to figure out how Comet ISON will fare when it gets closer to the Sun in November 2013. It could blow into pieces before arriving. It could break up when it gets close to the Sun. Or, it could live up to wildest expectations and shine so brightly you’ll be able to see it in daylight.

Veteran comet-gazers can name a few visitors that didn’t perform as well as predicted. Michael Mumma, who is with the NASA Goddard Space Flight Center’s solar system exploration division, was the lead for the agency’s scientific campaign on many comets of the past few decades. In an e-mail to Universe Today, he shared what made three comets less spectacular than predictions.

Comet Kohoutek (1973)

Comet Kohoutek in 1973. Credit: NASA/University of Arizona
Comet Kohoutek in 1973. Credit: NASA/University of Arizona

Billed by some as the comet of the century, Comet Kohoutek was predicted to pass close to the Sun after it was discovered in March 1973. NASA initiated “Operation Kohoutek” to keep an eye on the comet from a network of observatories in the sky, on the ground and even telescopes in mid-air.

Mumma joked that Kohoutek was a great career launcher for him, as a spectrometer that searched for ammonia ended up getting sustained funding for further development. But the comet was a visual disappointment, he acknowledged.

“The hype surrounding Comet Kohoutek was inspired by two predictions of its possible brightness, made by a recognized senior comet scientist. The NASA spokesman chose to promote the brighter of the two, that predicted the comet would become as ‘bright as the full Moon’. He usually mentioned (softly) that we couldn’t be certain it would actually brighten that much – but the press usually ignored that disclaimer,” Mumma wrote.

“Actually, the comet really did fizzle, failing to reach even the fainter estimate – probably because at discovery it was far from the Sun and activated by something other than water ice. Under those circumstances, any prediction was bound to be highly uncertain.”

Halley’s Comet (1986)

Halley's Comet in 1986. Credit: NASA
Halley’s Comet in 1986. Credit: NASA

Halley’s is the most famous periodic comet, meaning that it returns to the inner solar system over and over again. Its bright appearance made it show up repeatedly in the historical record, most famously in the Bayeux Tapestry after it arrived in 1066 shortly before William the Conquerer successfully led the Norman Conquest of England. However, astronomers in each era saw the comet’s appearance as separate, unpredictable events.

English astronomer Edmond Halley, in examining the astronomical record in 1705, supposed that a comet with similar properties that appeared every 75 years or so was probably the same comet. Ever since then, astronomers and the public alike eagerly await each appearance. The 1910 visit was particularly spectacular, making the press set high expectations for 1986. However, the comet was much further away from the Sun in the 1980s and was fainter.

According to Mumma, the comet did not actually fizzle. Many press reports just got the brightness of the comet wrong, leading the public to believe the comet was less spectacular than predicted.

“It was a bright comet, just as scientists predicted. However, it was much brighter in the southern hemisphere  than in the northern, as predicted. From Christchurch (New Zealand), and again from Cairns (Australia), it was large and the brightest object in the sky – easily seen with the unaided eye.”

As a scientific sidenote, Mumma’s team probed the comet with NASA’s Kuiper Airborne Observatory and, using infrared fluorescence spectroscopy that Mumma developed, found water for the first time in a comet.

Comet Austin (1990)

A negative image of Comet Austin. Credit: European Southern Observatory
A negative image of Comet Austin. Credit: European Southern Observatory

In 1989, Sky & Telescope published a cover article on Comet Austin with the eye-catching headline: “Monster Comet is Coming!” As with Halley, many people anticipated this would be a bright comet, easily visible with the naked eye. In the book Hunting and Imaging Comets, United Kingdom amateur astronomer Martin Mobberley pointed out it was a great object in telescopes or binoculars, but not so much with the eye alone.

“Austin was less bright than some had predicted, but it was bright enough to permit major scientific successes,” Mumma added in his e-mail to Universe Today. “My team detected CO (carbon monoxide) and methanol in that comet, among the first detections of these molecules in comets at infrared wavelengths.”

All in all, these comets show that it’s really hard to figure out what they look like when they get by Earth. This means that nobody knows exactly how ISON will behave until it’s almost upon us.