The Stories Behind the August Perseids

It's August, so for better or worse, two stories will be making the rounds in the blogosphere. First, is the Mars Hoax email that will not die. I've written about this before, so we won't go there again. Second, is the Perseid meteor shower, which occurs every August.

The Perseids are my favorite meteor shower because the late summer weather is a lot easier to tolerate than some of the cold winter nights other annual showers happen to occur, like the Leonids in November and the Geminids in December.

Every August that the Moon promises not to interfere, we are reminded that this will be the best meteor shower of the year. But there is more to the Perseids than meets the eye and there are some interesting stories behind the Perseids that don't generally get told. I'd like to share some of them with you here.

Let's start with where do meteor showers come from?

We know today that there are streams of particles, called meteoroids, in orbit around the Sun. When the Earth encounters one of these streams, these mostly dust sized particles are trapped by Earth's gravity and burn up as they descend through the atmosphere. The result of this flame-out is a meteor, or what people commonly refer to as "a shooting star."

But where do these streams of meteoroids come from?

In the 1800's Giovanni Schiaparelli was the first to suggest that meteor showers were associated with periodic comets. Schiaparelli noted the orbits of some periodic comets coincided with the orbits of streams of particles responsible for meteor showers. One of these, Biela's comet, discovered in 1826, was identified by Austrian astronomer Wilhelm Baron von Biela as a periodic comet that returned every 6.6 years.

Biela's comet put on quite a show on subsequent returns after its discovery. It was seen to break apart into two pieces in 1846, and in 1852 the two fragments returned as twin comets! The remains were assumed to have disintegrated entirely since the comet was never seen again. In 1872 and 1885, however, when Earth crossed the path of the comet’s known orbit, bright meteor showers known as Andromedids (or Bielids) were observed. This seemed to prove the idea that meteor showers are composed of fragments of disintegrated comets. Schiaparelli was also the first to show that the Perseid and Leonid meteor showers were associated with comets.

Today we know several annual meteor showers and their connection to periodic comets. The eta Aquarids and Orionids are associated with the famous comet 1P/Halley. The Southern Taurids are associated with comet 2P/Enke. I've already explained that the Andomedids come from comet 3P/Biela, the comet that broke into two pieces and then disintegrated entirely. The Leonids come from debris left behind from comet 55P/Temple-Tuttle and, getting back to our August astronomy story, the Perseids occur when Earth encounters the stream of dust left behind from comet 109P/Swift-Tuttle.

And here is another story within a story, the naming convention for comets. What is all this 1P/ and 2D stuff all about?

The exact rules for comet nomenclature are pretty involved, but essentially the letter P stands for a periodic comet with a period of less than 200 years. C stands for a comet that is not periodic, and D stands for a comet that has broken up or been lost, a dark comet. The names associated with comets are most often the discoverer or co-discoverers of the comet, although some of them, like Halley's Comet are named after the astronomers who first calculated their orbits rather than their original discoverers. The number corresponds to the order and number of discovery of that type of comet. 109P/Swift-Tuttle, the comet responsible for the Perseid meteor shower each year, is therefore the 109th periodic comet known, and it is named after its co-discoverers, Swift and Tuttle; which leads us to two more stories behind the story of the Perseid meteors.

Who were Swift and Tuttle?

Lewis Swift was born in Clarkson, New York on February 29, 1820. Swift was a farmer and hardware store owner by trade. Between 1866 and 1892 he discovered thirteen comets, making him one of the most prolific comet discoverers of all time. Lewis Swift's life story is one of pain, perseverance, disappointment, delight, fame and fortune.

According to Swift, he first really became interested in astronomy after observing the Great Comet of 1843. This comet was so bright it could be seen in broad daylight and its tail extended 40 degrees across the sky!

Swift made his first telescope, a 3-inch refractor with a lens purchased for $5.00 from the Spencer Optical Company. In 1858 the 3-inch was accidentally broken, so Swift purchased a 4 1/2-inch 'comet seeker' from the American optical craftsman Henry Fitz. This was the telescope he made all but one of his comet discoveries with. His early observatories weren't much more than small platforms built on the roof of his barn, accessed through a hole cut in the roof.

His first comet discovery, and the one he is most famous for, was actually quite by accident. Upon hearing of the discovery of a comet in the northern sky, near Polaris, the North Star, Swift decided to observe the comet one July evening in 1862. After less than five minutes he came across a beautiful comet that he took for granted was the comet he had been looking for. After following the comet for several nights it become clear that this was actually a different comet. In fact, it was the same comet that Harvard astronomer Horace Tuttle had independently discovered a few days after Swift.

It was in 1866 that Giavonni Shiaparelli announced that comet 1862III (Swift-Tuttle) traveled in an orbit virtually identical to the Perseid meteor stream. Swift and Tuttle argued bitterly for years over who deserved credit for this important comet's discovery.

In 1872, Swift moved to Rochester, New York and opened a hardware store. He became well known for discovering comets with his 4 1/2-inch telescope from the roof a local cider mill. As his popularity increased, he began to give lectures on astronomy and often held what we now call star parties, showing people comets and other celestial wonders in Lake View Park.

In 1879, Swift found a generous patron in Rochester patent medicine businessman Hulbert Harrington Warner, who financed the building of an observatory for Swift. Warner assured "Professor Swift" as he had become known, that if Swift could raise the money to purchase a large telescope, Warner would build an observatory for it. The original estimate for construction of the observatory was $20,000.

Swift was able to fulfill his part of the bargain by collecting donations for a 16-inch Alvan Clark & Son telescope from the people of Rochester, NY.  When the observatory opened it was the fourth largest telescope in the United States. The plans for the observatory also called for an astronomical library, an elevator and a residence for Professor Swift and his family. Ultimately, the Observatory cost Warner $100,000.

The observatory itself was the first observatory in the world to encourage visits by the general public. All a visitor had to do was buy a ticket for 25 cents at Warner's business on St. Paul Street. It became so well known that it was included in travel guidebooks.

Soon after the observatory opened in 1882, Swift closed his hardware store and became, for all intents and purposes, a professional astronomer. Swift's attentions soon turned from comets to nebulae. While Charles Messier had considered them to be mostly annoyances to the discovery of comets, Swift believed they were worthy of study in their own right. By the end of his career he had discovered over 1200 objects, ranking him third behind the Herschels and number one among American observers.

He was awarded an honorary Ph.D. from Rochester University, and received more medals than any other astronomer of his time, including three from the Imperial Academy of Science in Australia, four from the Astronomical Society of the Pacific, and the Laplace Medal from the French Astronomical Society. In 1897 he was the first person awarded the Jackson-Gwilt Medal of the Royal Astronomical Society.

In 1893, motivated largely by the construction of an Episcopal church next door to the observatory that blocked his view to the sky, Swift relocated the 16-inch telescope to Lowe Observatory on Echo Mountain in California, where he spent the remainder of his career. Swift discovered his last comet in 1899 at the age of 79. Although some of his comet discoveries can surely be said to be lucky, most were the result of persistent, systematic, tireless observations. Swift was known to remark often, "One cannot discover comets lying in bed."

The second half of the story behind the discovery of Comet Swift-Tuttle is Horace Parnell Tuttle. Born March 17, 1837 in Newfield, Maine, Horace's life story is a bit more of a mystery.

Charles Wesley Tuttle, Horace's older brother, was an amateur astronomer who constructed his own telescope, and upon visiting the Harvard Observatory so impressed observatory director, William Bond, he was hired as an assistant observer. This was Horace's connection and eventual inroad to working at Harvard Observatory later on.

Charles was eventually replaced at Harvard by his younger brother Horace as an observatory assistant. Horace became attached to the observatory's four-inch Merz comet seeker, which he used on the balconies of the observatory of the 15-inch refractor, spending night after night in search of new comets. While not as prolific as Swift, Horace Tuttle proved to be a successful comet hunter.

He discovered or co-discovered numerous comets, including 55P/Tempel-Tuttle, parent body of the Leonid meteor shower, 109P/Swift-Tuttle, parent body of the Perseid meteor shower, and the "Great Comet of 1860." Other comets that bear his name are 8P/Tuttle, parent comet of the Ursid meteor shower, 41P/Tuttle-Giacobini-Kresak and C/1861 Y1 Tuttle. In 1859 he was awarded the Lalande Prize of the French Academy of Sciences for discovering of two comets in one year (1858).

With the outbreak of the Civil War, Horace Tuttle enlisted in the 44th Massachusetts Volunteer Infantry and served at New Bern, North Carolina. He continued to make astronomical observations during the war, reporting on the appearance of Comet Tempel 1864 II.

The war had taken Tuttle out of comet seeking for three and a half years, so his discovery of comet 1866 I at the U.S. Naval Observatory on January 5th, 1866 must have felt pretty good after such a long hiatus. This was Comet Tempel-Tuttle, first discovered by the French astronomer, Tempel, more than two weeks earlier. Tuttle received a lot of press for this discovery since it was only the second comet ever discovered at the Naval Observatory.

In 1887 Tuttle obtained a 6.5-inch broken-back reflecting comet seeker, made for him by John Brashear. It was installed on the roof of the Naval Observatory, where he made his last comet discovery, a recovery of Comet 1888V Barnard.

Tuttle lived in the Washington, D.C. area from about 1884 until his death in 1923. In his final years he was feeble and blind. His gravesite is unmarked and its location is unknown.

Comet Swift-Tuttle itself is a pretty intersting story. It is the largest object known to make repeated passes near the Earth. It is also one of the oldest known periodic comets with sightings by the Chinese as far back as 68 B.C. The best estimate of when it will return is July, 2126.

The first attempt at computing a definitive orbit was made in 1889, when F. Hayn determined the orbital period to be 119.64 years. In 1971, Brian Marsden and Zdenek Sekanina took 212 positions obtained during the period of July 22 to October 22, 1862, applied perturbations by all nine planets, and came to a similar conclusion, 119.98 years.

A couple of years later, Marsden considered the possibility of linking Swift-Tuttle to an earlier comet. He found two in the 18th century that looked promising--1737 (Kegler) and 1750 (Wargentin). The 1750 comet appeared at just about the right time, but the 1750 comet seemed to be moving too fast to fit the orbital calculations. The 1737 comet actually exhibited a motion consistent with what would have been expected for Swift-Tuttle but the comet's period would have to have been some 10 years longer than was indicated by the observations in 1862.

Marsden made two predictions for a forthcoming return. First, using the definitive orbit calculated by Sekanina and himself, he suggested a perihelion date of September 16, 1981. Second, he suggested that if the link to the comet of 1737 was valid, Swift-Tuttle would most likely return to perihelion on November 25, 1992.

Initial searches for the comet began in 1980, which was within the error range given by calculations, and more rigorous searches were conducted in 1981 and 1982, but the comet was not recovered.

On September 26, 1992, Tsuruhiko Kiuchi, from Japan, discovered a comet and reported it to the National Astronomical Observatory in Tokyo. Several observers were able to confirm the comet within the next 24 hours and the direction and rate of motion were consistent with what would be expected for Swift-Tuttle. The long lost parent of the Perseid meteor shower had indeed returned.

After refining the calculations of its orbit and looking at predictions of its next return, there was some concern that Swift-Tuttle might actually collide with Earth in 2126!

The comet has a diameter of 10 kilometers, and if it did hit the Earth going 60km/sec, it would be catastrophic. The collision would be 1 billion times more powerful than the atomic bomb dropped on Hiroshima. An impact similar to this is believed to have caused the extinction of the dinosaurs. Swift-Tuttle crashing into the planet could create a cloud of dust that would block out the sun, killing all plant life, and causing an ice age.

What are the chances it will hit? It's difficult to estimate. The comet will only collide with the Earth if the two bodies occupy the same space within a narrow three and a half minute window of their orbits. A difference of one hour would cause the comet to miss the Earth by about 100,000 kilometers. Considering the last calculations were off by 10 years, you can imagine how difficult it is to be sure one way or the other.

We haven't heard the last of comet 109P/Swift-Tuttle. In less than 120 years our ancestors will be learning about Professor Swift's comet as it makes another pass through the inner Solar System, leaving a trail of meteoroids behind to delight another generation with August meteor showers for another 130 years.

1 comment:

Anonymous said...

Nice work! Now to go watch them...