- Education and Science
Five Comets of the Last 25 Years
Five Recent Comets
Since the beginning of mankind comets have amazed and mystified us. Over the past twenty years, our solar system system has witnessed some notable examples. These comets have been quite the spectacle for both astronomers and amateur stargazers alike. This five-page lens touches on some of the more important comets that have visited us over the past two decades.
Creative Commons photo courtesy Wikimedia/Sternwarte
Public domain animation courtesy Wikimedia/Anarchemitis
I can vividly remember observing the Hale Bopp Comet back in 1997. What an amazing thing it was to see. The comet was perhaps the most observed comet ever, having been visible to the naked eye for a period of 18 months. Coincidentally, it was also one of the brightest to have passed near our planet in recent years.
Creative Commons photo courtesy Wikimedia/Schnobby
The 1997 Hale-Bopp Comet was one of the brightest comets to reach the inner solar system in history.
Public domain photo courtesy Wikimedia/mkfairdpm
The Hale-Bopp Comet, also known as the Great Comet of 1997, was discovered separately by two amateur astronomers; Alan Hale, in New Mexico, and Thomas Bopp, located in Arizona, on July 23, 1995. The comet's extreme brightness was notable, as it was able to be seen by the naked eye for a record 18 months, more than twice as long as the previous record holder, the Great Comet of 1811.
During its visit, the Hale-Bopp Comet provided a massive amount of scientific information. NASA launched an unprecedented investigation of the comet, spearheaded by it's two most advanced observatories- the International Ultraviolet Explorer and the Hubble Space Telescope. Most astronomers thought that Hale-Bopp had a sizable nucleus up to 25 miles in diameter, while the average comet has a nucleus of about three or four miles in diameter or smaller. It is theorized that the asteroid, (or possibly comet), that struck Earth and led to the extinction of the dinosaurs around 65 million years ago, was roughly seven or eight miles in diameter.
Hale-Bopp was also notable because of it's properties. Scientists were shocked to discover that different types of ices that make up the comet's nucleus are somehow isolated from each other. The scientists also discovered that the comet had a third tail. While it is widely known that comets generally have 2 types of tails, dust and gas, this comet also had a sodium tail. This extremely long tail was found to contain neutral atoms. It was found to be located between the gas tail, which pointed away from the Sun, and the dust tail, which followed the comet's trajectory. Additionally, the Hale-Bopp Comet was also found to carry a huge amount of deuterium and argon. Argon had never before been discovered on any other comet.
Creative Commons photo courtesy Wikimedia/P. Salzgeber
Quick Comet Question
Did you observe Hale-Bopp in '97?
The Hale-Bopp Comet is now beyond the orbit of Uranus but is still visible by large telescope, and will be until about 2020. After that, it is not expected to make a return for another 2400 years.
More Hale-Bopp Vids
Creative Commons photo courtesy Wikipedia/Sternwarte
In early March 1996, Comet Hyakutake first became visible to the naked eye. By the middle of March, the comet was still relatively faint, but as it neared its closest distance to Earth, it quickly became brighter, and its tail grew greatly in length. By the end of March, Hyakutake was one of the brightest night sky objects, taking on a greenish-blue color.
On the 25th of March the Hyakutake would be at its closest point to Earth. The comet was moving so quickly across the night sky that its motion could be seen against the stars in just a matter of minutes; it moved the diameter of a full moon about every half-hour. The head of the comet was a greenish hue, due to the diatomic carbon emissions.
Hyakutake was only at its peak brightness for a few days. Because of this, the comet it did not have time to capture the public's attention in the way that Comet Hale-Bopp would the next year. Many people Europe were unable to see in comet at its best due to poor weather across the area during that period.
Comet Hyakutake, with the longest tail ever observed, will not return to our solar system for another 70,000 years.
Comet Hyakutake Video
Comet Shoemaker-Levy 9
In the above Hubble Space Telescope photo, the dark blotches show scars from Comet Shoemaker-Levy 9 fragments that struck the surface of Jupiter.
Public domain photo courtesy NASA/gov
Impact with Jupiter
The 1993 Discovery of Comet Shoemaker-Levy 9 is credited to astronomers Eugene and Carolyn Shoemaker and David Levy. Soon after its discovery, it was found to be a fragmented comet and possessed a decaying orbit around Planet Jupiter. SL 9 became fragmented in 1992 when it orbited close enough to be ripped apart by the Planets' gravitational forces.
The much anticipated impacts of the comet were witnessed between July 16, and July 22,1994. 21 impacts were observed over this period with the largest impact occurring on July 18, with a fragment causing a visible dark spot almost 9000 miles in diameter. This was the first time that astronomers had the good fortune to observe a collision of two space bodies.
The size of the original, unbroken comet vary, with some scientists estimating that the comet was somewhere between three and seven miles wide, with the largest impacting fragment somewhere around one mile in diameter.
S-L9 Impacts Jupiter
Due to its potential collision with Earth, the Swift-Tuttle Comet has been described by many experts as the single most dangerous object known to humanity.
Public domain photo courtesy NASA/gov
Comet's Threat to Earth
The CST is on an orbit which puts it near to the Moon and the Earth. After its 1992 re-discovery, the comet's date of closest passage was miscalculated by 17 days. It was then noticed that, if its next visit (August 14, 2126) is also off by another 17 days, the comet would almost certainly strike the Moon or Earth. Given the size of Swift-Tuttle, this was quite alarming. This influenced astronomer Gary Kronk to look for ancient versions of the comet. He found the CST was probably seen by the Chinese in 69 BC and 188 AD. This information led scientists to recalculate its orbit, and deduce that Swift-Tuttle's orbit is very predictable, and that there is no impact threat over the next two milleniums. Astronomers feel that in the 2126 visit it will likely be a fantastic naked eye viewing opportunity, similar to the very bright Comet Hale-Bopp (1997).
How many comets have you seen?
Comet Swift-Tuttle Video
Comet Tempel 1
This photo shows the head-on collision of the Tempel 1 Comet and the Deep Impact probe.
Public domain photo courtesy NASA/gov
Tempel 1 was first discovered on April 3, 1867 by astronomer Wilhelm Tempel, an while he was working in France. At that time, the comet's orbit was just under 5.7 years, and the Tempel 1 was also successfully viewed in 1873 and 1879. The comet did not become visible in 1884/1885 and it was believed to have broken up and disintegrated.
What astronomers in the late 1800s did not know is that Tempel1's orbit occasionally takes it close enough to Jupiter to affect it's orbit. It was not until 1967 when British astronomer Brian Marsden re-discovered the comet by recalculating the orbital path figuring in the influence of Jupiter's pull.
Tempel 1 is a fairly dim comet; its peak brightness so far has been too faint to be seen by the naked eye. Its size is roughly to be 8.5 by 2.4 miles, these calculations are based on measurements gathered by the Hubble Space Telescope.
Comet Tempel 1 was the target for NASA's Deep Impact space mission, where on July 4, 2005, a space probe was deliberately impacted on the comet's surface to gather scientific data. The next time Tempel 1 returned (in early 2011), it was once again studied closely from space, this time via the existing Stardust probe.
Tempel 1 was visited again in 2011 by the Stardust spacecraft. That was be the first time that a comet had been visited twice and was an opportunity to better observe the crater created by Deep Impact.