Taurids, Fireballs and Comet Encke
November 5, 2008 3 Comments
Over the past few weeks, a number of bright meteors and fireballs have been reported. On this blog, there have been reports from Colorado, New Jersey, Illinois, Florida, Michigan and Arizona. Over at the Fireball Report page of the American Meteor Society (AMS), multiple fireballs are being reported every night. So what’s going on?
There is a good chance that many of these fireballs are from the Taurid meteor complex. Most meteor showers are only visible for a few weeks to maybe a month. Often most of their activity is concentrated in a few night window around the time of their peak. The Taurids are different, though. Those who have paid close attention to my daily postings will notice that the Taurids have been active since late September. They will continue to produce meteors until the end of November. Another difference between Taurids and most other showers, is when they they take place. Most showers can only be observed after midnight. The Taurids can be seen at all hours of the night, whether morning or evening. As a result, they are active at a reasonable hour when many people are still awake and outside.So for the average person who is out and about in the evening, the months of October and November provide a greater than usual chance of spotting a nice meteor or fireball.
How can you see the Taurids?
The Taurids are visible at any time of the night. There are two separate branches of the Taurids, the Northern and Southern Taurids. Both showers are located within a few degrees of each other. In November, the radiants of both showers are located in the constellation of Taurus. Video data compiled by Sirko Molau find that the Southern Taurids are active from September 8 to November 30 with a broad peak around October 11. The Northern Taurids are active from October 8 to December 13 with a broad peak around November 14. Right now, both showers are active though more of the activity will be from the Northern branch.
The rates for the Taurids is fairly low. These are not major showers and at their best produce 10-15 meteors per hour from a dark site around midnight. For evening observers under suburban skies, the rates will be much lower. The thing to watch out for are evening fireballs. It is predicted that this year will see more fireballs than usual from the Taurids.
Taurid meteors look rather different from most meteors. Unlike the Orionids which are fast and only last a split second, Taurids are much slower and longer lasting. I have seen many early evening Taurids that appear as small green orbs or spheres that can take up to 3 seconds to cross most of the sky. Quite often they will be followed by a thin, short-lasting white tail or trail. They are definitely impressive.
Where do Taurids come from?
Taurid meteors are produced by Comet Encke which is one of the best observed comets in history. The reason for this is that it circles the Sun once every 3.3 years which provide lots of opportunities to observe it when bright. The fact that the comet is rather large and does not get very far from the Earth and Sun means that it is always observable by professional (and even some of the largest amateur) telescopes. But even after centuries of observation, Encke remains an enigma and continues to give professional comet researchers fits.
Comet Encke was first observed by the prolific French comet hunter Pierre Mechain on 1786 January 17. The comet was relatively bright (5th magnitude) but located deep in the bright twilight sky. After a few days, the comet moved to close to the Sun and was no longer observable. As a result, it became lost. Fast forward to 1795 November 7, Caroline Herschel (the first female comet discoverer and brother of William Herschel, the discoverer of Uranus) of Slough, England found a “new” comet which was observed for only a few weeks. There were not enough observations to identify the comet as a short-period comet. The 3rd “discovery” of Comet Encke occurred on 1805 October 20 when Jean-Louis Pons of Marseilles, France (until the last 15 years, Pons was the leading comet discoverer with 26 comets to his credit) discovered a comet which was followed for a month. Pons was so prolific at finding comets that he unknowingly found the same comet again on 1818 November 26.
One may be wondering why this comet is named Comet Encke rather than Comet Mechain-Herschel-Pons. Johann Franz Encke was a German astronomer and mathematician. In 1819, Encke calculated an orbit for Pons’ 1818 comet and noticed that it resembled the orbit of Pons’ 1805 find. Working the orbit, Encke quickly relaized that the two Pons comets were really the same comet returning every ~3 years. Further work identified Mechain’s 1786 comet and Herschel’s 1795 comet as previous apparitions of Pons’ comet. In honor of Encke’s work, the comet was named Comet Encke. It is rare for a comet to be named after the mathematician who computed its orbit rather than its discoverer but there are a few cases of this, especially centuries ago. The most famous example being Comet Halley. Edmund Halley did not discover Comet Halley but he was the first person to recognize that his namesake comet returned at regular intervals. Comets Lexell and Crommelin are other examples of comets being named after an orbit computer.
Since 1818, Comet Encke has been observed at every perihelion (closest approach to the Sun) except for one in 1944 at the height of World War II. Nowadays, it is rare for Comet Encke not to be observed at least once a year. In fact, Encke was the 2nd comet, after Halley, to be observed at more than 1 return, hence its official name of Comet 2P/Encke. The comet is currently located on an orbit that takes 3.3 years to circle the Sun. The comet’s orbit ranges from a perihelion (closest to the Sun) of 0.34 AU to an aphelion of 4.10 AU (farthest from the Sun). Its last perihelion was on 2007 April 19 and the next one will be on 2010 August 6. There is a short window before or after every perihelion when Encke is visible in small telescopes. The comet never gets bright enough for naked eye observations.
Where does Comet Encke come from?
Until a few years ago, it was theorized that all comets formed in the outer solar system beyond the orbit of Jupiter. As the outer planets migrated towards their current orbits, some comets were ejected into the Oort cloud, located out to a quarter of the distance to the nearest star, the Kuiper Belt, a belt of comets located just beyond Neptune’s orbit, or the Scattered Disk, located between the Oort cloud and the Kuiper Belt. Short-period comets, with periods less than ~20 years or so, spent billions of years in the Kuiper Belt or Scattered Disk before being kicked back into the inner solar system by the gravity of the outer planets. As a result, all comets go out as far from the Sun as the orbit of Jupiter or much further out. Encke only goes out to 4.1 AU. Since Jupiter is located at ~5 AU, Encke is safe from most of Jupiter’s gravitational interactions.
The question is how did Encke get there? Computer models find that it is very difficult to get an object from the outer solar system onto a Encke-type orbit. Though not impossible, it would require a very long amount of time. So long, in fact, that Encke should have burned out (run out of ice and volatiles) many 100,000s of years ago and should either appear as an inactive asteroid or perhaps have broken up into nothing more than dust by now. So what happened? It is possible that Encke took a very long time to get to its current orbit but was inactive, or dormant, for most of that time. If true, it would not have run out of ice and its cometary activity is a recent phenomenon. As we saw with the recent re-discoveries of Comets Giacobini and Barnard 3, comets can be inactive or barely active for many orbits.
Another possibility is that Encke is not from the outer solar system but rather from the Asteroid Belt between the orbits of Mars and Jupiter. It was long thought that the asteroids in the Asteroid Belt were all dead, dry objects, but recently a number of asteroids have been found that display the same activity as comets. Perhaps Encke was an asteroid from the Asteroid Belt until a collision or break-up event exposed a large area of ice resulting in cometary activity. Unfortunately, we could know a lot more about Encke but a planned mission to study Encke, and other comets, failed leaving Earth orbit. NASA’s COmet Nucleus TOUR (CONTOUR) mission was to fly-by Encke in 2003.