Comet 8P/Tuttle and the December Ursids:
A Christmas family reunion

During the Christmas and New Year Holidays, Earth received a visit from an old acquaintance: periodic comet 8P/Tuttle returned to the inner solar system after spending the last 13.6 years of its long orbital trek around the Sun beyond the orbit of Mars. This comet is no stranger to these parts; it was discovered in 1858, having first been seen in 1790, and since then has been observed on 11 returns to perihelion (the closest point of its orbit to the Sun). Only one predicted return went unobserved, that in 1953. Although not as well-known as other comets like Halley, Encke or Hale-Bopp, its nucleus size of 16km (10 miles across) rivals that of comet Halley, making it one of the largest comets of its type presently known. On this occasion the comet approached the Earth from the North Pole, passing at a distance of a quarter of an Astronomical Unit (the average Earth-Sun distance) or 37.8 million km (23.5 million miles) on New Year's Day 2008. During that month, the comet shone at 6th magnitude as it moved swiftly through the constellation of Cepheus, Cassiopeia and Andromeda. It continued on its southward course through Aries, Pisces and Cetus in early January when it was imaged by Armagh Observatory astronomers (see animation below). This is one of the closest approaches of any comet to the Earth and a record for this particular comet since its discovery. Many Earth-based and spaced-based Observatories took this opportunity to make detailed studies of the comet; an intriguing early result of these observations is that the comet's nucleus appears to be made of two parts in contact - a "contact binary" - rather than a single object.

Three-frame animation showing comet 8P/Tuttle racing through the stellar background in the constellation Cetus in the evening of 10th January 2008. The rapid motion of the comet (covering over one minute of arc during the 10 minutes spanned by the animation) is due to its closeness to the Earth, 45 million km or three tenths the Earth-Sun distance (Astronomical Unit or AU). The comet, currently shining at 6th magnitude, will reach perihelion on 27th Jan just outside Earth's orbit, having passed by the Earth itself at 0.25 AU a month earlier.

This, however, is not the whole story. Although the comet itself never comes dangerously close to the Earth, the particles of dust that it has shed during its many past passages through perihelion found their way into the Earth's atmosphere, appearing as meteors in the cold December sky. These meteors are part of the Ursid meteor stream created by comet Tuttle over the millenia of its existence. It is so named because the meteors appear to emanate from a point (the so-called radiant) within the constellation of Ursa Minor (or Little Bear) close to the Pole Star. The shower is active for about a week in late December every year, peaking on the 22nd and 23rd at a rate of 15-20 meteors visible per hour. Monitoring of the Ursids year after year, allows us a unique insight into the comet and its environment. This information is otherwise obtainable only through in situ study by spacecraft, a rather expensive undertaking. The Armagh Observatory meteor camera system, scanning the skies every night since 2005 for meteor activity, took full advantage of fair weather during the night of the Ursid maximum. Despite the presence of the Full Moon about a dozen meteors were recorded on that night, nine of which were positively identified as Ursids (see Animations 1, 2 below). This data, when combined with results from other observers throughout the globe (Link), will allow us to trace the path of the particles back to the time of their ejection from the cometary nucleus. This will tell us if the particles were ejected recently - in the last few centuries - or as long as several millenia in the past. Models of the stream's evolution also tells us that Ursid activity should vary considerably from year to year. For example, the shower rivalled the spectacular August Perseids in 1945, 1986 and as recently as 2004. Next year, the shower's activity is expected to be rather high, with up to 50 meteors per hour being visible by an average observer from a dark site with the radiant at the zenith, the highest predicted until 2016. Although these ideal conditions can all be met simultaneously only if one travels to the North Pole (!), the absence of the Moon combined with a 45-degree-high radiant at the predicted time of maximum (03:42 AM on 22nd December 2008) should result in a satisfying display for anyone prepared to brave the cold. She/He will be privileged to witness the demise of fragments of an ancient comet that has, in all probability, been gracing Earth's skies for over 10,000 years.

Animation 1
Caption cam1 23/12/07 040337 UT
An Ursid meteor captured by the Armagh Observatory's NE-facing meteor camera in the early morning of 23rd December. The meteor appears to move from left to right through the tail of Ursa Major, the prominent constellation of the Big Bear.

Animation 2
Caption cam2 23/12/07 040647 UT
This Ursid meteor was captured by the Armagh Observatory's NW-facing meteor camera three minutes later than the one above. The meteor appears to move slowly owing to its closeness to the "box" of Ursa Minor (the two stars to its right). It is heading almost directly towards the camera. The star at the lower left of the meteor is Polaris (aka the Pole star) forming the tip of the Little Bear's tail. The bright Full Moon shines at the left edge of the field.

Web references to comet 8P/Tuttle:
Gary Kronk's Cometography
Seiichi Yoshida's Site Web references to the December Ursids:
Meteor Showers Online
NASA Ames Research Center

Last Revised: 2010 February 22nd