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Updated February 14, 2004
Hyperlinks to the glossary to be added.
Orionids: October 15-29; Peak October 22, 2002
The Orionids rank among the year's most watched meteor showers. This is one of two showers, along with the eta Aquarids of early May, associated with the famous Comet Halley, making it an appropriate time to reflect on the contributions of Sir Edmund Halley. Amongst a lifetime of accomplishment, Halley predicted the return of the famous comet that bears his name, and was also the first to suggest that meteors might have an extraterrestrial existence after observing a fireball in 1686. Both insights were proven to be true long after Halley's death in 1742; today both can be observed simultaneously in the flash of a single Orionid. Indeed, this is the closest thing we'll get to Halley's Comet itself until its next return in 2061.
Comet Halley has a retrograde orbit which is steeply inclined to the ecliptic. The steep inclination means Halley doesn't come that close to Earth as it crosses the plane of its orbit on both the inward and outbound portions of each perihelion passage, with closest approaches of 22 million and 10 million km (0.15 and 0.065 astronomical units) respectively, suggesting Earth passes in two places through the outskirts of an extremely diffuse meteoroid stream left in the comet's wake. However, Halley's intersections of Earth's orbital plane were not always so distant, and since it has been extensively observed over the millennia this comet's orbital evolution is relatively well understood. A calculated orbit of the comet in 391 BCE compares very closely with the orbital path of the Orionid stream today. The stream itself seems to be very stable with meteoroids dispersed throughout its orbit, although it may best be considered as a bundle of filaments each in similar, stable orbits. These small scale belts result in variations in observed rates during each year's shower, an unusual feature shared with the eta Aquarids.
Because this shower is on the inbound portion of the meteoroid stream, the Orionids are much more a night-time shower than their kissing cousins the eta Aquarids. The shower radiant in northeastern Orion (to the upper left of the famous red giant star Betelgeuse in the warrior's left shoulder) rises in Alberta between 10 and 11 p.m., and passes due south around 6 a.m. Meteor counts can be expected to gradually rise throughout this period.
Because of the retrograde nature of their orbit, the meteoroids slam into Earth roughly head-on at very high speed of 66 km/s. This is very comparable to the eta Aquarids speed of 65 km/s, the difference presumably owing to Earth's own increased speed closer to perihelion in early January. Although it varies somewhat from year to year, the Zenith Hourly Rate (ZHR) is typically around 20, meaning a single observer under dark, clear skies might experience about 10 meteors per hour. The Orionids have a reputation of being colourful; observers are encouraged to be mindful of this aspect, and if possible keep a record of their impressions.
In 2002 the Moon will be full only 24 hours before the predicted peak at 2 a.m. MDT, Tuesday October 22, and will be high in the southern sky less than 60° from the radiant reducing the number of observable meteors by perhaps two-thirds to greatly spoil the show. With a duration of eight days, the Orionid peak is relatively flat meaning that roughly similar numbers can be seen in the nights just before and after the peak, however this year the Moon will be a constant irritant. To maximize one's counts the individual observer should look at an area of sky well away from the Moon, and if possible use a local obstruction block the Moon itself from the line of sight.
The Full Moon and chilly October mornings matter not at all to the radio observer of the Orionids. Indeed, the well-appointed remote sensing unit can allow the observer to 'observe' the Orionids (or more appropriately, the "Eyelids") from the comfort of his or her bed.
Radio observations have on occasion in the past confirmed an unusual distribution of shower meteors, occasionally featuring lulls near the expected maximum but increased rates two or three days later. This has been interpreted as confirmation of the filamentary structure of the meteoroid stream. It would be a worthy object of study to observe from roughly midnight to noon each night for the fortnight or so surrounding the maximum and study the resultant data for peaks and valleys in the distribution.
For further information
For a detailed account, visit Garry Kronk's comets and meteor showers website at: http://comets.amsmeteors.org/meteors/showers/orionids.html .
Or, visit NASA's report on the 2001 Orionids at: http://science.nasa.gov/headlines/y2001/ast17oct_1.htm