Ursids

Observer's Synopsis

      Occurring primarily between December 17 and 24, this meteor shower reaches maximum on December 22/23 (J2000 solar longitude=270.7 deg) with a radiant at RA=217 deg, DEC=+76 deg. The maximum hourly rate is usually between 10 and 15, except in 1945, when rates exceeded 100. Meteors belonging to this stream are typically faint.

History

      This meteor shower seems to have been discovered by William F. Denning, who, during several years around the turn of the century, observed a radiant at RA=218 deg, DEC=+76 deg, which endured during December 18-22. Further radiants have been listed by Cuno Hoffmeister (H1948) for 1914, 1931 and 1933. Coordinated studies did not commence until Dr. A. Becvar accidentally observed a strong display in 1945.
      Becvar was observing at the Skalnate Pleso Observatory (Czechoslovakia) on December 22, 1945, when he, and other observers, noted meteors falling at a rate of 169 per hour. Many meteors were photographed and M. Dzubak computed a preliminary radiant of RA=233 deg, DEC=+82.6 deg. A reinvestigation of the data by Zdenek Ceplecha revealed a ZHR of 108 and a photographic radiant of RA=217.1 deg, DEC=+75.9 deg.
      Coordinated studies of this meteor shower were finally begun in 1946. Becvár confirmed his detection of an active radiant, but found the maximum hourly rates to reach only 11, on the night of December 22. Bochnicek and Vanysek confirmed Becvár's observations and both observers also established radiants for December 22.9. Bochnícek estimated it as RA=213 deg, DEC=+75 deg, while Vanysek found it to be RA=217.8 deg, DEC=+76.7 deg.
      In 1947, visual studies of this stream were carried out by J. P. M. Prentice, of the British Astronomical Association. During 1 hour 43 minutes on December 22, only 1 meteor from the Ursid radiant was seen, but in 25 minutes on December 23, 8 meteors were detected---making the hourly rate about 20. Four of the eight meteors detected on the latter date were plotted and revealed a radiant of RA=207 deg, DEC=+74 deg. The radiant diameter was less than one degree.
      In addition to visual observations of this shower, 1947 was also notable for the first detection of the radiant by radio-echo observations. Aerials at Jodrell Bank first got a bearing on this shower at 3 hours UT on December 22. Beginning at 9 hours UT, hourly rates could be determined and, until 11 hours UT on December 23, these rates averaged 15 (equivalent visual rate of about 10). Thereafter, activity dropped sharply. The pointing of the aerial to different directions allowed the radiant to be determined as RA=195+/-8 deg, DEC=+78+/-5 deg.
      Radio-echo observations became the primary means of studying this shower during the period 1948 to 1953, with observers at Jodrell Bank determining the following details of what appears to be a very consistent shower---except for the observations in 1945:

Ursid Radio-Echo Radiants

      The Ursids went through another period of neglect after 1953, but this finally ended in 1970, when the British Meteor Society (BMS) began a period of annual observations. From observations spanning the period 1970 to 1976, the BMS found an average radiant of RA=217 deg, DEC=+76 deg. Maximum occurred at a solar longitude of 270.66 deg (about December 22), with the duration being established as December 17-24. Hourly rates were 10 in 1970, 22 in 1971, 16 in 1972, 18 in 1974, 9 in 1975 and 4 in 1976. The moon interfered in 1973, but during daylight hours on December 22, BMS radio observers detected a short, 1-hour burst of activity that produced a corresponding visual hourly rate of 30.
      Observations have been numerous in the United States, but they indicate the shower is far from what observers saw during the late 1940s and early 1950s. The following table lists analyses that have appeared in Meteor News.

      Observers in Japan tend to confirm the United States' finding of weak activity. They detected ZHRs of 5.7 in 1970 and 2.4 in 1971. There does, however, seem to be occasional strong displays of this shower. Observers in Sogne, Norway, noted a strong display during 2 hours of observations on December 22, 1979, with estimated ZHRs of 25 and 27. Veteran meteor observer, Norman W. McLeod, III (Florida) has commented that the Ursids "must be a compact stream like the Quadrantids. You have to be within 12 hours of maximum to see much."
      The first half of the 1980's showed no particularly impressive activity from the Ursids. Thus, it was particularly surprising that when an unexpected outburst of activity occurred on December 22, 1986, there were several observers monitoring the shower. Luc Gobin (Mechelen, Belgium) reported unexpected "very high rates" while operating radio equipment at 66.17 MHz. Gobin's equipment had detected average hourly echo rates of 60 to 68 during December 19, 20, 21, and 23, but found rates of 171 on the 23rd. This enhanced activity was also noted visually, and seems to have peaked during the nighttime hours over Europe. George Spalding (director of the British Astronomical Association Meteor Section) reevaluated his observations of December 22 and arrived at a ZHR of 87+/-29.
      Trond Erik Hillestad (Norwegian Meteor Section) reported the observations of Kai Gaarder and Lars Trygve Heen. The former observer detected 94 Ursids in 4 hours, including 37 in the hour following December 22.83 (ZHR=64+/-11), and reported an average magnitude of 1.90. The latter observer saw 75 Ursids in 2 hours, including 54 in the hour following December 22.88 (ZHR=122+/-17), and reported an average magnitude of 2.61. These two observers saw only 4 and 2 Ursids, respectively, during a one-hour interval on the night of December 23. Of the 175 Ursids seen, 17.1% possessed a persistent train. Of the 66 Ursids of magnitude 2 and brighter, 51.5% were white, 33.3% were yellow, 7.6% were red, 2.3% were green, and 5.3% were blue.
      As noted above, the Norwegian observers reported Ursid rates of only 2-4 per hour on December 23.8, 1986---thus, indicating a very rapid decline in activity. But it should also be pointed out that the rise to maximum was also very great. R. Koschack, R. Arlt, and Jürgen Rendtel (Arbeitskreis Meteore, East Germany) detected a ZHR of less than 5 on December 21.8, while R. Taibi (Temple Hills, Maryland) observed 2-3 per hour around December 22.4.
      From observations obtained early in the 20th century, Denning had suggested that this shower was associated with "Mechain-Tuttle's Comet"---now known simply as comet Tuttle. Upon Becvár's announcement of his discovery of the shower in 1945, he mentioned that a connection with periodic comet Tuttle "is highly presumable."
      The correlation between the observed activity rates of the Ursids and the perihelion passages of periodic comet Tuttle is particularly strange. In fact, the strong shower of 1945 actually occurred six years after the comet's perihelion passage---thus, placing the comet near aphelion! No rate estimates are available prior to 1945, and, unfortunately, observations virtually ceased during the latter half of the 1950's and early 1960's, so that the delay cannot be confirmed following the 1953 perihelion passage. Comet Tuttle next passed perihelion on March 31, 1967. No apparent observations were made in that year, but from 1968 onward, the rates generally remained between 10-15 per hour until 1973, when, as noted earlier, radio equipment operating in England detected activity corresponding to visual rates of 30 per hour during a one-hour period in daylight. This increase came six years after the comet's perihelion passage. The comet next passed perihelion on December 14, 1980. Hourly rates appeared normal in the following years, until December 22, 1986, when European observers detected very high rates while using both visual and radio-echo methods. Thus, three of the last four perihelion passages of comet Tuttle have been followed six years later by strong meteor activity.

Predictions and Observations for the 2000 display

      Comet Tuttle next passed perihelion during 1994. As in the past, the skies were well monitored during the next few years. Once again, no abnormal rates were reported. If everything worked as as before, a strong Ursid display might be possible in December of 2000.
      Following the successful predictions of enhanced displays of the Leonid meteor shower by several astronomers in 1999 and 2000, the webmaster began analysing the evolution of the Ursid meteor shower during the last half of November 2000. In the course of this analysis, amateur astronomer Richard Taibi published an announcement on December 1 that reiterated the 6-year delay between the comet's perihelion date and enhanced activity. Observing plans were now being made by amateur and professional astronomers around the world.
      I had finished my analysis on December 20 and posted the first part of a web paper on that day exclusively covering the history of Ursid observations. Interestingly, a press release was issued that same day by NASA's Ames Research Center stating that astronomer Peter Jenniskens and Esko Lyytinen were predicting "The shower is expected to last 2 to 3 hours, and possibly reach rates of one meteor per minute. Many of these will be faint meteors, so observers are encouraged to go to a dark location away from city lights for best viewing." Their study was based on an analysis of the meteor stream's evolution. The webmaster published his prediction the next day. The result was a prediction that because Jupiter perturbations in 1948 and 1960 would have dispersed the particles in this part of the Ursid orbit no abnormally large increase in activity would occur.
      The nights of December 22 and 23 were interesting. As expected, observations were made from around the world. But the final picture was a confusing one, primarily because of the introduction of observing techniques other than visual. Jenniskens ultimately announced a success in his modeling the stream and this success was very quickly published by the Central Bureau for Astronomical Telegrams on International Astronomical Union Circular No. 7548. Jenniskens also published all of the positive observations at the web site address of http://leonid.arc.nasa.gov/leonidnews29.html. But the fact was that there were more negative observations than positive and the contradiction came in two of the three observing techniques used: visual and radio-echo. The only observing technique not contradicted was the filming of the meteors using light intensified cameras and that was because Jenniskens was the only person in the United States using such cameras. These cameras apparently revealed an abundance of faint meteors when the Ursids reached maximum over the United States and Canada. They alone could explain why some extremely experienced observers with 20 or more years of observing experience failed to detect an increase in activity, including some in California and Oregon. But then why did Jenniskens team report high visual rates from their observing locations in California? This marks the first contradiction. The other contradiction comes from Europe. One observer operating a radio-echo system in Finland reported he detected the predicted Ursid outburst, yet Ondrejov Observatory, which is a much more advanced system capable of detecting meteors far below naked-eye visibility, detected no sign of an outburst.

Conclusions and Future Ursid Studies

      Very little is really known about the characteristics of the Ursid meteors and although it is possible to mathematically model the stream's evolution, astronomers need to learn more about the stream's members and density before accurate predictions of hourly rates can be made. Even the well-studied Leonid stream has not been modelled perfectly at this time, even though the mathematical models of its evolution have been supplemented by over a thousand years of visual observations.
      Whether or not an outburst really occurred in 2000 will only be realized after further observations of this stream are acquired. Jenniskens' video observations of the Ursid maximum hint at the fact that the Ursid stream might normally produce large numbers of faint meteors. The only way to determine whether this is really true is to maintain intensified observation programs for the next few years with as many techniques as possible. In this way we will recognize just what "normal" activity is for the entire population of Ursid meteors and be able to conclusively predict and recognize an outburst.

Orbit

      The Ursid orbit below is based on photographic meteor orbit collected from several sources. The orbit for periodic comet Tuttle was taken from the 1996 edition of the Catalogue of Cometary Orbits, written by Brian G. Marsden. Although there is a strong similarity between the orbits of the meteor stream and comet, notable differences in the perihelion distance, and minor differences in the other elements indicate this stream is not a recent product of this comet.

	Ursids	Tuttle
Argument of Perihelion () [J2000]	208.4 deg.	206.7 deg.
Ascending Node () [J2000]	268.0 deg.	270.5 deg.
Inclination (i) [J2000]	52.6 deg.	54.7 deg.
Perihelion Distance (q)	0.929 AU	0.998 AU
Eccentricity (e)	0.848	0.824
Semimajor axis (a)	6.112 AU	5.672 AU

      Using an orbit similar to the one above, Ken Fox (1986) computed the orbit of the Ursid stream for periods 1000 years in the past and future. Although Earth seems to have been in contact with the Ursid stream 1000 years ago, no contact will apparently be possible 1000 years in the future. In 950, the shower's maximum would have occurred six days later than at present from a radiant of RA=214.5 deg, DEC=+73.2 deg.

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