2003 EH1 is the Quadrantid shower parent comet

The Quadrantid meteor shower in early January is our most intense annual shower. The Quadrantids are named after the now defunct constellation Quadrant Murales where the radiant was located during its discovery in 1835. Its alternative name, the Bootids refers to the modern constellation of Bootes. The Quadrantid shower is hard to observe because the radiant is in lower culmination at midnight. Under good circumstances, however, when the peak of the shower is in the early morning and there is no disturbing moonlight, rates can increase up to Zenith Hourly Rate = 130 meteors/hr. In fact, viewing will be good for US-based observers this coming January 03. We plan to observe from Fremont Peak Observatory that night and invite you to join us.

For long, the Quadrantids were without known parent. The object should be a short-period comet, revolving around the Sun every 5-7 years. However, those tend to evolve rapidly due to frequent close encounters with Jupiter. During the first attempts to calculate the changes in the orbit it was discovered that the orbit rotates over a period of 1,500-4,400 years from a more typical low-inclination i = 13 degrees and low perihelion distance q = 0.10 AU, to the very high inclination i = 71 degrees and the q = 0.78 of the present orbit. Bruce McIntosh suggested in 1990 that comet 96P/Machholz has a sibling relationship with the Quadrantid shower. More recently, Williams and Collander-Brown concluded in that same vein that asteroid 5496 (1973 NA) is a possible candidate. The idea was that comet and stream could evolve at different rates and so spread out over time in a large region of the solar system.

Part of that assumption was based on rather imprecise orbits measured in the past. When we started our meteor program in 1994, a program in which SJAA members have played a big role in the past years, our fellow observers of the Dutch Meteor Society stumbled on a clear night on January 03 1995, with no disturbing Moon light. This rare occasion led to a rich harvest in multi-station photographed and video orbits, which were reduced by Hans Betlem and Marc de Lignie. I analyzed those results to find that all good trajectories clustered near the same radiant and speed, implying that this is a very young shower, no older than about 500 years. In our paper, published in Astronomy Astrophysics in 1997, I predicted that the comet was still among the meteoroids and now hidden from plain view by ceasing to be active and looking like a mere asteroid. Only if the age of the shower is very young may we expect to find the parent still among the meteoroids. Sadly, such an extinct comet nucleus is thought to be dark, and this one was in a high inclination orbit that only rarely put it in the same direction as other asteroids on the sky. Although the Quadrantids provide an approximate orbit, the position of the object in that orbit remained unknown.

In recent years, several large surveys for near-Earth (q < 1.3 AU) asteroids have produced a rich harvest of discoveries. At this moment, it is believed that about half of all large (D>1km) such asteroids have been found. While working on a book chapter on the Quadrantid shower two weeks ago, I came across my 1997 writings and decided to check the catalog of asteroid orbits again to see if a near-Earth asteroid had been found in an orbit close to that of the Quadrantids. To my great excitement, there was.

On March 6, 2003, the Lowell Observatory Near-Earth Object Survey - LONEOS telescope (Observer B. A. Skiff) discovered near-Earth asteroid 2003 EH1. I found that the aphelion of 2003 EH1 is precisely at the peak of the meteoroid distribution. The orientation of the orbit is close to that expected, with no significant discrepancy in the argument of perihelion and inclination, and only a slight offset in the rapidly evolving node. Indeed, the theoretical radiant and speed for a shower from 2003 EH1 falls in the middle of those measured for the Quadrantids by the Dutch Meteor Society observers.

2003 EH1 is now passing relatively far outside of Earth orbit (Figure 1). The minimum distance between comet orbit and Earth (0.213 AU) is larger than typical for other annual showers (<0.04 AU). However, backward integration of the orbits using the JPL/Horizons software shows that the orbit of 2003 EH1 evolved in the recent past from a much smaller perihelion distance in the same manner as found for typical Quadrantid orbits by authors in the past. The predicted decrease of the node over the past centuries is exactly that observed, as long as the meteoroids are ejected with small enough speed to not get trapped in the 2:1 mean motion resonance. The meteoroid orbits show a progressive scatter as a function of time since ejection, but overall follow the evolution of 2003 EH1, as required for this object to be still associated with the stream. By calculating the dispersion since 1600, and comparing with the observed dispersion from our photographic observations, I estimate the time of release of the particles occurred within a few hundred years prior to 1600.

There is no doubt that the meteoroids originate from a comet. The Quadrantids end as high in the atmosphere as the Lyrid meteors with similar entry speed but originating from a known comet, and higher than Geminid meteoroids, which have been sintered by a small perihelion distance, appearing more asteroid-like and penetrate deeper in the atmopshere.

Comet breakups can occur quite silently, but this one may have had a record. Ishiro Hasegawa calculated a parabolic orbit for comet C1490 Y1 from observations made in China, Korea and Japan between Dec 31.5, 1490 and Feb. 12.5, 1491, and pointed out the similarity with the orbit of the Quadrantids. Indeed, Iwan William and Zidian Wu first demonstrated that some backward integrated Quadrantids have orbital elements consistent with C1490 Y1 if that comet had an eccentricity of 0.77, rather than 1.00. Williams and Wu continued to proposed that a close encounter with Jupiter in 1650 ejected this bright comet into a much different orbit (leaving the Quadrantid shower in place), in order to explain that the comet has not been observed since. The age of the shower was estimated at 5,400 years, based on earlier meteoroid orbits that had a larger observational error. It now appears that the comet may still be there.

The identification of the Quadrantid parent was announced on an IAU Circular on December 08. The identification of the Quadrantid parent is more than just a curiosity. NASA's Deep Impact mission is scheduled to visit comet P/Wild 2 in July 2005 to probe the internal structure of that comet nucleus. The discovery of a cometary nucleus fragment in the orbit of a meteoroid stream makes it possible to investigate the mineralogical and morphological properties of cometary dust originating from much deeper inside a comet nucleus than is typically observed in meteor showers. Moreover, the identification of 2003 EH1 as an extinct comet nucleus could provide a new target for future missions. Hence, it becomes more important to study the shower as well as we can. We will therefore go out in early January and hope you will join us. Please contact Mike Koop for further information.

Editor's note: Space requirements shortened the printed version of this article. See the full article on the SJAA website at http://ephemeris.sjaa.net/0401/e.html.