Submitted to SJAA ephemeris (Dec. 08, 2003).

 

 

2003 EH1 is the Quadrantid shower parent comet

Peter Jenniskens*

SETI Institute, 2035 Landings Drive, Mountain View, CA 94043

 

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. Indeed, during the first attempts to calculate the changes in the orbit over time, S.E. Hamid and M.N. Youssef discovered in 1963, and later documented in more detail, 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 (discovered in 1985 and now with q = 0.12 AU) has a sibling relationship with the Quadrantid shower, part of a larger complex of dust including the Daytime Arietid and southern Delta-Aquarid created several thousand of years ago. 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 1995 January 03, 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 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.

 


 

 


 

 

 


Figure 1: Orbit and position of 2003 EH1 in January 04, 2004.

 

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 (RA = 229.9, DEC = +49.6, Vg = 40.2 km/s at lo = 282.9 - J2000) 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. Indeed, meteoroids ejected from an orbit in 1600 with a slightly longer orbital period, and calculated forward in time to the present epoch, show the expected small differences between comet and meteoroids. 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. The total mass in the stream is rather high, about 1x1013 kg. Comet 55P/Tempel-Tuttle, for example, only releases about 3x1010 kg of dust each orbit. Hence, I suspect that the shower was created during a breakup.

 

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.

 

Assuming an average albedo, 2003 EH1 is estimated to be only about 2.1 0.8 km in diameter. Although comet brightness and nuclear diameter are not well related, the comet's absolute magnitude of H10 = +5.4 suggests a much larger nucleus of up to 12 km diameter, or a mass of about 2x1014 kg. This is much more mass than is present in the Quadrantid shower. Therefore, it is possible that the object was in outburst, brighter by > 3 magnitudes, as a result of a breakup of a relatively gas-laden nucleus and 2003 EH1 is a remnant representing about 1x1012 kg, 1/10th of the mass in the shower.

 

Sadly, efforts to find a common orbit between 2003 EH1 and C1490 Y1 are complicated by close encounters with Jupiter and the Earth, that can change the result dramatically for very small differences in the initial orbit. By integrating 2003 EH1-like orbits back to 1600 and searching for perihelion times that might agree with a past perihelion in January of 1491, I found that a common orbit may exist, but tends to put the path in 1491 lower in the sky from the ideal trajectory deduced from the Chinese observations by Hasegawa, because of making q and i slightly too small. The comet was said to have had a five degree tail and some of that mismatch may come on account of the tail. On request, Brian Marsden of the Minor Planet Center made an effort as well and arrived at the same result. Most of the potential solutions yield 0.5 < q < 0.6 AU in 1491, and this is probably too small to fit the data used by Hasegawa. However, we both found that values in the more acceptable range 0.65 < q < 0.75 AU are possible, certainly with the help of a close approach to the earth or--more likely--the presence of nongravitational forces. Hence, we can not exclude that C/1490 Y1 was a prior sighting of the Quadrantid parent at the epoch when it created the shower. Further light could be shed on the problem by the recognition of precovery and/or recovery observations of 2003 EH1.

 

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.

 

Table 1: Orbital elements of possible parent objects of the Quadrantid shower (J2000).

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Object T q e a w W i

(UT) (AU) (AU) o) o) o)

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2003 EH 1 (2003) 2003 Feb 24.5 1.1924 0.6188 3.1277 171.368 282.938 70.798

0.0022 0.00035 0.0030 0.0030 0.0037 0.0021

Quadrantids 0.979 0.69 3.14 171.2 283.3 71.05+72.7

(Epoch 1995-Jan-04.15) variance: 0.002 0.03 <0.27 2.1 0.16 1.0

2003 EH1 (1995-Jan-04.15) 1.1979 0.6176 3.1320 171.19 282.952 70.68

Meteoroids ejected from 2003 EH1 in 1600:

(Epoch 1995-Jan-04.15) 1.157 0.628 3.114 173.38 283.08 71.24+72.4

variance: 0.064 0.020 0.041 1.20 0.11 0.56

Derived epoch of meteoroid ejection: -.- ~1400 -.- ~1300 ~1420 ~1290

C/1490 Y1

Hasegawa [13]: 1491 Jan. 08.9 0.761 1.000 -.- 164.9 280.2 73.4

2003 EH1 (1491)*: (1491 Jan. 08.9) 0.759 0.756 3.10 164.5 285.5 69.2

2003 EH1 (1491)**: 1491 Jan. 0.580 0.812 3.10 163.7 286.5 65.7

 

96P/Machholz 2002 Jan. 8.6 0.1241 0.9582 2.969 14.596 94.609 60.186

5496 (1973 NA) 2003 Sep. 28.0 0.8829 0.6373 2.435 118.124 101.109 68.003

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*) The most probable common orbit based on the evolution of 2003 EH1 like orbits in 1600-2003 timeframe.

**) A typical result by Brian Marsden, this one for initial epoch 2003 Dec. 27.0 TT = JDT 2453000.5, a = 3.1340203, e = 0.6194604, w = 171.36251, Node = 282.93072, i = 70.80067.



* E-mail: pjenniskens@mail.arc.nasa.gov