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).
_____________________________________________________________________________
Object T q e a w W i
(UT) (AU) (AU) o) o) o)
_____________________________________________________________________________
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
_____________________________________________________________________________
*) 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.