An Asteroid Family Among the NEAs?
Charles A. Wood, Ron Fevig and John Nordlie
Department of Space Studies
University of North Dakota, Grand Forks, ND 58202
Submitted to the twenty-fifth Lunar and Planetary Science Conference
March, 1994.
Abstract
A cluster of asteroids has been recognized near a=2.18 AU among the
Near Earth Asteroids. Named after its best studied member, the
1981QA family consists of 7 tightly clumped members that are
significantly paired according to the D discriminent. These core
members are surrounded by 13 additional asteroids with similar
orbital elements. Intriguingly, at aphelion members of the 1981QA
family pass near the Themis family in the main asteroid belt.
Background
The majority of main belt asteroids travel in independent orbits, but
a number of clusters have been discovered in which the members have similar
orbital elements. These so-called families are assumed to be fragments
from collisions in the asteroid belt; over time, perturbations should
disperse family members into 'sporadic' asteroids with totally independent
orbits. Apparently there has been little study of possible families
among the Near-Earth Asteroids (NEAs), but Drummond[1] discovered that
some NEAs have similar orbits and travel in streams that are probably
sources of meteors. Since Drummond's work many discoveries have increased
the number of NEAs (~280) sufficiently to allow more thorough searches for
families among the Earth approaching asteroids.
Procedure
Valsecchi and others[2] have reviewed definitions and techniques used in
prior searches for families among main belt asteroids. In terms of
orbital dynamics they note that, "an asteroid cluster family is constituted
by a group of asteroids whose proper elements a, e, and i
appear to form a cluster distinguishable from the background..." They
also added a constraint based on assumptions of family origins: "...these
asteroids are genetically related because they come from the disruption
of a larger parent body." Proper elements used in the main belt
investigations have had the affects of planetary perturbations removed,
but NEAs travel on chaotic orbits and have no meaningful long-term proper
elements[2]; thus we use osculating elements which change over time. Close
approaches to planets can rapidly randomize orbital elements of NEA family
members, thus if any NEA families did form by collisions they may only be
identifiable for times short compared to the dynamic lifetimes of NEAs.
Our listing of orbital elements of NEAs comes from a compilation by Helin (1993
reprint), augmented by more recent discoveries announced in the Minor Planet
Electronic Circulars. Actual identification of main belt families has
been accomplished by two means: formal computational searches and visual
inspection of a, e, i graphs. We first employed the latter technique
and then also calculated Southworth's D criteria [3] for each asteroid pair.
A problem with the D criteria is that no one knows what a limiting value is
in order to accept a clustering as a family. Linbald and Southworth [3] used
D=0.020; following their recommendation we corrected for sample size and
derive a critical value of D=0.036.
Examination of a-i and a-e distributions for 280 NEAs reveals a
number of apparent concentrations of two to four NEAs, some having D<0.036.
Most conspicuous, however, is a cluster of asteroids centered near a=2.18 AU,
e=0.43, and sin i=0.14. No other similar size cluster exists among
the presently known NEAs. Is this NEA clustering comparable to those accepted
for families in the main belt asteroids?
There is much disagreement of what constitutes a family for main belt asteroids,
but following the examples given by Chapman et al [4] we accept a group as a
family if their orbital elements cluster within a range of 0.07 AU, 0.04, and
0.025 in a, e, and sin i space, respectively. The NEA group near
a=2.18 contains 7 asteroids within this small volume, and 7 more closely
surround them in a larger volume defined by a range of a, e, and sin i
of 0.11 AU, 0.05, and 0.07. A more diffuse halo of 6 additional NEAs envelopes
these central groups. Six of the 7 asteroids of the inner core group are
involved in pairings where the D value <0.036. This core group contains one NEA
recognized by Drummond [1] as part of his looser stream, "Association I".
Conclusion
We propose that the innermost NEAs constitute a family (which we christen the
1981QA family, after the best known member); including the slightly dispersed
extra members makes it an extended family. The third, halo group, are second
cousins, maybe twice removed. The members of the central family and extended
family (below the line) are listed in the table; most are recent discoveries
and do not yet have measured physical properties. Other than brightness (H),
only 1981QA has any known properties. With the present lack of data it is
impossible to evaluate if the putative family members have similar spectral
properties, and hence could have collisionally evolved from a common parent. As
a challenge to observers we propose that the majority of these NEA family members
will have similar spectral types. Note that in the proposed nomenclature of
Farinella et al [5] our core group would be a "cluster" or a "type II family."
The existence of a family within the NEAs may be explained by a collision with
another asteroid when an original parent NEA passed though the main asteroid belt.
With an average a=2.18 and e=0.43, the proposed parent would have
Q=3.12 and thus would pass through nearly all of the main belt. Interestingly,
this Q is near the average a for the Themis family (2.14 AU) of
main belt asteroids. The obvious speculation that a single collision formed both
the Themis family and ejected the 1981QA family into its present orbit is flawed
by the observation that although 1981QA has a somewhat uncertain spectral type
(QRS; [6]), it is, however, distinctly different from the C types that dominate
the Themis family (C.R. Chapman, pers. comm. 1994).
The recognition of an asteroid family among the NEAs provides, if the reality of
the grouping is accepted, a family of much smaller diameter members than any known
in the main belt. Also, the 1981QA family is presumably much younger than the
main belt examples, because the dynamical lifetime of its members is only a few
hundred million years, and the rate of orbital randomization is higher. 1981QA
itself has a rotation period of 148 hrs - one of the longest known [6]. Binzel
et al [7] summarize reports that long periods may be due to a torquing by a
companion satellite, which is most likely to be captured during a collision. Thus,
the 1981QA family and extended family may represent the orbitally coherent
fragments from a relatively recent collision. The perihelion distances for these
Amor asteroids probably keep them from interacting with the Earth, but they may be
a contemporary source of meteorites on Mars.
References:
[1] Drummond, JD (1991) Icarus 89, 14-25.
[2] Valsecci, GB et al (1989) in Asteroids II (ed. by Binzel,
Gehrels, and Matthews), Univ. of Arizona Press, 368-385.
[3] Linbald, BA and Southworth, RB (1971) in Physical Studies of
Minor Planets (ed. Gehrels), NASA SP-267, 337-352.
[4] Chapman, CR et al (1989) in Asteroids II (ed. by Binzel,
Gehrels, and Matthews), Univ. of Arizona Press, 386-415.
[5] Farinella, P et al (1992) in Asteroids, Comets, Meteors 1991
(ed. by Harris and Bowell), 167-170.
[6] McFadden, L Tholen, DJ and Vedder, GJ (1989) in Asteroids II
Gehrels, and Matthews), Univ. of Arizona Press, 416-441.
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