Asteroidal object 1991 DA has an orbit of high inclination, crossing the planets from Mars to Uranus. This is unique for an asteroid, but not unusual for a comet of the Halley-type: it therefore seems likely that 1991 DA is an extinct or dormant comet. Previous CCD imaging has shown no indication of a coma; spectroscopic observations of 1991 DA which lack any evidence of strong comet-like emissions are reported. Numerical integrations of the orbit of this object were performed which show that is has been remarkably stable for the past approximately 20,000 yr, but chaotic before that. This may allow a new estimate to be made of the physical lifetimes of comets

Asher, David

Mcnaught, Robert H.

Steel, Duncan

English

NASA Technical Reports Server

Asteroids, Comets, Meteors 1991, pp. 573-576
Lunar and Planetary Institute, Houston, 1992
N93/ 1 247
1991 DA • AN ASTEROID IN A BIZARRE ORBIT
573
Duncan Steel and Robert H. McNaught,
Anglo-Australian Observatory,
Private Bag, Coonabarabran, NSW 2357, Australia; and
David Asher,
Department of Physics, University of Ozford,
Keble Road, Ozfard, OX1 3Rtt, England, U.K.
Asteroidal object 1991 DA has an orbit of high inclination, crossing the planets from Mars to
Uranus. This is unique for an asteroid, but not unusual for a comet of the Halley-type: it therefore
seems likely that 1991 DA is an extinct or dormant comet. Previous CCD imaging has shown no
: indication of a coma; here we report spectroscopic observations of 1991 DA which lack any evidence
of strong comet-like emissions. We also perform numerical integrations of the orbit of this object
which show that it has been remarkably stable for the past --_20,000 yr, but chaotic before that.
This may allow a new estimate to be made of the physical lifetimes of comets. _ / /=/ j
Introduction
The object denoted 1991 DA was discovered by the second-named author on a U.K. Schmldt
Telescope plate exposed on 1991 February 18. It is estimated to be about 5 km in size. Astrometric
observations soon showed it to have one of the most bizarre orbits known for any asteroid (Chapman,
1991): at 61.90 its inclination is the third highest of all known minor planets, and its eccentricity
of 0.866 is also the third highest amongst asteroids. Wlth a perihelion distance of 1.58 AU and
aphelion at 22 AU, 1991 DA crosses the paths of Mars, Jupiter, Saturn and Uranus, and therefore
would be expected to have a dynamical lifetime of at most .._ 105-s years. CCD imaging of 1991
DA by English and Freeman (IA U Circ. 5199) and by West and by Ryder (arAU Circ. 5208)
has shown no evidence of a coma despite the fact that the object was close to perihelion at the
time. Here we report spectral observations with the Anglo-Australian Telescope which lack any
comet-like emissions, backing up the conjecture that 1991 DA is asteroidal in nature. 1991 DA
may therefore be a totally de-volatilized comet, or a comet which has formed an insulating crust.
Numerical integrations may indicate the path by which the present orbit of 1991 DA has come
about, especially if non-gravitational forces have been negligible in the past. We have carried out
such integrations extending over the last 50,000 yr and find that the orbit has been remarkably
stable for at least the last ,_20,000 yr, prior to which nodal crossings near Saturn lead to large
changes in the orbit in close approaches to that planet. Before then the orbit is chaotic with longer
integrations being of statistical interest only. A more detailed discussion of such integrations, over
a longer time-base, is given by Hahn and Bailey (1991).
Spectroscopic Observations
We observed 1991 DA using the 3.9 m Anglo-Australian Telescope on the night of 1991 March
17. We had planned to obtain a high-dispersion spectrum covering the strong emissions from CN,
CH, C2 and C3 in the range 380-520 nm manifested by many comets but an instrumental fault
limited us to a spectrum from 530-1095 nm obtained with the Faint Object Red Spectrograph on
the AAT. This is shown in Figure 1 after subtraction of the sky background and ratioing against
the solar-analogue star 70 Virglnis. There is no evidence for strong comet-like emissions in the
spectrum, except possibly at 530-580 nm where C2 emission may occur. Atmospheric absorption,
https://ntrs.nasa.gov/search.jsp?R=19930010058 2020-03-17T08:17:52+00:00Z
574 Asteroids, Comets, Meteors 1991
l
particularly near 760 nm, is evident (the star and asteroid were at rather different zenith angles)
and longwards of 900 nm the atmospheric transmission was both low and variable, leading to the
great scatter in this ratio spectrum (and the strong spike, absent in another spectrum). On this
basis, therefore, we do not find any compelling evidence for cometary emissions from 1991 DA.
-- ' ' i i i v --_ -- t
0.5
14
0
._.
4,J
Low flux,
variable
atmospheric
0.3 _ transmission
" " __d ,.,
0.1
I I I , I t • ._ L
530 Wavelength (nm) 1095
Figure 1: Ratio spectrum from 530-1095 nm for 1991 DA / 70 Virginis after subtraction of
atmospheric background. No strong cometary emission lines are found. Above 900 nm the spectrum
is noisy due to low signals and variable atmospheric transmission.
Numerical Integrations
We used a 7th order Runge-Kutta-NystrSm integrator developed by Dormand and Prince (1978). It
is a variable step-length method shown by Fox (1984) to be very efficient for high-eccentricity orbits.
Fox (1984, personal communication) provided a Fortransubroutine implementing the meth0d. Our
steplengths ranging from a little below 0.1 yr at r = 1.6 AU to about 3 yr at_r --22AU me_anthat
we are not solving the equations of motion to anywhere near the limit of machine accuracy, but this
is probably sufficient given tfie-t_nc_taintiesin-st_art_g-e!ements and the¢ven_u-al-cha_tic_nh_ure
_iL_ °__-_=. _ _ :..::__-:_=--=--_._-=-__-...._-_:-:._.:=-/ -:-._- _ - •
-W_rmcfuded the six planets from E_xrth out to Nept_L up-_]_i_-_t]ae_ 0rbltal eieme_ts _ery
500 yr using smoothed ehments from Qniun (1991, personal communlcation)_de/= |red= _om the
planetary integration of Quinn et 02. (1991). Jupiter and Saturn dominate.... the effect on the sec-
ular change in dements of 1991 DA, and the long-term effect of precessing planetary dements is
noticeabh. The initial dements of 1991 DA were taken from MPC 18437: v/z. a = 11.8721271 xu,
e = 0.8670227, i = 61.88558 °, 1"1= 313.40412 °, _o = 191.25159 °, M = 8.95122 ° at !_9_ !.9_.
In Figure_2 we._s_ho_w- the history of the semi-major axis of 1991 DA for six _fferent sets of
starting dements; the topmost plot is with the eiements just listed, and then descen_ng we modify
a by +0.0001 xu, e by +0.000001, and i, fl and _o by +0.001 ° (one only of these changes in turn
from the nominal orbit). These represent smaller uncertainties in the orbit than exist due to the
short are over which 1991 DA has been observed.
Asteroids, Comets, Meteors 1991 575
14
13
12
11
-4x 10 4 -3x 10 4 -2x 10 4 - 10 4 0
0
I-i
0
I
i . i .... i .... t .... i .... i
16
14
12
-4xlO 4 -3xlO 4 -2xlO 4 -10 4 0
14
13
12
11
i .... i • • , ' I .... | .... I •
-4x|O 4 -3xlO 4 -2xlO 4 -10 4 0
16
15
14
13
12
-4x 10 4 -3x 10 4 -2x 10 4 - 10 4 0
12 .... i .... i .... i .......
11.5
11
0.
I0 -4xlO 4 -3x10 4 -2xlO 4 -10 4 0
18
16
14
12
I0 i i , I i , , , l .... I
-4x 10 4 -3x 10 4 -2x 10 4 - 10 4
Year A.D.
0
Figure 2: Results of numerical integrations of 1991 DA backwards over 50,000 yr from the present
for slightly different starting parameters: for details see text. Only the semi-major axis for each of
the six hypothetical orbits is shown. The plots do not start to diverge appreciably until _20,000 yr
ago when close approaches to Saturn become possible. The oscillatory parts of each plot are due
to commensurabilities with Jupiter, which are described in the text.
576 Asteroids, Comets, Meteors 1991
The asteroid is currently close to the 7:2 resonance with Jupiter and this and other commen-
surabilities are clearly seen in Figure 2. In particular the long-term oscillatory phases seen in the
second, third and fourth plots are due to the 4:1 resonance (a = 13.11 Au); in the fifth plot the
3:1 resonance (a : 10.82 AU); and in the sixth plot the 9:2 resonance for -38,000-32,000 A.D. (a
: 14.18 Au) and 6:1 for -47,000-45,000 A.D. (a = 17..17 Au)_In_the_st-pl0t, using the nominal
elements, the 7:2 resonance (a = 11.99 AU) occurs at --32,000-26,000 A.D., and the 10:3 resonance
(a = 11.61 AU) at --38,000-34,000 A.D. It appears to be due to the action of these resonances that
1991 DA owes its remarkable stability; for a more detailed discussion see Hahn and Bailey (1991)
and Steel_ad Asher (1991). : :-
There is little divergence between these six orbits until about20,OOOyr ago_ At that time, w
attains values of around 2200 and nodal intersections with Saturn occur. Since the exact distances
of the close approaches which occur in that epoch depend critically upon the starting elements, the
six integrations begin to differ markedly at that point and there is no likelihood Of determining the
exact orbital history prior to that time. The integrations are thus of statistical interest only.
In view of the increasing awareness Of the effects upon the terrestria]environment of large
comets and asteroids such as this, and also the integrations of Hahn and B_ey (1990)=for 2060
C_on w]lich showed that this massive asteroid/c0met maywe_'h-avehad an Ea_r_-crossing orbit
in the past, it is of interest tonote here that of thes_p_xate integrationsrepresented in Figure 2
two of them attain a perihelion distance q < 1.0 AU within the last 50,000 yr, and another reaches
q < 1.1 AU. These results are presented in more detail by Steel and Asher (1991). The original
comet (if this is the source of the remnant leR as 1991 DA), which may have been over 20-25 km in
size if the asteroid represents only the silicate fraction, could therefore have left a very substantial
amount of material in an Earth-crossing orbit in the astr0nomlcally-recent past.
References
Chapman, C.R. (1991). A way-out 'asteroid'. Nature, 350, 556.
Dorm_d_J.R. andPrince, P.J. (1978). New Runge-Kutta Algorith_ for Numerical Simulation
in Dynamical Astronomy. Celestial Mech., 18_ 223"2_32.
Fox, K. (1984). Numerical Integration of the Equations of Motion of Celestial Mechanics.
CelestiaIMech" 33, 127-142 :.......
Hahn, G. and Bailey, M.E. (1990). Rapid dynamical evolution of giant comet Chiton.
Natur.______e,34 ._88,132-136 ........ ::
Hahn, G. and Bailey, M.E. (1991). Long-Term Evoluti-on of 1991 DA: A Dynamically Evolved
Extinct Halley-Type Comet, these proceedings.
Quinn, T._., Tremaine,S__and Duncma' M. (1991)' A Three Million Year Integration of the Earth's
Orbit. Astron. J., 10___!,2287-2305. : =
Steel, D.I_ and Asher, D.J. (1991). The past and future orbit of (extinct comet?) 1991 DA.
To appear in International Workshop on Periodic Comets (J.A. FernAndez and H. Rickman,
eds.), University of the Republic, Montevideo, Uruguay.
Acknowledgements: This work was supported by the Australian Research Council and the U.K.
Science and Engineering Research Council. We thank the Director of the AAO, RUssell Cannon, for
the allocation of time on the AAT; Raylee Stathakis and Kevin Cooper for assistance in collecting
the data; and David Allen for information on cometary spectra. Discussions with M.E. Bailey and
G. Hahn on the numerical integrations were appreciated, as was a preprint of their paper.


1991 DA: An asteroid in a bizarre orbit

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