Central Bureau for Astronomical Telegrams
IAUC 5402: 1991 VG
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Circular No. 5402
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
Telephone 617-495-7244/7440/7444 (for emergency use only)
TWX 710-320-6842 ASTROGRAM CAM EASYLINK 62794505
MARSDEN or GREEN@CFA.BITNET MARSDEN or GREEN@CFAPS2.SPAN
1991 VG
Further to IAUC 5401, R. M. West also reports: "CCD observations
were obtained during Dec. 2.12-2.19 UT under very good conditions
(seeing 0".7-0".8; long trails of the object were recorded on
unguided V- (total 47 min) and B-frames (16 min). The trails are
characterized by very rapid variations; due to the high rate of
motion the time resolution is about 2 s (during which time the object
moved 1"). After careful subtraction of the adjacent sky by
interpolation, the lightcurve shows 1.8 mag peak-to-peak variations
between V = 16.9 and 18.7; mean V = 17.7 +/- 0.1, mean B = 18.2 +/-
0.2; B-V = 0.5 +/- 0.2, i.e. solar. Although there are differences
between the individual trails, we recognize deep minima on all of
them, lasting about 20-30 s, interspersed by peaks at maximum
brightness of about the same duration; the full 1.8-mag change
happens in 15 s. Superimposed on the trails are three flashes at mag
16, each lasting < 2 s; they are quite unlike cosmic-ray events and
appear real. It has not been possible to determine the period with
certainty, although 7.5 min appears to fit most trail segments. The
curve is reminiscent of that of a rapidly rotating satellite, with
highly reflective sidepanels. Assuming an albedo of 0.5, the
reflecting area is of the order of 30 m**2. These observations
therefore support the interpretation of 1991 VG as an artificial object
(cf. IAUC 5387), probably rotating around more than one axis."
Although the seeing was poor (about 4"), CCD photometry of an image
(deliberately trailed in declination) for 7 min on Nov. 29.2 UT by
W. Wisniewski with the Steward Observatory's 2.3-m reflector on Kitt
Peak seemed to rule out peak-to-peak variations > 0.3 mag with peak
widths of more than about 22 s; a series of tracked CCD exposures
revealed a rather typical asteroidal variation of amplitude 0.2 mag
over a period of about 100 min.
Possible space debris candidates for 1991 VG during 1974-1975
(and also around the previous earth encounter in the late 1950s)
have been mentioned by J. McDowell, Marshall Space Flight Center,
and by R. Rast, Johnson Space Center. Among them are the rockets
involved with the Helios A launch in Dec. 1974, the Pioneer 4 launch
in Mar. 1959, and various Luna missions. Identification with such
space debris requires the action of nongravitational forces (course
corrections, fuel leaks, radiation pressure) on 1991 VG; experimental
computations by P. Chodas, Jet Propulsion Laboratory, and by B.
G. Marsden, Harvard-Smithsonian Center for Astrophysics, confirm
these as possibilities, but the situation is far from conclusive.
1991 December 13 (5402) Daniel W. E. Green
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