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2006 & 2007 Light Curves
2005 My First Observation
Asteroid 86279 is probably a member of the Hungaria
family of asteroids (based on the orbital radius and eccentricity). Its
diameter is estimated to be 1.9 km (1.7 to 3.1 km), which is based on
an albedo assumption of 23 % (30% to 9%). My light curve measurements
have established a rotation period of 8.84856 hours. The peak-to-peak
brightness variation is 0.26 magnitude (±12.7% variation about
the average). The rotational light curve is close to sinusoidal,
meaning that the asteroid's shape is close to an oblate spheroid. The
brightness variation requires that the solid angle varies by 27% (max
to min), and this implies that the ratio of longest axis to shortest is
at least as great as 1.27. The rotational axis is in the northern
ecliptic hemisphere (using the right-hand rule) since it rotates in a
Thanks, Dave Healy and Jeff Medkeff, for assigning my name to this asteroid.
When someone names an asteroid for you it's incumbent upon the
recipient to try to determine as much about that asteroid as possible.
Since I like observing challenges this asteroid's faintness, at ~18th
magnitude, typically, was an added incentive for me to hone my
observing and analysis skills. This web page describes what a dedicated
amateur can learn about a faint asteroid if the motivation exists.
Asteroid "86279 Brucegary" was discovered in 1999 by Jeff Medkeff and
David Healy using the Junk Bond Observatory (in Sierra Vista, AZ). It's
orbital elements and standard (1 a.u.) brightness are:
a = 1.931865 a.u. (semi-major axis)
i = 21.9853 degrees (inclination)
e = 0.0679895 (eccentricity)
H = 15.9 (16.2 my suggested revision)
Based on the two orbital parameters a and i this asteroid can be
identified as a member of the "Hungaria" family. The smallness of this
asteroid family is shown in the following scatter diagram of 10,592
asteroids observed by SDSS (reported by Ivezic et al, 2002).
. Scatter diagram of "sine(inclination" versus
semi-major axis for 10,592 asteroids observed with the SDSS telescope
and reported by Ivezic et al, 2002. Each asteroid dot is colored to
represent its SDSS g, r, i, z colors. Asteroid 86279 has
sine(inclination) = 0.374 and semi-major axis = 1.93 a.u., which places
it within the upper-left group in this diagram (close and highly
inclined). (Note: Mars orbits at 1.52 a.u., and at its farthest is at
As of mid-2004 there were 3375 Hungaria asteroids tabulated, which
represented 1.6% of all 214,044 categorized asteroids (Faure, 2004).
What is the asteroid's size? We need to estimate the asteroid's albedo
in order to convert its brightness to a diameter. The next plot is a
summary of measured albedos versus semi-major axis.
Figure 2. Albedo versus semi-major axis (Plot created by Piotr Deuar, appearing in Wikipedia).
In this figure there are 7 Hungaria asteroids, and their median albedo
is 23% (range is 9 to 30%). As shown below, the asteroid's standardized
brightness is V-mag = 15.76 (estimated SE = 0.05 mag). This corresponds
to a diameter of 1.9 km (range is 1.7 to 3.1 km).
2006 and 2007 Light Curve Observations
Here's a chi-squared "solution" for the rotation light curve shape
and period based on adjusted observation dates: 20060128, 20060131,
20071008, 20071011, 20071101 and 20071107 (UT).
The "adjustments" consist of two things: 1) magnitude offsets to
compensate for distance and sun-asteroid-earth geometry (from an
ephemeris), and 2) time shifts caused by the same sun-asteroid-earth
angle and hypothetical rotation period. This second adjustment causes a
similar rotational aspect to be observed either earlier than or later
than an average periodic interval depending on whether the asteroid is
before or after opposition. If the rotational vector pointed at the
ecliptic north pole, for example, before opposition a given aspect
would be viewed before a uniformly spaced schedule and after opposition
it would be later. This means the asteroid's rotation direction,
prograde versus retrograde, can be determined. For the above solution
only the prograde hypothesis produced a good solution.
Based on these observations it appears that the asteroid's
rotation period is 8.84856 ± 0.00003 hours. The average
peak-to-peak brightness variation is 0.26
magnitude, implying that the asteroid's projected solid angle varied by
27% (i.e., the largest area projection was ~27% greater than the
smallest). This assumes a uniform albedo and similar shape for all
sides. However, since the two minima are different the ends must have
different shapse. The peaks are about the same, so the opposite sides
probably have the same projected area and albedo.
The measured magnitude offsets from ephemeris values predicted a
magnitude average -0.14 magnitude. In other words the average
V-magnitude is brighter than the ephemeris values by ~14%. This
requires that the ephemeris H value be changed from 15.9 to 15.76
(assuming G = 0.15 is correct and my all-sky calibration is
Image Subtraction Analysis
[Note: this section describes an image processing
procedure that removes a background star field image from individual
asteroid images that produces a residual image showing the asteroid at
100% of its true value and stars at ~1% of their original value. Since
I wrote this I have made small improvements in analysis procedure, but
the basic concept is the same. Later, when I settle on a final
procedure, I'll update this section.]
An image subtraction technique was used to process observations made
2006.01.28 UT with my 14-inch Celestron. Downslope winds cause seeing
to vary from bad to poor, with FWHM ranging between 4.5 to 6.0 "arc
(for 4-minute exposures).
Let's begin with an image of the star field showing the path of the asteroid during the 3-hour observing period.
Figure 2. Path of asteroid during the 3-hour observing period
of 2006.01.28. At the beginning (lower-left) the asteroid is just north
of a 19.1 magnitude star. The asteroid does not appear in this image
because it is a median combine of 21 images during which the
astreroid's location changed. The faintest stars have CV magnitudes of
22.2 (SNR=3). The brightest star (saturated in this 8-bit image, but
not saturated in the 16-bit FITS image) has CV = 14.59 with FWHM = 6.0
"arc. The FOV is 8.6 x 6.6 'arc (cropped from the original 15 x 10 'arc
image). North up, east left. [Total exposure = 84 minutes.]
The next image is a median combine of the first 3 images (cropped the same as the previous image).
Figure 3. Median combine of first 3 images, showing asteroid (cicled) at the beginning of its path for the night.
The asteroid is close to a 19.1 magnitude star, located to its south
within the sky reference annulus. Any attempt to measure the asteroid's
brightness would be affected by the nearby star. This is when "image
subtraction" is useful.
Figure 4. Same star field after image subtraction, showing the asteroid without interference from nearby stars.
Let's show the before and after versions of the previous two images for a smaller FOV.
Figure 5. Before and after image subtraction (for a smaller FOV) for the first 3-image set.
Notice that the 19th magnitude star to the asteroid's south is
completely removed after image subtraction. Only the 14.6 magnitude
star is present as a ghost feature with an approximate brightness
comparable to the 18.7 magnitude asteroid. Thus, the image subtraction
processing achieved a star subtraction of about 4 magnitudes (40-fold
If you want to learn more about the image processing procedure used on this data go to IS for 6128
Animation of Asteroid Motion
Figure 6. Animation of asteroid's 3-hour motion, using cropped versions of the image subtraction images.
Notice that in this animation there is no hint of other stars besides
the ghost of the bright 14.6 magnitude star. This suggests that the
image subtraction process did what it was suppsoed to do.
Here's an animation from "smoothed" versions of the same images (only 10 frames).
Figure 7. Same animation but with smoothing applied.
2005 January Observation
The discovery of "86279 Brucegary" was
made 1999.10.17 by Jeff Medkeff and Dave Healy. They used Dave's Junk Bond Observatory 20-inch
Ritchey-Chritien (loaner) telescope in Sierra Vista, AZ (3 miles from my place).
The asteroid has the following known properties:
Semi-major axis = 1.932 a.u.
Eccentricity = 0.068
Inclination = 21.8 degrees (unusually large)
Absolute magnitude = 15.9 (my suggested revision
calls for 15.8)
The brightness implies a diameter of ~3.0 km (based on the H value and an albedo of 0.15).
Of course, I had to take a "picture" of it as soon as I learned about the official naming.
Stack of 48 one-minute exposures, using the
predicted position of the asteroid for alignment. Unfiltered brightness
corresponds to V-mag = 20.6 +/- 0.13 (SNR = 8). [Celestron 14-inch,
prime focus, f/1.86, SBIG ST-8XE; 2005.01.29; Hereford, AZ]
Measuring the brightnessof a stationary object with a V-magnitude of
20.6 is easy for amateurs, but a moving object of the same brightness
is more difficult. This asteroid was moving at the rate of 3.8
"arc/minute, which meant that exposures could not exceed ~1 minute. The
asteroid's motion was known, so I added an "artificial star" to each
image that was carefully offset from a nearby bright star by an amount
that corresponded to the asteroid's motion. I then used this artificial
star for alignment of groups of 4 images while performing a median
combine. Since there were 48 one-minute images, this led to a set of 12
median combined images, each of which was cleansed of any artifact
blemishes (such as cosmic ray features). These 12 images were averaged,
again using the artificial star for alignment. Voila! At the predicted
location there appeared a 20.6 magnitude object which I assume is the
asteroid. It was located 1.6 pixel to the east and 0.1 pixel to the
south of the predicted location.
Nearby Tycho stars were used to establish the magnitude scale, which I
estimate allowed me to establish a magnitude scale accurate to 0.07
mag, SE. The ephemeris predicted a magnitude on the observing date of
20.3, so based on my measured magnitude I am suggesting that the
asteroid is 0.3 magnitude fainter than assumed by the ephemeris
program. In other words, I am suggesting that H = 16.2 (instead of
15.9). Of course, there are two other alternative reconciliations of
this brightness difference. The asteroid rotates, and I observed it
near minimum, and the assumed opposition effect differs from the
adopted value describing the fall of of brightness with increasing
sun-object-observer angle (G = 0.15). My observation was made close to
the time when the asteroid was on the opposite side of the sun from the
Earth. It was ~2.9 a.u. from Earth, whereas at opposition it will be
~1.3 a.u. away, and will be at 18.8 magnitude (at declination +42
Faure, Gerard, http://www.astrosurf.com/aude/map/us/AstFamilies2004-05-20.htm
Ivezic, Z., Lupton, R. H., Juric, M., Tabachnik, S., Quinn, T., Gunn,
J., Knapp, G. R., Rockosi, C. M., Brinkmann, J., "Color Confirmation of
Asteroid Families," A. J., 124
, 2943-2948, 2002 November.
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Webmaster: Bruce L. Gary. Nothing on this web page is copyrighted. First created: 2005.02.13. Last