Minor Planet (1951) Lick

On March 7, 2003 I took 14 successive 60 second images of an area the size of about one quarter of the full moon in Hercules, near the borders of Draco and Bootes. Each image was taken with a delay of 60 seconds, except for the second image, which was taken after a delay of about 4 minutes. The movie below depicts this sequence, in reality lasting a little more than 30 minutes, accelerated 480 times.

Look closely, and you'll find asteroid (1951) Lick moving in front of the background stars. This asteroid was discovered on July 26, 1949, by Carl A. Wirtanen (Lick Observatory, CA, USA, I guess that's why it was named Lick). He also discovered comet 46P/Wirtanen, that originally would be visited by the European Rosetta spacecraft, before it was delayed and a different target was chosen. Unlike many other asteroids, it does not move around the Sun in an orbit that lies between those of planets Mars and Jupiter, but one that is a little closer to the Sun and one that does not lie in the ecliptic plane, but is inclined 39° [1]. It has an eccentricity of 0.0615. The distance of Mars to the Sun is about 1.52 AU (228 million kilometers), whereas that of the asteroid is up to 1.39 AU (209 million kilometers) [1]. Because the Earth moves around the Sun too (at a distance of 1 AU, or 150 million kilometers), the distance between the asteroid and Earth varies a lot, and at the time of these images, it was about 0.74 AU (111 million kilometers). The positions of the planets and the asteroid at the time of imaging, is depicted in the following graph. The planets and asteroid (the square) move clockwise in this orientation.

The bright star in the images is the V magnitude 8.5 HIP 78421, which has a distance of 492.68 light-years (4661 trillion kilometers) [2]. The star below the center of the image is magnitude 14.43 GSC 3497:1453. Using the latter star for unfiltered differential photometry, I determined that the magnitude of the asteroid was about 16.4 at the time. I estimate the accuracy of this to be only about 0.5 magnitude, as a result of a combination of severe vignetting (this was before I exchanged the poor focal reducer), and field rotation (ALT-AZ mount). Nonetheless the measured magnitude matches the computed magnitude 16.5 well.

This inaccuracy did also not allow me to determine a light curve for the asteroid. It is known to have a rotational period of 5.3016 hours [3;4;5;6] (secure result with no ambiguity, full lightcurve coverage), and a V magnitude variation of 0.17-0.27. Using GSC 3497:1453 (mag 14.43) and GSC 3497:1319 (mag 14.09) as C and K stars, I found the variation of C-K to be of the order of 0.4 magnitude, which is larger than the expected variation of the asteroid. Of course, half an hour of imaging would be too short for confirming this rotational period anyway.

The asteroid moved 57.02 arcseconds between the first and last image, which corresponds to an apparent speed of 0.0313 arcsecs/sec (taking half exposure as the time of an image, the first and last image were 1824 seconds apart). This is slightly less than the computed 0.0382 arcsecs/sec (computed RA rate of 0.0294 and dec rate of 0.0244). This difference is likely caused by the small change in image scale during imaging (clearly visible in the above movie).

Using my measured value of 0.0313 arcsecs/sec, and the computed distance of 111 million kilometers, this means the asteroid was moving at a speed of at least 16.8 Km/sec relative to the Earth.

The parallactic angle (angle between the top of the image and true North) of the aligned images is 273.43°. Combined with the positional differences between the first and last images, I found the angle of the asteroid move to be 329.67°.

The last image is an averaged combination of all 14 images. It shows fainter stars, up to about magnitude 17.5, and a trail for the asteroid (in case you still didn't find it). The uneven background is a result of vignetting and processing.

2003-03-06: 14 minute exposure (14x60s). Camera cooled to -20°C.

Bibliography

[1] Minor Planet Center Orbit Database (MPCORB); 2003.
[2] Hipparcos Catalog.
[3] IAU Minor Planet lightcurve parameters list; 2003-12-15.
[4] Velichko, F.P., Krugly, Yu.N., Lupishko, D.F., Mohamed, R.A.; 1989, Astron. Tsirk. 1546, 39-40.
[5] Wisniewski, W.Z., Michalowski, T.M., Harris, A.W., McMillan, R. S.; 1997, Icarus 126, 395-449; 1995, Lunar & Planetary Science XXVI, 1511-1512.
[6] Pravec, P., Wolf, M., Sarounova, L.: 2003, posted on WWW; http://sunkl.asu.cas.cz/~ppravec/neo.htm.