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RSAA News of the Month: July 2003

Asteroid Named "Mount Stromlo"

 

On May 1st, 2003, the Minor Planet Centre (MPC) of the International Astronomical Union (IAU) announced that asteroid 1996 PA7 had been numbered 17640 and named "Mount Stromlo". In the early 1980's asteroid 1979 MD4 had been numbered 2343 and named "Siding Spring". Both RSAA observatory sites now have asteroids named after them.

1996 PA7 was first noticed by Rob McNaught at Siding Spring Observatory on August 15, 1996, using photographic plates taken with the UK Schmidt Telescope. As Rob's main interest is the potentially-dangerous Near-Earth Asteroids (NEAs or NEOs), he passed this "ordinary" asteroid's coordinates over to Canberra amateur astronomer Jack Child who did the careful observations and measurements needed to pin down the orbit.

The Siding Spring Schmidt telescopes
On the left, the 1.2m UK Schmidt, discoverer of thousands of asteroids.
On the right, the 52cm Uppsala Schmidt, hunter of potentially-dangerous Near-Earth Objects.

Here is Rob's story of the discovery:   "When scanning UK Schmidt plates for NEOs, I would inevitably come across many unknown main-belt asteroids, some quite bright. I had a system of checking these unknown objects to see if they were a main-belt object, or could potentially be more exotic. If the latter, I'd make follow-up observations with the Uppsala Schmidt or the 40inch, but otherwise the "ordinary" objects were ignored. Initially they were ignored because the investment of time in measuring these main-belt objects was simply too high to be justifiable when we were searching for NEOs. The change came when I developed a program to take crude measurements of the asteroid position from the plate and calculate a range of possible orbits. The measurements were made with a plastic ruler, took only a matter of seconds and gave an accuracy of an arcminute or so. To derive the direction and rate of motion required a little more care. The trail length was measured with a graticule eyepiece and the direction of motion estimated. This took several more seconds! From the time and duration of the exposure, the velocity of the asteroid is obtained. This is more than adequate for follow-up in small field instruments for several days afterwards".

Most small asteroids, including Mount Stromlo and Siding Spring, look similar to asteroid Gaspra.
This NASA image was taken by the Gallileo spacecraft on its way to Jupiter in October 1991.

"Main-belt asteroids in the direction of the object can have only a specific range of motions, so objects that fell within this range were highly unlikely to be NEOs. Thus all the basic work was done to pass these observations on to some amateur to follow-up the object and secure its orbit. Such asteroids north of the equator, went to Japanese amateurs (via Syuichi Nakano) and for those in the south to Jack Child. By arrangement with the MPC, the credit for the discovery went to the collaborating amateur with me as co-discoverer. The eventual naming rights would go to the amateur collaborator".

Jack did the hard work of observing and doing accurate astrometry on the object for a sufficient time to allow a reliable orbit to be derived. He uses a 32cm f/4.8 telescope and an ST-6 CCD camera. Once a highly reliable orbit was determined, the object was given a permanent number and could be named. The number 17640 was assigned by the IAU and Jack suggested the asteroid be named "Mount Stromlo".

The orbit of "Mount Stromlo" is of moderately high eccentricity and inclination and lies in the inner main asteroid belt. It is related to the Phocaea family of asteroids whose orbits form a discrete group, isolated due to the gravitational influence of Mars. The asteroid is around 8km diameter and can get as bright as magnitude 15, well within the reach of amateur telescopes with CCD cameras.

UK Schmidt images of asteroids Mount Stromlo (left) and Siding Spring (right).
The asteroids are the streaks near the centres of the images.
During the exposures the telescope is tracking the stars.
Since the asteroids are moving in different directions and at different speeds, they show as streaks.
Measuring the length and angle of the streak allows Rob to tell if the asteroid is a Main Belt Object or a Near-Earth Object.

Since it began operation in the mid-1970's, the UK Schmidt has identified thousands of asteroids and photographed tens of thousands that could not be followed up. The first of its asteroids to be numbered and named was 1979 MD4, discovered on June 25, 1979 by Drs. E F Helin and B J Bus. This was given the MPC number 2343 and was named "Siding Spring" after the RSAA observatory at which the UK Schmidt is located.

For more images of asteroids, click here.

For a list of IAU asteroid (minor planet) names and numbers, click here.

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