Texas Minor Planet Project and Hubble Space Telescope Astrometry

The story of laser ranging at McDonald Observatory and its role in operational satellite tracking at UT can be traced back to Harlan Smith, a pioneering administrator who foresaw the critical role that ground-based observatories would play in supporting space exploration. He successfully convinced NASA, the National Science Foundation, and UT Austin to jointly fund the 107-inch telescope at McDonald Observatory in the sixties. This telescope became the primary ground-based facility for the pioneering lunar laser ranging experiments and was classed as a component in the overall Apollo program. He also served on major national committees, including the National Academy of Sciences committee that helped lay the groundwork for the Hubble Space Telescope.

Hubble was good example of how McDonald integrated into the space age, particularly the structural role McDonald played in Hubble astrometry and the parallels with McDonald’s role in geodesy. Both astrometry and geodesy are concerned with fundamental reference measurements, of the celestial and terrestrial realms respectively. For Hubble, McDonald provided precise ground-based tracking data for calibrating the spacecraft's instruments via a direct integration between the Texas Minor Planet Program and the Hubble astrometry team. The primary astrometric instruments aboard Hubble were the fine guidance sensors. To achieve sub-millisecond of arc precision, the astrometry team had to map and correct severe optical field angle distortions caused by the telescope's mirrors and refractive optics. While static distortions were mapped by observing distant, fixed star clusters like M35, the team also needed to perform dynamic calibrations, such as tracking time-varying mechanical flexures, scale changes, and the alignment between the three independent fine guidance units. To do this, they needed to track moving celestial targets with highly predictable paths. Minor planets, or asteroids, were selected as the ideal moving targets. [1]

For the fine guidance calibration to work, the orbital paths of these minor planets had to be known to a higher degree of accuracy than the guidance sensor measurement precision itself. To achieve this, Paul Hemenway and Raynor Duncombe initiated the Texas Minor Planet Program in 1978. The intro photo of this post shows Paul on the left and Ray on the right. Between, left to right, are Bill Jefferys, Fritz Benedict, and Pete Shelus.

Using the long-focus Cassegrain focal plane of the 82-inch telescope at McDonald, the team targeted thirty-four specific minor planets, twenty of which were suited for HST observation. Over the course of the project, researchers captured more than a thousand glass photographic plates of these asteroids. An innovation of the program was the crossing-point method. By taking observations exactly where the projected paths of two different minor planets intersected, researchers could compare the objects against the same local background stars at different times, effectively isolating and eliminating systematic star catalog errors from their orbit reductions.

The precise ground-based observations from McDonald were processed by orbit determination specialists at UT Austin's Center for Space Research. By feeding the McDonald data into complex dynamical models, the team refined the asteroid orbits so that their orbital uncertainty was reduced to near-zero. Because these ground-truth reference trajectories were so precise, any deviations or residuals recorded by the fine guidance sensors could be attributed directly to their own instrumental distortions rather than the asteroid's actual motion.

This integration of McDonald Observatory's minor planet data and Hubble astrometry was used to resolve broader issues in celestial mechanics. The combined data allowed researchers to measure the unknown rotation of the European Hipparcos satellite's instrumental coordinate system, effectively linking the stellar optical reference system to a dynamical, gravity-based reference frame. These results were directly incorporated into the celestial reference frame, much as the McDonald geodetic measurements were incorporated in the terrestrial reference frame. Both of these research fields, astrometry and geodesy, can be traced directly to the inspiration from Harlan Smith to focus on participating as fully as possible in NASA’s space missions, tying McDonald and the West Texas Chihuahuan Desert together with the realm of satellites and spacecraft.

Harlan Smith and Ray Duncombe. Many count themselves fortunate to have known Harlan and Ray.

The 82-inch telescope at McDonald, where glass-plate photographs of asteroids were made on many a Chihuahuan Desert night.

Operator's console on the observer's deck.

[1] HST Astrometry Team