Mapping the Invisible Moon

by G. Randy Gladstone, Ph.D.      image of PDF button

Dr. Randy Gladstone, a senior research scientist in the Instrumentation and Space research Division, came to the Institute in 1993 from the Space Sciences Laboratory at the University of California at Berkeley, where he conducted studies in planetary science. His primary interest is modeling the scattering of UV light and photochemistry in the upper atmospheres of the planets. Obtaining the necessary data for his models requires him to be an active observer on many space missions; in addition to Astro-2, he has participated in the International Ultraviolet Explorer (IUE), Hubble Space Telescope (HST), Extreme Ultraviolet Explorer (EUVE), and Röentgensatellite (ROSAT) missions.

The first global images of the Moon at ultraviolet (UV) wavelengths were recently made during the flight of the NASA space shuttle Endeavour, carrying the Astro-2 payload. The 16-day mission, completed March 17, was designed to explore the universe at UV wavelengths, examining diverse astronomical objects, from planets to exploding stars to intergalactic matter.

Three Southwest Research Institute scientists from the Instrumentation and Space Research Division — the author, Dr. Chan Na, and Dr. Alan Stern — led efforts to obtain the lunar images. Such pictures cannot be taken from the Earth’s surface, because its atmosphere screens out UV light. Previously, the only UV data available for the Moon were spectra covering 10 percent of the surface from the Apollo program.

In turn, this would provide evidence that UV observations can be useful in the study of other, more difficult to reach, atmosphereless solar system bodies such as the planet Mercury, asteroids, and the smaller satellites or moons associated with the outer planets, by allowing more detailed studies of their geological history.

A number of problems had to be overcome to ensure a successful experiment. Only 250,000 miles away, the Moon is so large that its image, taken at a close distance, barely fit within the wide field of view of the Astro-2 camera, known as the Ultraviolet Imaging Telescope (UIT). The Moon’s light was also almost too bright for the UIT to look at, because it was designed to register fainter, more distant images of galaxies and star clusters.

The large field of view of the UIT was essential to make a complete UV image of the Moon. For comparison, if a complete picture of the Moon were composed using the UV cameras of the Hubble Space Telescope, with its much smaller fields of view, it would require a mosaic of more than 900 frames.

In addition, the Astro-2 payload was designed to point at fixed and distant astronomical targets, while the Moon’s proper motion around the Earth each month causes it to move in a way that can produce unacceptable blurring of images in seconds. This problem is exacerbated by the parallax produced by the motion of the space shuttle around the Earth at approximately five miles per second, causing the apparent position of the Moon to wobble back and forth by about four lunar diameters during each 90-minute shuttle orbit.

The first two observations of the less-than-full Moon were successful, but when the time came to take the final images of the full Moon, the cameras were accidentally misdirected. Within hours, a decision was made by mission managers at the Marshall and Johnson Space Flight Centers, after checking with the astronauts, to add an extra orbit to the mission solely to procure the images of the full Moon that were missed 12 hours before. This time, the cameras were aimed correctly, and the full Moon became the last as well as the most difficult target to be observed by the Astro-2 telescope.

The Moon has been extensively studied at visible wavelengths, primarily through data gathered from the Apollo missions, and more recently from the Clementine mission. These studies provided a large body of “ground truth” data on the age, composition, and particle distribution of major lunar geological units. The new UV data are important not only for extending these studies to shorter wavelengths, but also because the Moon provides an ideal known target to evaluate the usefulness of far-UV (between 1,000 and 2,000 angstroms) imaging as a tool for remote sensing of other airless solar system bodies that are harder to reach.

In particular, laboratory studies have shown that UV reflectivity of different surface regions provides a useful indicator of the degree of “space weathering” that has occurred. While most materials eventually become darker with increasing exposure to space radiation, this process occurs more rapidly at UV wavelengths than at visible wavelengths. This is why more recent lunar features, such as the debris churned up by the formation of the Tycho impact crater about 100 million years ago (at bottom center in the accompanying UV and visible wavelength images), appear much brighter than the older lunar highlands and maria.

The primary research objective of image analyses to be carried out at the Institute is to obtain lunar surface UV reflectivity or albedo maps and to correlate and compare such maps with existing mineralogical and cartographic data acquired from earlier space missions. This should confirm that UV measurements of albedo and scattering properties provide a useful indication of the age and mineral composition of different regions on the surface of the Moon.

Published in the Summer 1995 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.

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