John Caldwell of York University in Toronto, Canada, got the ball rolling. He planned to use Hubble for taking spectra of Saturn’s moon Titan as well as the Galilean satellites of Jupiter. To better understand the spectra of sunlight reflected off these objects, however, he needed to test Hubble’s capabilities on another target. The sun was definitely out as a test subject; it shines far too brightly to have Hubble point anywhere near it. So Caldwell chose an object closer to home – the moon.
It’s safe to say that if an outside observer had suggested viewing the moon with Hubble, the proposal wouldn’t have stood much of a chance. But Caldwell had a built-in advantage: He belongs to that rare breed known as Guaranteed Time Observers. As the name implies, these people are assured access to the orbiting telescope to observe virtually whatever they want. It’s then up to the technicians at STScI to devise a way to accomplish the observing program, if it’s at all possible.
When Caldwell announced his plan in 1997, Tony Roman and Andy Lubenow of STScI got to work and figured out a way to track the moon as it moves across the sky. Hubble typically keeps an object centered in the field of view by using its Fine Guidance Sensors, which lock onto fixed guide stars and tell the telescope exactly where it is pointing. It’s reasonably straightforward to track objects outside the solar system because Hubble’s motion around Earth is small enough that they remain stationary relative to the stars.
It becomes trickier inside the solar system because planets, moons, asteroids, and comets are much closer and the spacecraft’s orbital motion requires an onboard correction. This can be done when the telescope’s Fine Guidance Sensors and gyros work in concert and the object’s changing position is known precisely. Tracking the fast-moving moon, however, raised this challenge to a whole new level because the onboard correction would overflow Hubble’s computer.
Caldwell chose as his target Mare Imbrium, the huge impact basin that spans some 780 miles (1,250 kilometers) and dominates the northwestern quadrant of the lunar nearside. When Storrs heard that the Space Telescope Imaging Spectrograph would be observing Imbrium, he suggested imaging the moon’s surface with the Wide-Field/Planetary Camera 2 at the same time. Observing with filters that let only a small fraction of the moon’s light pass through and keeping exposure times to less than a second, the team managed to diminish the moon’s glare enough to get the pictures shown here.
The team targeted an area near the southern margin of Mare Imbrium that stretches from the prominent crater Copernicus west to the smaller crater Kepler. Eight images were taken on November 6, 1998, just two days past full phase, and combined to produce the mosaic shown on the opposite page. (Hubble would have to take 130 images to create a mosaic of the entire lunar disk.) The image shows little topographic relief because the sun lay almost directly overhead at the time, so shadows were just about nonexistent. Still, the detail is extraordinary. Theoretically, Hubble can deliver images with a resolution of 280 feet (85 meters) – slightly better than that of the Clementine spacecraft, which orbited the moon in 1994. In practice, however, the ground controllers weren’t able to correct precisely for the moon’s changing position, so the resolution wasn’t quite that sharp.
The giant crater Copernicus spans 58 miles (93 km) and shows a bright rim and deep, terraced walls. This relatively young crater formed when an asteroid more than a mile wide slammed into the moon some 800 million years ago. A group of central peaks rises nearly 4,000 feet (1,200 m) above the crater’s flat floor, which digs down 12,300 feet (3,760 m) below the level of the surrounding terrain. Another relatively recent impact created the 20-mile-wide (32 km) crater Kepler, seen at the very bottom of the mosaic. The impacts that created Copernicus and Kepler blasted out tons of material, forming a pair of bright ray systems that adds to the craters’ prominence around the time of full moon.
The imaging team constructed a mineralogical map (below, left) of part of this region from exposures taken through three different filters. The map is centered near the crater Milichius, which lies about 185 miles (300 km) west of Copernicus and 155 miles (250 km) east-northeast of Kepler. (You can find the crater some 45 percent of the way along, and slightly above, a line from Kepler to Copernicus.) Milichius is much smaller than Copernicus and Kepler, only 8 miles (13 km) across, and significantly older. The bright ray systems of the younger craters interact here.
The yellow color in the mineralogical map represents aged ejecta from the crater as well as the surrounding mare material. The bright blue of the crater itself and its neighbor to the west, Milichius A, depicts fresher deposits exposed when landslides carried material down the walls of the craters. The centers of both craters remain yellow because the fresher material could not slide that far.