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In setting out to have the first spacecraft encounter with an asteroid, the Galileo Project determined several goals. Foremost, the scientists wanted to characterize the asteroid (its size, shape, and cratering record), to learn about its composition, and to survey the surrounding environment. From the initial results received, the Gaspra (see photo) encounter was a resounding success.
The Galileo spacecraft flew past the tiny asteroid on October 29 at 2:37 P.M. (PST), traveling at a relative velocity of 8 kilometers per second (17,895 miles per hour). At closest approach, Galileo was just 1.5 seconds and 5 kilometers (3 miles) from the aim point--a bull's-eye in planetary distances. Dr. Michael Belton, Solid-State Imaging Experiment Team Leader, said, "The key word out of this encounter is precision. It was like taking a picture of a large house in San Francisco from Los Angeles." In fact, the pointing was dead-on--Gaspra was in the central image of the nine-image mosaic taken about 30 minutes before closest approach.
"The team has been extremely ambitious about going for all they can with the Gaspra encounter. It's been a shot in the arm," emphasized Project Manager Bill O'Neil. In fact, because the spacecraft is currently constrained to low data rates, the Project had planned not to have any data available until either the High-Gain Antenna (HGA) is deployed or November 1992. Because of the excellent targeting by the Flight Team, four images have already been retrieved. These four images comprise one full-color image.
For the first Gaspra images to send back, the team members selected a frame that, based on their calculations, had well over a 95 percent chance of containing Gaspra's image. These images have a resolution of about 170 meters per picture element; the best image, with about a 50 meter resolution per picture element, remains on the tape recorder awaiting playback next year. "The camera is operating absolutely to specs; it's first rate. We were overjoyed when we saw the first 12 lines of the image and realized we got the asteroid right on target," noted Dr. Belton.
The first portion of these images came back on November 5. At Galileo's distance from the Earth, downlink data rates via the low-gain antenna (LGA) were limited to 40 bits per second. At this rate, approximately 72 hours are required to transmit a full, 800-line imaging frame. At this Gaspra-spacecraft range, the asteroid was contained in just an eighth of the entire frame; thus, only 100 lines had to be transmitted to receive the entire image. Once the HGA is open, the time to transmit one full, 800-line image will be just one minute.
Another adjustment due to the low data rate was that Galileo was targeted to pass closest to Gaspra on its shadowed, rather than sun-lit, side. That trajectory offered the best chance to view craters and other surface features in crisp relief along the day-night terminator. Unfortunately, such a path precluded spectral and photometric observations over as large a range of lighting angles as would be achievable using a bright-side passage.
From ground-based observations, quite a bit of information was known about Gaspra before the encounter. Discovered in 1916 by astronomer Grigori Neujmin at Simeis Observatory in the Ukraine, Gaspra was named for a scientists' resort on the Crimean Peninsula. Gaspra, an S-type asteroid, is believed to be made up of metallic and rocky minerals, including iron, nickel, olivine, and pyroxene. This asteroid is just one of more than 5000 comprising the "main belt"--a large doughnut- shaped region midway between Mars and Jupiter. Gaspra is located about 331 million kilo-meters (206 million miles) from the Sun, near the inner edge of the main belt. Asteroids are also known as minor planets; many are much larger than Gaspra, ranging up to more than 800 kilometers (500 miles) in diameter.
Gaspra is believed to be a "collisional fragment of a larger parent body, a survivor of a series of catastrophic events," noted Joe Veverka, a member of the Imaging Team. In fact, Gaspra's current size may be only a tenth of its original size.
Investigating Gaspra will be important to scientists because asteroids may contain important clues to processes in the early solar system. Asteroids are believed to have formed during the earliest stages of the solar system and thus are primitive bodies that could shed clues about the evolution of the Sun and its planets. Asteroids could have been spawned when the other planets were formed in the early solar nebula or they could be the remnants of an incomplete planet formation between Mars and Jupiter.
Another significant reason for encountering Gaspra was to help calibrate ground-based observations. Since all previous observations of asteroids have been limited to ground-based viewing, Galileo's encounter provided a unique opportunity to verify our assumptions and analyses based on our limited information.
Project scientists have been able to ascertain many facts about Gaspra from the initial data received. Gaspra is 20 kilometers (12.4 miles) long and 12 kilo-meters (7.4 miles) wide, as well as 11 kilometers (6.8 miles) thick.
Because the asteroid is small, its gravitational force (0.0005 g) is two thousand times smaller than the Earth's gravity, yielding an escape speed of a scant 10 meters per second (22.3 miles per hour). Since the gravitational force is so low, when something impacts with Gaspra, most of the dust and material from the resulting collision fly off the asteroid. With these conditions, scientists expected to see sharply defined craters and ridges, since there should have been little, if any, surface rubble (called regolith). What is surprising about Gaspra, then, is its subdued appearance. There may be a small layer of regolith on the surface. As yet, scientists cannot tell how deep this layer may be, but initial estimates are from a few centimeters to a few meters. Scientists did note, with relief, that the Dust Detector registered no dust impacts during the encounter, indicating a very clean environment around the asteroid.
The degree of cratering seen on the asteroid's surface can tell volumes about Gaspra's origin and evolution. Project Scientist Dr. Torrence Johnson noted that the images of the surface were "better than expected. We had hoped to see the asteroid's shape and may-be a couple of craters. While we know quite a bit about this object statistically from ground analysis and observations, there is still lots of controversy about details."
What is discernible about the surface are major depressions, indentations, ridges, and craters. Some of these craters are as wide as 1.5 kilometers--indicating, for example, an impact with a body 100 meters (330 feet) in size, traveling 5 kilometers per second (3.1 miles per second). Gaspra has probably existed in its present form for the last 200 to 500 million years. Another initial observation is that the albedo is relatively homogeneous, although there are noticeable variations in albedo and color at about the 10 percent level.
The question of what materials actually comprise Gaspra is one that cannot be answered until the rest of the data are received by Project scientists. The main tool in revealing Gaspra's composition involves spectral scans by the Near-Infrared Mapping Spectro-meter (NIMS). Dr. Robert Carlson, the NIMS Principal Investigator, noted that the low-resolution data he has received so far suggest compositional differences between Gaspra's northern and southern hemispheres. The clearest evidence of what constitutes the asteroid's interior might be found if further NIMS data reveal fresh craters that expose subsurface material.
As for the future, the Project scientists are eagerly awaiting the rest of the data from the encounter, including an additional 15 images of Gaspra. Slated to be returned either when the HGA is deployed or in November 1992, the information stored on board Galileo will answer many of the questions we have about asteroids and early solar system materials. Undoubtedly, many questions will also be raised by the brief glimpse we will be getting of this one asteroid.
The information still on board Galileo includes the remaining NIMS data, which should reveal a plethora of facts about Gaspra's composition. In addition, one of the images yet to be transmitted has a resolution about 3.5 times better than the full-color image already received. With a resolution of about 50 meters per pixel, combined with lower Sun angles (which yield longer shadows) in these images, more details will become apparent. Referring to several features that are almost resolved in the images received so far, Dr. Veverka noted, "The images we've seen give us high confidence that the rest of the images will be excellent. We're at the limit of our resolution with these first images. We cannot tell if these are ridges or are grooves caused by an impact that didn't quite break up the asteroid." In the later image, these features should be resolved.
One of the potential plans the Project has to gain a better understanding of asteroids is to repeat Galileo's recent performance by flying past asteroid Ida in August 1993. The success of the team and the spacecraft at Gaspra will certainly be considered by the Project as it develops future plans. However, the final decision will be made in the summer of 1992 and will consider the spacecraft's status and propellant needs at Jupiter as well. The comparison of these two S-type asteroids would help scientists to further calibrate and verify ground-based observations of asteroids, as well as to analyze the range of characteristics that may exist in similar asteroids.
Joe Veverka summed up the Imaging Team's enthusiasm. "We are reading another page in the exciting history of this asteroid." A second chapter on asteroids, entitled Ida, may still be to come.
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