In Simple Terms
In 1991, a spacecraft called Galileo rushed past an asteroid named Gaspra and became the first spacecraft to visit one. It took close-up pictures of the lumpy, crater-covered rock and gathered clues about what it is made of. Galileo could not measure the asteroid’s mass, but the brief encounter showed that asteroids could be explored up close—not just seen as distant points of light.
A Brief Encounter in the Asteroid Belt
Galileo flew past Gaspra on October 29, 1991, while on its way to Jupiter, the largest planet in the solar system. Gaspra orbits near the inner edge of the asteroid belt, the region between the orbits of the inner planets and Jupiter. Galileo had crossed into the belt about two months before reaching the asteroid, and its position and trajectory created a rare opportunity to examine one of the belt’s rocky inhabitants at close range.
The spacecraft came within about 990 miles, or 1,593 kilometers, of Gaspra. Another account puts the distance at just under 1,000 miles, or about 1,601 kilometers. These are very similar estimates of the flyby, not fundamentally different descriptions of the encounter. Galileo was traveling at about 5 miles per second, or 8 kilometers per second, relative to the asteroid; its speed is also given as nearly 18,000 miles per hour.
This was no leisurely approach that allowed the spacecraft to linger near its target. Galileo was moving through the area at great speed, so it had only a limited window in which to point its instruments at Gaspra and complete its imaging and observations. A flyby like this differs from landing on an object or entering orbit around it: the spacecraft sweeps past, gathers data as it crosses the object’s vicinity, and then continues toward its main destination.
Images of an Unfamiliar World
The images showed that Gaspra was not a regular sphere but an elongated, uneven object. Its dimensions have been described as about 12 miles long and 7 miles wide. A more detailed estimate gives measurements of about 20 kilometers by 12 kilometers by 11 kilometers, or roughly 12.4 by 7.4 by 6.8 miles. Together, these figures help convey its overall shape: a rocky body with axes of different lengths, rather than a smooth, round world.
The images also revealed a surface pocked with craters. Each crater is a mark left by an object striking the asteroid, so their distribution offers scientists clues to Gaspra’s violent past. NASA described the asteroid as complex and irregular in shape, with a thin layer of dust and rocky debris on its surface. The pictures were more than attractive snapshots: they offered the first close-up view of an asteroid’s surface features and gave scientists a chance to compare what they could see directly with what had previously been inferred from distant observations.
Joseph Veverka, a member of Galileo’s imaging team, said Gaspra’s appearance suggested that it had undergone considerable upheaval over the course of its history. He proposed that the asteroid might be a surviving fragment left behind by earlier catastrophic collisions. That idea fits with the evidence of impacts shown by the craters, but it remains an interpretation of the scene, not a complete history that can be recovered from a single set of images. The asteroid preserves traces of past events; scientists must use those traces to infer what may have shaped it.
What Galileo Could Measure—and What It Couldn’t
Galileo did more than take pictures in visible light. It also used a spectrometer to map near-infrared radiation—a form of light invisible to the unaided eye. The instrument can reveal information because different materials respond to this light in different ways. Its data allowed scientists to investigate Gaspra’s chemical composition and some of its physical properties. In this way, the spacecraft added clues about the material making up the asteroid to what the images revealed about its surface.
But gathering information did not mean that every property of the asteroid could be determined. Galileo could not measure Gaspra’s mass because it passed too far away for the asteroid’s gravity to produce a measurable change in the spacecraft’s path. Mass is commonly inferred from the strength of an object’s gravitational effect on something nearby. When that effect is too small to show up clearly in a spacecraft’s motion, the method cannot provide a measurement with the required precision.
This limitation illustrates an important point about space exploration: a successful mission does not answer every question in a single pass. Images can describe an object’s shape and craters, and instruments can analyze signals linked to its materials, while measuring its mass requires suitable observations and a favorable close approach. Galileo’s flyby made Gaspra the first asteroid visited by a spacecraft, but it did not make the asteroid fully understood. It was the beginning of close-up data collection, not the end of the mystery.
A Stop on the Longer Journey to Jupiter
Galileo launched in October 1989 on a journey that took six years to reach Jupiter. Along the way, it flew past Venus and then Earth in 1990 before encountering Gaspra the following year. The asteroid flyby came roughly halfway through the journey, as a scientific side trip within a mission whose main goal was to study Jupiter and its system.
After the Gaspra encounter, Galileo flew past Earth for a second time in 1992 and then approached the asteroid Ida in 1993. In July 1994, it happened to be in a good position to observe Comet Shoemaker-Levy 9 as it broke apart and then struck Jupiter. Galileo finally reached Jupiter in 1995. This sequence shows that the spacecraft did not travel in a straight line from Earth to Jupiter. Its route included encounters with and observations of several different objects, and Gaspra was the first asteroid it met along the way.
Because Gaspra came before Ida, the encounter mattered beyond the data collected about that one asteroid. It demonstrated that a spacecraft headed for a distant planet could make a scientific flyby of an asteroid along the way, and that imaging an object from about 1,000 miles away could reveal details unavailable from remote observations. The target was small and the time was short, but the encounter served as a practical test of using interplanetary routes to study other objects.
From First Images to Hands-On Asteroid Exploration
Asteroid exploration changed after Galileo. Later spacecraft reached the surfaces of these bodies; one example is the NEAR spacecraft, which landed on Eros. Other missions collected samples from asteroids and returned them to Earth, including NASA’s OSIRIS-REx and Japan’s Hayabusa spacecraft. In a different kind of mission, DART deliberately crashed into an asteroid to test techniques that could help protect Earth from the threat of a possible impact.
These later achievements do not diminish the importance of Galileo’s flyby. Instead, they show how much the possibilities have expanded. Exploration progressed from photographing an object during a quick pass to landing, bringing back samples, and testing the effects of a spacecraft impact. Gaspra was the first asteroid visited by a spacecraft, and the encounter offered an early demonstration of what a close flyby could—and could not—reveal.
The Bottom Line
On October 29, 1991, Galileo made history by flying past Gaspra, the first asteroid ever visited by a spacecraft. It came within an estimated 990 miles, or 1,593 kilometers; another account describes the distance as just under 1,000 miles, or about 1,601 kilometers. The spacecraft was moving at roughly 5 miles per second, or 8 kilometers per second, and close to 18,000 miles per hour. Its images showed an elongated, irregular asteroid covered in craters. The reported dimensions vary by estimate, while a near-infrared spectrometer examined its composition and some of its properties. Galileo could not measure Gaspra’s mass because the asteroid’s gravity did not affect the spacecraft’s path strongly enough. Galileo’s journey began with its launch in October 1989 and ended with its arrival at Jupiter in 1995, but its brief encounter with Gaspra left a lasting mark: it opened the way to close-up asteroid exploration and helped usher in an era of imaging, landing, sample return, and testing planetary-defense techniques.