On Oct. 29, 1991, NASA’s Galileo spacecraft flew by the asteroid Gaspra at a distance of 990 miles, marking the first time a spacecraft had ever visited an asteroid. The probe whizzed past the rocky body at a relative speed of about 5 miles per second, or roughly 18,000 mph, serving as a mid-journey milestone during its six-year trek to Jupiter.
The historic encounter offered planetary scientists their first close-up look at an asteroid in the inner edge of the asteroid belt. Galileo spent about two months traveling through the asteroid belt before reaching Gaspra, capturing multiple photos and gathering compositional data using its Near-Infrared Mapping Spectrometer.
Galileo Encounters Gaspra on the Way to Jupiter
NASA launched the Galileo spacecraft toward Jupiter in October 1989. Before reaching Gaspra, the probe executed gravity-assist flybys of Venus and Earth in 1990. The Gaspra encounter occurred near the midpoint of the mission’s primary cruise phase.
During the flyby, Galileo approached to just under 1,000 miles from the asteroid. Because the spacecraft remained too far away for Gaspra’s weak gravity to exert a measurable pull, instruments could not determine the asteroid’s total mass.
Images returned by the probe revealed an irregular, elongated body measuring about 12 miles long and 7 miles wide, or approximately 20 by 12 by 11 kilometers. NASA’s mission overview noted that the surface featured a thin covering of dust and rubble alongside heavy cratering.
35 years ago today, we got our first ever up-close look at an asteroid! On Oct 29, 1991, NASA's Galileo spacecraft—two years into its journey to Jupiter—flew by Gaspra, an asteroid roughly 10 miles (~17 km) long, taking this series of images on its approach. pic.twitter.com/aejgBvHjS8
Gaspra Likely Survived a Series of Catastrophic Collisions
Data from the Near-Infrared Mapping Spectrometer allowed researchers to study the chemical composition and physical properties of the asteroid’s surface.
“The picture we get of Gaspra is one of something that has suffered a lot during its history,” Joseph Veverka, a Galileo imaging team member, said at the time, according to the UPI press service. “We suspect that this is the survivor of a series of catastrophic collisions in the past.”
Subsequent Flybys and Legacy of Asteroid Research
Following the Gaspra encounter, Galileo continued its complex trajectory through the inner solar system. The spacecraft completed a second Earth flyby in 1992 and swung past the asteroid Ida in 1993 before finally arriving at Jupiter in 1995. While en route, Galileo observed Comet Shoemaker-Levy 9 break apart and impact Jupiter in July 1994.
Space agencies subsequently developed missions to orbit, land on, and sample asteroids. These efforts include NASA’s NEAR mission to asteroid Eros, sample-return projects like NASA’s OSIRIS-REx and Japan’s Hayabusa probes, and kinetic impact tests such as NASA’s DART mission.
Frequently Asked Questions About the Gaspra Flyby
How close did the Galileo spacecraft get to asteroid Gaspra?
Galileo passed approximately 990 miles, or 1,593 kilometers, away from Gaspra during the Oct. 29, 1991, flyby while traveling at a relative speed of about 5 miles per second.
What were the dimensions of asteroid Gaspra determined by Galileo?
Images and data from the spacecraft revealed that Gaspra is an irregular, elongated body measuring roughly 12 miles long and 7 miles wide, or about 20 by 12 by 11 kilometers.
Why couldn’t Galileo measure Gaspra’s mass?
The spacecraft remained too far away from the asteroid for Gaspra’s gravitational field to noticeably affect the probe’s trajectory, preventing scientists from calculating the body’s mass during the encounter.
Keep reading