This week will bring us the Galileo at 30 meeting, a retrospective of the Galileo mission to Jupiter and the Jovian system:
https://www.kiss.caltech.edu/special_events/2025_Galileo_at_30.html
Monday marks 30 years since the orbit insertion maneuver. Let’s not forget, aside from the Jovian system, that Galileo observed asteroids up close for the first time. If anything, this would be more groundbreaking than Jupiter studies, since Galileo had been preceded there by two Pioneers and two Voyagers. The later mission flew by Gaspra (Oct. 29, 1991) and Ida (Aug. 1993).
It’s hard nowadays to appreciate the Galileo mission and its advances, on multiple fronts. Before Galileo, the closest we had seen to a “small body” was Phobos, and to a lesser extent Deimos, from Viking orbiters. (Of course, one theory of their formation is asteroid capture, but that’s as uncertain now as it was back then.) A reasonable person might suspect that asteroids would look like Phobos/Deimos, but who knew- nature has a way of surprising us when we least expect.
Before Galileo, speculation on the morphologies of asteroids included no regolith (not enough gravity), dust particles in rings instead of regolith, asteroids as contact binaries and trinaries, etc. It was the Wild West in space pedagogy. In terms of composition, we weren’t sure of the metal contents- some asteroids might be regolith-free since they were chunks of metal, and resistant to fragmentation by meteors.
Data trickled back from Galileo, since the main antenna was stuck. In fact, the flyby data was stored on the onboard tape drive, then replayed when Galileo approached Earth again in its looping trajectory. We would have to wait still more.
After Gaspra, we had one example that was, broadly, Phobos- and Deimos-like, though Gaspra appeared to be somewhat dense and likely a bit metal-rich, unlike the lighter Martian satellites. Gaspra was a coherent body, not a rubble-pile asteroid, since it held up ridges and peaks against meteor impacts.
After Ida (and another downlink delay, until Galileo neared Earth again and allowed faster radio links), we were surprised in a different way. Ida showed a satellite, Dactyl- a binary system. The ability to measure- even roughly- a satellite’s orbit gave the mass of Ida, and it was a fairly low mass. Ida’s makeup resembled ordinary-chondrite meteorites, with only secondary metal content. But the real discovery was space weathering- Ida did not look like the ordinary chondrites because old surfaces were aged by space exposure- solar wind, micrometeorite bombardment, and lesser radiation. Only the floors of fresh craters looked like ordinary chondrite meteorites.
The legacy of Galileo is all the more important and vital since CRAF got canceled. CRAF (Comet Rendezvous-Asteroid Flyby) would have been Cassini’s brother… and Rosetta’s preemption. Instead, Cassini (though flying by (2685) Masursky at long distance) was delayed and alone, while Rosetta was over a decade behind any CRAF notional launch.
Rest in peace, Galileo- applicable to the man, and the craft. We salute your accomplishments.