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From Earth, the asteroids are unresolved (that is, much less than one pixel, even in the largest telescopes). The only reasonable way to measure their sizes and shapes is with stellar occultations. In the occultations, the asteroids pass in front of a distant star, blocking out their light. By measuring the exact time that the star disappears and re-appears, you can measure the exact size and shape of the asteroid.
I was involved with two occultations for the New Horizons mission, to Colombia and South Africa. This particular occultation was very similar. The star was a bit fainter, requiring a larger telescope (16-inch vs. 14-inch). The occultation was about 4.4 seconds long. Lucy is a mission run by SwRI; I'm not involved with it but was happy to be invited to observe on this trip.
See Jason's video showing our setup and a brief tour of the sky with our hosts.