Seconds after impact. Click for movie, taken by amateur
astronomer Bruno Payet from the Réunion Island.
Scientists today published five papers outlining their results from the impact of Dimorphos by DART, summed up as follows:
- Dimorphos’s density is about half that of Earth’s, illustrating its rubble pile nature.
- The orbital period around the larger asteroid Didymos was changed by 33 minutes.
- The ejection of material from Dimorphos during the impact had a greater effect on the asteroid’s momentum than the impact itself
- The mass ejected was only 0.3 to 0.5% of Dimorphos’s mass, showing that the asteroid was not destroyed by the impact.
- The impact turned Dimorphos into an active asteroid, with a tail like a comet.
The data not only tells us a great deal about this asteroid binary itself, it suggests that this impact method might be of use in defending the Earth from an asteroid impact. There are caveats however. First, the orbital change was not to the system’s solar orbit, the path that would matter should an asteroid threaten the Earth, but to Dimorphos’s orbit around its companion asteroid. We don’t yet know the effect on the solar orbit. Second, the impact did not destroy this small rubble pile asteroid, which means such an asteroid might still be a threat to the Earth even after impact. Third, in order for an impact to be the right choice for planetary defense, detailed information about the target asteroid has to be obtained. Without it such an impact mission might be a complete waste of time.
The irony to all this is that we knew all this before the mission. DART in the context of planetary defense taught us nothing, so NASA’s claim that this mission was to learn more about planetary defense was always utter bunkum. The mission’s real purpose was the study of asteroids, but selling it that way was hard. The sizzle of planetary defense however was a better lobbying technique, and it worked, even if it was dishonest.
That the press was also fooled by it, and continues to be fooled by it, is a subject for a different essay.
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