Are Impacts Into Small Bodies Strength Or Gravity Controlled? What Does Deep Impact Tell Us?

Details Are Impacts Into Small Bodies Strength Or Gravity Controlled? What Does Deep Impact Tell Us? Are Impacts Into Small Bodies Strength Or Gravity Controlled? What Does Deep Impact Tell Us?

Sep 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006dps....38.6504h&link_type=abstract

American Astronomical Society, DPS meeting #38, #65.04; Bulletin of the American Astronomical Society, Vol. 38, p.615

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Scientific paper

The response of small bodies to impacts is governed by either gravity or material strength, depending on their relative magnitudes. On Earth, cratering at small scales is strength dominated, but at large sizes is gravity dominated. That transition occurs for craters of about 100 m, and scales as the inverse of the gravity for other bodies. For the catastrophic disruption of asteroids, the estimates for such a transition vary widely: from bodies with diameters as low as 100's of meters up to bodies with diameters of several 10's of km.
Deep Impact adds a new twist to this uncertainty. It has been widely reported that it was gravity dominated, based on observations of the ejecta plume. However, I conclude that there is no reason to suggest that the crater was gravity dominated. Furthermore, most of the outcome of Deep Impact is at odds with the conventional picture of cratering, including the ejecta velocities and the time scale of the plume. And the momentum imparted to Tempel 1 was from 20 to 200 times that of the impactor! It is more likely that the impact triggered the intrinsic energy of a comet outburst event.
This strength/gravity question is especially important regarding the protection of Earth from impacts of asteroids or comets. For small bodies a breakup is a distinct possibility. That threshold could be as low as the gravitational binding energy, which, for a 100 m object, is less than 102 erg/g. Or, if it is held together with strength, that energy could be several 106 erg/gm. In fact, the 370 kg Deep Impact impactor could break up a 600 m diameter object with mass density of 2 g/cm3 if that object were only gravitationally bound!
I shall discuss these issues.
This research was sponsored by NASA Grant NAG5-11446

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