Once more, in the March 9, Wall Street Journal (NASA's Asteroid- Defense Skills Draw Plaudits', p. A3), we were treated to the premature exuberance and optimism of a major media outlet, rivalling even the 2022 NY Times codswallop that 'NASA Can Spare Us From The Dinosaur's Fate'). But in either media's eagerness, insufficient account was taken of the difference in size and mass.
To wit, the DART mission evidently deflected the path of one asteroid (Dimorphos) of a pair (Dimorphos, Didymos) which came in at a mass of 4.4 x 10 9 kg and widths of 525 ft. to 2,300 ft. The end result of 3 years' worth of analysis - published March 6 in the Journal Science Advances, i.e.
disclosed the impact by the 1,300 lb. DART 'bus' changed the orbit by 11.7 microns (about 1/10th the width of a human hair). Translating that to the change over a period of 1 year resulted in a 2/10ths of a mile difference. As the WSJ piece gushed:
"Despite doomsday movie plots about Earth-ending collisions, this key test of NASA's planetary defense capabilities successfully changed the path of a two-asteroid pair"
So there is no doubt that if this DART mission were a genuine physically realistic test of an asteroid deflection it would be of immense relevance and importance. Indeed, on the Torino scale for potential asteroid impacts, the Dimorphos-Didymos system would fall into the category of a 'mass- extinction impact' - applicable to asteroid dimensions 330'- - 3300' diameter. The resulting explosive release between 100 and 100,000 megatons.
But impressive though it is, it isn't in the same Torino class as the Chicxulub impactor which took out the dinosaurs in the KT boundary event. That object was estimated to be between six and nine miles across with a mass of between 1 x 10 15 kg and 5 x 10 16 kg. Or a mass of roughly 10 million times that of the Dimorphos-Didymos pair. (In more concrete terrestrial terms this would be about 1,000 times more than the mass of a small car to a bumble bee.)
Could a bumble bee, no matter how fast it flew, displace a small car - say a Volks? Nope, not even by a micron. Which is why the DART media cheerleaders would do well not to conflate the two cases of radically different magnitudes, including of explosive energy released. In the case of the Chicxulub impactor case we are looking at an explosive equivalent of 100 million megatons, rightly termed a "planet killer". Fortunately such objects are believed to strike only every 15 million years or so.
In 2009, I attended a conference sponsored by the Dynamical Astronomy Division of the American Astronomical Society that featured a paper entitled; ‘Analytic Theory of the YORP Effect for Near –Spherical Objects. At that time torques of the form:
dt = r x F dS
were considered, where r is the radius vector and F the force supplied. The element of asteroid surface area is dS. The research condition was confined to the special case where the impinging solar radiation was at right angles to the asteroid’s spin axis. Three separate detections of the effect were announced, including for a nearly spherical object (1998 KY), and on two more irregular objects, (1862 Apollo, and 25143 Itokawa).
In the case of the Apollo object the observed effect was approximately 3.0 x 10 -4 deg/day, vs. the theoretically –predicted YORP effect magnitude of 2.6 x 10 -4 deg/day. This constitutes a real measurable magnitude and deflection for a serious asteroid – given Apollo objects are the most likely to inflict serious devastation on Earth since their orbits intersect Earth's. (Note: The DART deflection cannot be measured in deg/day given the interaction is not of sufficient magnitude.)
More than 90% of potentially hazardous "mountain-size" asteroids have been identified, according to NASA. But only about 40% of potentially hazardous asteroids with diameters of 460 feet or more are believed to have been identified. There may be as many as 25,000 of these smaller asteroids, each of which might lay waste to an entire region - say like Siberia - if it were to strike our planet. In order for even a smaller asteroid - or pair (like Dimorphos-Didymos) to be theoretically taken out would likely require at least a year of advanced warning to get a deflector spacecraft in position to do its job. Even then, there is no certainty it would succeed - a) hitting the intruder, and b) deflecting it enough to change its path to avoid striking Earth.
In other words, one DART mission success with a lesser asteroid pair (in deflecting them), only shows a well-prepared in advance project could succeed. It doesn't prove any and every such attempt would, especially if the craft has to be designed and built from scratch and the threatening asteroid orbit still must be correctly computed.
These points ought to be considered before the media again goes 'gah gah' over a space effort like DART, concluding it "saved us" from the fate of the dinosaurs. Granted, there is a temptation to put out 'clickbait' these days, but it's best to temper headlines and content before making exaggerated claims
See Also:
- Asteroids: The Celestial 'Swords of Damocles'
- And:
- NASA’s DART Mission Changed Orbit of Asteroid Didymos Around Sun - NASA
- And:
- After Last Week We Can Truly Say The Asteroid Peril Isn't just Science Fiction
- And:
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