Adeldor t1_itm7t8u wrote
Reply to comment by burtzev in Deflecting asteroids is not enough — we need to know when they approach by burtzev
> I suspect such a system was chosen precisely because the degree and even direction of movement was unknown and unpredictable.
This target was chosen because the orbital parameters of Dimorphos about Didymos were precisely known, and its short orbital period of hours made it relatively quick and easy to determine the magnitude of change.
ETA: Also, Dimorphos' relatively small size made the changes larger, thus easier to measure.
burtzev OP t1_itmirq1 wrote
I'm sure that there has to be more than one factor in choosing an appropriate target when there are thousands of possible choices. The size and orbital period of Dimorphos certainly come into play - once you have first decided to look at binary systems as the safest bet. Here's what Science Magazine had to say about the mission. I quote the relevant paragraph:
>NASA chose to conduct the test on a binary asteroid system for two reasons. First, even though the pair was not on a course to hit our planet, the 780-meter-wide Didymos served as a gravitational anchor during impact, ensuring that Dimorphos wasn’t inadvertently ricocheted toward Earth. And second, having a pair of space rocks locked in orbit made it easier for scientists to measure the asteroid’s deflection relative to its partner.
Ease of measurement is reason number 2. Safety is reason number 1. Like the old slogan says, "Safety First". Space missions aren't always planned so responsibly.
Adeldor t1_itmwtun wrote
Curious. That doesn't gel with what I've read. For example, in this Wikipedia entry:
> "The Didymos system is not an Earth-crossing asteroid, and there is no possibility that the deflection experiment could create an impact hazard."
Further, in this preliminary presentation (PDF) by the planners of the DART mission, and in their subsequent paper (PDF), there's no mention of the system's binary nature being selected specifically for safety. The ease of measurement afforded by it being binary and its well understood parameters are the reasons given.
It's a shame Science Magazine didn't include a reference for the claim:
> "Didymos served as a gravitational anchor during impact, ensuring that Dimorphos wasn’t inadvertently ricocheted toward Earth."
To avoid future misunderstanding, I'd love to see a more direct reference for that if you know of one!
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