burtzev OP t1_itm4x18 wrote
Reply to comment by MankindsError in Deflecting asteroids is not enough — we need to know when they approach by burtzev
I'd agree that size is a great factor. There's , however, another thing about this that lurks in the background. Simply 'moving' an object isn't the same thing as moving the object to a precise and planned place. The mission did change the orbit of a smaller asteroid around a larger one. I suspect such a system was chosen precisely because the degree and even direction of movement was unknown and unpredictable. Nobody would want to 'accidentally succeed' by pushing a harmless target onto a dangerous (and once more unpredictable) trajectory.
I've read about the numbers associated with the success ie it changed the orbital period of the target around its babysitter. But I've seen no evidence that the precise change was predictable. After showing that it is possible to hit a small target very far away and move it there's still the steep hill of moving it where you want to left to climb.
MankindsError t1_itm5v1i wrote
I guess that's where distance and velocity come in? So if you only move it a small amount, over a great distance it would be enough to make a hit a miss? Say we have 18 months a millions upon millions of miles before impact. We push it successfully a short distance which makes its trajectory a miss for earth. We could then plan out where it will be in the future and in necessary do it again? I don't know much about the subject or the math that goes into something like this. It's just really interesting
bestest_name_ever t1_itn8b87 wrote
Yes, this is pretty much spot on. Accuracy isn't actually important, because in a theoretical mission where we're saving earth, well, it was already going to hit. We can't accidentally make it double hit, so the only thing that matters is changing the orbit enough so it no longer hits. And because asteroids are inert, it doesn't even matter if the new course isn't perfectly safe and would still hit earth next time it comes around, because when we know of an asteroid we can track it, so that case would buy sufficient time to do another deflection mission.
Adeldor t1_itm7t8u wrote
> 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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