FAQ : The trajectory of DART was frontal collision not side on of with the orbit - wondering why this was chosen? Likely discussed elsewhere(?) Would side on glance risk controversy ( as evidenced by many panic questions even here) ? I would have thought DART pushing impact in direction of orbit would be easier? Maybe the doom panic crowd had a say? Thanks everyone.
No doom panic crowd influence, just good experimental design. The experiment being done was really to look at momentum transfer during collision and how that translates into shifting a big chunk of rock that might really just be an aggregated pile of rubble. In this case, a more or less head-on collision that could slow the orbital period of that tiny asteroid moon by a small but measurable amount (modeled at a 10 minute shift in a 11 hour orbital period, easily measurable) is the best way to gather the data of interest.
But, there's more than that when we consider the practical application. There was a time when I was particularly interested in this asteroid deflection stuff, and at that time (years ago) I read up and worked through a bit of the math. Some key learnings I came away with in terms of actually using robotic impactors to prevent an asteroid colliding with Earth:
1. Time is our friend. The longer advance notice we have, the earlier we can tweak the orbit of the asteroid, and the less of a tweak is needed to miss Earth.
2. Think of the Earth as a 7900 mile wide target, moving in its orbit around the sun at about 18.5 miles per second. This boils down to the Earth moving one Earth diameter every 7.1 minutes. So, for any given trajectory of asteroid heading toward earth (more or less within the ecliptic plane) the Earth will only be "there" for 7.1 minutes. If course, this assumes math of where the asteroid is falling directly in. More practical trajectories have it crossing at an angle, that either prolong or shorten the time where Earth is in the crosshairs, depending on the relative directions involved. But, on average, 7.1 minutes.
3. You can either deflect the asteroid (up, down, or to either side) to miss where Earth will be at that time, or you can slow down or speed up the asteroid so that it reaches earth's orbit a bit too early or too late to hit us.
4. If the asteroid was on "final approach" so to speak, the only practical way to reach it is pretty much head-on on a (gravitationally curved) line between earth and the asteroid. It wouldn't be energetically favorable to loop behind it and hit it from the back, or to the side or top or bottom. Any of those maneuvers would waste precious delta-V.
5. If the asteroid is years out, even decades out, we have a lot more options for various trajectories that could impact from different angles to tweak it in different directions, but generally you get the most bang for the buck with something that is essentially head on, slowing the asteroid. If the original trajectory is dead-on-center, you only need to tweak it by 3.55 minutes which is a very small change over a timespan of years.
This is all very neat stuff.
that the positives of this mission far outweigh the negatives.
