The DART spacecraft will ultimately be a kinetic energy impactor, using its 500 kg mass at a relative velocity of over 6000 m/s to slightly change its target 65803 Didymos's companion Dimorphos's velocity. The degree of success will be the momentum transfer fraction (actual vs theoretical maximum) as observed from Earth by the slight change in the orbital period of the pair.

Dimorphos (aka "Didymos B", "Didymoon") has a diameter of about 170 meters as measured by delay-doppler radar by Arecibo (R.I.P.) and careful photometric light curve analysis and modeling of the pair.

Trying to implement a near-direct hit of a 170 meter target for maximum momentum transfer as it wobbles circling every 12 hours around another asteroid while approaching at an angle with a relative velocity of >6000 m/s (>21600 kph) is certainly quite a challenge!

Many asteroid missions have established near-osculating intercept orbits, then fallen in to gravitationally bound orbits about the asteroid, then successively lowered those orbits.

This is the opposite of that, you come up on a tiny rock at heliocentric velocity, so fast that it's as wide as the Moon (0.5 degrees) only 3 seconds before impact. This is roughly ABM or ASAT-like performance!

But it's in deep space, millions of kilometers from Earth and by a ion-engine propelled spacecraft.

Question: Will DART's intercept and impact trajectory represent the highest level of deep space marksmanship ever, if successful?

If so, is there some way to express just how much higher of an intercept challenge this poses than anything before?

Here's an attempt to find some previous examples of spaceflight marksmanship or ways of calculating it. Since the target is only 85 meter in radius and the goal would be to hit somewhere near the middle the target B or impact parameter here is probably 10 or 20 meters!

Fig. 21. Definition of target or arrival B-plane coordinates click for larger

Fig. 21. Definition of target or arrival B-plane coordinates

Source Interplanetary mission design handbook. Volume 1, part 2: Earth to Mars ballistic mission opportunities, 1990-2005 page 20, found in @MarkAdler's answer


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The DART spacecraft will, in my opinion, be the most precise mission that NASA has ever launched so far. Even anti-missile and anti-satellite rockets would not even come close to the precision that DART will have to accomplish.

Every single asteroid mission so far has used various techniques so that it could pseudo-achieve a high level of precision. Because DART is approaching an asteroid system at a ludicrous speed, the previous techniques simply cannot work. To achieve this level of precision, DART uses a gridded ion thruster which has a tiny amount of thrust (236 mN of force). DART will use this to "fine-tune" its approach and achieve a small ellipse.

The only craft that is comparable to this is the ISEE-3 satellite. It used tiny engines to fine-tune its trajectory in order to achieve the most complicated trajectory ever. This site also has an image to illustrate the trajectory that was completed.

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    $\begingroup$ What does "pseudo-achieve a high level of precision" mean? $\endgroup$ Commented Nov 25, 2021 at 12:36
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    $\begingroup$ You should say something about the numerical precision actually needed and predicted, or you haven't answered the "by how much" part of the question. $\endgroup$
    – Ryan C
    Commented Nov 25, 2021 at 12:56
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    $\begingroup$ IMO, it would be too simplistic to explain the technical challenge of DART mission by the precision of the ion thruster technology alone. The Optical Navigation and the autonomous algorithm ("SmartNav") are equally important, if not more. $\endgroup$
    – Ng Ph
    Commented Nov 25, 2021 at 17:00
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    $\begingroup$ Are you sure it will not use its hydrazine thrusters for trajectory correction? $\endgroup$
    – uhoh
    Commented Nov 25, 2021 at 17:57
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    $\begingroup$ Thank you to all the people who replied to my post, I'll try my best to answer those questions. Brendan, by "pseudo-achieve" I mean that since the ion engine has a very low TWR, DART can change its course by a very small amount by burning for a couple of seconds. Ryan, I did some rough calculations and DART's orbit's eccentricity cannot vary by -2.23377005347*10^-12. Ng, I will definitely have to agree with you there as well. uh oh, This sounds quite crazy but the hydrazine thrusters have a greater thrust than the ion engine on DART. 4.44822 N to 236 mN of thrust respectively. $\endgroup$
    – Endqrmqn
    Commented Nov 25, 2021 at 18:32

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