NASA looking at 'hitchhiking' across solar system on speeding comets

If my harpoon allows me to accelerate/decelerate at 10g for 1 minute, I'd gain 5.9km/s. This requires a tether that's 176km long. That would be a pretty long tether; that can take a lot of weight.

But lunar orbit insertion requires a delta-v of only 2km/s. Maybe one day, on the far side of the moon there will be a great magnetic hook that space ships will shoot their harpoon at in order to quickly insert into moon's orbit.

Seems quite odd to me. Last (and first) time something landed on a comet it didn't go as planned. Harpoon's fault. Maybe a fishcakes grab isn't the best solution, maybe done magnetic one?