Is there some reason to think that the only way to get higher energy collisions is by scaling the accelerators? Is there any possibility that we may be able to get to higher energies with earth-scale experiments?

Now that you mention this, would an accelerator in space need walls? Could you make it from a series of big circles (maybe solenoids?) at distances of a million kilometers apart or so? Neptune's orbit is about 15 billion km long, so, you'd need about 15k of such loops. In free fall they would just go around the Sun, so you'd only need to maintain their perfect alignment with some minute adjustments.

Um... doesn't it produce synchrotron radiation while operating? I'm not sure that you want to be driving on that...

Hollywood has vastly overplayed the actual density of asteroid fields. It’s kind of disappointing.

If you build something in a vacuum then you don’t necessarily need a single continuous piece to create a vacuum. If you specced an accelerator a million miles in diameter, the diameter matters from a standpoint of whether we can accelerate the particle sideways fast enough to keep it in the track, but they are also saying they need 3.1 million miles of accelerator hardware. They are implying they can’t lay it out as a crazy straw, but is that because of the lateral acceleration, the interaction between the coils, or the limits of manufacturing?

What if you built a collider in a spherical arrangement, accelerating in three dimensions at once, but 1/3 the diameter? What if the accelerator were broken into sections to dodge asteroids, with a cumulative segment length that added up to the desired total? What if you laid it out like a Spirograph? What if you laid it out like a truncated Spirograph (just bits of the outer circumference)? What if you laid it out like a 3 dimensional truncated Spirograph?