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The Emperor (may he live forever) plans to visit the Solar System on a rare royal visit in ten years' time. The Bureau for Interplanetary Tidying have decided that the Asteroid Belt is an eyesore that shouldn't sully the eyes of His Mightiness and needs to be cleaned up. The obvious way to do this is to form them into a single body.

Clearly, moving every single asteroid individually would be incredibly energy expensive. Is there a way to start a domino type reaction so that the asteroids assemble themselves over a period of 10 years or so?

Here is what I have in mind when I talk about a domino reaction. https://youtu.be/5JCm5FY-dEY?t=32

Assumption: There's enough mass in the target asteroid belt to make a planet the desired size.

As amazingly powerful as gravity is, it's quite a bit weaker than kinetic energy unless there's a whomping lot of mass nearby. Consider, for example, the impact of a baseball on a bat. Eventually it falls back to Earth — but for a moment, it's free.

Now let's go to space where we're trying to bring asteroids together and, especially at the beginning, you don't have enough mass to guarantee that the chunks of asteroid that fly off when two come together return to the central mass. What are your options?

You don't have any. This process would be slow.

• You can use an net the size of your final planet to sweep around the orbit and collect stuff. The amount of energy needed to do this is so great that it begs the question "why are we doing this, again?"




• You can plop thruster packs on asteroids to speed them up a little, thereby causing them to come together (hopefully gently) with the stuff in front of them. Somewhere around a third the way through^{A complete guess} you could probably dispense with the thruster packs and wait. The final planet would need to swing around the orbit for a very long time (veeeerrrrrryyyyyy looooooonnnngggggg ttiiiiiiimmmmeeeeee) to sweep up the debris.




• You can get them moving faster, but this means debris winging off into space. You'll build the basic planet faster, but it'll take longer to sweep up the debris to finish the job (meteors are not your friend, you must clean up).

But, even if you did these, how long before you have a planet? Pressure would build, heat would form... eventually you'd get the right kind of core, etc. You'd likely need to drop a bunch of methane/water asteroids on the thing to kickstart an atmosphere....

Conclusion

Ten years? The uneducated reader would probably never think twice about it, but it's totally unbelievable. I can't see how you could do it in less than centuries, maybe millennia. You're dealing with so much space, so much mass, so much energy... there's reasons why spacey things happen slowly. When they happen fast, they tend to be catastrophic.

Edit: I didn't think to ask Chasly what's to become of this planet(oid)? Derived from our own asteroid belt, it won't be big enough to develop heat or hold an atmosphere and might not be enough mas to stick together as a cohesive ball. In other words, the dirt clods may simply hang together as dirt clods in space until gravitic distortions from the orbits of other planets start spreading them out again. In astronomical lengths of time, they wouldn't stay together very long.

With sufficient effort, this could be done. It would require the manual alteration of the orbit of each asteroid.

Bear in mind that the total mass of the asteroid belt is around 4% that of the moon, and that around half of that mass is already contributed by only 4 asteroids. The remainder are tiny and barely significant.

It would be an awful lot of work for basically no gain.

Short answer, no.

The asteroid belt is incredibly sparse, where your chance of seeing another asteroid from the surface of one with the naked eye is pretty low.

There's no simple way to overcome the momentum of every asteroid. They would all require manual adjustments to their orbits, many of them requiring multiple burns to reposition them and collide them with Ceres (the largest asteroid).

Probably not all in one go, but along the way why not have some fun, save your job and give the Emperor something to smile about.

Say you have a sixteen space tugs to work for you - first thing first, order 12 of them to pop out to the Kuiper belt and drag in a dozen matching 100 Km diameter KBO's (essentially comets), whilst this is happening, get the other 4 to grab Ceres, Vesta, Pallas, and Hygiea from the belt , and in the most efficient way place them equidistant around the inner edge of the belt.

Meanwhile the other tugs are draging their loads into equidistant positions between the aforementioned roids.

The tugs can then proceed to push all these objects at optimal speed around the belt to gather mass as they go, gradually increasing the diameter of orbit as they clear debris.

"But, what if the Emperor comes before the job's finished?" I hear you cry in alarm.
Worry not, approaching from the Pole of the solar system (on the Sun's axis of rotation) his magnifience will see: - centre of his view, dark save for the sun shining like the twinkel of humour in his eye, then the inner planets, the sharp inner edge of the belt carved and delineated by the 4 planetisimals and the comets then like a great iris with their tales radiating outward.

The wise and benevolent one will recognise (with judicious prompting) the magnificent sight of a likeness of his own eye gazing benignley upon the universe, and revel in his subjects rejoicing and adulation.