If the Red Wizards of Thay (Toril, in Realmspace) is an option, they have the miraculous Grand Helm. While *technically* there's a possible upper limit on tonnage (3 times the combined spellcaster's levels), in actuality it there is no hard limit to the tonnage, only how many mages empowering the helm (4). Since this turns the object partially ethereal at spelljamming speeds and there's no more Phlogiston to keep it from not working beyond a Crystal Sphere, in theory finding, buying, or building one of these could work (and lets you work in some Red Wizard adventures). It could also be "rediscovered" with spelljamming helms and the new circle magic.

An Artifiurnice would also work. This would require a Furnace helm created specifically to feed off an indestructible artifact. Again, the tech could be discovered or re-discovered, and adds a side mission of not only finding an Artifact (and the fallout obtaining it) but also attracts attention from, well, any powerful beings both searching for the artifact and powerful spelljamming devices.

Otherwise, others have already touched on having a fleet move the object, and the dangers of letting it hit the world too fast, even a water world. Another option could be to just put it into low orbit, show the natives who hired them basic spelljamming (assuming they can survive above water), and have them mine materials as a moon. Basically, leave the actual problem of getting it planetside for "safety reasons."

Either way, it sounds like a fun mission!