[Rosie] NO, not a celestial object
[Raak] NO, see above
[Superman] YES, it's down under
[Tuj] YES, there can be only one.

Yup, that was easy enough. Ayers Rock did finish a little-known Australian SF story called The Mountain Movers by being revealed as a stone space ship. Nevertheless Rosie gets there first and will take this boomerang-shaped baton for the next round. Well done!

Talking of boomerangs, I made one in 1974 from instructions in the New Scientist, which explained the aerodynamics. I've still got it; it's probably under a pile of junk in the garage somewhere. It really works but you need an area the size of a football pitch and preferably no wind. Boomerangs are chiral, i.e. -handed and mine, to suit, is a southpaw. Having said all that, it's now deduction time.

The next object, which has but the most tenuous link with steam engines, is:

VEGETABLE and MINERAL

Is it a potato iron?

(KagomeSuko) Ace silliness, but wide of the mark by some distance.
(Raak) Well-known dehydrating agent? NO.
(Boolbar) Lemon battery? NO. Nor a Van de Grapefruit machine.
(Superman) NOT a fossilised tree.
(Softers) The flesh door? NO, not even when steam-hauled, alas.