Hi, I tried to put 'constant' for the 'constants' answer. And 'space agency' for the space programmes.

Enjoyable grid with an overall theme too!

Incidentally, Webb, Hubble, Planck and Newton are also all telescopes! While Newton is a type of telescope, all the rest are space telescopes in their own right. Bit ambiguous as to what the answer would be; I ended up answering Telescopes and getting it wrong.

Constellations should be an accepted answer.

Fun grid, with a good theme. I had the same problem as K.

I put astrophysicists - is that wrong? I think it is almost impossible to put the correct answer in this grid