Across:
   4. Vital message finally goes to press (3)
   8. Place where an emperor was brought down to a lower arrangement? (8)
   9. see 15
  10. Somewhere great travellers are launched, and not one compensated! (3)
  11. Early examples: polar wandering (into disaster)? (11)
  13. We're told to remember, the fifth and third of each minor chord have the same vibrato when strumming (8)
  15,9. That place, strange land... pet project of humanity is to get to it! (3 3 6)
  16. Mrs. America initially included as world fourth hottest (4)
  17. Smart roof features cleverly save on energy consumption (5)
  19. Shifting top... and 5 ended up in this type of trap (of course?) (4)
  21. Lament being damaged in the head (6)
  24. Reformed 6s perhaps inure son to change (8)
  25. Opening with muddled input (or typo) for 16 11 14 - B (11)
  27. Some dirigible craft (3)
  29. Bob's showy vehicles (6)
  30. Heartless reactions ruined situation (8)
  31. Drunk ate, as usually drunk (3)
Down:
   1. Unfortunate 21 state of a pair on a breakdown (8)
   2. Very dark knave, and French! (3)
   3. Broken bell fixed, mostly, but 20 (8)
   4. Earth's ultimately precious natural resource? (4)
   5. Life gone, it's R.I.P. for 16 11 14 - A (6)
   6. Using ridiculous spin, Earth P.R. creates alliance (11)
   7. Mileage to me, running back, essentially far! (6)
  12. Salt is one Na compound which is amazing (11)
  14. Deluxe vehicle to have discoveries in essentially? (5)
  18. Spacecrafts closed, lest damaged (8)
  20. No riding around for blimp (8)
  22. Getting cross during broken sleep as 2 does 28 (6)
  23. Most recent trial on West Coast? (6)
  26. Asian revolutionary, one escaping American agency (4)
  28. Laugh with N₂O perhaps (3)