Level select:
     Press  Up, Left, Down, Right, Up, Right, Up, Down, Left, Up, L1 +
R1, L2 + R2 at the level selection screen.

View credits:
     Press  Left, Right, Up, Down, Left, Right, Up, Down, Left, Right,
Up,  Down, Left, Right, Up, Down, Left, Right, Up, Down at the mission
briefing screen.

Secret missions:

     1A  -  Finish  mission  1 in under five minutes, two seconds with
more than 40 percent accuracy.

     4A  - Finish mission 4 in under six minutes, 44 seconds with more
than 40 percent accuracy.

     5A - Destroy half of the energy plough and pull the other half in
with the tractor beam.

     5B  -  Look  for an info unit between the two asteroids somewhere
near the broken shuttle and pull it in with the tractor beam.