It's a two-player game. The players were represented as an orangle
triangle and a blue triangle, in a field of asteroids / planets. They
took it in turns to fire a missile (direction and speed controlled
with mouse) which then followed an N-body simulation until it hit a
planet, a player, or escape velocity (possible because energy wasn't
conserved). Obviously the aim was to hit the other player.
Thanks for reading.
A friend pointed me at http://en.wikipedia.org/wiki/Gravity_Wars. Was
there an Archimedes version of this, or is it another member of the
same subgenre?
The game was called Stellar on the Archimedes.