Jed Margolin (ex-Atari) has the definitive history of the Atari vector games and how the hardware works:

http://www.jmargolin.com/xy/xymon.htm

http://www.jmargolin.com/vgens/vgens.htm

Asteroids was a humble 6502 running at 1.5 MHz with a PROM-driven pseudo-CPU acting as the vector generator.