Page "Mariner 4" Paragraph 18

During the early part of the mission, roll error signal transients occurred frequently, making the spacecraft lose its lock on Canopus.

The lock was lost six times in less than three weeks, each time requiring radio commands to reacquire the lock.

Analysis concluded that it was probably due to small dust particles that scattered sunlight at the same brightness as Canopus.

If the brightness of the object exceeded the maximum limit of eight times the brightness of Canopus, the spacecraft would search for a new star.

A radio command was sent on December 17, 1964 to remove this limit ; the limit was initially implemented to prevent a lock on Earth, but no longer necessary.

No further loss of the Canopus lock was experienced, although roll transients occurred 38 more times before the encounter with Mars.