Earth is 1au from the sun. Mars is 1.666au from the Sun at Aphelion. That makes it .666 from Earth at its Aphelion.
“Aphelion” is the term for the point in the orbit of a planet, asteroid or comet at which it is furthest from the Sun. Mars is at a distance of 249,232,432 kilometers or 1.666 AU when it is furthest from the Sun. (An AU is an astronomical unit. In simple terms, 1 AU is the average distance between the Sun and Earth.)
By NASA in the middle of the page
The first photograph of Mars taken from its surface was taken by Viking 1 on July 20 1976. Exactly 7 years after the Moon landing July 20 1969.
Subject: Mars is .666 astronomical unit from Earth at Aphelion 
Fri Jun 22, 2018 2:08 pm
