One possible explanation for NASA not having used Mars' gravity to propel a probe to the outer planets is that Mars might not have been in the right alignment to do so. You see, some times Mars is on the opposite side of the Sun from Earth. For example, lets suppose that NASA wants to send a probe to Uranis while Earth and Uranis are on one side of the sun and Mars is on the other. Mars cannot be used for gravitational assist because it is on the opposite side of the Sun, it's in the opposite direction from Uranis in spite of the fact that its orbit is closer to the Sun than Jupiter's or Saturn's. Mars is simply not "on the way" to Uranis. For that mater, Jupiter and Saturn might not be on the way either! That's why NASA's launch windows are so critical... so that they CAN use gravity as means for propulsion.[/i]