42 according to Wikipidia

42 (forty-two) is the natural numberimmediately following 41 and directly preceding 43. The number has received considerable attention in popular culture as a result of its central appearance in The Hitchhiker’s Guide to the Galaxy as the “Answer to The Ultimate Question of Life, the Universe, and Everything“.

NASA’s Opportunity Mars Rover completes 42 km marathon milestone in 11 years

Washington: NASA’s Opportunity Mars Rover has recently completed the first 42 kilometers marathon milestone on Mars in 11 years and two months, after its January 2004 landing in Eagle Crater.

The vehicle surpassed marathon distance of 26.219 miles (42.195 kilometers) with a drive completed on March 24, 2015, during the 3,968th Martian day, or sol, of Opportunity’s work on Mars.

John Callas, Opportunity project manager at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, said that this was the first time any human enterprise has exceeded the distance of a marathon on the surface of another world and it happens only once.