German astronomer Gustav Witt discovered the asteroid Eros (image at top of post) on Aug. 13, 1898. Eros was both the first asteroid found to orbit entirely outside of the Main Belt of asteroids between Mars and Jupiter and the first known planet-crosser; its path crosses the orbit of Mars. Eros orbits the Sun in a little more than 643 days. Eros and Earth pass nearest each other – about 14 million miles apart – every 81 years.

In March 1966, Eugene Smith, an engineer with Northrop Space Laboratories in Hawthorne, California, presented a paper outlining a piloted Eros flyby mission at the Third Space Congress in Cocoa, Florida. In it, he wrote that Eros exploration might help scientists to understand Main Belt asteroids and small planetary moons (for example, the martian satellites Deimos and Phobos). He noted that Eros – which he described as “brick-shaped” – would pass within 14 million miles of Earth on Jan. 23, 1975, its closest approach of the 20th century.

At the time Smith presented his paper, NASA and its contractors devoted considerable effort to studies of piloted free-return Mars and Venus flyby missions based on Apollo and Apollo Applications Program technology. The first of these was expected to depart Earth for Mars in late 1975. Among other expected benefits, a Mars flyby would provide interplanetary flight experience ahead of 1980s piloted Mars landings. Smith noted, however, that a Mars flyby mission would likely be so heavy that placing all of its components and propellants into space would need either a Saturn V rocket with a nuclear-thermal upper stage or multiple all-chemical Saturn Vs followed by assembly through multiple dockings in Earth orbit. He called instead for a 1975 piloted Eros flyby that would provide experience applicable to Mars landings, yet could depart Earth on a single uprated Saturn V rocket.