Outside the Moon are found tiny ships left by an alien race. volunteers get in them and press a button which cause the ship to "warp" to a distant location in the galaxy, then push another button to return. I believe they get a share of what they find. Sort of like Stargate with space ships
In the main part of the series, the frontier has moved away from Venus after explorers discovered an asteroid orbiting perpendicular to the ecliptic plane, filled with cobalt blue tunnels, and hundreds of small Heechee spaceships. The asteroid, named Gateway by the discoverers, is occupied by the powerful nations of the world, who subsequently form the Gateway Corporation to administer the object.
By happenstance, one of the asteroid explorers enters a ship and hits a button, activating the vehicle and sending him on a thirty-day journey to another solar system. Upon his return, the Gateway Corporation decides to allow explorers (called Prospectors) to take trips on the mostly still-functioning ships. Prospectors who find valuable materials or make discoveries are rewarded with substantial bonuses. Ships on the asteroids come in three sizes and are defined as a "one", "three", or "five" based upon the number of passengers that they can (for the most part uncomfortably) carry. Not every vehicle returns and there are other great hazards to the explorers.