This story was told in two parts;
1) An alien civilisation (very advanced indeed) are trying to get an object to a distant Point A in space. They achieve this by firing two microscopic objects at near-lightspeed (a needle and a cylinder) with millimetric precision from two nearby systems. They almost collide and the interaction of the two, I believe the needle passes through the cylinder, sets in process a chain of molecular events culmimating in the cylinder turning into a space station, then receiving the mind-states of two scientists who beam down to a nearby planet to make contact with the locals.
2) On the surface of the planet, the two scientists make contact with the nearest authority figures. They use their advanced knowledge of science to convince them that they're aliens and try to convince them to stop a major project, some kind of mining operation that will destroy the last remains of an ancient race that used to occupy the planet. The precursor civilisation's remains consist of some kind of mathematical proof stored on tablets(?). Ultimately they're successful in getting a good look at these and are able to upload the info back to their own civilisation.
Much is made of the precursors being looked down upon by the locals "If they were so advanced, how come they're all dead?"
There are multiple mentions of the aliens being able to kill themselves at will if they're kidnapped or mistreated.
The aliens are able to use their space station to convince the locals that they're aliens.
The bodies are essentially disposable. If memory serves, they leave avatar bodies on the space station to die.
The scientists may have been lovers?