The light from the fake star runs through a long tube into a prototype of the daughter craft, and also continues on to the mother craft at the far end of the room.

While the technical challenges of StarLight are tremendous, the budget for building a prototype is not. StarLight's budget was $14 million last year. The team is asking for $29 million in the coming year. While this is not pocket change, it is a fraction of the cost of the real mission. Combined, a project like this is what an engineer would call a one-off of the grandest proportions. There is no assembly line, no second version. Just a prototype and then a spacecraft.

So Blackwood's engineers do their designing on borrowed computers. They store equipment in racks that once served the Cassini mission. They order optical parts out of a catalog, and make do with what's available whenever possible, avoiding costly special orders. And they do most of the work themselves; very little is farmed out. The StarLight team wrote the specs for the building in which they work, including its not-so-stylish black-plastic walls. They drew up the plans for the prototype.

"Then we order the pieces, machine it. Beg, borrow ..." Blackwood stops, smiles. "This is not flight hardware. This is all prototype. We go as cheap as we can."

It's a common refrain at JPL, where the real money goes into the final product that will have to endure the rigors of radiation and frigid temperatures in outer space. For StarLight, there's all that and then the delicate act of formation flying.

"I believe that's the easy part," Blackwood said. "It's very doable."

In space, at least, there is no air to disturb the flight. And StarLight would orbit around the Sun, matching Earth's orbit but trailing behind our planet at a distance of about 10 million miles (17 million kilometers). It's a "very quiet" part of space within our Solar System, Blackwood says.

The real challenge is in building a two-legged interferometer that still works.