I think it was "impossible" given the year it was proposed. In 1990 the tech required didn't exist.

It is about the end-result of this program that was considered "impossible" - a celestial map consisting billions of stars with detailed information about each of them, including position and movement. It seems that such a task was considered so difficult in the nineteens, that when they requested it they didn't expected to end up with a solution that would completely cover all the requirements.

Probably just the progressive changes: the accuracy and number of stars. They're saving 1000 terabytes of data from the gigapixel sensor. These missions always take very long to design and build so these numbers probably were thought impossible ten years ago.

> Could someone with better knowledge of astronomy/aerospace explain (with less hyperbole) which part is the "impossible" part here?

Hyperbole is the operative term. When first proposed it would have been impossible with then-current hardware, but since such missions take time to germinate, sometimes people use the term "impossible" to add a sense of drama and daring, but with the expectation that the goal will become possible in time.

Also, according to Arthur C. Clarke, "When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong."

gerry gilmore's ability to generate a bankable quote is impossible for normal human beings.

(he's a nice guy, when i knew him a long time ago, but he's a smooth talker.)