Ever since the wildly optimistic projections of progress from the mid-20th century—robot companions, extra-orbital sojourns, 200-year life spans—scientists have been gently dialing back the public's expectations. So, perhaps it is no sobering surprise that many of the hopeful predictions for scientific advancements for the new millennium have yet to come to pass.

"We were supposed to have cities under domes, jet packs and space planes that could take commuters from Los Angeles to Tokyo in two hours," Paul Milo, author of Your Flying Car Awaits: Robot Butlers, Lunar Vacations, and Other Dead-Wrong Predictions of the Twentieth Century (Harper, 2009), told The Wall Street Journal earlier this month.

Of course science thrives on a powerful elixir of optimism and challenge. So, enthusiasm for lofty goals can grease the wheels of inquiry—not to mention funding. And even if crucial breakthroughs remain unachieved for now, little ones help to pave the way toward a new solution. That's in science's accretionary nature.

Even so, hopes for some of the big missed marks—from climate regulation to cognitive cures—were high even as late as the 1990s. So, in the spirit of hoping that science can save the world a little more expeditiously in the next decade, here are 10 scientific disappointments of the "aughties" as seen through the eyes of the editorial staff at Scientific American.

View Slide Show: 10 Science Letdowns of the New Millennium