The U.S. space agency launched its newest X-ray space telescope Wednesday on a mission to hunt for black holes and other hard-to-see objects lurking in the Milky Way and other galaxies.

"It's a terrific day," assistant launch director Tim Dunn said.

'If you had high-energy X-ray eyes and you stared up out of the galaxy, what you would see is the glow of all the massive black holes sprinkled throughout the cosmos'—Fiona Harrison, California Institute of Technology

NASA decided to air-launch the $170 million mission, instead of rocketing off from a launch pad, because it was cheaper.

The telescope was boosted into orbit by a rocket released from a aircraft that took off from the remote Kwajalein Atoll, a Pacific island halfway between Hawaii and Australia.

The Nuclear Spectroscopic Telescope Array, or NuStar for short, focuses high-energy X-rays to peer through gas and dust in search of supermassive black holes in the centre of galaxies, remnants of exploded stars and other exotic celestial objects.

While black holes are invisible, the region around them gives off telltale X-rays. NuStar will observe previously known black holes and map hidden ones.

Scientists hope to better understand how galaxies form and evolve.

Hunt for remains of ancient supernovae

"We can view black holes and galaxies even if they're enshrouded with dust and gas. If you had high-energy X-ray eyes and you stared up out of the galaxy, what you would see is the glow of all the massive black holes sprinkled throughout the cosmos," chief scientist Fiona Harrison of the California Institute of Technology said earlier this week.

NuStar will also hunt for the remains of ancient supernovae, stars that exploded in past centuries. If it's lucky, it'll witness a star's death throes, but such events don't happen often, and the telescope will have to be pointed at the right place at the right time.

Scientists expect sharp images from the mission, which is many times more sensitive than previous space telescopes that have looked in this part of the electromagnetic spectrum.

The launch comes at a trying time for NASA's astrophysics division. Last week, the space agency killed an X-ray telescope mission because it failed to come in on budget. That mission, called GEMS, was supposed to launch in 2014 and would have observed many of the same targets as Nustar.

NASA is pressing ahead with its flagship astrophysics mission — the budget-busting James Webb Space Telescope, considered the successor to the Hubble Space Telescope. It has the capability of peering deeper into the universe and back in time than ever, and is expected to launch in 2018 with an $8 billion price tag.