The Hubble Space Telescope (HST) hit the 100,000-orbit mark today, nearly two decades after it launched into space.

The beloved observatory has been faithfully circling Earth since its April 1990 launch, offering us Earthlings glimpses of the cosmos as we've never had before.

Now, after travelling around Earth at nearly five miles per second for 100,000 orbits, Hubble's odometer reads about 2.72 billion miles — that's roughly 5,700 round trips to the Moon.

To mark the event, scientists turned Hubble's camera eye toward part of a nebula near the star cluster NGC 2074, which is about 170,000 light-years from Earth near the Tarantula nebula.

"That's a lot of orbits and that represents a lot of miles and a lot of time," said HST deputy senior project scientist Malcolm Niedner. "It's been just a fabulous long journey of scientific discoveries, with more to come."

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Still going strong

Despite wear and tear from micrometeorite impacts and temperature extremes in orbit, the telescope is still going strong after 18 years in orbit, and has contributed to thousands of scientific discoveries.

"It's explored entirely new grounds in terms of the ability to see things in detail, and what has resulted from that is just marvelous," said Bob O'Dell, an astronomer at Vanderbilt University in Tennessee, who started as a NASA project scientist for Hubble 19 years before its launch, helping to get the project off the ground. "Something like the Hubble Deep Field, which has penetrated far back in time, is the kind of thing that we'd always hoped to be able to do."

Because of its unprecedented contribution to science, the telescope holds a special place in many scientists' hearts.

"For me it's special because I used this to discover the farthest planet that has ever been discovered," said Kailash Sahu, an astronomer at the Space Telescope Science Institute who used Hubble to observe exoplanets 26,000 light-years away. "My own program was to discover extrasolar planets at the farthest ranges we can see. HST is the only one which can do that."

Because it views from space, beyond Earth's atmosphere, Hubble can beat ground-based observatories much larger than it in terms of resolution.

"Personally, my work in black holes would not have been possible without HST, showing that nearly all galaxies contain a massive black hole in their center and that the black hole is an essential component for why galaxies look the way they do," said University of Texas astronomer Karl Gebhardt.

A great ride

The telescope has endured ups and downs, especially an early post-launch scare that it would never see as it was supposed to, after scientists discovered that its main mirror was ground to the wrong shape.

But astronauts were able to correct the flaw during a space shuttle repair mission in 1993, and subsequent tune-ups have kept Hubble highly functional most of the time.

"It's been a great ride," O'Dell said. "We're still doing good science with it, and what appears to threaten its lifetime is not the spacecraft itself but rather our ability to send vehicles up for maintenance."

The observatory is set to have its fifth, and final, face-lift in October, when the space shuttle Atlantis visits the orbiting scope. Astronauts plan to install new equipment and repair broken instruments during five spacewalks.

The tune-up should extend Hubble's life until at least 2013. By that time, NASA's shuttle fleet will likely be retired and the telescope could face destruction by burning up in the atmosphere during a controlled dive down to Earth.

Big shoes to fill

When it finally does retire, it will leave big shoes to fill.

"I would say its biggest legacy would be that wherever it looked with its really broad range of capabilities, it found new things, many of which people couldn't have even dreamed of," Niedner said. "It's been an exciting, unpredictable journey with great surprises."

And while Hubble's aid to science has been enormous, its effect on the public's appreciation of science may be even more noteworthy.

"What is arguably the most important contribution has been the inclusion of the public in the science from HST," Gebhardt told SPACE.com. "The real contribution from HST has yet to be realized, as it has hopefully inspired a generation of scientists to be leaders for the U.S. and the world."

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