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The Hubble Space Telescope has been sending home images of the universe for more than thirty years, but none of its work would have been possible without the many servicing missions that kept it up to date.

Hosted By: Hank Green

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Hubble Space Telescope has now clocked more than thirty years of service in the skies above Earth. That's more than double its original intended mission length, and it's an impressive legacy of incredible science and breathtaking images. However, it almost didn't happen.

 The Problem (0:18)

None of Hubble's achievements would have been possible without the servicing missions that have kept it going all these years. They've been described as some of the most challenging crude space missions ever attempted, and they have brought Hubble back from the brink of disaster on more than one occasion.

Hubble was developed over about 20 years, at a cost of 1.5 billion dollars, and it was eventually launched in 1990 on board the Space Shuttle Discovery.

But almost as soon as the observatory came online, scientists realized there was a problem. Images that should have been sharp and crisp were instead fuzzy with a halo of phantom light.

It turns out that Hubble's 2.9 meter curved mirror, despite being one of the most precisely polished mirrors ever made, had been polished to the wrong shape. It was too flat at its edges by a mere 2.2 microns, producing what is known as a spherical aberration in the images. In other words, Hubble was near-sighted.

Fortunately, Hubble's designers knew that by building a Space Telescope intended to last at least 15 years, they were in it for the long haul. They knew that things would need repairing and replacing and they had designed the observatory to be serviceable in orbit.

For a start, the space telescope was placed in low Earth orbit, easily accessible from the space shuttle launch site at Cape Canaveral, though that meant that the Earth would obscure the telescope's view of an object for around half of its orbit.

The telescope's various scientific instruments were designed so that they could be easily slid out and replaced by others of a similar modular design. And all over the school bus-sized telescope the engineers had built in grapples and handholds that would make it easier for spacewalking astronauts to hang on.

 Servicing (1:59)

So detailed was the plan that Hubble already had a fixed servicing schedule in place when the problem with its mirror was discovered. So engineers got to work designing a fix that could fly on Servicing Mission One in December of 1993. So at four thirty a.m. local time on December 2nd, Space Shuttle Endeavor launched from the Kennedy Space Center.

Endeavor performed a series of burns to catch up to the Hubble Space Telescope over the course of several days. When the two spacecraft were less than 10 meters apart, the astronauts onboard used Endeavor's robotic arm to grab hold of Hubble and birth it upright in the shuttle's cargo bay.

Once it was secure, which is no easy feat in itself, the maintenance could begin. Over five days, pairs of astronauts suited up and headed out to remove and replace Hubble's faulty equipment.

Now these were long space walks. The astronauts were out there working for six or seven hours at a time. Though many of their tasks were completed faster than planned, like one replacement that was scheduled to take 4 hours actually only took 40 minutes.

So over their five long spacewalks, the astronaut maintenance crew got through a lot. They installed a fix for the primary mirror's nearsightedness called the Corrective Optics Space Telescope Axial Replacement, or COSTAR for short.

 Repair (3:13)

This instrument was the size of a telephone booth and it consisted of five sets of corrective mirrors that refocused the light ahead of three of the telescope's instruments. Kind of like, gave the Hubble glasses. It was an innovative solution, though it came at the cost of losing one of the telescope's other instruments to make room for it.

Next, the telescope's Wide Field Planetary Camera was replaced with a second generation version. It had an improved ability to see in ultraviolet, and it had built-in corrective optics to account for the spherical aberration.

The astronauts also replaced other components like the solar arrays which were designed to expand and retract in response to the sun's heating but which ended up sticking and slipping unevenly, causing a jitter in the telescope's images. These were replaced with a simpler spring design that allowed the arrays to expand and contract more smoothly.

And the Space Shuttle also used its own thrusters to boost the telescope to a slightly higher circular orbit to reduce the drag caused by the atmosphere even all the way out in space.

With all the maintenance complete, Hubble was released from the robotic arm on the Shuttle's ninth flight day. Endeavor used small jets to move away gradually, and then headed home.

 Conclusion (4:19)

Incredibly, the engineers could not know if their maintenance had worked until after Hubble was released, but thankfully, it did. Now this first Servicing Mission was a huge success, and it set a precedent for future on-orbit maintenance of this advanced space telescope.

Four more scheduled Servicing Missions followed in 1997, '99, 2002, and 2009. Each time, the spacewalking maintenance crews breathed new life into Hubble, upgrading instruments, replacing and even eventually fixing components, all while weightless in orbit.

But since the retirement of the Space Shuttles in 2011, it is no longer possible to service Hubble. The space vehicles we use these days just don't have the same capabilities as the Shuttle.

Without periodic boosting of its orbit, atmospheric drag is expected to make Hubble de-orbit sometime between 2028 and 2040. And it's big enough that parts of it could make it all the way to Earth's surface, which is why the final Servicing Mission included a sadder task: installing equipment to hopefully help de-orbit Hubble safely when its service comes to an end.

2028 is not that far away, and I am not ready. But as we make this video, the Hubble Space Telescope is still going, thanks to the hard work, innovation, and bravery of everyone involved in its Servicing Missions.

And to commemorate that amazing achievement, this month's SciShow pin of the month depicts Endeavor and Hubble in orbit together. This little space meet-cute single-handedly enabled Hubble's incredible scientific legacy and the pin is pretty cute too.

Preorders for the pin begin when this video is uploaded and we will be taking them through the end of the month. After that, we'll close down orders and start making the pins and you will not be able to get this one ever again.

But we are going to be bringing you more space mission pins in 2022, so keep your eyes out for those.