Friday, April 24, 2020

Hubble Thirtieth Anniversary

Hubble's Thirtieth Anniversary

Lake Michigan seen from the International Space Station on a uncommonly clear day.
Canada is toward the bottom of the image. Image Credit: NASA

The Earth is a pretty cool place to live. It’s got tons of fresh air, drinkable water, awesome trees, and a whole bunch of people to hang out with.

Dramatic storm clouds on the edge of typhoon at sunset as seen from the International Space Station.
Basically imposssible to do astronomy through. Image Credit: NASA

The Earth kinda stinks if you’re an astronomer. It’s got tons of air above you that bounces light all around, water that just appears from thin air to cloud up the whole place, trees that don’t care one bit if they’re in your way, and there’s all these people everywhere who want to light the whole place up when it gets dark out. It’s really kind of a crummy place to set up a telescope.

Observatory telescopes at Mauna Kea, Hawaii.
Image Credit: Public Domain
But what are you going to do? You’re stuck here, for now. So diligent astronomer that you are, you haul your telescope away from all those people and trees. You carry it up the side of a mountain, so high up you need to bring extra oxygen just to breath, and up above the clouds. And yet, still there’s so much air still above you, rarified as it is, that it still bounces your image around. There’s just no escape from all that air.

And then there’s the planes. And the satellites. And then you look at your phone and some billionaire is planning to put… 12,000 more satellites up there?! Maybe 30,000 more after that?!

Being stuck on the Earth as an astronomer is a real downer. Where you’d really like to be is up there, up above all of that air and water and billionaires with their tens of thousands of satellites. If you could put your telescope up there, it wouldn’t have to worry about the light of the image being bounced around. It would be able to see those wavelengths of light that get absorbed by the air. You could do real science that you just can’t do from the surface.

That’s the dream of the space-based telescope. An instrument immune to the obstacles of the Earth-based observing, and open up the ultraviolet and infrared universe to scientific exploration. It’s a dream that astronomers held since the earliest days of rocketry. Many telescopes have since been launched into space, and each of them has expanded the frontier of human understanding in exciting and unique ways. None has been as successful, famous, or iconic as the Hubble Space Telescope.

The Hubble Space Telescope is held in position above the service bay of
the space shuttle Endeavour by the shuttle's robotic arm. Image Credit: NASA

The Hubble Space Telescope launched in April 24, 1990 aboard the space shuttle Discovery. The telescope sports a 2.4 meter primary mirror, and the entire telescope is slightly larger a school bus. Because Hubble orbits above the atmosphere, it’s equipped with sensors that can see into the ultraviolet and infrared spectrums, light which would be absorbed by air and invisible to the eye.

The Pillars of Creation in the Eagle Nebula. The pillars
are immense columns of dust being compressed and forming new stars.
Image Credit: NASA, ESA/Hubble and the Hubble Heritage Team
Beyond the gorgeous images Hubble returned over the last 30 years, the telescope is infamously known for being blurry. After deployment, engineers discovered that Hubble’s primary mirror was ground to the wrong shape. The telescope became a punchline, and NASA feared it might be labeled a gigantic boondoggle. And who can blame them? The idea that the brightest minds at NASA made the mirror the wrong shape is inherently funny. What most people joking about it don’t realize is that the shape of the mirror was too flat at the edges by about 2,200 nanometers, or about one hundredth the thickness of a human hair. It might have been the wrong shape, but it was the most precisely machined mirror in history.

The precision eventually allowed Hubble to be repaired. Because the defect in the mirror was so well known and characterized, scientists developed sophisticated techniques to compensate for the error, and still produce usable images. They also designed corrective optics, new mirrors that would interrupt the light path to the instruments that could also compensate for the flaw. Essentially, these would be like glasses for the enormous telescope. The corrective optics were installed in Hubble in 1993 on the first space shuttle mission to service the telescope in orbit.

The Hubble Ultra Deep Field. The image contains nearly 10,000 galaxies
in an area of the sky the size of a tennis ball held 100 yards away.
Image Credit: NASA, ESA, and S. Beckwith (STScI) and the HUDF Team.

This is itself worthy of note, because the Hubble Telescope was designed to be serviced by the space shuttle. Ultimately, astronauts worked on the telescope five times before the shuttle was retired in 2011. Each mission worked to increase and improve the capabilities of the telescope, furthering our understanding of the universe.

Over the last 30 years, Hubble has contributed immensely to our scientific knowledge. Some of the important discoveries that Hubble produced include:
  • Constraining the age of the universe to 13.7 billion years
  • Constraining the rate of expansion of the universe, including the discovery of dark energy
  • The discovery of additional moons around Pluto
  • Determining the mass and size of the Milky Way galaxy
  • Discovery of the connection between galaxies and supermassive black holes
  • Observing protoplanetary disks around stars forming in the Orion Nebula

Hubble is still working well today, but the writing is on the wall for the great space telescope. Like all near Earth satellites, Hubble is subject to drag forces from the incredibly thin outer edge of the Earth’s atmosphere. Originally this wouldn’t be an issue. During each of the space shuttle servicing missions, Hubble’s orbit was boosted using the shuttle’s rocket engines, counteracting the effects of atmospheric drag. But the space shuttle was retired in 2011, and there are no other vehicles capable of boosting Hubble in service today.

Moreover, no spacecraft today could ferry astronauts to service the telescope. Keeping the telescope pointing precisely and steady is obviously important for taking images of faint and distant objects. Hubble uses a set of high performance gyroscopes to maintain its pointing. These gyroscopes fail over time, and can no longer be replaced, meaning at some point Hubble may not be able to maintain pointing at its imaging targets. This would render the telescope unable to do science.

The Hubble Space Telescope above the limb of the Earth.
Image Credit: NASA

Eventually, whether or not the gyroscopes fail, Hubble will return to Earth. NASA estimates that the Hubble Space Telescope will reenter the Earth’s atmosphere and burn up some time between 2028 and 2040. When this happens, it’s likely that portions of the telescope, including parts of the mirror, will survive to the surface. A spectacular end to such an intrepid tool of discovery.

Test engineers stand infront of the James Webb
Space Telescope primary mirror.
Image Credit: NASA/Goddard/Chris Gunn
Successors to Hubble are already in progress. NASA’s next great space observatory, the James Webb Space Telescope, will have more than 5 times the light gathering area than the Hubble. It will be able to see further and fainter than Hubble ever could. Webb will orbit the L2 Earth-Sun Lagrange point, a point in space about 1.5 million kilometers from the Earth where the gravity from the Earth and Sun balance against the centrifugal force experienced on an object orbiting the Sun. This will be so far away that servicing it will be out of the question. But it will also be so far away it will be free from the influence of Earth’s atmosphere and should be able to operate for a very long time. The James Webb Space Telescope is scheduled to launch in 2021, but the project has been plagued with delays and could be pushed back further.

In the meantime, we’ll continue to use the Hubble Space Telescope to probe the depth of the universe and expand the frontier of human knowledge.