Hubble Space Telescope: Mission and Discoveries

Point a good telescope at the night sky from your terrace and the stars twinkle. That twinkling is not romantic, it is a problem. The Earth’s atmosphere is a churning ocean of air that bends and blurs every ray of light passing through it, which is why even the best ground observatories struggle to see fine detail. The fix is almost absurdly simple in principle: put the telescope above the atmosphere. That is the whole idea behind the Hubble Space Telescope, a school-bus-sized observatory that has been orbiting about 540 kilometres above your head since 1990, sending back the sharpest pictures of the universe humans had ever seen.

Here is what trips up almost everyone. Hubble is not famous because it is the biggest telescope, it is not. Its main mirror is 2.4 metres across, smaller than several ground-based giants. It is famous because it sits where the air cannot smudge the view, and because it very nearly failed on day one before becoming the most productive scientific instrument in history. Get those two facts straight and the rest of Hubble’s story falls into place.

What the Hubble Space Telescope actually is, and why it lives above the sky

The Hubble Space Telescope (HST) is a large space-based optical and ultraviolet telescope, launched on 24 April 1990 aboard the Space Shuttle Discovery, and run as a joint project of the American space agency NASA and the European Space Agency (ESA). It orbits Earth in low Earth orbit, completing one lap roughly every 95 minutes, well above the layer of air that distorts starlight.

That altitude is the entire point, so it is worth translating. Down here, light from a distant galaxy has to fight through turbulent air, water vapour, and pollution before it reaches a lens. The atmosphere also flatly blocks most ultraviolet and much infrared light, so whole slices of the sky are simply invisible from the ground. Lift the telescope clear of all that and two things happen at once: images get razor-sharp, and wavelengths that never reach the surface suddenly open up. If you have revised the electromagnetic spectrum, this is that theory made concrete. Hubble sees the visible, the near-ultraviolet, and the near-infrared bands that ground telescopes either blur or lose entirely.

The telescope is named after Edwin Hubble, the American astronomer who showed in the 1920s that the universe is expanding, which is a neat piece of foreshadowing given what his namesake would later measure. As satellites go it is not small: about 13 metres long and 11,000 kilograms, powered by two solar panels, and designed from the start to be visited and repaired by astronauts. That last design choice, being serviceable, is the single feature that saved the mission. If you are comparing it with the orbits and types of satellites you have studied, Hubble is the rare science satellite built to be reached by human hands.

The flaw that nearly sank it, and the 1993 rescue

Within weeks of launch, Hubble’s first images came back disappointingly fuzzy, and engineers traced the blur to a manufacturing error in the main mirror. The 2.4-metre primary mirror had been ground to the wrong shape, its edge too flat by about 2.2 micrometres, roughly one-fiftieth the thickness of a human hair. That tiny error was enough to smear the light and rob the telescope of its whole advantage. The flaw is called spherical aberration, and for a while it looked like a multi-billion-dollar embarrassment orbiting the planet.

Here is the part worth sitting with, because it is easy to assume they replaced the mirror. They did not. You cannot swap a mirror in orbit. Instead, engineers worked out the exact error and built a set of small corrective mirrors to cancel it out, the optical equivalent of prescribing spectacles for a telescope. The device was called COSTAR, and it was carried up on the first servicing mission (SM1) in December 1993 aboard the Space Shuttle Endeavour. Astronauts spent five spacewalks installing COSTAR and a new camera, and when the corrected images came down, Hubble finally saw the way it was meant to.

That rescue set the pattern for the mission. In all, astronauts visited Hubble on five servicing missions between 1993 and 2009, swapping out cameras, gyroscopes, batteries, and instruments, each time leaving the telescope more capable than they found it. The final visit, SM4 in May 2009, was flown by the Space Shuttle Atlantis, and after that no spacecraft has been able to reach it, because the shuttle fleet retired in 2011. So the working lesson of Hubble is not just “put the telescope above the air.” It is that a repairable instrument outlives a perfect one, because nothing launches perfect.

The discoveries that rewrote the textbooks

Hubble’s real legacy is not one pretty picture, it is a run of results that changed the numbers in astronomy textbooks. Start with the biggest one: how old the universe is. Before Hubble, estimates of the universe’s age ranged wildly, from about 10 to 20 billion years, because nobody could pin down how fast the universe is expanding, a rate called the Hubble constant. By carefully measuring the distances to galaxies, Hubble narrowed that rate down and helped settle the age of the universe at close to 13.8 billion years. That single act of measurement is why the telescope earned its name twice over.

Then there is the image that changed how people think about scale, the Hubble Deep Field. In 1995, astronomers did something that sounds reckless: they pointed the telescope at a tiny, apparently empty patch of sky near the Big Dipper, smaller than a grain of sand held at arm’s length, and left the shutter open for ten days. That “empty” patch turned out to hold around 3,000 galaxies, each one a city of billions of stars. The follow-up Hubble Ultra Deep Field (2004) pushed even further, capturing about 10,000 galaxies and light that had travelled for over 13 billion years. If a pinprick of blank sky holds thousands of galaxies, the number of galaxies in the whole universe runs into the hundreds of billions. No graph teaches that lesson the way that photograph does.

Hubble also cracked open two frontier topics. It made the first detection of the atmosphere of a planet orbiting another star, an exoplanet called HD 209458b, in 2001, by watching how starlight filtered through the planet’s air as it crossed its star. And its careful observations of distant exploding stars, Type Ia supernovae, were central to the 1998 discovery that the universe’s expansion is not slowing down but speeding up, driven by a mysterious something now called dark energy. That finding won the 2011 Nobel Prize in Physics, and it is the concrete evidence behind the dark matter and dark energy chapter that otherwise reads as pure abstraction. Line these up next to the Big Bang and expanding universe theory, and Hubble is the instrument that turned that theory from a good idea into measured fact.

Hubble versus James Webb: partners, not rivals

The most common confusion now is whether the James Webb Space Telescope (JWST), launched at the end of 2021, has replaced Hubble. It has not, because the two telescopes look at different kinds of light and sit in completely different places. Treat them as teammates covering different beats, not as an old model and its upgrade.

The cleanest way to hold the difference is a short table.

The wavelength split is what matters. Hubble’s strength is sharp visible and ultraviolet vision, which is ideal for the light young, hot stars pour out. Webb sees mainly in the infrared, which lets it peer through dust clouds and catch light from the very first galaxies, stretched into infrared as the universe expanded. So Webb sees further back in time and through more dust, while Hubble delivers the crisp visible and ultraviolet views Webb cannot. The two often study the same object in their own bands and hand astronomers a fuller picture than either could alone.

The orbits explain the servicing gap too. Hubble sits close enough that a shuttle could dock with it, which is why it could be repaired five times. Webb sits about 1.5 million kilometres away at a gravitational balance point, far beyond any crewed reach, so it had to work perfectly on the first try, with no rescue mission possible. If you keep one contrast in your notes, make it this one: near and fixable versus far and untouchable.

The legacy, and what happens when it finally falls silent

More than three decades after launch, Hubble is still working, and its record is the reason it belongs in the story of modern science, not just astronomy. It has made well over 1.5 million observations and fed data into tens of thousands of published research papers, which makes it one of the most productive scientific instruments ever built. For most people alive today, the mental image of a glowing nebula or a field of distant galaxies is a Hubble image, and that shift in how the public pictures the cosmos is a legacy in itself.

The telescope is not immortal, though, and this is where honesty matters. Hubble has no engine to boost its own orbit, so the thin drag of the upper atmosphere is slowly pulling it lower, and sometime in the 2030s it is expected to re-enter and burn up unless a future mission raises it. Its gyroscopes and instruments are also ageing, with occasional glitches that engineers now nurse rather than fix in person, because no crewed spacecraft can currently reach it. So the plain judgment is that Hubble is in a long, dignified decline, still doing real science but running on borrowed time.

None of that dims what it proved. India’s own space programme under ISRO is now building space-based observatories in the same spirit, and every one of them rests on the lesson Hubble taught first: get above the air, and the universe comes into focus.

How to lock Hubble into your notes

The efficient way to remember Hubble is to build it around five anchors and hang the detail off each: what it is and where it orbits, the mirror flaw and the 1993 fix, the headline discoveries, the contrast with Webb, and its ageing legacy. Almost every question worth asking is a variation on one of those five.

Start with the “why above the atmosphere” fact, because it is the concept the whole mission rests on and the one that connects to physics you already know. If you can explain in one line that the atmosphere blurs light and blocks ultraviolet, you have the reason Hubble exists. Then attach the numbers that examiners love as clean data points: launched 1990, 2.4-metre mirror, NASA and ESA, about 540 km up.

Keep the Hubble-versus-Webb contrast on one page, because that is the comparison most likely to be tested and most easily muddled. Optical and ultraviolet versus infrared, near orbit versus a distant balance point, serviceable versus not. Finally, tie each discovery to a “so what”: the Hubble constant fixed the age of the universe, the Deep Field showed how crowded it is, and the supernova work gave dark energy its first solid footing. Facts anchored to a consequence survive revision. Loose ones evaporate.

Frequently Asked Questions

When was the Hubble Space Telescope launched and by whom?

It was launched on 24 April 1990 aboard the Space Shuttle Discovery. It is a joint mission of NASA and the European Space Agency (ESA), and it is named after the astronomer Edwin Hubble.

Why is the Hubble Space Telescope placed above the atmosphere?

Earth’s atmosphere bends and blurs light and blocks most ultraviolet radiation, which limits ground telescopes. Orbiting about 540 km up, Hubble sees far sharper images and captures ultraviolet light that never reaches the ground.

What was the mirror flaw and how was it fixed?

Hubble’s 2.4-metre main mirror was ground very slightly to the wrong shape, an error called spherical aberration, which blurred its early images. Astronauts fixed it in December 1993 by installing corrective optics called COSTAR, effectively giving the telescope spectacles, rather than replacing the mirror.

What are Hubble’s most important discoveries?

It helped pin down the expansion rate of the universe and its age at about 13.8 billion years, produced the Deep Field images revealing thousands of distant galaxies, made the first detection of an exoplanet’s atmosphere, and gathered key evidence for dark energy and the accelerating expansion of the universe.

How is Hubble different from the James Webb Space Telescope?

Hubble sees mainly visible and ultraviolet light from low Earth orbit and can be serviced by astronauts. James Webb sees mainly infrared light from a point about 1.5 million km away and cannot be reached for repair. They complement each other rather than compete.

Is the Hubble Space Telescope still working?

Yes, more than three decades after launch it is still making observations, though its instruments are ageing and its orbit is slowly decaying. It is expected to re-enter the atmosphere sometime in the 2030s unless a future mission raises its orbit.

What does the Hubble constant mean?

The Hubble constant measures how fast the universe is expanding, specifically how quickly distant galaxies move away from us with distance. Measuring it accurately is what allowed astronomers to estimate the age of the universe.

Practice Questions

1. The Hubble Space Telescope is primarily designed to observe in which region of the spectrum?

a) Radio waves
b) X-rays and gamma rays
c) Visible (optical) and ultraviolet light
d) Microwaves

Answer: c

2. The Hubble Space Telescope is a joint project of which two agencies?

a) NASA and ISRO
b) NASA and ESA
c) ESA and Roscosmos
d) NASA and JAXA

Answer: b

3. The defect discovered in Hubble’s main mirror soon after launch was:

a) A crack from the launch vibration
b) Spherical aberration from an incorrect mirror shape
c) A coating that reflected too little light
d) Contamination by space dust

Answer: b

4. Which of the following is a key difference between Hubble and the James Webb Space Telescope?

a) Webb observes mainly in infrared, while Hubble observes mainly in visible and ultraviolet
b) Both orbit at the same altitude
c) Hubble cannot be serviced, but Webb can
d) Webb orbits closer to Earth than Hubble

Answer: a

5. The Hubble Deep Field image is significant because it:

a) Showed the surface of a distant planet
b) Revealed thousands of galaxies in a tiny, apparently empty patch of sky
c) Was the first colour image of the Moon
d) Detected radio signals from another galaxy

Answer: b

Mains-style questions

1. Explain why space-based telescopes such as Hubble offer advantages over ground-based observatories, with reference to the atmosphere and the electromagnetic spectrum. 2. Discuss how the servicing history of the Hubble Space Telescope illustrates the value of designing scientific instruments to be repairable. 3. The Hubble and James Webb space telescopes are often described as complementary rather than competing. Examine this statement with reference to their capabilities and orbits. 4. Evaluate the contribution of the Hubble Space Telescope to our understanding of the age and expansion of the universe. 5. “A repairable instrument can outlast a perfect one.” Discuss this idea using the example of the Hubble Space Telescope’s mirror flaw and its correction.

Hubble earns its place in your preparation not because it is the largest telescope, it is not, but because it bundles so many threads into one story: the physics of why we go above the atmosphere, an engineering rescue that turned failure into method, and a set of measurements that fixed the age of the universe and put dark energy on the map. Learn it as that bundle, not as a single launch date, and it stops being a name you can confuse with James Webb and becomes a mission you can actually explain.