The Hubble Space Telescope has reshaped virtually every branch of astronomy since its launch in April 1990, producing observations that pinned down the age of the universe, revealed that cosmic expansion is accelerating, and captured light from galaxies formed less than a billion years after the Big Bang. Orbiting roughly 540 kilometers above Earth’s surface, its 2.4-meter primary mirror collects light free of atmospheric distortion, giving it a sharpness that ground-based telescopes of the era could not touch. What makes Hubble’s story so compelling is not just the science but the engineering drama: a crippling manufacturing flaw, daring spacewalk repairs, and an ongoing question about how much longer the aging observatory can survive.
A Flawed Mirror and the Fix That Saved the Mission
Within weeks of launch, astronomers realized something was badly wrong. Images that should have been pinpoint-sharp came back fuzzy. The primary mirror had been ground to an extraordinarily precise but incorrect shape, introducing spherical aberration that spread starlight into a halo instead of focusing it tightly. The error was tiny in absolute terms but devastating for a telescope that was supposed to deliver diffraction-limited images.
The solution came during the first servicing mission in December 1993, when astronauts installed the Corrective Optics Space Telescope Axial Replacement instrument, known as COSTAR. Five specially shaped mirrors inside COSTAR compensated for the aberration in front of three of Hubble’s scientific instruments: a camera and two spectrographs. Each corrector paired a field mirror with an aspherical reimaging mirror that cancelled out the flawed optics.1Applied Optics. Reflective correctors for the Hubble Space Telescope axial instruments The result restored Hubble’s angular resolution in the camera and recovered its ability to collect useful spectra in crowded star fields.2PubMed. Hubble Space Telescope COSTAR asphere verification with a modified computer-generated hologram interferometer Later replacement instruments, such as the Wide Field Camera 3 and the Space Telescope Imaging Spectrograph, had their own built-in corrective optics, making COSTAR eventually unnecessary. It was removed in 2009 during the final servicing mission and is now on display at the Smithsonian.
Servicing Missions and the Shuttle Connection
Hubble was deliberately designed so that astronauts could reach it by Space Shuttle, swap out failed or outdated hardware, and give the telescope a new lease on life. That design choice turned out to be one of the most consequential decisions in the observatory’s history.3SAE International. Lessons Learned from Hubble Space Telescope ExtraVehicular Activity Servicing Missions Five servicing missions flew between 1993 and 2009, and each one involved multiple spacewalks in which astronauts replaced gyroscopes, solar arrays, batteries, and entire scientific instruments while floating beside a school-bus-sized telescope moving at about 27,000 kilometers per hour.
Servicing Mission 4, the last one, almost did not happen. After the Columbia disaster in 2003, NASA initially cancelled it because Hubble’s orbit was too far from the International Space Station to serve as a safe haven if the shuttle were damaged. Public pressure and a change in NASA leadership reversed that decision, and in May 2009 astronauts installed the Wide Field Camera 3 and the Cosmic Origins Spectrograph, repaired the Space Telescope Imaging Spectrograph and the Advanced Camera for Surveys, and replaced all six gyroscopes along with batteries and a fine-guidance sensor. That overhaul is the reason Hubble is still functioning more than fifteen years later.
Nailing Down the Expansion Rate of the Universe
The telescope is literally named after Edwin Hubble, the astronomer who established that distant galaxies are receding from us and that the universe is expanding. Fittingly, one of the observatory’s flagship programs was a sustained, decades-long effort to measure the rate of that expansion, known as the Hubble constant.
The HST Key Project, which ran through the 1990s, used Hubble’s sharp vision to identify pulsating Cepheid stars in dozens of galaxies out to about 25 megaparsecs. Those Cepheids served as distance markers, calibrating a ladder of secondary methods that extended much farther. The final Key Project result placed the Hubble constant at about 72 kilometers per second per megaparsec, with an uncertainty of roughly 10 percent.4The Astrophysical Journal. Final Results from the Hubble Space Telescope Key Project to Measure the Hubble Constant That number implied a universe about 13 to 14 billion years old, resolving a long-running debate that had seen estimates swing between 10 and 20 billion years.
Subsequent work tightened the measurement considerably. A 2009 analysis combining improved Cepheid distances with a large set of Type Ia supernovae found a value of about 74 kilometers per second per megaparsec, with less than 5 percent total uncertainty.5The Astrophysical Journal. A Redetermination of the Hubble Constant with the Hubble Space Telescope from a Differential Distance Ladder By 2018 the precision had improved to about 2.3 percent, yielding a value near 73.5 kilometers per second per megaparsec.6The Astrophysical Journal. New Parallaxes of Galactic Cepheids from Spatially Scanning the Hubble Space Telescope: Implications for the Hubble Constant
That exquisite precision created a new problem. Measurements of the cosmic microwave background by the Planck satellite, combined with the standard cosmological model, predict a value closer to 67. The disagreement between the two approaches now exceeds three standard deviations and has resisted every attempt to explain it away as a calibration error. Known informally as the “Hubble tension,” it is one of the most active puzzles in cosmology, and Hubble data sit right at the heart of it.
Dark Energy and the Accelerating Universe
Hubble also played a central role in one of the most startling discoveries in modern physics: the universe’s expansion is speeding up rather than slowing down. Ground-based surveys first noticed the trend in the late 1990s by measuring distant Type Ia supernovae, but Hubble pushed the observations to much higher redshifts where the signature becomes clearest.
A key program discovered 16 Type Ia supernovae at redshifts above one, including six of the seven most distant such objects known at the time. Those data provided the first conclusive evidence that the early universe was actually decelerating before the current phase of acceleration took hold, a transition sometimes called the “cosmic jerk.” The statistical confidence was above 99 percent.7The Astrophysical Journal. Type Ia Supernova Discoveries at z > 1 from the Hubble Space Telescope: Evidence for Past Deceleration and Constraints on Dark Energy Evolution The implication is that some unknown form of energy, making up roughly 70 percent of the total energy content of the universe, is pushing space apart. More than two decades later, the nature of that dark energy remains one of the deepest open questions in physics.
The Deep Fields
In December 1995, Hubble stared at a seemingly blank patch of sky in the constellation Ursa Major for ten consecutive days. The resulting image, the Hubble Deep Field, contained roughly 3,000 galaxies in a region of sky smaller than a grain of sand held at arm’s length. It was one of the most striking images in the history of science, and it fundamentally changed how astronomers thought about galaxy formation.
Analysis of the Deep Field revealed that the rate at which stars formed across the universe rose sharply from the present day back to a peak around a redshift of about 1.5, roughly 10 billion years ago, then declined at higher redshifts.8The Astrophysical Journal. The Evolution of the Global Star Formation History as Measured from the Hubble Deep Field That pattern, a steep rise by about a factor of twelve from today to the peak and then a gradual falloff, became one of the foundational measurements in the field of galaxy evolution.9arXiv. The Hubble Deep Field and the Early Evolution of Galaxies The Deep Field also enabled researchers to use color-selection techniques to identify star-forming galaxies at redshifts up to about 4.5, pushing the observational frontier back to within about 1.5 billion years of the Big Bang.10Monthly Notices of the Royal Astronomical Society. High-redshift galaxies in the Hubble Deep Field: colour selection and star formation history to z ∼ 4
Later deep-field campaigns, including the Hubble Ultra Deep Field in 2004 and the Hubble eXtreme Deep Field in 2012, pushed even fainter, catching galaxies at redshifts above 10. Those observations set the stage for the James Webb Space Telescope to pick up where Hubble left off, probing the earliest epochs of cosmic history at infrared wavelengths.
Supermassive Black Holes and Galaxy Centers
Hubble’s resolution proved essential for studying what lurks at the centers of galaxies. By measuring how fast stars and gas orbit the cores of nearby galaxies, astronomers could weigh the objects hiding there. A study of 105 galaxies within about 100 megaparsecs, spanning a wide range of galaxy types, used Hubble’s imaging spectrograph to set tight upper limits on central black hole masses. Those limits tracked closely with the well-known relationship between black hole mass and the velocity dispersion of stars in the host galaxy’s central bulge, confirming the pattern across a broad population.11The Astrophysical Journal. Upper Limits on the Masses of 105 Supermassive Black Holes from Hubble Space Telescope/Space Telescope Imaging Spectrograph Archival Data
Hubble surveys also revealed that a majority of low- and intermediate-luminosity galaxies, somewhere between half and 80 percent, contain a compact stellar nucleus at their center regardless of the galaxy’s shape. The masses of these nuclei follow the same scaling relation obeyed by supermassive black holes: roughly 0.2 percent of the total galaxy mass. The finding suggests that galaxy formation generically produces a dense central object, whether that turns out to be a black hole or a tightly packed star cluster, depending on the galaxy’s overall mass.12The Astrophysical Journal. A Fundamental Relation between Compact Stellar Nuclei, Supermassive Black Holes, and Their Host Galaxies
Watching a Comet Hit Jupiter
Not all of Hubble’s greatest hits involve deep space. In July 1994, the telescope had a front-row seat for one of the most dramatic events ever observed in our solar system: the collision of Comet Shoemaker-Levy 9 with Jupiter. The comet had broken into more than 20 fragments, and Hubble tracked them from July 1993 all the way through the week-long bombardment. Spectroscopic observations captured an outburst in magnesium-ion emission as the first fragments approached Jupiter, possibly triggered by electrostatic charging of cometary dust as it crossed into Jupiter’s magnetic field.13PubMed. The Hubble Space Telescope (HST) observing campaign on comet Shoemaker-Levy 9
The impacts themselves were spectacular. Hubble images showed plumes of debris rising 3,000 kilometers above Jupiter’s cloud tops at ejection speeds around 10 kilometers per second. Atmospheric waves rippled outward from impact sites at roughly 450 meters per second. Some of the dark scars left behind were larger than Earth and persisted for months.14PubMed. HST imaging of atmospheric phenomena created by the impact of comet Shoemaker-Levy 9 Ultraviolet observations added another layer, revealing that impact debris darkened over two to three hours and extended to higher altitudes than the visible-light absorbers, indicating that ultraviolet-absorbing gases or aerosols had been lofted to extreme heights. Transient auroral emissions were also spotted near the magnetic conjugate point of one impact site.15PubMed. HST far-ultraviolet imaging of Jupiter during the impacts of comet Shoemaker-Levy 9 The event gave planetary scientists their first direct look at what a large-scale impact does to a giant planet’s atmosphere.
Probing Exoplanet Atmospheres
Hubble was not designed with exoplanets in mind. The first confirmed planet around a Sun-like star was not found until 1995, five years after launch. Yet the telescope became one of the most productive tools for studying exoplanet atmospheres, thanks to a technique called transmission spectroscopy: when a planet crosses in front of its star, a tiny fraction of the starlight filters through the planet’s atmosphere, and Hubble can measure what wavelengths get absorbed.
For the hot Jupiter HD 189733b, Hubble’s data revealed a transmission spectrum consistent with Rayleigh scattering from a high-altitude haze that blankets the planet across the entire optical range and into the near-ultraviolet. The haze appears to be a globally dominant atmospheric feature, which would give the planet a high reflectivity at shorter wavelengths.16Monthly Notices of the Royal Astronomical Society. Hubble Space Telescope transmission spectroscopy of the exoplanet HD 189733b: high-altitude atmospheric haze in the optical and near-ultraviolet with STIS For the hot Jupiter HD 209458b, infrared observations with the Wide Field Camera 3 detected water absorption of about 200 parts per million near the 1.38-micrometer water band.17The Astrophysical Journal. Infrared Transmission Spectroscopy of the Exoplanets HD 209458b and XO-1b Using the Wide Field Camera-3 on the Hubble Space Telescope
More recently, Hubble has been applied to smaller, cooler worlds. Observations of TOI-270 d, a temperate sub-Neptune, showed evidence for molecular absorption consistent with a hydrogen-rich atmosphere at four-sigma significance relative to a featureless spectrum, with water vapor as the strongest individual absorber at about three-sigma significance.18The Astronomical Journal. Hubble Space Telescope Transmission Spectroscopy for the Temperate Sub-Neptune TOI-270 d: A Possible Hydrogen-rich Atmosphere Containing Water Vapor If confirmed by further observations, that would make TOI-270 d one of the smallest and coolest exoplanets with detected atmospheric features. These Hubble results established the playbook that the James Webb Space Telescope is now using to characterize exoplanet atmospheres at longer infrared wavelengths.
Stars Being Born and Stars Dying
Hubble’s resolution made it possible to resolve individual protoplanetary disks around young stars in the Orion Nebula, one of the nearest active star-forming regions. An atlas compiled from Advanced Camera for Surveys images cataloged 178 externally illuminated protoplanetary disks (known as proplyds), 28 silhouette disks seen in absorption against the bright nebula, and additional objects showing jets or bipolar outflows.19The Astronomical Journal. The Hubble Space Telescope/Advanced Camera for Surveys Atlas of Protoplanetary Disks in the Great Orion Nebula Seeing so many disks at once, each in a slightly different stage, gave astronomers a statistical picture of how planetary systems begin to form around newborn stars.
At the other end of stellar life, Hubble transformed the study of planetary nebulae, the shells of gas that low- and intermediate-mass stars shed as they die. Before Hubble, the conventional assumption was that these nebulae were mostly round. High-resolution images revealed the opposite: round nebulae are the exception. Jets, disks, tori, and other asymmetric features dominate, showing that the ejection process is far more complex than simple radiation pressure on a spherical shell.20Galaxies. AGBs, Post-AGBs and the Shaping of Planetary Nebulae For the famous butterfly-shaped nebula NGC 6302, Hubble images taken years apart allowed astronomers to measure the actual motion of gas knots, revealing a largely radial velocity field flowing outward from the central star and aligned with the nebula’s bright structural features.21Monthly Notices of the Royal Astronomical Society. The expansion proper motions of the planetary nebula NGC 6302 from Hubble Space Telescope imaging
How Hubble and Webb Complement Each Other
A common misconception is that the James Webb Space Telescope simply replaces Hubble. The two observatories see different parts of the electromagnetic spectrum, and each catches things the other misses. Hubble was designed to work from the ultraviolet through the near-infrared, roughly 0.1 to 1.6 micrometers, while Webb operates in the infrared from about 0.6 to 28 micrometers. That distinction matters because ultraviolet and optical light traces young, hot, massive stars, while infrared light reveals older stellar populations, dusty star-forming regions, and the very high-redshift universe that Hubble cannot access.22arXiv. We Must Preserve Hubble given its Unique Complementarity to Webb, Roman, and Euclid
Recent work combining Hubble’s ultraviolet imaging with Webb’s mid-infrared data for galaxies at moderate redshifts illustrates the point: Hubble maps where unobscured star formation is happening, while Webb’s MIRI instrument reveals star formation hidden behind dust. The two views together produce a far more complete picture of how galaxies build their stellar mass than either telescope alone.23The Astrophysical Journal. CEERS: Spatially Resolved UV and Mid-infrared Star Formation in Galaxies at 0.2 < z < 2.5: The Picture from the Hubble and James Webb Space Telescopes If Hubble goes dark, there is no other operational telescope that can pick up its ultraviolet capability from space. That gap is a real concern for the astronomical community.
Orbital Decay and How Long Hubble Has Left
Hubble has no propulsion system. Its orbit slowly decays due to atmospheric drag, and the rate of decay depends on solar activity, which heats and expands the upper atmosphere. A 2012 NASA analysis estimated that without intervention, Hubble would descend to about 500 kilometers altitude around 2027 and eventually make an uncontrolled reentry, carrying a calculated risk of fatality of about 1 in 240.24Elsevier / Journal of Space Safety Engineering. An updated re-entry analysis of the Hubble Space Telescope At 500 kilometers, atmospheric effects would start to exceed the telescope’s ability to maintain stable pointing, with a maximum tolerable rotation rate of about 0.22 degrees per second per axis.
NASA and SpaceX studied a potential mission to boost Hubble’s orbit using a Dragon spacecraft, which would buy additional years of science operations and reduce reentry risk. As of mid-2025, no boost mission has been approved, though Hubble continues to operate with reduced gyroscope capacity: the telescope shifted to single-gyroscope mode in 2024 after several of its six gyroscopes failed or became unreliable. Single-gyroscope mode limits how quickly Hubble can slew between targets and restricts some types of observations, but the core science capability remains intact. Solar Cycle 25 has been more active than initially forecast, increasing drag and potentially shortening the timeline. Whether Hubble gets a boost, a graceful deorbit, or simply falls silent as its hardware gives out remains one of the open questions in space-science policy.