HST

The Hubble Space Telescope (HST) is one of the most recognizable scientific instruments ever built, a large optical observatory that has been circling Earth continuously since its launch in April 1990. Catalogued under NORAD ID 20580 and international designator 1990-037B, it remains in low Earth orbit and continues to conduct astronomical observations decades after its deployment. More than any other space-based instrument of its era, HST transformed both the practice of professional astronomy and the broader public's relationship with the cosmos, producing imagery and data that have fundamentally altered scientific understanding of the universe.

Mission and Purpose

The Hubble Space Telescope was conceived as a solution to a persistent limitation of ground-based astronomy: the distorting effect of Earth's atmosphere. By placing a large, precise optical telescope above the atmospheric boundary, scientists could capture light — across ultraviolet, visible, and near-infrared wavelengths — without the blurring and absorption that ground-based observatories must contend with. The result would be images of exceptional sharpness and sensitivity, capable of resolving objects at cosmological distances.

Named in honor of Edwin Hubble, the American astronomer whose observations in the early twentieth century established that galaxies exist beyond the Milky Way and that the universe is expanding, the telescope carries a legacy tied directly to the largest questions in astrophysics. Those questions — about the age and expansion rate of the universe, the life cycles of stars, the structure of galaxies, and the existence of exoplanets — have all been addressed, in some measure, through HST's observing campaigns.

HST is classified within NASA's Great Observatories program, a series of four large, complementary space-based telescopes each sensitive to a different region of the electromagnetic spectrum. While the other Great Observatories targeted gamma rays, X-rays, and infrared light, HST's primary coverage of optical and ultraviolet wavelengths made it uniquely suited to a vast range of astronomical targets, from nearby planets to the most distant galaxies yet observed.

Mission operations are divided between two institutions. The Goddard Space Flight Center, located in Greenbelt, Maryland, serves as the spacecraft operator — managing the engineering health of the observatory, conducting communications, and overseeing orbital maintenance. The Space Telescope Science Institute, located in Baltimore at Johns Hopkins University, is responsible for the scientific side: selecting observation targets, scheduling the telescope's time, and processing and archiving the scientific data returned. This division of responsibility has allowed each organization to focus on its core competency while maintaining a highly productive observatory over a multi-decade operational lifetime.

The telescope has undergone a series of in-orbit servicing missions carried out by Space Shuttle crews, which replaced and upgraded instruments and corrected the infamous spherical aberration discovered in its primary mirror shortly after launch. These servicing missions — a capability that no other large space telescope has enjoyed to the same degree — have kept HST at the forefront of observational astronomy far longer than its original design life would have allowed.

Orbit and Tracking

HST occupies a nearly circular low Earth orbit with an apogee of 487 km and a perigee of 485 km, giving it an exceptionally low eccentricity. This tight, stable orbit keeps the telescope at a consistent altitude, which simplifies thermal management and reduces variability in the observing environment. The inclination of 28.5° reflects the latitude of Kennedy Space Center in Florida, from which it was launched — a common characteristic of payloads sent to orbit from that facility without requiring the fuel expenditure of a high-inclination insertion.

At this altitude, HST completes one full orbit of Earth approximately every 94.2 minutes, meaning it circles the planet roughly 15 to 16 times per day. The telescope's ground track covers latitudes between 28.5° north and 28.5° south, and at an orbital speed consistent with low Earth orbit, the spacecraft moves across the sky relatively quickly when visible from the ground.

The orbit is not entirely self-sustaining. Atmospheric drag, even at nearly 500 km altitude, is sufficient to cause a slow orbital decay over time. Historically, servicing missions included reboost maneuvers to raise HST's altitude and extend its orbital lifetime. Without such reboosts, the telescope will eventually reenter the atmosphere, though the timeline for natural decay from its current altitude spans many years. Planning for a controlled or managed reentry at end of life has been a topic of ongoing discussion given the spacecraft's mass of 12,247 kg, which would make an uncontrolled reentry a matter of public safety concern.

For tracking purposes, HST is monitored continuously through the global network of radar and optical sensors that contribute to the public satellite catalog. Its relatively large cross-section and well-known orbital parameters make it one of the easier large payloads to track with precision.

Design and Operator

HST was manufactured by PerkinElmer, the company responsible for fabricating and polishing the telescope's optical components, including its 2.4-meter primary mirror. The overall spacecraft integration involved multiple contractors and NASA centers, but PerkinElmer's role in producing the optical heart of the observatory was central to its design. The discovery after launch that the primary mirror had been ground to the wrong prescription — off by a fraction of a hair's width — led to the development and installation of corrective optics during the first servicing mission, restoring the telescope's full resolving power.

The spacecraft has a total mass of 12,247 kg, making it one of the heavier payloads ever deployed by the Space Shuttle. Its cylindrical body, roughly the size of a large school bus, carries the main optical tube assembly along with a suite of scientific instruments, solar arrays for power generation, and pointing and stabilization systems that allow it to lock onto a target with extraordinary precision.

Operational authority rests with the Goddard Space Flight Center, which functions as the managing NASA center for the mission. The United States is the owning country of the spacecraft, and the observatory is operated as a national and, in practice, international scientific resource — observing time has been allocated to researchers from institutions around the world throughout its history.

Legacy and Current Status

HST's contributions to astronomy are extensive and well-documented in the scientific literature. Among its most consequential results are observations that helped establish the accelerating expansion of the universe — findings tied to the discovery of dark energy, which earned a Nobel Prize in Physics in 2011. The telescope's measurements of Cepheid variable stars in distant galaxies provided some of the most precise early determinations of the Hubble constant, the parameter describing the universe's expansion rate. Its deep field images revealed the universe to be populated by hundreds of billions of galaxies, a result that reshaped understanding of cosmic structure.

Beyond the science, HST's public impact has been significant. Images produced from its data — rendered in false color to represent wavelengths invisible to the human eye — have become some of the most widely reproduced photographs in the history of science. They have appeared in educational materials, public exhibitions, news media, and popular culture across the globe, making HST a touchstone for public engagement with space and astronomy.

As of the time this entry was prepared, HST remains in orbit and operational status is not definitively characterized in the public satellite catalog for this object. The telescope has outlasted its original design life by a considerable margin, a testament to both the engineering quality of its construction and the maintenance investment of successive servicing missions. NASA has studied options for the telescope's long-term future, including possible assisted reentry to control where debris might fall when the spacecraft eventually deorbits.

How to Spot It

HST is large and travels in a relatively bright, predictable orbit, making it visible to the naked eye from the ground under favorable conditions. With a mass of over 12,000 kg and large solar panels extending from its cylindrical body, the telescope reflects sufficient sunlight to be seen as a steadily moving point of light in the minutes after dusk or before dawn, when the satellite is illuminated by the sun but the observer below is in darkness.

Because its orbital inclination is 28.5°, HST passes over a swath of Earth between roughly 28.5° north and south latitude on each orbit, though its ground track shifts westward with successive passes. Observers at latitudes within or near this band have the best opportunities for overhead or near-overhead passes, which produce the brightest apparitions. Those at higher latitudes may still observe it crossing lower in the sky.

Satellite tracking tools — including the real-time predictions available on this site — can provide precise pass times, directions, and brightness estimates for any given location. Given the 94.2-minute orbital period, multiple passes may occur on a single evening, and the satellite can traverse the visible sky in a matter of minutes. No optical aid is necessary for bright passes; binoculars will reveal no additional detail but can help confirm the object's identity by showing the characteristic brightness variation as the satellite's solar panels rotate relative to the observer's line of sight.