Hubble Space Telescope 23rd anniversary
To celebrate its 23rd year in orbit, the NASA/ESA Hubble Space Telescope has released a stunning new image of one of the most distinctive objects in our skies: the Horsehead Nebula. This image shows the nebula in a whole new light, capturing plumes of gas in the infrared and revealing a beautiful, delicate structure that is normally obscured by dust.
This year marks the 23rd year of observing for the Hubble Space Telescope. Alongside cutting-edge science, the orbiting observatory has produced countless stunning astronomical images. Some of the most striking and beautiful subjects of Hubble’s images have been nebulae — vast interstellar clouds of gas and dust.

This new Hubble image, captured and released to celebrate this milestone, shows part of the sky in the constellation of Orion (The Hunter). Rising like a giant seahorse from turbulent waves of dust and gas is the Horsehead Nebula, otherwise known as Barnard 33. The nebula formed from a collapsing interstellar cloud of material, and glows as it is illuminated by a nearby hot star.
The gas clouds surrounding the Horsehead have already dissipated, but the jutting pillar is made of stronger stuff — thick clumps of material — that is harder to erode. Astronomers estimate that the Horsehead formation has about five million years left before it too disintegrates.

This nebula is a very well-known object and a popular target for observations, most of which show the Horsehead as a dark cloud silhouetted against a background of glowing gas. This new image shows the same region in infrared light, which has longer wavelengths than visible light and can pierce through the dusty material that usually obscures the nebula’s inner regions. The result is a rather ethereal and fragile-looking structure, made of delicate folds of gas — very different to the nebula’s appearance in visible light.
We cannot see infrared radiation with our eyes or with standard cameras, which are designed to detect optical light. To observe these objects, we have infrared-sensitive telescopes or instruments — for example, Hubble’s high-resolution Wide Field Camera 3, fitted in 2009. Hubble’s pairing of infrared sensitivity and unparalleled resolution offers a tantalising hint of what we will be able to achieve with the upcoming James Webb Space Telescope, set for launch in 2018.

History: The Spherical Aberration Problem
Hubble’s main mirror being polished before installation. The edges of the were polished very slightly too flat, leaving the telescope unable to focus perfectly.
We have come to take the excellent performance of the Hubble Space Telescope for granted. However, immediately after launch people were reminded that Hubble was not just an ordinary satellite, but a complex piece of innovative engineering and, as such, liable to experience teething problems.
The most serious and notorious of these was an optical defect called spherical aberration, which was caused by the malfunction of a measuring device used during the polishing of the mirror made of Zerodur glass (Schott Glasworks). As a result, Hubble could not achieve the best possible image quality, although still outperforming ground-based telescopes in many ways. Analysing the problem and developing an optical correction was a masterpiece of optical engineering and an outstanding example of the valuable collaboration between engineers and scientists from both America and Europe.
During the first Hubble Servicing Mission in December 1993, a crew of astronauts carried out the repairs necessary to restore the telescope to its intended level of performance.
The Wide Field and Planetary Camera 1 (WFPC1) was replaced with a second-generation camera, Wide Field and Planetary Camera 2 (WFPC2), which was designed to precisely counteract the error in the main mirror’s shape. This led to a dramatic increase in image quality.
The astronauts also removed the High-Speed Photometer instrument and replaced it with COSTAR (Corrective Optics Space Telescope Axial Replacement). COSTAR was not a science instrument but a collection of movable curved mirrors which could correct the path of light as it passed into the other three remaining scientific instruments: the Faint Object Camera (FOC), the Faint Object Spectrograph (FOS), and the Goddard High Resolution Spectrograph (GHRS). COSTAR in effect acted like a giant pair of spectacles, refocusing the light and letting the instruments see clearly again.
In subsequent missions, the Faint Object Camera, Faint Object Spectrograph and Goddard High Resolution Spectrograph were all replaced by new instruments with built in optical correction, like Wide Field and Planetary Camera 2. This meant that, by the time of the final servicing mission in 2009, COSTAR was no longer needed. Removing COSTAR made room for a new scientific instrument to be installed, the Cosmic Origins Spectrograph.

This new Hubble image, captured and released to celebrate the telescope’s 23rd year in orbit, shows part of the sky in the constellation of Orion (The Hunter). Rising like a giant seahorse from turbulent waves of dust and gas is the Horsehead Nebula, otherwise known as Barnard 33.
This image shows the region in infrared light, which has longer wavelengths than visible light and can pierce through the dusty material that usually obscures the nebula’s inner regions. The result is a rather ethereal and fragile-looking structure, made of delicate folds of gas — very different to the nebula’s appearance in visible light.
Credit: NASA, ESA, and the Hubble Heritage Team (AURA/STScI)