NASA officials have told Congress that the Hubble Space Telescopes successor, the James Webb Space Telescope, is currently on-time and budget after early delays and cost overruns threatened to doom the project. The JWST offers a number of improvements and enhancements over the Hubble, though NASA has said publicly that it prefers to think of the next-generation telescope as a successor rather than a replacement.

The JWSTs primary mirror is far larger than Hubbles, at over 21 feet in diameter, compared with about 8 feet for Hubble. The new telescopes collecting area is more than 7x larger, and it uses a new type of mirror thats never been deployed at this scale in a space-based telescope. Each pane of beryllium (there are 18 in total) can be adjusted independently of the others. Hubble, in contrast, deployed a conventionally ground mirror.

Despite the vast difference in size, Webbs mirror actually weighs a fraction of what Hubbles does. The conventional mirror aboard the Hubble clocks in at 2,200 lbs, compared to just 1,375 lbs for the JWST.

Webbs mirror design and larger gathering area will allow it to peer farther back in time all the way back to 100 million to 250 million years after the Big Bang, compared with 800 million to 1 billion years for Hubble. The Webb isnt optimized for optical wavelengths its specialization is the infrared spectrum but itll be capable of imaging in spectrums friendly to the human eye, and with far greater resolution than what Hubble can achieve. The image below is a simulation of how the Webb is expected to perform compared with Hubble.

The James Webbs detectable spectrum is also much larger than its predecessors. While still tiny next the largest Earth-based telescopes, the deep infrared capabilities of the Webb telescope and its location should allow it to assist in the ongoing search for habitable planets and even alien life.

The James Webb telescope is years behind schedule, and substantially over its originally proposed budget, which raises the question of whether the telescope can overcome these initial obstacles and achieve successful results. Many of these issues originally plagued the Hubble telescope. Originally, the HST was meant to launch in 1983, but a similar morass of technical issues and the unexpected loss of the Challenger delayed deployment until 1990. After launch, NASA discovered that the HSTs main mirror had been incorrectly ground. For three years, Hubbles mission was severely curtailed, as NASA built a corrective lens to be installed over the improperly ground mirror.

With the James Webb, there will be no such margin for error. Unlike Hubble, which was placed in Earth orbit and designed to be serviced by the shuttle, the Webb will be placed in Sun-Earth L2 LaGrange points. LaGrange Points are positions in space where the gravitational pull of large masses equal the centripetal force required for a small object (in this case, a telescope) to move with them. An object in Earths L2 point remains in approximately the same position relative to our planet and the Sun, and can maintain its location with minimal use of fuel.

More than 20 years after deployment, the Hubble Space Telescope has become one of the most enduring missions NASA has ever launched. For many people, the images Hubble beams back of our universe are the public face of NASA, and thats a mission every bit as critical as scientific research. The James Webb telescope will inherit that mantle when it launches in 2018 heres hoping for a smooth debut and a long, storied career.

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NASA confirms Hubble telescope successor is on-time, on-budget for 2018