The largest space telescope ever built is the James Webb Space Telescope (JWST). It is set to launch in 2021.
The JWST has a primary mirror that is 6.5 meters (21.3 feet) in diameter, which is over 2.5 times larger than the primary mirror of the Hubble Space Telescope. The JWST is an infrared telescope, meaning it is designed to detect infrared radiation, which is light with longer wavelengths than what the human eye can see.
This allows it to see through dust clouds and study some of the earliest galaxies in the universe as well as study exoplanets and their atmospheres. It is a collaboration between NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA).
The JWST will be positioned at the second Lagrange point (L2), a stable point in space about 1.5 million kilometers (about 930,000 miles) from Earth, where it will be able to observe the cosmos without the interference of the Earth’s atmosphere or its own heat signature.