Design and build a space telescope is something that takes a long time. In fact, when the 2.0 Hubble was launched into space, astronomers had already been several months working on the Hubble 3.0, which so far has been dubbed ATLAST.
Scientists have plans to equip Hubble with a mirror 3.0 quite large, since the larger the mirror leads fainter the images will be obtained. How big is this mirror? To give you an idea, the Hubble 2.0 has a mirror diameter of 6.5 meters, while the largest ground-based telescope is one of 13 meters, but the Hubble mirror diameter 3.0 will be 16 meters and actually be formed by a group of several mirrors. With this mirror, the ATLAST images will be up to 10 times better than his brother.
Besides this, the Hubble 3.0 will be used to take data from the surface and atmosphere of potential habitable exoplanets that are even 200 light years away.
