The European Space Agency (ESA) has chosen to focus its next two large missions on "The Hot and Energetic Universe", and the "The Gravitational Universe".
In order to pursue the theme of the hot and energetic universe, ESA is expected to launch an X-ray telescope, such as the Advanced Telescope for High-Energy Astrophysics (ATHENA), around the year 2028. ATHENA, which would observe from space, would be able to detect X-rays, which have extremely high energies. This is ideal for studying hot, X-ray emitting gas. This hot gas is the dominant form of regular matter in galaxy clusters, which are the largest structures in the universe, containing hundreds or thousands of galaxies. Information about this hot gas can help us learn about how these enormous clusters formed. ATHENA will also be great for detecting the X-ray emission from the regions around the supermassive black holes that live in the centers of most galaxies. It is expected to be able to observe the earliest supermassive black holes, helping us learn about how black holes, and the galaxies they live in, evolve and grow over time.
A new X-ray observatory is great news for those who study the high energy universe. NASA currently has no future X-ray telescope missions planned, and the two X-ray telescopes it has now are aging. NASA's Chandra X-ray Observatory was launched in 1993, and Swift, a space observatory with an X-ray telescope on board, was launched in 2004. ATHENA ensures we will have the capability to observe the high energy universe when current X-ray telescopes cease operations.
ESA also plans to focus its efforts on detecting gravitational waves. Just like we have light waves due to the electromagnetic force, scientists expect there to be waves from the force of gravity that manifest as ripples in spacetime. Strong gravitational waves are produced in systems where two astronomical objects orbit around each other very close and fast, or from astronomical objects merging. eLISA, or the evolved Laser Interferometer Space Antenna, would be able to detect these gravitational waves from space. eLISA is actually comprised of three spacecraft that will orbit the Sun in a triangle formation. Lasers will connect the three spacecraft, and if a gravitational wave passes through, eLISA will detect a small change in the distance between particles in the spacecraft. We have yet to actually detect a gravitational wave, meaning that eLISA could fundamentally affect our understanding of gravity. eLISA is actually a reboot of the formerly planned LISA mission, on which NASA was a partner until funding limitations required they back out. The eLISA collaboration will be launching the LISA Pathfinder spacecraft in 2015 to test the technologies that eLISA will rely on in a space environment.
It's important to note that these mission launches are still far off in the future. ATHENA wouldn't be ready to launch for at least 15 years, while eLISA is planned for launch around the year 2034. Missions of this caliber can take decades to plan and develop, and unforeseen issues could push these scheduled launch dates back even further. However, this kind of forward thinking is essential for insuring that the next generation of astronomers has the tools they need to carry the field forward.