The three astronauts of China's Shenzhou 21 mission currently have no safe way to get home from the Tiangong space station in the event of an emergency. But that situation is likely to change soon.
Quasars acting as strong gravitational lenses are among the rarest finds in astronomy. Out of nearly 300,000 quasars catalogued in the Sloan Digital Sky Survey, only twelve candidates were identified, and just three confirmed. These systems are exceptionally valuable because they allow astronomers to precisely measure the mass of a quasar's host galaxy, something that is normally impossible given that the overwhelming brightness of the quasar itself drowns out its surroundings.
