The space age began in 1957 with the launch of Sputnik 1, the first artificial satellite, by the Soviet Union. Cosmonaut Yuri Gagarin became the first human in space in 1961, followed later that year by American astronaut Alan Shepard. The crowning achievement of the early era came on July 20, 1969, when NASA's Apollo 11 mission landed Neil Armstrong and Buzz Aldrin on the Moon. Today, astronauts live and work aboard the International Space Station (ISS), a modular laboratory in low Earth orbit that has been continuously occupied since 2000. The boundary of space itself is often taken to be the Kármán line, about 100 km above Earth's surface. Above the atmosphere, two common orbits dominate: low Earth orbit (LEO), up to about 2,000 km altitude, where the ISS flies, and geostationary orbit (GEO), a circular path at about 35,786 km where satellites match Earth's rotation and appear fixed above a point on the ground. Space debris, the accumulating junk from defunct missions, is an increasing concern for all of these activities.
Robotic probes have revolutionized our understanding of the Solar System. NASA's Voyager 1 and Voyager 2, launched in 1977, performed grand tours of the outer planets and continue to send data from interstellar space, with Voyager 1 now the farthest human-made object from Earth. The New Horizons mission flew by Pluto in 2015, while the Parker Solar Probe is flying closer to the Sun than any previous spacecraft. On Mars, the Curiosity rover has explored the surface since 2012, and Perseverance, which landed in 2021, is searching for signs of past life and caching samples for a future return to Earth. Mission planning relies on concepts like delta-v, the velocity change needed for a maneuver, the rocket (Tsiolkovsky) equation linking velocity to mass and exhaust velocity, and gravity assists, in which a spacecraft steals a tiny bit of a planet's orbital momentum to change course and speed.
Beyond our Solar System, astronomers have confirmed more than 5,000 exoplanets, planets orbiting other stars. Detection methods include the transit method, which observes tiny dips in stellar brightness as a planet crosses in front of its star, the radial velocity method, which measures the wobble a planet induces in its host star, microlensing, which uses gravitational distortion of distant starlight, and direct imaging, which blocks out the host star's glare. Of particular interest is the habitable zone around a star, the region where temperatures could allow liquid water on a planet's surface. Saturn's moon Enceladus and especially Titan, with its thick atmosphere and liquid hydrocarbon lakes, are tantalizing astrobiological targets within our own Solar System.
The possibility of life beyond Earth is formalized by the Drake Equation, a probabilistic estimate of the number of detectable communicating civilizations that considers factors such as the star formation rate, the fraction of stars with planets, the fraction of habitable planets where life emerges, the fractions that develop intelligence and technology, and the expected lifetime \(L\) of such civilizations. The Fermi Paradox highlights the apparent contradiction between a high probability of alien life and the lack of evidence for it. The Search for Extraterrestrial Intelligence, or SETI, scans the skies for artificial signals, while the panspermia hypothesis suggests that life, or its building blocks, may have been delivered to Earth from elsewhere. Underlying all of this, astrobiology studies the origin, evolution, and possible distribution of life in the universe. Closer to home, the Sun's roughly 11-year cycle of magnetic activity drives space weather, including solar flares, coronal mass ejections, and geomagnetic storms. These can produce beautiful auroras when charged particles stream into Earth's magnetic field, are funneled by the Van Allen radiation belts, and sometimes disrupt satellites and power grids on the ground.