An interstellar probe is a space probe that has left—or is expected to leave—the Solar System and enter interstellar space, which is typically defined as the region beyond the heliopause. It also refers to probes capable of reaching other star systems. There are five interstellar probes, all launched by the American space agency NASA: Voyager 1, Voyager 2, Pioneer 10, Pioneer 11 and New Horizons. As of 2019, Voyager 1, Voyager 2 and Pioneer 10 are the only probes to have actually reached interstellar space. The other two are on interstellar trajectories. The termination shock is the point in the heliosphere where the solar wind slows down to subsonic speed. Even though the termination shock happens as close as 80–100 AU (astronomical units) the maximum extent of the region in which the Sun's gravitational field is dominant (the Hill sphere) is thought to be at around . This point is close to the nearest known star system, Alpha Centauri, located 4.36 light years away. Although the probes will be under the influence of the Sun for a long time, their velocities far exceed the Sun's escape velocity, so they are leaving forever. Interstellar space is defined as the space beyond a magnetic region that extends about 122 AU from the Sun, as detected by Voyager 1, and the equivalent region of influence surrounding other stars. Voyager 1 entered interstellar space in 2012. Currently, three projects are under consideration: Chinese Interstellar Express, NASA's Interstellar Probe, and StarChip from the Breakthrough Initiatives. Overview Planetary scientist G. Laughlin noted that with current technology a probe sent to Alpha Centauri would take 40,000 years to arrive, but expressed hope for new technology to be developed to make the trip within a human lifetime. On that timescale the stars move notably. As an example, in 40,000 years Ross 248 will be closer to Earth than Alpha Centauri. One technology that has been proposed to achieve higher speeds is an E-sail. By harnessing solar wind, it might be possible to achieve 20-30 AU per year without even using propellant. List of interstellar probes Functional spacecraft Voyager 1 (1977–)Voyager 1 is a space probe launched by NASA on September 5, 1977. At a distance of about as of , it is the farthest manmade object from Earth. It was later estimated that Voyager 1 crossed the termination shock on December 16, 2004 at a distance of 94