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. As of 2024, there are five interstellar probes, all launched by the American space agency NASA: Voyager 1, Voyager 2, Pioneer 10, Pioneer 11 and New Horizons. Also as of 2025, Voyager 1 and Voyager 2 are the only probes to have actually reached interstellar space. The other three are on interstellar trajectories. Contact to Pioneer 10 and 11 was lost long before they reached interstellar space. The termination shock is the point in the heliosphere where the solar wind slows down to subsonic speed. In this context, sonic refers to the speed of sound in a plasma like that found in interstellar space. Calculating the speed of sound in interstellar space is complex due to high variation in density which is coupled to other plasma physics parameters. In rough and relatable magnitudes, sonic speed in interstellar space is around 225,000 miles/hour. In comparison, the speed of sound (in air) at sea level on earth is around 767 miles/hour. 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 230,000 astronomical units (3.6 light-years). 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: CNSA's Shensuo, 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 162.755 AU (2.435×1010 km) as of 2025, 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 AU from the Sun. At the end of 2011, Voyager 1 entered and discovered a stagnation region where charged particles streaming from the Sun slow and turn inward, and the Solar System's magnetic field is doubled in strength as interstellar space appears to be applying pressure. Energetic particles originating in the Solar System declined by nearly half, while the detection of high-energy electrons from outside increases 100-fold. The inner edge of the stagnation region is located approximately 113 astronomical units (AU) from the Sun. In 2013 it was thought Voyager 1 crossed the heliopause and entered interstellar space on August 25, 2012 at distance of 121 AU from the Sun, making it the first known human-manufactured object to do so. As of 2025, the probe was moving with a relative velocity to the Sun of about 16.95 km/s (3.58 AU/year). If it does not hit anything, Voyager 1 could reach the Oort cloud in about 300 years.
Voyager 2 (1977–) Voyager 2 crossed the heliopause and entered interstellar space on November 5, 2018. It had previously passed the termination shock into the heliosheath on August 30, 2007. As of 2025, Voyager 2 is at a distance of 141.43 AU (2.116×1010 km) from Earth. The probe was moving at a velocity of 3.25 AU/year (15.428 km/s) relative to the Sun on its way to interstellar space in 2013. It is moving at a velocity of 15.2 km/s (55,000 km/h) relative to the Sun as of November 2025. Voyager 2 is expected to provide the first direct measurements of the density and temperature of the interstellar plasma.
New Horizons (2006–) New Horizons was launched directly into a hyperbolic escape trajectory, getting a gravitational assist from Jupiter en route. By March 7, 2008, New Horizons was 9.37 AU from the Sun and traveling outward at 3.9 AU per year. It will, however, slow to an escape velocity of only 2.5 AU per year as it moves away from the Sun, so it will never catch up to either Voyager. As of late 2025, it was traveling at 2.87 AU/year (13.68 km/s) relative to the Sun. On July 14, 2015, it completed a flyby of Pluto at a distance of about 33 AU from the Sun. New Horizons next encountered 486958 Arrokoth on January 1, 2019, at about 43.4 AU from the Sun the furthest object ever explored by a spacecraft. The Heliosphere's termination shock was crossed by Voyager 1 at 94 astronomical units (AU) and Voyager 2 at 84 AU according to the IBEX mission. If New Horizons can reach the distance of 100 AU, it will be traveling at about 13 km/s (29,000 mph), around 4 km/s (8,900 mph) slower than Voyager 1 at that distance.
Inactive missions
Pioneer 10 (1972–2003) The last successful reception of telemetry from Pioneer 10 was on April 27, 2002, when it was at a distance of 80.22 AU, and the last signal from the spacecraft was received on January 23, 2003, at a distance 82 AU from the Sun traveling at about 2.54 AU/year (12 km/s).
Pioneer 11 (1973–1995) Routine mission operations for Pioneer 11 were stopped September 30, 1995, when it was 6.5 billion km (approx 43.4 AU) from Earth, traveling at about 2.4 AU/year (11.4 km/s).
Probe debris New Horizons' third stage, a STAR-48 booster, is on a similar escape trajectory out of the Solar System as New Horizons, but will pass millions of kilometers from Pluto. It crossed Pluto's orbit in October 2015. The third stage rocket boosters for Pioneer 10, Voyager 1, and Voyager 2 are also on escape trajectories out of the Solar System.
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