Interstellar probe

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: Voyager 1, Voyager 2, Pioneer 10, Pioneer 11 and New Horizons. As of 2015, Voyager 1 is the only probe to have actually reached interstellar space. The other four 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, 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 Sun's escape velocity, so they will eventually leave forever.

Interstellar Probe (1999)

Interstellar Probe is the name of a proposed NASA space probe intended to travel out 200 AU in 15 years, studied in 1999. This 1999 study by JPL is noted for its 200-meter 1g/m2 solar sail as a propulsion method combined with a 0.25 AU flyby of the Sun to achieve higher solar pressure after which the sail is jettisoned at 5 AU distance from the Sun. Solar sails work by converting the energy in light into the momentum of spacecraft, thus propelling the spacecraft. Felix Tisserand noted the effect of light pressure on comet tails in the 1800s. Note that this is talking about actual electromagnetic light, not the particles in the solar wind.

It proposed using a solar sail to accelerate a spacecraft to reach the interstellar medium, and was designed by the NASA Jet Propulsion Laboratory as a study. It was planned to reach as far as 200 AU within 10 years at a speed of 14 AU/year (about 70 km/s, and function up to 400+ AU. A critical technology for the mission is a large 1 g/m² solar sail. The probe itself hoped to use an advanced RTG for electrical power, Ka-band radio for communication with Earth, Delta 2 for Earth launch, and a 25 kg instrument package using 20 watts.