Of the major spacecraft, Voyager 1, Voyager 2, and New Horizons are still functioning and are regularly contacted by radio communication, while Pioneer 10 and Pioneer 11 are now derelict. In addition to these spacecraft, some third stages and de-spin weights are leaving the Solar System, assuming they continue on their trajectories.
These objects are leaving the Solar System because their velocity and direction are taking them away from the Sun, and at their distance from the Sun, its gravitational pull is not sufficient to pull these objects back or into orbit. They are not impervious to the gravitational pull of the Sun and are being slowed, but are still traveling in excess of escape velocity to leave the Solar System and coast into interstellar space.
Although other probes were launched first, Voyager 1 has achieved a higher speed and overtaken all others. Voyager 1 overtook Voyager 2 a few months after launch, on 19 December 1977. It overtook Pioneer 11 in 1983, and then Pioneer 10—becoming the probe farthest from Earth—on February 17, 1998.
Depending on how the "Pioneer anomaly" affects it, New Horizons will also probably pass the Pioneer probes, but will need many years to do so. It will not overtake Pioneer 11 until the 22nd century, will not overtake Pioneer 10 until the end of that century, and will never overtake the Voyagers.
The table below is arranged starting from the farthest probe from the Sun. For comparison, Pluto's average distance (semi-major axis) is about 40 AU.
(as of 2020)
Solar escape velocity is a function of distance (r) from the Sun's center, given by
where the product G Msun is the heliocentric gravitational parameter. The initial speed required to escape the Sun from its surface is 618 km/s (1,380,000 mph), and drops down to 42.1 km/s (94,000 mph) at Earth's distance from the Sun (1 AU), and 4.21 km/s (9,400 mph) at a distance of 100 AU.
Voyager 1 and 2 speed and distance from the Sun
Pioneer 10 and 11 speed and distance from the Sun
New Horizons speed and distance from the Sun.
Every planetary probe was placed into its escape trajectory by a multistage rocket, the last stage of which ends up on nearly the same trajectory as the probe it launched. Because these stages cannot be actively guided, their trajectories are now different from the probes they launched (the probes were guided with small thrusters that allow course changes). However, in cases where the spacecraft acquired escape velocity because of a gravity assist, the stages may not have a similar course and there is the remote possibility that they collided with something. Some objects in heliocentric orbit have been reidentified with a telescope, and so their trajectories can be confirmed. The stages on an escape trajectory are:
In addition, two small yo-yo de-spin weights on wires were used to reduce the spin of the New Horizons probe prior to its release from the third-stage rocket. Once the spin rate was lowered, these masses and the wires were released, and so are also on an escape trajectory out of the Solar System. None of these objects are trackable (they have no power or radio antennae, spin uncontrollably, and are too small to be detected), and their exact positions are unknowable beyond their projected Solar System escape trajectories.
The third stage of Pioneer 11 is thought to be in solar orbit because its encounter with Jupiter would not have resulted in escape from the Solar System.[better source needed] Pioneer 11 gained the required velocity to escape the Solar System in its subsequent encounter with Saturn.[dubious ]
The only objects to date to be launched directly into a solar escape trajectory were the New Horizons spacecraft, its third stage, and the two de-spin masses. The New Horizons Centaur (second) stage is not escaping; it is in a 2.83-year heliocentric (solar) orbit.
In order to leave the Solar System, the probe needs to reach the local escape velocity. After leaving Earth, the Sun's escape velocity is 42.1 km/s. In order to reach this speed, it is highly advantageous to use the orbital speed of the Earth around the Sun, which is 29.78 km/s. By passing near a planet, a probe can gain extra speed with a gravity assist.
On January 19, 2006 the New Horizons spacecraft to Pluto was launched directly into a solar-escape trajectory at 16.26 kilometers per second (58,536 km/h; 36,373 mph) from Cape Canaveral using an Atlas V version with 5 of the AJ-60A SRBs and the Common Core Booster, Centaur upper stage, and Star 48B third stage. New Horizons passed the Moon's orbit in just nine hours.
Propulsive units for New Horizons, which was launched directly to a solar escape velocity from Earth: