On December 14, 1962, Mariner 2 became first spacecraft to visit another planet, and it was Venus. Then spacecrafts became visitors to the red planet. Many orbited Mars, and many landed on it.

As soon as we started space exploration, we started sending spacecrafts towards other planets in the solar system. Keeping in mind that the space era began in 1957 with the first artificial satellite launched by the Soviet Union, the first spacecraft to pass near the Moon was Luna 1 of the Soviet Union in January 1959.

With this, we started exploring the solar system. On December 14, 1962, Mariner 2 became the first spacecraft to visit another planet, and it was Venus. Then spacecrafts became visitors to the red planet. Many orbited Mars, and many landed on it.

Outer planets of solar system were difficult to reach due to their distances, but we launched spacecrafts towards them. This article is about those spacecrafts that were launched towards outer planets of solar system and now have escaped the system of planets, travelling in the outer reaches of the solar system, sometimes called interstellar space.

Pioneer 10

It was launched on March 3, 1972, towards Jupiter. Its systems were powered by its onboard RTG (radioisotope thermoelectric generator). It is an electricity generator using heat from the decay of radioactive elements.

It was the first ever spacecraft to cross the asteroid belt (a region of rocks between Mars and Jupiter) and reach Jupiter. In November 1973, it started photographing Jupiter. It sent 500 images of Jupiter and also analysed the asteroid belt and the solar wind (the stream of charged particles emitting constantly from the Sun). It carried many instruments and cameras to do its scientific job.

Like a routine mechanism, Pioneer 10 also carried onboard fuel for course correction. All spacecrafts carry fuel, and if they are a little off target, scientists command them to use fuel to orient themselves back on the correct path.

After passing by the planet Jupiter, it continued its motion but didn’t pass near any other gas giants. On June 13th, 1983, it crossed the orbit of Neptune (but did not come near the planet) and became the first spacecraft to do so. On March 31, 1997, the mission officially ended while the spacecraft was still in contact with mission control.

The final signal was received on January 23, 2003. As of 2023, the Pioneer 10 spacecraft is heading towards the constellation Taurus, which is currently 135 AU (Astronomical Units) from Earth.

One AU is the distance between Earth and the Sun. Spacecraft also carried a plaque (a plate) with information about humans and the position of Earth written on it. This is placed on a spacecraft in case it is captured by some extraterrestrial beings, and they wonder what this object is and who sent it.

Pioneer 11

It was launched on April 5, 1973, to study the asteroid belt, Jupiter and Saturn, and the solar wind. It became the first spacecraft to pass near Saturn. It passed near Jupiter in November-December 1974. It obtained details about the planet and its great red spot. It passed near Saturn on September 1, 1979. On November 24, 1995, the last signal was received from the spacecraft.

This was the first spacecraft to closely observe the largest satellite of the planet Saturn, called Titan. Titan became a target for later spacecraft and proved itself to be a very interesting object in the solar system. It contains liquid lakes (the only place to have lakes after Earth).

As of 2023, Pioneer 11 is 112 AU from Earth and going towards the constellation Scutum.

Pioneer anomaly

Both Pioneers showed a slight deceleration (slowing down) effect. This became famous as a pioneer anomaly. This anomaly even sparked discussion about re-understanding the nature of gravity, which means a new theory of gravity.

But later it was discovered that this slowing down or braking is due to heat emitted by RTGs of pioneers into space in the opposite direction of their motion. RTGs generate heat, which is converted into electricity, but this heat is more than we need on spacecraft, so we discard it into space. This heat emission causes action and reaction effects on spacecraft.

Voyager 1

Voyager 1 is a famous spacecraft launched by NASA (NASA also launched Pioneers) towards the planets Jupiter and Saturn of solar system on September 5, 1977. Currently, it is at 159 AU from Earth, and after 45 years since its launch, it is still in contact with mission control on Earth.

It is also an RTG-operated spacecraft with many instruments to observe planets and detect solar wind and magnetic fields in space. Currently, its radio signal takes 22 hours to reach Earth (as all electromagnetic waves travel with the same velocity as light) while travelling at 3,00,000 km/s.

When Voyager 1 passed near Saturn and its satellite systems, NASA had a choice of either doing a Titan flyby mission or changing the path of the spacecraft and leaving the Titan flyby for a future Pluto flyby mission. The experts choose Titan! Had this spacecraft been sent to Pluto, it would have been the first spacecraft to visit Pluto. This title is now carried by the New Horizons spacecraft.

It made its closest Jupiter flyby on March 5, 1979, and then its closest flyby of Saturn on November 12, 1980. Its RTG carried Plutonium-238 oxide spheres. It was Voyager 1 that discovered active volcanoes on Jupiter’s satellite “Io”. Voyager 1 made detailed images of volcanoes and surface features on other satellites of Jupiter.

On November 12, 1980, voyager 1 passed near Saturn. It imaged the planet with great detail, including its ring system and winds. Voyager 1 also measured the helium percentage in the atmosphere of Saturn and Jupiter. It showed that in the atmosphere of both gas giants, the primary gas is still hydrogen, while helium has a lower percentage. Titan’s hazy atmosphere was detected by the spacecraft.

After passing near Saturn and its satellite systems, Voyager 1 continued moving outward without any planned flyby of any of the objects. It is still measuring solar wind and is now considered the farthest human-made object. It has crossed the heliosphere (the region where the solar wind flows outward) and is now travelling in interstellar space. It is believed that a spacecraft entered interstellar space on August 25, 2012.

It is expected that spacecraft will remain in contact until 2025, after which RTG will not provide enough energy to power up instruments, including communications. It also carries a golden record with many voices, images, and other information about humanity in case it is captured by any extraterrestrial intelligent beings.

Voyager 2

Voyager 2 has the pride of being the only spacecraft to visit Uranus and Neptune. It also visited Jupiter and Saturn, hence another pride of visiting all four gas giants by a single spacecraft! The visit of Voyager 2 is often called the “grand tour”. After visiting all four gas giants, Voyager 2 continues its journey out of the solar system and is currently 133 AU from Earth.

It was launched on August 20, 1977. After launch, it passed close to Jupiter on July 9, 1979; close to Saturn on August 26, 1979; Uranus on January 24, 1986; and Neptune on August 25, 1989. It discovered 11 moons and rings around Uranus. It also discovered six new moons and rings around Neptune.

Having the same power source as Voyager 1 and almost the same launch time, it will be out of contact sometime after 2025 due to the decay of RTGs. It also contains a golden disc containing information in the form of sounds, pictures, and text telling a small story of humanity. It has now overtaken Pioneer 10 and become the second most distant object humans have launched.

All the close-up images of Neptune and Uranus we see in all books were obtained from the Voyager 2 spacecraft. Neptune proved to be a little more active than Uranus. Uranus is the most quiet planet in the solar system. Voyager 2 also discovered the great dark spot, or Neptune.

Both voyagers are now part of general knowledge and discussion in all types of gatherings. They are the most distant objects still in connection with mission control and have contributed a lot to understanding the size of the heliosphere and the limits of the solar system.

New Horizons

The New Horizons spacecraft was on a mission to explore Pluto. It was launched on January 19, 2006. It encountered Jupiter on February 28, 2007 for a gravity assist (gravity assist, or gravity slingshot, is a procedure to increase the speed of a spacecraft using a planet’s gravity).

It then went directly to Pluto for an encounter many years ahead. On July 15, 2015, it passed very close to Pluto and its satellite system. It flew only 12500 km above the surface of Pluto, imaging its atmosphere, surface, and satellites. Pluto has five satellites, of which four are very small and the largest is 1212 km in diameter, called Charon.

It is amazing how this small spacecraft remained in good health. It flew past Pluto and gave us amazing views of the object. It is also surprising that after its launch, the International Astronomical Union declared Pluto not to be a planet. Also, after its launch, we discovered four satellites of Pluto using the Hubble Space Telescope.

New Horizons is also powered by RTG, which has pyroxen in it. It is really amazing that the element named after Pluto is powering up a Pluto bound spacecraft.

After the Pluto encounter, the spacecraft continued to move towards the Kuiper Belt and encountered another object called Arrokoth. It is an asteroid and orbits the sun. New Horizons passed Arrokoth on January 1, 2019. Spacecrafts have also observed other dwarf planets like Makemake, Eris, and Quaoar.

New Horizons is currently 55 AU from Earth. It will never take over Voyager 1 and 2. As it is still active, this spacecraft can do much science even years after its launch. It may remain in contact until 2030 and beyond.

Some Last words

All these spacecrafts that have escaped the solar system are currently working, and among these three, New Horizons will work for a longer time as it is a younger spacecraft.

These missions gave us much engineering knowledge to build a new fleet of spacecrafts, which may leave the solar system for studying much deeper parts. Suggestions are most welcome for new kinds of spacecraft, as one of them is to make one with a target distance of 1000 AU in less time. Visiting our remote dwarf planets would be amazing, for which we must develop plans.