Launched in 1977, spacecraft Voyager 1 celebrates 40 years in space on Tuesday. 

Its twin spacecraft, Voyager 2, celebrated the same anniversary last month on Aug. 20.

Here's a look back at some of the many discoveries made by the space probes.

Active volcanoes

Voyager 1 flew within 277,400 kilometres of Jupiter on March 5, 1979, while Voyager 2 came within 650,180 kilometres on July 9, 1979.

The Voyagers took images that showed plumes from volcanoes on the surface of Io, one of Jupiter's moons.

It was the first time an active volcano was observed in our solar system beyond Earth.

NASA says Io is "the most volcanically active body in the solar system," with hundreds of active volcanoes on its surface.

NASA Voyager

An image taken by the Voyager 1 spacecraft showing a volcanic plume on the Jupiter moon Io. (NASA via AP)

Subsurface oceans

The Voyagers also took images of Europa, another one of Jupiter's moons.

Those images showed cracks on the surface, which were filled in with a dark, icy material. This was the first hint for scientists of water underneath the surface of Europa.

The Galileo spacecraft mission was launched in 1989, and entered orbit around Jupiter in 1995. Its measurements showed the planet's magnetic field was disrupted in the space around Europa, which scientists believe is caused by an "electrically conductive fluid" beneath the moon's surface — most likely an ocean of salt water.

Because of this, many scientists believe Europa is the most likely place to find life outside of Earth in our solar system.

Alien Life

This image, released Nov. 12, 1996, by the Jet Propulsion Laboratory in Pasadena, Calif., shows Jupiter's moon, Europa. (Jet Propulsion Lab via AP)

Erupting geysers

On Aug. 25, 1989, Voyager 2 took the first images of Triton, Neptune's largest moon.

The photos showed geyser-like eruptions shooting nitrogen gas and dust particles several kilometres into Triton's atmosphere.

Due to Triton's relatively high density and retrograde orbit — orbiting in the opposite direction to Neptune's rotation — scientists determined Triton, like Pluto, was likely not originally from our solar system, and had been captured into Neptune's orbit. 

Earth-like atmosphere

On Nov. 12, 1980, Voyager 1 flew by Saturn, recording data on the planet and its many moons.

It found that one of the moons, Titan, had a thick atmosphere mostly composed of nitrogen, like Earth's atmosphere, although the surface pressure was about 1.6 times higher.

According to NASA, "The chemistry in Titan's atmosphere may strongly resemble that which occurred on Earth before life evolved."

In fact, the Planetary Habitability Index — developed by scientists and published in the journal Astrobiology in 2011 — listed Titan as the second most habitable place in our solar system, behind only Earth.

titan moon

Saturn's moon Titan is seen in this image taken by Voyager 1 on Nov. 12, 1980. It shows the haze that covers Titan. (NASA/JPL)

Interstellar space

NASA says that in August 2012, Voyager 1 became the first and, as of today, only spacecraft to enter interstellar space.

Interstellar space is defined as the region between stars — and beyond the reach of the sun's constant flow of material and magnetic field, known as the heliosphere.

The probe informed scientists that cosmic rays — or highly energetic atomic nuclei travelling at almost the speed of light — are four times more abundant in interstellar space, which suggests the heliosphere essentially functions as a radiation shield for planets in our solar system. 

It will be at least another 40,000 years before Voyager 1, now more than 20 billion kilometres from Earth, encounters another star.

Voyager 1 was more than 18.5 billion kilometres away from the sun when it entered interstellar space. Voyager 2 is currently almost 18 billion kilometres from the sun and is expected to enter interstellar space in the next few years.

Voyager 1 leaves solar system5:40