November 25, 2019

This month, scientists at the United States National Aeronautics and Space Administration (NASA) released their latest findings from decoded transmissions sent from the space probe Voyager 2 in interstellar space (the space between the stars). About a year ago, Voyager 2 became the second spacecraft (following its twin, Voyager 1) to enter interstellar space, exiting the heliosphere, a vast, teardrop-shaped region of space containing electrically charged particles given off by the sun.

Voyager 2, seen here passing Neptune in 1989, entered interstellar space in late 2018. Credit: © Mark Garlick, Science Source

The sun and all the planets are inside the heliosphere. Scientists estimate that the nose (blunt end) of the heliosphere is about 9 billion to 15 billion miles (15 billion to 24 billion kilometers) from the sun. Voyager 1, launched in September 1977, crossed the boundary from heliosphere to interstellar space in 2012. The crossing was marked by a steady drop in temperature and an increase in the density of charged particles known as plasma. Voyager 1 also detected an abundance of cosmic rays (particles accelerated by exploding stars) in interstellar space and provided evidence that the heliosphere protects Earth and the other planets from much interstellar space radiation.

This artist’s depiction shows the approximate locations of the two Voyager spacecraft relative to the sun, the bright spot in the center, in the mid-2010′s. In 2012, Voyager 1, shown as the upper probe in the image, sailed beyond the heliopause and into interstellar space. Voyager 2, the lower probe in the image, crossed the heliopause in 2018. Credit: NASA/JPL-Caltech

Voyager 2 was launched a month before Voyager 1 in August 1977. Slightly slower than its twin craft and following a different course, Voyager 2 reached the heliopause (the edge of the heliosphere) in November 2018. Voyager 2 also detected changes in temperature and plasma and cosmic ray density, but the heliosphere at Voyager 2′s crossing point appeared to be sharper and thinner. This could be explained by Voyager 2 crossing the heliopause at a different location or at a less angled trajectory or by crossing during a period of lower solar activity than that experienced by Voyager 1 in 2012. The sun goes through a roughly 11-year cycle of high and low activity, theoretically causing the heliosphere to expand and contract or thicken and thin. Both spacecraft found that particles from the sun are trickling through the somewhat porous heliopause into interstellar space. Voyager 2 also confirmed Voyager 1′s detection of similar magnetic fields on both sides of the distant boundary.

This artist’s impression shows Voyager 1 passing through the heliopause in 2012. Credit: NASA/JPL-Caltech

Both Voyager space craft are powered by slowly decaying plutonium. In 1977, scientists did not know exactly how long the space probes would continue to operate, nor did they know if, when, or where they would reach interstellar space. Now that the probes are there, scientists hope to learn more about the distant realm before the Voyagers power down sometime in the next few years. Voyager 1 is currently more than 13.6 billion miles (22 billion kilometers) from the sun, and Voyager 2 is about 11.3 billion miles (18.2 billion kilometers) away. After they lose power, scientists expect both to continue sailing through space for billions of years.

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The path of  Voyager 2 is shown in red. Voyager 2 flew past Jupiter in 1979, Saturn in 1981, Uranus in 1986, and Neptune in 1989. Credit: WORLD BOOK illustration by Ken Tiessen, Koralik Associates

September 10, 2012

Bearing greetings from Earthlings now more than 11 billion miles (18 billion kilometers) away, the Voyager 1 spacecraft continued its epic trek to the edge of the solar system on September 5, 35 years after its launch into space. NASA had launched a companion craft, Voyager 2, on August 20, also in 1977. Together, the two probes have lasted longer and traveled farther than any other space probes in history. Voyager 2 is now about 9 billion miles (15 billion kilometers) from the sun, going south, the opposite direction from Voyager 1.

The space probe Voyager 2 was launched on Aug. 20, 1977. Its path through the solar system is shown in red. Voyager 2 flew past and photographed Jupiter in 1979, Saturn in 1981, Uranus in 1986, and Neptune in 1989. (World Book illustration by Ken Tiessen, Koralik Associates)

Initially, the two spacecraft were given a five-year mission to explore the gas giants Jupiter and Saturn. The data they sent back to Earth have answered many questions about the planets’ atmospheres, interiors, rings, magnetospheres (surrounding zones of strong magnetic fields), and satellite systems. With this part of the Voyager mission completed, engineers at NASA’s Jet Propulsion Laboratory in Pasadena, California, in 1981 decided to push the two craft farther into space. Voyager 2 became the first–and so far–only spacecraft to visit Uranus and Neptune. They found evidence of geologic activity on two previously known moons—volcanoes on Jupiter’s moon Io and icy geysers on Neptune’s moon Triton.

Since 1989, the two probes have been exploring the heliosphere, a vast, tear-drop-shaped region of space that contains the solar system, the sun’s magnetic field, and the solar wind (electrically charged particles given off by the sun). In late 2004, Voyager 1 crossed a shock wave called the termination shock, becoming the first craft to reach the region of space that lies just inside the heliopause (the final boundary of the solar system). The crossing occurred at a distance of about 8.7 billion miles (14 billion kilometers) from the sun. In 2007, Voyager 2 crossed the shock in a different area and at a distance of about 7.8 billion miles (12.6 billion kilometers) from the sun. By detecting the shock at different distances from the sun, the two craft confirmed scientists’ belief that the solar system is not perfectly round.

The two Voyager spacecraft explore the outer edges of the heliosphere, a vast, tear-drop-shaped region of space that contains the solar system, in an artist’s illuustration. (NASA/JPL-Caltech)

Scientists are not sure when Voyager 1 and 2 will leave the heliosphere and go where no spacecraft has gone before–interstellar space. Some signals from Voyager 1 suggest that the craft’s escape from the solar system may occur in the near future. Regardless, the two spacecraft will have enough electrical power to continue collecting data and communicating it back to Earth through 2020, and possibly through 2025.

Additional World Book articles:

• Space exploration
• The Thirty-Year Journey of Voyagers 1 & 2 (A Special Report)
• Astronomy (1979) (A Back in Time article)
• Astronomy (1989) (A Back in Time article)
• Space Exploration (1977) (A Back in Time article)
• Space Exploration (1979) (A Back in Time article)