NASA created history at the beginning of November when its Voyager 2 probe entered interstellar space.
According to NASA, the spacecraft left the heliosphere, the protective bubble created by solar winds, to become only the second man-made object to enter interstellar space. However, unlike the previous man-made object to enter interstellar space, Voyager 1, Voyager 2 is still functioning and communicating back to base.
The probe is now more than 18 billion kilometres away from the Earth.
The boundary between the heliosphere and interstellar space is known as the heliopause. It is the place where tenuous, hot solar wind meets the cold, dense interstellar medium.
The location of Voyager 2 as it entered interstellar space in November 2018. Public domain image created by NASA
Voyager 2 was launched in 1977, 16 days before its twin probe Voyager 1. The primary aim of the Voyager missions were to collect information about two planets, Jupiter and Saturn. The spacecraft were designed to function for just five years, but after making several discoveries as they flew past the two planets, their missions were extended. The twin probes then continued their journey to explore Uranus and Neptune.
While Voyager 1 became inoperative several years ago, the Voyager 2 probe is still serving NASA, 41 years after its launch, and providing scientists with a glimpse of what might lie beyond the heliopause, just beyond the solar system.
Voyager 1 was the first man-made probe to enter interstellar space in August 2012. But, Voyager 2 has something that its twin didn't have when it crossed over the heliosphere - a working Plasma Science (PLS) experiment, which will provide the world's "first-of-its-kind observations of the nature of this gateway into interstellar space", according to NASA.
The PLS on Voyager 1 stopped working in 1980.
"Even though Voyager 1 crossed the heliopause in 2012, it did so at a different place and a different time, and without the PLS data. So we're still seeing things that no one has seen before," said John Richardson, principal investigator for the PLS instrument.
The Voyager mission team can still communicate with the probe, but signals between Voyager 2 and Earth take about 16.5 hours. The communication link is established using NASA's Deep Space Network, which consists of three clusters of antennas located in California, Spain, and Australia.
While both Voyager probes have entered interstellar space, they have not yet left the solar system, and are not expected to do so in the near future. The boundary of the Solar System lies beyond the outer edge of the Oort Cloud - a collection of small objects that move under the influence of the Sun's gravity.
"I think we're all happy and relieved that the Voyager probes have both operated long enough to make it past this milestone," said Suzanne Dodd, Voyager project manager at NASA's Jet Propulsion Laboratory.
"This is what we've all been waiting for. Now we're looking forward to what we'll be able to learn from having both probes outside the heliopause."
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