This is a big moment in human history, though like many moments it probably won’t sink in for years to come. Nevertheless, humanity – with the reach of the incredibly long-lived Voyager 1 space probe – is now a civilization of the stars.
The official announcement came from NASA yesterday (September 12, 2013) after careful data analysis from Voyager 1’s Plasma Wave instrument: the spacecraft has passed though the heliopause (the bubble that separates the inside of our solar system from the outside) into the interstellar medium.
To watch the full press conference, hit play here:
NASA scientists were able to confirm Voyager 1’s exit from the heliopause by measuring a significant change in the plasma density that surrounds the probe. And interestingly, the change they noted was a marked increase in density. This may seem counter-intuitive, but as the plasma temperature of the interstellar medium is significantly cooler than the solar system’s plasma, the increased density makes sense – and was in fact expected.
(Warning, the following is a simplification: All things being equal, cool objects have a higher density than warm objects, as their molecules have less energy and pack closer together. This is why solids are denser that liquids; liquids denser than gases; gases denser than plasma – in each state of matter transition energy is added, which causes a decrease in density.)
“Exploration is in our nature. We began as wanderers, and we are wanderers still. We have lingered long enough on the shores of the cosmic ocean. We are ready at last to set sail for the stars.” ― Carl Sagan
The other interesting thing to note is that Voyager 1 , it appears, actually entered interstellar space on August 25, 2012 – more than a year ago! But it took until now to make sense of the data the probe was sending back. But I kind of like this. When we as a species are going somewhere and doing something no one has ever done before, it takes time for us to understand what we’re seeing and what’s happening. Exploration and discovery doesn’t happen in 20 seconds – it takes years (and historically speaking, many years) of concerted human effort to accomplish these amazing firsts – whether we’re talking about crossing the oceans, inventing medicines, sailing around the globe, building landmarks, landing on the moon, and now leaving the solar system.*
Voyager 1 is right now about 19 billion kilometers from the Earth, or 125 times further from the Sun than we are, and getting ever further away at a speed of about 60,000 kilometers per hour – or roughly 500 million km per year! A little perspective: we think of the dwarf planet Pluto as “far away” but Voyager 1 passed the orbit of Pluto in 1989 – 24 years ago!
And let’s not forget about Voyager 2 either. It is still within the heliopause, and will also cross into interstellar space in the coming years. Voyager 2 is on a different – slower – trajectory than Voyager 1 and so it is about 5 billion kilometers behind its twin sister, on a southbound course away from the Sun.
The Voyager mission will, I expect, to continue to deliver more discoveries. As the probe continues to sail into the unknown I look forward to it sending back new data that will continue to answer questions – and perhaps more importantly – raise new questions, in order to push our understanding of the universe forwards.
To read more about the Voyager mission, check out these other posts I’ve written on the subject over the last couple years.
* While Voyager 1 is now travelling through interstellar space, it technically hasn’t left the solar system. I know this might sound bizarre, but it’s due to the fact that the Sun’s gravitational influence reaches far beyond the influence of its plasma. There are (at minimum) billions of chunks of rock and ice in the Oort Cloud that orbit our Sun over thousands of years that are still many times further away from the Sun than Voyager 1 is. It may be in fact thousands of years before Voyager 1 passes the last of the Oort Cloud objects. All that having been said, the heliopause is accepted as the limit between inside our solar system and “amongst the stars” in interstellar space, just as Oort Cloud objects are “among the stars” even while being bound by gravity to our Sun. Phil Plait also wrote on this subject here.