Launched 40 years ago, the Voyagers are our longest-lived and most distant spacecraft. Voyager 2 has reached the edge of the heliosphere, the realm where the solar wind and the Sun’s magnetic field live. Voyager 1 has already left the heliosphere and entered interstellar space! A new movie, The Farthest, celebrates the Voyagers’ journey toward the stars:
What has Voyager 1 been doing lately? I’ll skip its amazing exploration of the Solar System….
Leaving the realm of planets
On February 14, 1990, Voyager 1 took the first ever ‘family portrait’ of the Solar System as seen from outside. This includes the famous image of planet Earth known as the Pale Blue Dot:
Soon afterwards, its cameras were deactivated to conserve power and computer resources. The camera software has been removed from the spacecraft, so it would now be hard to get it working again. And here on Earth, the software for reading these images is no longer available!
On February 17, 1998, Voyager 1 reached a distance of 69 AU from the Sun — 69 times farther from the Sun than we are. At that moment it overtook Pioneer 10 as the most distant spacecraft from Earth! Traveling at about 17 kilometers per second, it was moving away from the Sun faster than any other spacecraft. It still is.
That’s 520 million kilometers per year — hard to comprehend. I find it easier to think about this way: it’s 3.6 AU per year. That’s really fast… but not if you’re trying to reach other stars. It will take 20,000 years just to go one light-year.
Termination shock
As Voyager 1 headed for interstellar space, its instruments continued to study the Solar System. Scientists at the Johns Hopkins University said that Voyager 1 entered the termination shock in February 2003. This is a bit like a ‘sonic boom’, but in reverse: it’s the place where the solar wind drops to below the speed of sound. Yes, sound can move through the solar wind, but only sound with extremely long wavelengths — nothing you can hear.
Some other scientists expressed doubt about this, and the issue wasn’t resolved until other data became available, since Voyager 1’s solar-wind detector had stopped working in 1990. This failure meant that termination shock detection had to be inferred from the other instruments on board. We now think that Voyager 1 reached the termination shock on December 15, 2004 — at a distance of 94 AU from the Sun.
Heliosheath
In May 2005, a NASA press release said that Voyager 1 had reached the
heliosheath. This is a bubble of stagnant solar wind, moving below the speed of sound. It’s outside the termination shock but inside the heliopause, where the interstelllar wind crashes against the solar wind.
On March 31, 2006, amateur radio operators in Germany tracked and received radio waves from Voyager 1 using a 20-meter dish. They
checked their data against data from the Deep Space Network station in Madrid, Spain and yes — it matched. This was the first amateur tracking of Voyager 1!
On December 13, 2010, the the Low Energy Charged Particle device
aboard Voyager 1 showed that it passed the point where the solar wind flows away from the Sun. At this point the solar wind seems to turn sideways, due to the push of the interstellar wind. On this date, the spacecraft was approximately 17.3 billion kilometers from the Sun, or 116 AU.
In March 2011, Voyager 1 was commanded to change its orientation to measure the sideways motion of the solar wind. How? I don’t know. Its solar wind detector was broken.
But anyway, a test roll done in February had confirmed the spacecraft’s ability to maneuver and reorient itself. So, in March it rotated 70 degrees counterclockwise with respect to Earth to detect the solar wind. This was the first time the spacecraft had done any major maneuvering since the family portrait photograph of the planets was taken in 1990.
After the first roll the spacecraft had no problem in reorienting itself with Alpha Centauri, Voyager 1’s guide star, and it resumed sending transmissions back to Earth.
On December 1, 2011, it was announced that Voyager 1 had detected the first Lyman-alpha radiation originating from the Milky Way galaxy. Lyman-alpha radiation had previously been detected from other galaxies, but because of interference from the Sun, the radiation from the Milky Way was not detectable.
Puzzle: What the heck is Lyman-alpha radiation?
On December 5, 2011, Voyager 1 saw that the Solar System’s magnetic field had doubled in strength, basically because it was getting compressed by the pressure of the interstellar wind. Energetic particles originating in the Solar System declined by nearly half, while the detection of high-energy electrons from outside increased 100-fold.
Heliopause and beyond
In June 2012, NASA announced that the probe was detecting even more charged particles from interstellar space. This meant that it was getting close to the heliopause: the place where the gas of interstellar space crashes into the solar wind.
Voyager 1 actually crossed the heliopause in August 2012, although it took another year to confirm this. It was 121 AU from the Sun.
What’s next?
In about 300 years Voyager 1 will reach the Oort cloud, the region of frozen comets. It will take 30,000 years to pass through the Oort cloud. Though it is not heading towards any particular star, in about 40,000 years it will pass within 1.6 light-years of the star Gliese 445.
NASA says:
to wander the Milky Way.
That’s an exaggeration. The Milky Way will not last forever. In just 3.85 billion years, before our Sun becomes a red giant, the Andromeda galaxy will collide with the Milky Way. In just 100 trillion years, all the stars in the Milky Way will burn out. And in just 10 quintillion years, the Milky Way will have disintegrated, with all the dead stars either falling into black holes or being flung off into intergalactic space.
But still: the Voyagers’ journeys are just beginning. Let’s wish them a happy 40th birthday!
My story here is adapted from this Wikipedia article:
• Wikipedia, Voyager 1.
You can download PDFs of posters commemorating the Voyagers here:
• NASA, NASA and iconic museum honor Voyager spacecraft 40th anniversary, August 30, 2017.
Azimuth 
“But still: the Voyagers’ journeys are just beginning. Let’s wish them a happy 40th birthday!”
Something about that conclusion is really poignant. Sort of like the end of Silent Running.
Thanks. :-)
You’re welcome. I’ve never seen that movie, and watching just the final scene I still have no idea what it’s about.
+Todd Trimble: yes, true it is poignant = ergreifend .
+ John Baez : Thanks, too. I had a poster in my room, I think from 1979 or 1980 , showing the Red Spot on Jupiter – a detailed picture taken by Voyager 1 .
Illuminating! :-)
But a teeny factcheck: Reaching within 1.6 light years of Gliesse 445 in 40,000 years??? Unless Veeger (ST reference) dramatically accelerates that should be more like 340,000 years surely? (~17 LY to Gliesse 445 x 20000 years for 1 LY, not accounting for the expansion of the Universe in that time!)
love.
Here’s the missing puzzle piece: Gliese 445 is approaching us rapidly. From the Wikipedia article on this star:
My (Human) tendency to consider the Universe only from a single perspective! Thanks for the Additional info John. One observation though – it would appear that some stars can move towards our own and that might imply that we can travel’ to other stars in much less time than it would take to fly to one if they were in a ‘fixed’ or expanding away from us situation. :-)
love
Some of the images on Voyager’s Golden Record have disappeared into a copyright black hole.
Regarding the broken “solar wind” instrument — I worked on Pioneer 10/11 as a teenager(!) – it had a number of “particle telescopes” on it. They’re called “telescopes” because they are collimated, directional, although there is no lensing of any sort. They were optimized for different kinds of particles: e.g. heavy charged nuclei – oxygen, carbon, nitrogen – and telling these apart so that you know what the mix is. The detector is tuned to work best at typical solar-wind energies. But there are other detectors; one for low-energy protons, one for high-energy protons, one for electrons, one for gamma rays. Each of these are mildly sensitive to some of the other particle types. I assume the Voyager has a mix of these, also.
I’m guessing that the “solar wind detector” is the one that can distinguish the different kinds of nuclei. If that’s broken, then you won’t know the abundance of each, but you can still measure the overall charged particle flux and direction with the other instruments.
Linas wrote:
Cool! I should be able to read about exactly what broke on Voyager 1; I just haven’t gotten around to it. Here’s something on Voyager 1 that was working on October 2011, after that other thing broke:
The caption on Wikipedia says
The dropoff of these protons is apparently due to passing through the termination shock (although I’m a bit confused about the timing here).
I kind of know nothing at all about space plasma physics, but have a strong “male answer syndrome”. Anything below might be wrong and is totally fictionalized. Still, I think its 100% true. So….
The >0.5MeV/nucleus cut is probably the limit of detector sensitivity, rather than an intentional cut. I think the detectors are a stack of thin sheets of doped silicon or germanium, and they look for a current of electron/hole pairs when the nucleus goes through. Less than 500 KeV does not generate enough current for the charge amplifiers to detect. Don’t know if the 511 KeV cut is coincidental, or has something to do with electron mass. Maybe there is random noise from pair annihilation???
“Principally protons” means that they lost the detector that can tell apart protons from heavier nuclei, but based on earlier experience, they can guess. This detector does not tell you what the nuclear charge Z is.
A 0.5MeV particle is not relativistic. Since
I can approximate the kinetic energy by assuming that the momentum
is small, so that the velocity 
is approx 
for a 1 MeV proton. (Sadly, I dropped a factor of 
in the above. I’m lazy.) So, plugging in the speed to light in above gives v = 300 km/sec which is still very fast. If I type in “speed of solar wind” into the search bar, then NASA tells me that “The solar wind streams off of the Sun in all directions at speeds of about 400 km/s (about 1 million miles per hour). The source of the solar wind is the Sun’s hot corona. The temperature of the corona is so high that the Sun’s gravity cannot hold on to it.” I conclude that a 1 MeV proton is a little bit slower than the solar wind. A 0.5 MeV proton is less than half the speed of the solar wind.
In the heliopause, I expect protons to go from solar-wind-speeds—again —400 km/sec or about 1-2MeV to… what?? Maybe 1/10th the speed of solar wind?? That is, maybe 0.1 MeV, which is a lot less than 0.5MeV of the detector limit. There might be a lot of protons (and helium nuclei, and nitrogen and oxygen and carbon) floating around up there, but, at speeds of “only” a hundred-thousand miles per hour, these are much too slow for the detector to register them. From the detectors point of view, the slow protons/nuclei are invisible. Thus, the count drops to almost zero.
Homework assignments: find the factor of
above. Make an estimate for the energy of helium and carbon at solar-wind speeds. Assume that the heliopause is a perfect gas, having a Boltzmann distribution of velocity, and estimate the percentage of protons travelling faster than 0.5MeV. The above graph suggests that only about 1 out of 25 protons are moving faster than 0.5 MeV. What is the temperature of this gas? How does it compare to the temperature of the corona?
At this point one should mention that it seems that more exactly at the same time of the above dropoff the Sorce Time series:
http://lasp.colorado.edu/lisird/sorce/sorce_ssi/ts.html
display a gap for all(as far as I can tell, I didn’t check all) frequencies, as if there was some very energetic burst, which was cut out because it was an outlier.
Scary?
The link to the above image at
which is
http://voyager.gsfc.nasa.gov/heliopause/heliopause/v1la1.html
is broken.
OK. I found some scatterplots under another NASA adress, namely at:
https://omniweb.gsfc.nasa.gov/coho/form/voyager1_s.html
John, I’ll send you a screenshot via Email.
I had falsely misiinterpreted the 2011 at the beginning of the x-Axis in that image you show in
so I had seen the dropoff at End August/Beginning Sept. 2011, but the dropoff is in 2012 not 2011.
And the interactive app at https://omniweb.gsfc.nasa.gov/coho/form/voyager1_s.html shows the same dropoff time. The dropoff is by the way apparently the heliospause and not the termination shock. The termination shock was in 2004: https://en.wikipedia.org/wiki/Heliosphere#Termination_shock
But the Sorce mission “outcut” which I has seen was in Sept.5 2011.
However by looking at the Voyager data there was actually indeed a “burst” (and not some instrumental malfunction) in end august/beginning sept 2011 (Radial distance around 118 AU).
It doesnt look giant, but it is clearly visible. In that screenshot one can see another burst around Sept 26, 2010…..and I looked now again at the Sorce data (http://lasp.colorado.edu/lisird/sorce/sorce_ssi/ts.html)….and again there is an “outcut” (i.e. no data) exactly at that time Sept 26 2010 for all (or at least a lot) of frequencies.
The bursts look like some really big hydrogen flares and if they were so noticable so far out then this might explain, why the Sorce mission had an “outcut” then.
Not sure what you are saying, @nad. Data loss in space-science is not uncommon. The motors that move the deep-space dishes jam. Amplifiers burn out. Distant spacecraft often need electronics upgrades to receive the now-weak signals, and the upgrade is not done on time. Graduate students who drop screwdrivers where they should not go, and delete files that should not be deleted. Add to that bad weather in space and bad weather on the ground… this is the stuff of water-cooler gossip. Was the stuff of water-cooler gossip. Now we have facebook for that, I suppose.
Yep. Some things get inevitable lost.
I had written a comment here yesterday, which explains more.
But I hope it is not inevitable lost. John, what happened to the comment?
Hi – for some reason I needed to approve your comment before it showed up; this usually happens when someone posts for the first time. Maybe you changed something like your email address or… something.
No I didn’t change anything. Apparently all of my comments need now to be approved…..
Anyways thanks for approving. Unfortunately my comment is not so easily to understand without the scatterplot. I wasn’t sure how safe it is to post the plot. For the use of data it was mentioned that one can use it if one mentions the source (i.e. NASA), but for the scatterplots? Did you understand what I was talking about in the comment?
The Earth itself travels a bit less than 6.3 AU per year, but mostly keeping the Sun just at a safe-ish distance without leaving her too far behind. The Voyagers’ pace of just over half that much is remarkable for being still that fast so far away, and mostly straight “up”. …