It’s heartening to consider that the two Voyager spacecraft, though built for a 4 ½ year mission, have continued to function ten times longer than that. This fact, and data from other missions, will help us get a handle on longevity in spacecraft systems as we contemplate pushing out beyond the heliosphere with a spacecraft specifically designed for the job. Mission longevity is mysterious for it often seems to surprise even the designers, who would like to have a more concrete sense of how to ensure operations continue for decades.
Voyager 2 broke Pioneer 6’s record of 12,758 days of operation way back in 2012, but we can also consider spacecraft like Landsat 5, launched in 1984 and carrying two instruments, the Multispectral Scanner System (MSS) and the Thematic Mapper (TM). Managed by the U.S. Geological Survey (USGS), Landsat 5 completed over 150,000 Earth orbits and sent back more than 2.5 million images of Earth’s surface, with operations lasting almost three decades. Design life for Landsat 5 was estimated at three years, but it became, as Guinness World Records labels it, the ‘longest-operating Earth observation satellite.’
While the Landsat accomplishment is significant, the two Voyagers have actually taken us into a new realm, with Voyager 2 joining Voyager 1 beyond the heliosphere on November 5, 2018. Outside the protective ‘bubble’ blown by the stream of particles and magnetic fields from the Sun known as the solar wind, these craft are now the subject of five new research papers in Nature Astronomy describing the data Voyager 2 has returned since the crossing. Have a look at the relative position of the two spacecraft.
Image: This illustration shows the position of NASA’s Voyager 1 and Voyager 2 probes, outside of the heliosphere, a protective bubble created by the Sun that extends well past the orbit of Pluto. Voyager 1 crossed the heliopause, or the edge of the heliosphere, in August 2012. Heading in a different direction, Voyager 2 crossed another part of the heliopause in November 2018. Credit: NASA/JPL-Caltech.
Note the plasma flow lines both inside and outside the heliopause. Plasma is a gas composed of charged particles, a ‘wind’ that differs in direction depending on whether its source is the Sun or the interstellar medium itself. The Voyagers have two instruments returning data on plasma at the borderline between the Sun’s domain and interstellar space. The data show hot and sparse plasma inside the heliosphere, while interstellar plasma is colder and denser. We learned from Voyager 1 that the heliosphere protects the Solar System from about 70 percent of the incoming cosmic ray radiation, which is made up of particles accelerated by exploding stars.
While Voyager 1 showed higher than expected plasma density just outside the heliosphere (an indication, researchers say, of compression), Voyager 2’s findings demonstrated slightly warmer plasma than expected, while confirming the compression at the edge of the heliosphere. Meanwhile, the spacecraft’s particle instruments (two of the five still operating instruments can detect particles in different energy ranges) showed some particles slipping across the boundary into interstellar space, indicating a more porous boundary in Voyager 2’s location outside the ‘flank’ of the heliosphere, as opposed to Voyager 1’s exit at its front.
Magnetic field issues still raise questions. Voyager 1 had shown that the magnetic field just beyond the heliopause is parallel to the magnetic field inside the heliosphere. Voyager 2’s magnetometer confirms this finding of field alignment. Ed Stone (Caltech) is the all but legendary project scientist for Voyager:
“The Voyager probes are showing us how our Sun interacts with the stuff that fills most of the space between stars in the Milky Way galaxy. Without this new data from Voyager 2, we wouldn’t know if what we were seeing with Voyager 1 was characteristic of the entire heliosphere or specific just to the location and time when it crossed.”
Having two spacecraft leaving the heliosphere has been a tremendous boon for science. Voyager 1 and Voyager 2 exited the heliosphere at different locations as well as at different times in the 11-year solar cycle. The latter keeps the solar wind mutable and frothing, something to be borne in mind when we consider spacecraft designs that could ‘sail’ on this wind, and affects the shape of the heliosphere itself, whose boundaries vary with solar changes. We learn from the new papers that neither Voyager is in undisturbed interstellar space, but rather in a churning transitional region, outside the heliosphere but still affected by its presence.
The papers, all of them in Nature Astronomy’s October 2019 issue, are Richardson et al., “Voyager 2 plasma observations of the heliopause and interstellar medium”; Krimigis et al., “Energetic charged particle measurements from Voyager 2 at the heliopause and beyond”; Stone et al., “Cosmic ray measurements from Voyager 2 as it crossed into interstellar space”; Burlaga et al., “Magnetic field and particle measurements made by Voyager 2 at and near the heliopause”; and Gurnett & Kurth, “Plasma densities near and beyond the heliopause from the Voyager 1 and 2 plasma wave instruments.”
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I wonder what the implications of this are for the chips on handkerchiefs plan to explore nearby systems in a century. Is there enough density to affect their trajectories? Do we need to build extended arrays of lasers out by Pluto’s orbit to keep them aligned right?
One thing that interests me about the voyager probes is that in a few years they will pass the point where, if they’d been launched at 10 percent the speed of light, and in the right direction, they would be entering the Centauri system. While their current speed is in no way as dangerous as 10 percent that of light, it does show that we can build a probe that can last the necessary time to reach another star system, using propulsion techniques that are almost within reach.
Was it fortuitous that the 2 Voyagers were heading towards the bowshock and not in some other direction? If it was, when was it determined that they were headed towards it?
Yes, I think it was simply by chance. The Voyagers’ trajectories were determined by the needs of their planetary encounters, with the decision to alter Voyager 1’s path made because of the desire to have a closer look at Titan. I don’t know when Voyager scientists started thinking seriously about the spacecraft lasting long enough to exit the heliosphere, but somewhere along the line the Voyager Interstellar Mission took hold as a concept and then as a fact.
Since it arises from the movement of the Sun around the galaxy and the movement o the Local Interstellar Cloud, probably its location was first predicted when the LIC was first detected and studied.
Doing some digging, it appears that we knew the direction of the sun’s movement through the local cloud at about the same time as the launch of the Voyagers in 1977.
That the planetary alignment for the missions was in the direction of the movement was serendipitous given the unexpected longevity of the probes.
Solar system caught in an interstellar tempest
Decades-Long Changes of the Interstellar Wind Through Our Solar System
CHECK THIS OUT! “Scientists Chat Interstellar Probe Project in New York City.” currently up on the Space.com website. The plan is to use SLS to launch a probe which would get to the “heliosphere fishbowl” boundary(90 billion miles, or ~1,000 AU) in the SAME TIME FRAME as Voyager 2, from launch to present! That begs the question: could some sort of rudimentary small space telescope be attached to such a probe to conduct the FIRST EVER solar gravitational lensing observations? 36 years to get to 1,000 AU means 500 AU in just 18 years! THIS IS WAY TOO BIG A POSSIBILITY TO PASS UP!
The link to the above Space.com article:
Now Scientific American has an article on this as well.
Proposed timeline for the mission: 2030-2080.
That is such a stretch of time that they might as well say 3000 A.D.
As the Sun orbits the galaxy, are there times [on a geological scale] when its velocity relative to the interstellar medium becomes very low? Are there any interesting effects when a star matches the speed of sound in the interstellar medium? (or if it becomes stationary?)
Will China follow in the Voyager probes’ space steps next? And rather soon at that?
A bit off-topic, but I found this article about the Voyagers of 2013 very much fascinating, and also a later one (which I am unable to locate right now) wherein it was speculated about using Voyager’s last few drops of fuel for one last burn before its batteries gives out, in order to make an encounter with a star some time very far into the future more likely.
At the time we didn’t know enough about the trajectories of stars in our neighborhood to speculate much beyond 40000 years from now, and burning the little fuel any one of the Voyagers have left would certainly make no difference at all on that timescale.
Which is why I was very much interested in this article I saw yesterday. I just thought I’d share it here since comments have been turned of for the 2013 C-D article.
Thank you, Tiens. I’m glad you’re interested in the idea, which I think is fascinating. It dates back ultimately to Sagan, although he didn’t develop it, and was later picked up by Jim Bell in his book The Interstellar Age. Haven’t been able to get any response on it from the Voyager team at this point, but I think it’s a viable concept.
Having re-read the article, I noticed the following:
1. The closest encounter listed for V1 is still 0.3 parsec or almost a light-year, and none closer than that for V2 in the next 5 million years. So one will have to look farther ahead than that, apparently… Ouch :)
2. Gaia recorded the velocities of “the brightest 150 000” apparently. So one can assume that there may be objects like the Luhman 16 system which aren’t included in Gaia’s 3D map, or some red dwarves which are currently not close to Earth but may be headed our way. So we have to be patient and wait for more observations.
I also wonder if Bailer-Jones and Farnocchia thought about checking on Oumaouma and 2I/Borisov’s future close encounters . Might add a little romantic selling point to do a mission to chase after one of these interstellar visitors if it is on its way to another system.