Voyager 1 Has Outdistanced the Solar Wind


The venerable Voyager spacecraft are truly going where no one has gone before. Voyager 1 has now reached a distant point at the edge of our solar system where it is no longer detecting the solar wind. At a distance of about 17.3 billion km (10.8 billion miles) from the Sun, Voyager 1 has crossed into an area where the velocity of the hot ionized gas, or plasma, emanating directly outward from the sun has slowed to zero. Scientists suspect the solar wind has been turned sideways by the pressure from the interstellar wind in the region between stars.

“The solar wind has turned the corner,” said Ed Stone, Voyager project scientist based at the California Institute of Technology in Pasadena, Calif. “Voyager 1 is getting close to interstellar space.”

The event is a major milestone in Voyager 1’s passage through the heliosheath, the turbulent outer shell of the sun’s sphere of influence, and the spacecraft’s upcoming departure from our solar system.

Since its launch on Sept. 5, 1977, Voyager 1’s Low-Energy Charged Particle Instrument has been used to measure the solar wind’s velocity.

When the speed of the charged particles hitting the outward face of Voyager 1 matched the spacecraft’s speed, researchers knew that the net outward speed of the solar wind was zero. This occurred in June, when Voyager 1 was about 10.6 billion miles from the sun.

However, velocities can fluctuate, so the scientists watched four more monthly readings before they were convinced the solar wind’s outward speed actually had slowed to zero. Analysis of the data shows the velocity of the solar wind has steadily slowed at a rate of about 45,000 mph each year since August 2007, when the solar wind was speeding outward at about 130,000 mph. The outward speed has remained at zero since June.

“When I realized that we were getting solid zeroes, I was amazed,” said Rob Decker, a Voyager Low-Energy Charged Particle Instrument co-investigator and senior staff scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. “Here was Voyager, a spacecraft that has been a workhorse for 33 years, showing us something completely new again.”

Scientists believe Voyager 1 has not crossed the heliosheath into interstellar space. Crossing into interstellar space would mean a sudden drop in the density of hot particles and an increase in the density of cold particles. Scientists are putting the data into their models of the heliosphere’s structure and should be able to better estimate when Voyager 1 will reach interstellar space. Researchers currently estimate Voyager 1 will cross that frontier in about four years.

Our sun gives off a stream of charged particles that form a bubble known as the heliosphere around our solar system. The solar wind travels at supersonic speed until it crosses a shockwave called the termination shock. At this point, the solar wind dramatically slows down and heats up in the heliosheath.

A sister spacecraft, Voyager 2, was launched in Aug. 20, 1977 and has reached a position 8.8 billion miles from the sun. Both spacecraft have been traveling along different trajectories and at different speeds. Voyager 1 is traveling faster, at a speed of about 38,000 mph, compared to Voyager 2’s velocity of 35,000 mph. In the next few years, scientists expect Voyager 2 to encounter the same kind of phenomenon as Voyager 1.

The results were presented at the American Geophysical Union meeting in San Francisco.

Source: NASA

59 Replies to “Voyager 1 Has Outdistanced the Solar Wind”

  1. It is hard to comprehend there is boundary out there 10.6 billion miles out where vacuum meets a vacuum. Particles from the sun must be miles a part! I mean what did solar wind particle hit to slow it down?

    1. Emilio, I think the following may have happened:

      The solar wind carries momentum, so it can be modeled as having a pressure. When it meets the interstellar medium it will eventually slow down and disperse that momentum.

      The Voyager 1 isn’t there yet, AFAIU. (So the article heading is a tad misconstrued, IMO.)

      First Voyager 1 passed the termination shock in 2004. That is the shock region where the solar winds slows below the surrounding media “speed of sound”. (A rather theoretical concept for such a dilute media.)

      Now it is in the heliosheath, where the solar wind is not yet dispersed but turbulent. If I understand the article correctly, Voyager 1 has entered a turbulence region where the wind is entirely “from the side”. But it is still there.

      The wind will abate fully first when Voyager 1 enters the heliopause.

      1. I goofed. Reading again, I should have realized it is the wind that has the shock and not the media, so it is the solar wind that is slowed to sub-sonic speed. The shock is presumably what makes the wind in the heliosheath turbulent.

    2. A simple model can be found in your kitchen sink. Put the drain plug in and start filling the sink with a modest water flow. As the sink fills up a little bit there is a disk or crater region where you see water flowing away from where the falling stream hits the bottom of the sink. The edge of this disk or crater is where that flow ends. This is similar to what happens with the solar wind. It flows outwards and diffuses in density as 1/r^2. At the point the density and pressure drops below the diffuse interstellar gas that is where you get this bow shock or heliosheath.


  2. Nancy wrote “17.3 billion km (10.8 billion miles)”, then every other figure quoted afterwards was just in miles or mph (miles per hour). For quoting such distances, wouldn’t simple astronomical units be far more appropriate? (I..e. It can be visualised in the mind.)
    Also I’d had also thought that kilometres per second would have been more appropriate (km.s^-1)?

    The conversions for the rest of us are;
    “…10.6 billion miles (17.1 billion kilometres or 114 Astronomical Units or 114 AU) from the sun.”

    “Analysis of the data shows the velocity of the solar wind has steadily slowed at a rate of about 45,000 mph (72,000 kph) [20 km.s^1] each year since August 2007, when the solar wind was speeding outward at about 130,000 mph (209,000 kph) or 58 km.s^-1]. The outward speed has remained at zero since June.”

    [The original figures quoted were likely all in kilometres per second, and were converted into miles!! It calculates out as 20.0000… km.s^-1]

    “A sister spacecraft, Voyager 2, was launched in Aug. 20, 1977 and has reached a position 8.8 billion miles (14 billion kilometres or 94.5 AU)from the sun. Both spacecraft have been traveling along different trajectories and at different speeds. Voyager 1 is traveling faster, at a speed of about 38,000 mph (61,200 kilometres per hour) or 17 km.s^-1), compared to Voyager 2?s velocity of 35,000 mph (56,300 kilometres per hour or 15.6 km.s^-1). In the next few years, scientists expect Voyager 2 to encounter the same kind of phenomenon as Voyager 1.

    1. Give it a rest Mr. Crumb. Arguments about what system to use outside of “scientific peer reviewed publications” here in the USA will continue long after you and I are forgotten.

      1. Thanks Nancy for putting doing distances in miles and writing in English. I fly out of airports not aerodomes.

      2. @ tripleclean
        The word “aerodromes” is really only used by the British. As far as I know, it usually only used in airfields. Even the British have abandoned the term. I.e. It is Heathrow Airport, etc. and not Heathrow Aerodrome.

        As for “writing in English”, I assume here you mean American English. I.e. ‘color’ or ‘honor’, and not the correct spelling in English of ‘colour’ and ‘honour’, etc.

        (Do Americans only do this just so it just force non-american readers to have you in their every future forethoughts? Isn’t it bad enough that every time you open a damn word processing document, you always have to change the default back to other forms of English so your can read and spell. Ever time you write HTML your forced to use American English. Every operating system has some American English words in some menu.
        I’d really think most of non-americans, who are in the vast majority, are getting quite feed up with such uncompromising biassed imperialistic ways. Why is that?)

      3. @Crumb

        Having lived in the UK, as I have, I know just how schizoid Brit’s are. You buy your petrol by the liter, your beer by the pint and you drive on roads that have speed limits in miles per hour. Don’t sound so pompous about Americans. After all, a Brit invented HTML (Berners-Lee) and HE decided on the use of American English spellings for its syntax while working at CERN, so sorry if he did not deign to use the British method of spelling. Had Great Britain invented Microsoft and their word processing program, Word, then I am sure you would have some level of hubris to express, but as they did not…

      4. @ wjwbudro
        Sorry mate. I don’t think, nor many others reading these articles throughout the world, would consider imperial units like miles, etc. comprehensible nor relevant. This significant issue is that quoting numbers in articles should be done so people will understand the storyline. This has nothing to do with “scientific peer reviewed publications”, it is just a good means of communication. It is easy enough to give BOTH units, so why not do it?

        Also your positively grating words “…here in the USA…” quite wrongly imply some inherited superiority, where, in fact, none actually exist. Only three countries do not use the metric system (being also Liberia and Myanmar) — the Système International d’Unités or (SI). Really crazy statements like your last one just show your narrow inconsiderate attitude towards others. Pity.

      5. There you go again Mr. Crumb. Superiority was far from implied. A simple reference to my geographic locale was the intent. Now, as long as I’m in the USA, miles will be my (and a few others) method of measuring distance even though I have adjusted to the transition and can readily convert w/o the use of slide rule anymore (I’m from a couple generations back btw).

      6. Again. It is easy enough to give BOTH units, so why not do it?

        The basic question is why have to “adjust” at all just to read an interesting article?

      7. “Superiority was far from implied.” Eh?
        Have you ever heard of irony?
        So don’t you dare go overboard with the gratitude, will you!

      8. I do believe, as the Americans are the ones who built, launched and now share the data free with the world for the various Voyager spacecraft, they can bloody well use whatever method of Olde English measurements they wish, whether that be rods, chains, or miles, I don’t care. As a scientifically literate person, I merely convert in my head. Perhaps rather than waste your short span on this Earth complaining, you may wish to learn to do so as well, it really isn’t all that difficult.

      9. Have I ever heard of irony? I am forced to endure it every time I read one of your posts here @ UT Today.

      10. “As a scientifically literate person, I merely convert in my head.”
        Fine, but not every one is scientifically literate. What about the average Joe who just wants to learn and understand?
        Distances are usually a piece of cake. OK, how about converting energy from ergs to watts or vice-versa. Sometimes this process just isn’t easy as you make out.
        As for “complaints”, well that is my business. Don’t like it, don’t read it.

      11. here in the USA

        Fair enough.

        But wouldn’t a web article, even if it has a dominant US readership (but has it?), benefit from “neutral position”? That is, using the international system for ease of comparison and add US conversions for ease of reading. (It would also help shortening the time until a nationalistic perspective dissolves.)

        Otherwise it could as well be “US universe Today”, couldn’t it?

        [Also, IMHO the “unhelpful” conversions uncovered by HSBC, which originates from NASA, reveals the clunkiness of an un-neutral position.

        Mark my words, one day a space mission will get into trouble because someone forgets to convert between international and national units! (-_-‘) ]

    2. We have an article about the pending arrival of man made machines into interstellar space, the first our species, or any we know of, has been able to achieve. We have built a machine that can travel 114 times further from the sun than we are, and continues to communicate with us. We are making our first tiny mark on the galaxy around us, marking our progress toward an amazing future of development for our kind.

      But that isn’t nearly as provocative as the correct usage of measurement units in an article on the internet.

      Forget the threat of nuclear apocalypse. We won’t develop because humankind’s energy will be sapped while furiously critiquing the grammar and punctuation of random people on the internet.

      Such is the human condition.

    3. Thanks for the AU distance… helps put things in perspective. Voyager really hasnt gone that far at all in the grand sceme of things.

  3. Astronomical Units, I like it.

    It takes 8.333 minutes for the light to travel 1 AU. That makes 949.962 minutes or 15 hr 50 min for the light to travel for 114 AU. And that is how long it takes for the Voyager signal to get here.

  4. This is very cool. 33 years on and the Voyagers are still providing us with valuable science- and may they continue for many years to come!

  5. Fantastic. Can’t wait ’til we get the pronouncement that Voyager 1 truly sails in interstellar space.

    Distance measurement:

    The league is the distance that a horse, or a person, can walk in 1 hour. It is divided into 3 miles. The statute mile is divided into eight furlongs; each furlong is ten chains; each chain is four rods (also known as poles or perches); and each rod is 25 links.

    Imperial measures: Sensible.

    That’s just my two threepence, anyway.

    1. Using furlongs, chains and rods… Eh? Are you talking about distances or are these the methods also used by the Spanish Inquisition here? As for “leagues”, all I know is Jules Verne used 20,000 of them., but then, I never owned a horse nor walked that far in ages.

      At least the distance here wasn’t in kilomiles!!

  6. Wow, it’s truly amazing that these probes are not only traveling at these speeds over all these years, but that we still can maintain contact with them at these grand distances, science is great!! 🙂

  7. First, this is great news and very exciting because this craft is at the very frontier (in a physical sense) of humanity’s presense in the universe.

    Next, I’m going to come to Crumb’s defense (although I admit it is somewhat anal for him to be pointing it out): Metric is far more science-friendly than imperial. I thought everyone knew that. The only reason at all that Nancy and the others bother with showing miles is for those who do not understand metric (Ams, Brits). Even most American scientists use metric in the labs. NASA does too.

    Finally, an “airport”, according even to US law, is a facility where aircraft land and take off that also has Air Traffic Control, while an “aerodrome” can be anywhere that aircraft land and take off , either monitored or unmonitored, and is registered with (in the USA) the FAA.

    1. The applicable portion of US law, namely the Code of Federal Regulations, Title 14, Part 1, Section §1.1 (commonly known as “The FARs” or Federal Air Regulations) states:

      Airport means an area of land or water that is used or intended to be used for the landing and takeoff of aircraft, and includes its buildings and facilities, if any.

      Aerodrome is not mentioned in the FARs.

      See for yourself:

  8. Apologies for the anti-American sentiments here, but I’ve been under an enormous strain. Well after the United States deliberately attacked my home city and my country with its most deadliest weapon in its whole arsenal — Oprah Winfrey — what else do you expect?
    Surely using these kind of weapons are in the highest level of “crimes against humanity”! Poor defenceless Australia will never be the again same! (and much worse, she says she is coming back.) Next will be the imposition of your imperial units! You dastardly mean Americans really are a bunch of dirty rotten miscreants, aren’t you!! Shame on your terrible cruelty!

  9. Mr. Crumb, you can keep Oprah. It’s our free gift to you. You’re welcome in advance.

  10. For such a tiny craft at such a huge distance, the radio signals from Voyager 1 must be very weak. Could someone say what the strength of the signal is and at what distance it will be too weak to receive?

  11. The vast majority of British people use the imperial system on a day-to-day basis. Even the pupils at the school where I’m a lab tech travel in miles, at miles per hour. They buy produce in pounds even if the shops sell in pounds AND kilograms. Strangely enough they measure their height and weight in metres and kilos! The lab apparatus might be in a cupboard six feet away where they will find a 30 centimetre ruler[ asking them if they want a foot ruler will be greeted with blank stares.]. On the occasions I buy petrol I have absolutely NO idea how many gallons [UK ones] I’m putting in the car or how much per gallon I’m paying! The Metrication Board which was set up under a Labour government in the 1960’s never really met with any success. Metrication has sort of slithered into use unnoticed and definitely unloved.

    1. Paul,
      “pounds AND kilograms”. Well that is the point. Youngsters in my part of the world have not been exposed very much to the imperial system, and they just look confused with anything else. If they read units they don’t understand, then all thy do is switch off. In articles like this one, well they will just read something else.
      UT has great stories, and I don’t have to buy S&T anymore just to get the news in the world of astronomy and space science. The more who read it, the better the understanding of the science.

      (I note Nancy hasn’t replied to the debate here, but if anything was a quite serious point. getting others to read these articles only improves how interesting astronomy and space science is.)

      1. “Most” kids here in the US (superiority not implied) must learn both systems esp. if they plan to pursue higher learning or even plan to extensively travel or work abroad.
        Nancy is probably laughing too hard to type.

      2. ““Most” kids here in the US (superiority not implied) must learn both systems…”

        I didt know that, but I thought that the local education boards could do so at their own discretions. There is a lot of useful sites on this; I.e. Teaching the Metric System for America’s Future

        They say;

        “In today’s global environment, metric measurements are prominent in workplaces, consumer products, and news reports. Almost every other country in the world uses the metric system of measurement. The European Union, Japan, and Korea have passed legislation limiting international commerce to products measured in metric units. If the United States is to continue to play a leading role in international business, using metric measurement is imperative and U.S. workers at all levels must be knowledgeable about the Système Internationale (SI), the international name for the metric system.”

        Also these guys have their head together. I.e.

        “However, the Council recognizes the leadership responsibility of schools to ensure that all students have experiences that enable them to measure in both the metric and the customary systems as well as to solve problems related to measurement in either system.

        Irony or not, this reinforces the basis of my arguments. (Nancy laughing or not!)

        This, frankly, reinforces my argument what BOTH values should be used.

        BTW. In my country, they only just learn the metric system, as far as I know.

        Thanks for the useful response here.

      3. … by the way;

        I’d suggest you might like to be a part of the The U.S. Metric Association (USMA), Inc..

        This is;

        “….a national non-profit organization that was founded in 1916. It advocates U.S. conversion to the International System of Units, known by the abbreviation SI (ess-eye) and also called the modern metric system. The process of changing measurement units to the metric system is called metric transition or metrication.”

        Let’s see if I can get a sponsorship for “Universe Today” for their initiatives. (We could even have a competition of a “Think Metric Mug”!!! Who says the USA isn’t progressive! (These guys have been around since 1916!!

        I do love the slogan “Go Metric”!!

      4. This is a no win situation Mr. Crumb. We in the USA are stuck with this duality and we seem to have manage quite well with it, thank you very much.

  12. To answer Sam Bartlett’s question using some quick back of the napkin math here:

    -Voyager I has a transmit power of 18 watts (43dBm) max at roughly 8420 MHz and has a carrier suppression of 6dB
    -The Voyager dish gain is 48dBi
    -The 35m deep space network dishes have gains around 80dBi

    Friis equation for path loss in dB is 32.44 + 20log(dist in km) + 20log(freq in MHz)
    32.442 + 20log(17300000000) + 20log(8420) = 315.7dB

    EIRP for Voyager I is 43dBm + 48dBi – 6dB = 85 dBm

    so the received power would be [NASA DSN antenna gain] + [EIRP] – [path loss] = 80dBi + 85dBm – 315.7dB = -150.7dBm = 8.51e-19 Watts!

    A good phase locked receiver has a sensitivity of around -157dBm. That gives us a margin of just over 6dB. So to answer the second question, Voyager I will become too weak to receive with today’s radio equipment at a distance of 28200000000 km or 188.5 AU. So we can go another 74.5 AU before she’s out of range. That will take 20.79 more years.

    1. The above analysis appears to connect up with:

      given the logarithmic definition of antenna gain. I am not sure where the factor of 20 enters into the logarithms or where the 32.44 dB enters in as well. I am not an electrical engineer, so this is a bit outside my experience. However, modulo these fiddle factors which I might figure out with some reading this analysis looks reasonable.

      By calculating in my head we receive signal from only about a million photons per second.


      1. Lawrence,

        32.44 is just a correction factor that equals 20log(c / [4*pi*1e3 meters *1e6 Hz]) that accounts for the conversion to km and MHz.

        The level in decibels is 10 times the logarithm of the ratio of two power levels in any non-logarithmic unit. dB = 10 log (P/P ref)
        When using the decibel to refer to voltage gain in dB as I did, you use dB = 20 log (V/V ref) due to the property of logarithms that squaring the argument is the same as multiplying the whole thing by 2 (2 x 10 = 20).

      2. If I take the logarithm of the equation for the ratio of the power received and transmitted by the two antennas

        (P_r/P_t) = G_rG_t(L/4piR)^2

        I get

        Log(P_r/P_t) = Log(G_rG_t) + 2Log(L/4piR).

        The base of the logarithm is probably 10, since we are working the dBs, and multiplying by 10

        10*Log(P_r/P_t) = 10*Log(G_rG_t) + 20*Log(L/4piR).

        So the bit about multiplying by 10 I did not realize. The 32.4 is a conversion factor. The gains of the two antennas also contribute to the EIRP, where the numbers used are the 10*Log(G_rG_t). So this all makes sense.


  13. Way to go Voyager!
    I think that its power will decrease faster than the decrease in signal strength from it. AFAIR the life-expectancy of the Voyagers (power-wise) is till ~2025ish or so… this means only 15 years from now.

    And speaking of life expectancy, I have to mention that every time that I see a MER Mars picture here on UT, my heart skips one beat in hope that it is the announcement that Spirit has come back alive. I’m still waiting for that to happen 🙂 (Opportunity was always my favorite, but I would love to see Spirit back alive 🙂 )

    To come to Crumb’s defense (and not only), using a more proper distance unit (AU in this case) would indeed help the readers that are not used in calculating distances in imperial units (just as I promote using a common time reference for space related events and milestones (i.e. GMT)) instead of local time of the event (e.g. EST time for Florida launches);

    Moreover, even NASA is using the km on their Voyager homepage (and the imperial units are given just as reference on the spacecraft’s homepage):

  14. Nancy, Please edit this article too reflect Voyager 1’s distance from the sun:
    Change 17.3 billion km to 115.6433572152441 +/- .5 AU
    There, that should do it.

  15. I believe american English (ie “color”, “harbor” — no u’s) was started by Daniel Webster when he wrote his first dictionary. It was in the 1790’s, just after the American War for Independence (also incorrectly referred to as the “American Revolution”) and Webster wanted to stick it to the Brits.

    Personally, I like the “u’s”, have no use for metrics (since I live in the US), but do think that AU is the most appropriate when referencing distance in the Solar System.

    1. So your alright with a fractional AU. Off the top of your head, can you tell me the mean earth/moon separation in AUs? Would you be more comfortable with 0.002569575 AUs or 384,403 kilometers (238,857 miles). Just asking.

      1. The AU is useful within the Solar System. i.e. Between planets, etc. Moon and satellite, the best distance is in km (+ miles.) Frankly a 1.8 billion kilometres or miles means nothing if you have no useful scale to adjudge it by.
        The whole issue is just to avoid big and small numbers (needing scientific notation), as they are not imaginable or convertible in the mind.
        When I use to teach astronomy, I’ve leant the distances scale should be always brought down in scaleable terms. I.e. An Australian $2 coin is 2cm across, so I related the planets distances or sizes to this scale. Therefore, if the Earth was a 2cm. $2 coin, the moon would be 7.5 metres away, and the Sun would be 2.2 metres across, etc.
        Another example is in galactic terms; stars in the night sky are in light years or parsec, globular clusters and deep-sky objects are in kiloparses, galaxies are in megapasecs, etc. Again the number reduce to comprehensible scales and head maths.

      2. I should have said, the same analogy works between a US one cent coin and a five cent coin, being 1.9cm and 2.1cm, respectively. A US quarter is just under one inch (24.6cm).

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