The Voyager 1 recently entered the interstellar space.

The Voyager group said that the Voyager 1 has left the Heliosphere and entered the interstellar space. Charge particles and plasma activities increased about 40X as compared to readings inside the Heliosphere.

Proof of having crossed the Heliopause should be signaled by

  • a sharp drop in the temperature of charged particles,
  • a change in the direction of the magnetic field, and
  • an increase in the amount of galactic cosmic rays

Did it really cross the Heliopause?

  • $\begingroup$ The question is OK, but I'm not really sure your own answer draws any clear conclusions. For the time being, we have measures by one probe and in one direction. If you look at any termination shock boundary, you'll notice it's hardly a clear cut line to be drawn and we'd average it out to have a finite value to work with. And you'd obviously need more than a single measure to calculate the average. So we could also just say the answer is: NASA says so. And so they did, about a humpteen number of times by now. Pick any answer between yes and no. ;) $\endgroup$
    – TildalWave
    Commented Sep 25, 2013 at 8:33
  • $\begingroup$ My last paragraph says no. I should edit it to make it more clear. $\endgroup$ Commented Sep 25, 2013 at 8:46
  • 1
    $\begingroup$ Part of the problem is we don't really know what interstellar-space is going to be like. Voyager is the first probe that's close, so we're kinda learning as we go as to what the boundary actually looks like. There's some pressure to declare that Voyager made it to interstellar-space (as it is a noteworthy milestone); but in many ways we won't know if it has until other probes return similar results. $\endgroup$
    – john3103
    Commented Sep 25, 2013 at 14:54

3 Answers 3


The heliopause is not expected to be a sharp cutoff, but instead a turbulent and fluctuating area, outside of which there is one norm, and inside another.

The transition could be much wider than we expect, hence the IBEX mission has been sent to understand this further, but no one is doubting that the data shows something is happening at the heliopause. NASA have data indicating that various shock boundaries have been crossed - initially, expectations were that this may be well defined, but it makes equally as much sense for there to be any number of layers, vortices, shocks etc.

So if your question is does it exist, yes, it does, by its very definition. Has Voyager crossed it - maybe not yet, but it is certainly at or in it.

  • $\begingroup$ Voyager is in interstellar region. The region of plasma and gas in ionic, atomic, and molecular form, dust, and cosmic rays. And the "sharp drop" is as mentioned in Wikipedia: en.wikipedia.org/wiki/Heliopause_(astronomy)#Heliopause and none of the statement by the Voyager Group confirm its existence. $\endgroup$ Commented Sep 25, 2013 at 9:39
  • $\begingroup$ If you follow some of the information and links from the question @Tildal pointed at ( space.stackexchange.com/q/1934/38 ) I think it shows that NASA do confirm that Voyager is somewhere in the Heliosheath and moving towards the Heliopause. $\endgroup$
    – Rory Alsop
    Commented Sep 25, 2013 at 10:23

Yes. On the 25th of August, 2012, Voyager 1 measured very abrupt changes in two out of three of the criteria listed in the question. This was in fact Voyager 1 crossing the heliopause. In a single day.

The kicker was that the magnetic field direction only changed by 2°. This was a puzzlement.

By sheer luck, a large coronal mass ejection from the Sun tickled the plasma around Voyager 1 a few months later. The resulting ringing allowed them to measure the plasma density, which matched what was expected for the interstellar space in the vicinity of our solar system. They were able to find earlier, weaker oscillations, extrapolated back, and found that the density increased very rapidly in, you guessed it, August 2012. That provided the confirmation they were looking for.

The magnetic fields were expected to have different directions since they have different sources. There would be no reason for them to have the same direction, so that would be unlikely.

But, not impossible.

In short, there was a very sharp change when crossing the very real heliopause. The determination that that really was the heliopause was delayed by an unexpected approximate alignment of magnetic fields.

News flash:

On the 5th of November, 2018, Voyager 2 crossed the heliopause entering into interstellar space.


From the Nasa missions page:

"We literally jumped out of our seats when we saw these oscillations in our data -- they showed us the spacecraft was in an entirely new region, comparable to what was expected in interstellar space, and totally different than in the solar bubble," Gurnett said. "Clearly we had passed through the heliopause, which is the long-hypothesized boundary between the solar plasma and the interstellar plasma."

and in Wikipedia:

In May 2012, Voyager 1 detected a rapid increase in such cosmic rays (a 9% increase in a month, following a more gradual increase of 25% from Jan. 2009 to Jan. 2012), suggesting it was approaching the heliopause. In the fall of 2013, NASA announced that Voyager 1 had crossed the heliopause as of August 25, 2012.

All the articles suggest that Voyager1 has crossed the heliopause, meaning there is no chance of Voyager encountering it again. But none of them have confirmed its existence.

From Wikipedia:

The current mission of the Voyager 1 and 2 spacecraft is to find and study the termination shock, heliosheath, and heliopause. Voyager 1 reached the termination shock on May 23–24, 2005, and Voyager 2 reached it on August 30, 2007 according to NASA. Meanwhile, the Interstellar Boundary Explorer (IBEX) mission is attempting to image the heliopause from Earth orbit within two years of its 2008 launch. Initial results (October 2009) from IBEX suggest that previous assumptions are insufficiently cognisant of the true complexities of the heliopause.

The Change in the number of Charged particles and plasma has been gradual and not abrupt. So the fact denies the existence of Heliopause.

  • $\begingroup$ Huh? The Heliopause is there, there was an abrupt change, and we know exactly when and where Voyager 1 crossed it. $\endgroup$
    – Mark Adler
    Commented Nov 6, 2013 at 17:39

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