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On February 5, 2023 I will celebrate my 70th birthday. Yay! As I do every year during this birthday month, I run a campaign to raise money to support my work here at Behind The Black. I do not run ads. My only support comes from my readers, which leaves me utterly free to speak my mind openly about space, culture, and politics. Please consider supporting me in this work by giving either a one-time contribution or a regular subscription, in any one of the following ways:


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Scientists: Plow the solar system through a dense-enough interstellar cloud and the heliosphere would no longer protect the Earth

The Earth's orbit outside the heliosphere

The uncertainty of science: Using a computer simulation, scientists have determined that if the solar system had two million years ago passed through one of the known nearby interstellar clouds within the relatively empty Local Bubble of space, it would have shrunk the Sun’s heliosphere enough so that the Earth would no longer be inside it, thus exposing the planet to interstellar space.

The image to the right comes from that simulation, and is figure 1 of the scientist’s paper [pdf]. The red line marks the Earth’s orbit (tilted sideways slightly to make it obvious), the yellow blob the shrunken heliosphere.

From the paper’s abstract:

There is overwhelming geological evidence from 60Fe and 244Pu isotopes that Earth was in direct contact with the ISM [interstellar medium] 2 million years ago, and the local ISM is home to several nearby cold clouds. Here we show, with a state-of the art simulation that incorporate all the current knowledge about the heliosphere that if the solar system passed through a cloud such as Local Leo Cold Cloud, then the heliosphere which protects the solar system from interstellar particles, must have shrunk to a scale smaller than the Earth’s orbit around the Sun (0.22).

Using a magnetohydrodynamic simulation that includes charge exchange between neutral atoms and ions, we show that during the heliosphere shrinkage, Earth was exposed to a neutral hydrogen density of up to 3000cm-3. This could have had drastic effects on Earth’s climate and potentially on human evolution at that time, as suggested by existing data.

This model is just one possible explanation of the presence of 60Fe and 244Pu isotopes on Earth. Another popular hypothesis is that a supernova occurred about 30 light years away, close enough to expose the Earth to interstellar space but not so close as to cause the total extinction of life.

With both theories, the event could also be an explanation for the significant climate changes two million years ago — such as the beginning of the most recent and now-ending ice age (no SUVs required) — as well as major evolutionary changes that occurred at that time among the ancestor species of humanity.

All is uncertain however. The scientists have no evidence the Earth actually entered a local dense cloud two million years ago. All they are doing is postulating that if such a thing happened, the dense cloud could shrink the heliosphere so much the Earth would be exposed to the interstellar medium.

Since we also do not yet have evidence of a specific nearby supernovae event either, neither theory can be favored. In fact, both could have been happened at different times in the past. Or neither.

Hat tip to reader Phil Berardelli, author of Phil’s Favorite 500: Loves of a Moviegoing Lifetime.

Conscious Choice cover

Now available in hardback and paperback as well as ebook!


From the press release: In this ground-breaking new history of early America, historian Robert Zimmerman not only exposes the lie behind The New York Times 1619 Project that falsely claims slavery is central to the history of the United States, he also provides profound lessons about the nature of human societies, lessons important for Americans today as well as for all future settlers on Mars and elsewhere in space.

Conscious Choice: The origins of slavery in America and why it matters today and for our future in outer space, is a riveting page-turning story that documents how slavery slowly became pervasive in the southern British colonies of North America, colonies founded by a people and culture that not only did not allow slavery but in every way were hostile to the practice.  
Conscious Choice does more however. In telling the tragic history of the Virginia colony and the rise of slavery there, Zimmerman lays out the proper path for creating healthy societies in places like the Moon and Mars.


“Zimmerman’s ground-breaking history provides every future generation the basic framework for establishing new societies on other worlds. We would be wise to heed what he says.” —Robert Zubrin, founder of founder of the Mars Society.


All editions are available at Amazon, Barnes & Noble, and all book vendors, with the ebook priced at $5.99 before discount. The ebook can also be purchased direct from my ebook publisher, ebookit, in which case you don't support the big tech companies and I get a bigger cut much sooner.


Autographed printed copies are also available at discount directly from me (hardback $24.95; paperback $14.95; Shipping cost for either: $5.00). Just email me at zimmerman @ nasw dot org.


  • Jay

    Good paper. Shrinking the heliosphere from 130 A.U. down to 0.1 A.U., what a thought. I wonder if two or multiple lobes of the heliopshere would be created depending on size of the cloud and the angle of plane would break it up.
    Wow, I have never looked at this 244Pu isotope before. I always thought any isotope after 242Pu had a half-life of days or less, this one is 8 × 10^7 years. We can’t make that one in a controlled environment.

  • pawn

    Measuring the half life of something in the millions of years range. I need to read up on how that is done. With a near 100% chance of splitting into 240U and an alpha particle that is some radical metrology. They even have the percentage of it decaying with a pair of beta particles at 7.9 X10E-9. Holy cow!

  • John

    Great, now we have to worry about the heliosphere shrinking. As if magnetic field reversal wasn’t enough!

    We’ll definitely have to validate this model next time the sun moves through a cloud.

    The model is no doubt very sophisticated, but maybe the sun had a hiccup and the climate and heliosphere changed.

  • Frank

    Heliosphere change deniers will get Heliosphere justice

  • wayne

    “Battle Beyond The Sun” (1959)
    Roger Corman / Francis Ford Coppola

  • wayne


    “New Measurement of the Fe-60 Half-Life”
    ->August 2009

  • Jay

    Thanks for the link. Good paper, and that is a good approach at the end on allowing the comments at the end either supporting or refuting the citations.

  • Star Bird

    Somewhere out there is the Jupiter 2 and V-Ger

  • pawn


    To borrow a meme:

    “I saw Heliosphere Justice open for Sun Ra at the Zappos Theater in ’94.”

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