Leaving Earth cover

In 2019 I obtained from my former publisher the last 30 copies of the now out-of-print hardback of Leaving Earth. I sold about half of these, and with only a handful left in stock I have raised the price. To get your own autographed copy of this rare collector's item please send a $75 check (includes $5 shipping) payable to Robert Zimmerman to

Behind The Black, c/o Robert Zimmerman
P.O.Box 1262
Cortaro, AZ 85652

I will likely raise the price again when only ten books are left, so buy them now at this price while you still can!

Also available as an inexpensive ebook!

Leaving Earth: Space Stations, Rival Superpowers, and the Quest for Interplanetary Travel, can be purchased as an ebook everywhere for only $3.99 (before discount) at amazon, Barnes & Noble, all ebook vendors, or direct from my ebook publisher, ebookit.


Winner of the 2003 Eugene M. Emme Award of the American Astronautical Society.

"Leaving Earth is one of the best and certainly the most comprehensive summary of our drive into space that I have ever read. It will be invaluable to future scholars because it will tell them how the next chapter of human history opened." -- Arthur C. Clarke

Worlds without end

Last week’s fly-by of Pluto by New Horizons illustrated forcefully once again the power of exploration on the human mind, and how that exploration always carries surprises that delight and invigorate us.

First of all, the images from that fly-by demonstrated clearly that the decision by the International Astronomical Union (IAU) to declare Pluto a non-planet was very much premature. Even project scientist Alan Stern himself enthusiastically noted at the start of Friday press conference that Pluto-Charon was a “double planet system”.

The IAU definition itself was faulty and difficult to apply. The clause that required a planet to have “cleared the neighborhood around its orbit” made little sense in the real universe, as even the Earth has not successfully cleared its orbit after several billion years. Was the IAU suggesting the Earth was not a planet?

New Horizons’s discovery last week that even a small object like Pluto, orbiting the Sun on its own with no gas giant nearby to provide tidal heating, can still exhibit significant and on-going geological activity, shows that our understanding of what defines a planet is at this time quite limited. We simply don’t know enough about planetary evolution and formation to definitively define the term. Nor do we have enough knowledge to determine if Pluto falls into that category, though the data strongly suggests that it does.

Are planets made up of only gas giants, rocky terrestrial planets like the Earth, and dwarf planets like Ceres and Pluto? Or are there numerous other as yet unknown categories?

For example, Ceres, the largest object in the asteroid belt, is in the inner solar system, orbits by itself like Pluto, but it also shows evidence of some geological activity. It is spherical, which means it is massive enough for gravity to force it into the most efficient gravitational shape. But it is small, and somewhat similar to the Moon. To what planetary category does it really belong? The term dwarf planet seems very insufficient.

Then there is Vesta, another asteroid belt object and the second largest object in that belt. We might call it a dwarf planet or a very large asteroid, or even a failed planet. Which is it? Though the spacecraft Dawn gathered a great deal of information about it, we do not yet have a large enough sampling of similar objects to properly categorize or even understand it.

Pluto and Charon, small but spherical but showing signs of geological and meteorological activity, are located far from a star in a region where there is little energy to stimulate such activity. Will we be forced to create an additional planetary category for this type of planet? Who knows? Right now they are the only such objects we’ve gotten a close look at. Maybe the Kuiper Belt will reveal a variety of planetary types, some like Pluto, some like Vesta, some like Ceres, and some entirely different than them all.

Though we have discovered several thousand exoplanets, we know practically nothing about them other than their size and mass. Today’s announcement of more Earth-like exoplanets, including one 60% larger than the Earth, underlines this point. Categorizing exoplanets at this time is impossible. We might discover that the word planet encompasses dozens of types, most of which we presently do not even know exist.

In essence, we really don’t know what defines a planet, and New Horizons and Pluto proved that quite obviously last week. It is simply too early in our exploration of the galaxy to try to pin down the meaning of this term.

New Horizons’ Pluto fly-by also demonstrated something even more profound, the importance of exploration itself to the human race.

To children, everything they do and see is new and exciting. They squeal with delight at the simplest things, because those simple things are unexpected and have never before been experienced. Thus they are naturally attracted to the idea of space exploration, as it epitomizes this daily experience but extends it to the limit of all human knowledge. It is the future, and the future does belong to them.

Adults however lose this joy as they mature, since the things around them are increasingly less new or unexpected. In fact, things become so mundane and expected that many adults come to find it difficult to even get excited about real discoveries, such as those announced last week at Pluto.

Pluto and New Horizons, however, gave us all a chance to be kids again. For a few days the world was eagerly glued to our modern equivalent of the television, watching our computers and smart phones and tablets in breathless anticipation of seeing those first Pluto images. And when those images were finally unveiled we were all like children, seeing something fresh and exciting that was new and different. And like little kids, everyone was thrilled, overjoyed, and enthusiastic about life because of it.

And the show has only begun. Those first images were only a taste of the hundreds of images and data that New Horizons gathered as it zoomed past Pluto and Charon. For the next year, New Horizons will bit-by-bit transmit its treasure trove of knowledge back to Earth, and beginning tomorrow, Friday, the New Horizons science team will do the first of many press conferences, unveiling that data and those images to us, bit-by-bit.

Who knows what those images will unveil? Fortunately, none of us really know, and the experience each time will allow us all to be kids again.

Pioneer cover

From the press release: From the moment he is handed a possibility of making the first alien contact, Saunders Maxwell decides he will do it, even if doing so takes him through hell and back.

Unfortunately, that is exactly where that journey takes him.

The vision that Zimmerman paints of vibrant human colonies on the Moon, Mars, the asteroids, and beyond, indomitably fighting the harsh lifeless environment of space to build new societies, captures perfectly the emerging space race we see today.

He also captures in Pioneer the heart of the human spirit, willing to push forward no matter the odds, no matter the cost. It is that spirit that will make the exploration of the heavens possible, forever, into the never-ending future.

Available everywhere for $3.99 (before discount) at amazon, Barnes & Noble, all ebook vendors, or direct from the ebook publisher, ebookit.


  • Joe

    No matter what it’s classified as, Pluto will almost always be include on charts of the outer planets, much as Bermuda is almost always included in charts of “Caribbean” islands, even though it has been known to be a very different animal since soon after its discovery.

  • Edward

    It seems to me that when astronomers discovered that objects in the Kuiper Belt could be large enough to be classified as planets, they realized that there could be far too many for schoolchildren to memorize. It is hard enough to memorize nine planets, much less dozens or hundreds.

    So many astronomers panicked that future schoolkids would not have the same interest in astronomy and voted to redefine what constituted a planet (after many of the astronomers left the conference to go back home). Unfortunately, in their rush, they did not think through the new definition and what it meant. One part of the definition is that a planet has “cleared the neighborhood” of its orbit of other objects, but there are plenty of other objects in the orbital neighborhood of Earth, and most of the other planets.

    My father sent me a link to this article, which says that there is a growing demand that Pluto be reinstated as a planet.

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