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Many of us delight in space adventurers like Star Trek, Star Wars, …. Many also look forward to mankind colonizing Mars and ultimately exoplanets (planets beyond our Solar System). The grim reality is other star systems are vastly remote — farther than we could reach in any realistic space voyage. Voyager 2, our unmanned spacecraft with the greatest terminal velocity, would take about 75,000 years to reach Proxima Centauri, the star nearest our Sun. Even if we achieved the most efficient propulsion system that nature’s laws allow (antimatter annihilation), a one-way 100-light-year journey would take 104 years of Earth time and require 100 times the payload mass in antimatter fuel. Due to relativistic time dilation, the crew would age only 18 years, since most of the journey would be at 99% of light speed. But, with antimatter costing about one trillion dollars per pound, I can’t imagine anyone funding this. Two UCLA astrophysicists, B. Hansen and B. Zuckerman, propose extreme patience as the solution to space travel. If we wait long enough, they say, some stars will come close to us, and at least one will probably come just in time. No planet can be habitable forever. Very massive stars have brief lives (a few million years) and die in catastrophic explosions. Middle-mass stars burn steadily for a few billion years before greatly expanding into red giants and ultimately collapsing into white dwarfs. Low-mass stars, such as M-dwarfs, burn very slowly for much longer, up to trillions of years, before becoming white dwarfs. Middle-mass stars, like our Sun, provide the best environment for life to develop. But as these stars expand into red giants, their radiated energy soars, incinerating previously habitable planets. We believe this demise will befall Earth in 600-800 million years. Eventually, after the Sun becomes a white dwarf, none of its planets will be habitable. For the human race to survive the Sun’s end-stage, we must establish colonies elsewhere. Hansen and Zuckerman estimate a suitable home will likely come near enough, soon enough. 
This image shows our stellar neighborhood. All these stars are moving, and some are coming our way. They say the average star density in the Sun’s part of our Milky Way galaxy is about 1 star per 350 cubic light-years, equivalent to the next nearest star being an average of 3.5 light-years away. (Proxima is about 4.2 light-years away, which is in the same ballpark.) They also say the average stellar velocity (net of overall galactic rotation) is 48 km/sec in the Sun’s neighborhood. From the star density and relative velocities, they say 1400r2 stars per billion years pass within r light-years of any particular location, such as Earth. (From a moving star’s viewpoint, passing with distance r means hitting a circle of radius r, hence the r2). The authors choose a target distance of r=0.032 light-years, which is 2000 times Earth’s distance from the Sun and 130 times closer than Proxima. They say one star will pass this close every 700 million years, and will remain at about that distance for some 400 years, on average. Most stars are low-mass M-dwarfs with very long lifetimes, so we probably wouldn’t have to migrate again any time soon. However, such stars pose severe challenges. Because they are very dim, only planets extremely close to their star could be habitable (about 15 times closer than Mercury is to our Sun). These planets are tidally locked, with one side always facing the star and the other side always in darkness. Additionally, such stars produce more violent and more frequent stellar flare and mass ejections (many millions of times worse than we experience on Earth). These challenges may preclude life from originating near low-mass stars, but an advanced civilization could reside on a planet’s dark side, shielded from its violent star, while collecting energy and supplies from the eternally illuminated side. If we can achieve greater terminal velocities with more aggressive gravity-boosts, the travel time with conventional rockets could be reduced to several centuries. With more advanced propulsion technologies, the journey might be made within a human lifetime. It will still require dramatic engineering advances to make sending a small group of human colonists to another star system conceivable. NASA says it now costs $10,000 to lift one pound up to low Earth orbit, and that on long voyages, each person needs 3500 pounds of food and water per year. You do the math. One might imagine humanity establishing a new colony in a new stellar system every billion years for the indefinite future. Indeed, other life forms may have been star hopping for billions of years. Best Wishes, Robert 
August 2021 Note: Previous newsletters can be found on my website. | 
