This article, the first of two parts, by physicist Eric Hedin is reprinted with permission from Evolution News. Here’s the second part.

Reporting on any newly discovered exoplanet, especially if it has even the barest overlap of Earth-like parameters, is standard fare for science news outlets. While these discoveries are exciting from an astronomical point of view, the attention-getting angle often relies upon the presumption that such planets may offer a future home for humans. The notion of extraterrestrial life pullulating on any planetary surface either not hellishly hot or stuck in the frozen outer regions of a distant star system also tends to feed the interest in such reporting.

For example, a recent article at Science News reports, “Potentially Habitable Earth-Sized Exoplanet Discovered Around Gliese 12.” It describes the discovery of a planet orbiting an M-type dwarf star located about 40 light years (lyrs) from Earth. The planet, tagged Gliese 12b, is so close to its host star that its complete orbit takes only 12.76 days. For a planet orbiting one of these common dwarf stars, several obstacles to habitability prevail, including tidal locking.

However, the tidal force that a star exerts on such closely orbiting planets is extreme. A planet orbiting an M dwarf close enough to possibly possess surface liquid water will be tidally locked, with one of its hemispheres always facing its star (just as one hemisphere of the Moon always faces Earth).

An Alluring Hope

What about the alluring hope of future generations of humans traveling across the interstellar oceans of space to establish Earth outposts on such extrasolar planets? Astronomers have determined the number of possible stellar destinations in our “close” galactic neighborhood. There are roughly 2,000 stars within 50 lyrs of Earth, compared to approximately 200 billion stars in the Milky Way galaxy (one millionth of one percent lie within 50 lyrs of Earth). We can set some boundaries on the feasibility of interstellar space travel (either for humans or putative aliens) by applying the known laws of physics.

Using special relativity, let’s imagine what it would take in terms of raw energy to accelerate a spaceship to the speed necessary to make a 40 lyr journey (say to Gliese 12b) in 5 years as measured by the astronauts onboard. First, we need to find the speed, relative to Earth that this spaceship would have to attain to generate the proper time dilation for the astronauts to arrive in 5 years by their clocks. A bit of algebra with Einstein’s time dilation formula yields a required speed of 0.99c, where c=300,000 km/sec is the speed of light through vacuum.

Since time flows at different rates for the reference frame of Earth compared to the reference frame of the spaceship moving at 0.99c, we find that the journey which takes 5 years for the astronauts takes 40.3 years according to observers back on Earth. This would be somewhat inconvenient, but perhaps manageable. But when we calculate the energy required to accelerate a modest-sized spacecraft, outfitted with everything that a team of astronauts would need for a multi-year expedition to explore an unknown destination, we find that the answer is daunting.

Using Einstein’s special relativity formula for kinetic energy, and assuming a mass for the spaceship equivalent to that of a fully loaded 747 aircraft (m=400,000 kg), we find that the energy needed to accelerate the spaceship to the cruising velocity of 0.99c is 70 quadrillion kWh. Assuming that the astronauts want to stop at the distant star system, we’ll need to double this amount to account for deceleration, for the spaceship to make the one-way journey from Earth to the distant star. 

So, just how much energy is 140 quadrillion kWh? Believe it or not, it’s equivalent to 4,800 times the total energy consumption of the United States in the year 2022. This means all the electricity, petroleum, natural gas, and any other form of energy used to power everything in the U.S. for one year would be 4,800 times too small to get our modest-sized spaceship to a relatively nearby star in a reasonable amount of time. I think it’s fair to say that interstellar space travel isn’t even remotely possible with our current understanding of physics and technology.

Here’s the second part of this two-part series, “But why not dream?”