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rocket propulsion

How Far Are Stars?

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Photo by Michael J. Bennett

This week’s post is a bit of a distraction from the usual business, based on a question I wondered about. Rather than ask Google, I dug in like a nerd to get a more complete picture.

One of my frequent spiels is about the vastness of space, in the context that we can dismiss fantasies about humans traveling to the stars. I do throw in an old-school calculation at the end to reinforce this point, but until then we’ll entertain ourselves with a sense for the scale of the sky we see with our eyes.

When we consider a scale model in which the sun is reduced to the size of a sand grain (about 1 millimeter), the closest neighbor star is about 30 km away. One light year at this scale is about 7 km. But how typical is this yawning gulf in our region of the galaxy? And how far away are the stars we lay eyes on in the night sky, typically?

Before getting to those questions, just how many stars can we see, naked-eye? It depends on the darkness of your sky. According to the Hipparcos catalog, rounding apparent visual magnitudes to the nearest integer, there are two −1 magnitude stars: Sirius and Canopus. Eight more join at magnitude zero; 12 at first magnitude; 71 at second; 192 at third; 622 at fourth; 1909 at fifth; and 5976 at sixth—at which point our eyes run out of steam. A suburban sky might allow fourth magnitude, or roughly 1,000 stars (not all at once, since only half are up at a time). At fifth magnitude, we get about 3,000 (all-sky). At the limit, we tally about 9,000 stars. About half this number would be above the horizon at any given time.

Incidentally, going to space hardly does a thing to improve visibility: the atmosphere is pretty impressively transparent at visible wavelengths (only “eating” about a tenth of a magnitude). I was excited to see the night sky from Mauna Kea on my first observing trip there as a graduate student. Being above 40% of the Earth’s atmosphere, it’s the closest I had been to space. The thing is, low oxygen levels impair visual sensitivity, so when I first went outside it really sucked: I could barely see a thing (eventually dark-adapted, but way slower than at lower elevations). Space is even worse on the oxygen front.

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Space-Based Solar Power

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A solar panel reaps only a small portion of its potential due to night, weather, and seasons, simultaneously introducing intermittency so that massive storage is required to make solar power work at a large scale. A perennial proposition for surmounting these impediments is that we launch solar collectors into space—where the sun always shines, clouds are impossible, and the tilt of the Earth’s axis is irrelevant. On Earth, a flat panel inclined toward the south averages about 5 full-sun-equivalent hours per day for typical locations, which is about a factor of five worse than what could be expected in space. More importantly, the constancy of solar flux in space reduces the need for storage—especially over seasonal timescales. I love solar power. And I am connected to the space enterprise. Surely putting the two together really floats my boat, no? No.

I’ll take a break from writing about behavioral adaptations and get back to Do the Math roots with an evaluation of solar power from space and the giant hurdles such a scheme would face. On balance, I don’t expect to see this technology escape the realm of fantasy and find a place in our world. The expense and difficulty are incommensurate with the gains.

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Stranded Resources

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A few weeks back, I made the case that relying on space to provide an infinite resource base into which we grow/expand forever is misguided. Not only is it much harder than many people appreciate, but it represents a distraction to the message that growth cannot continue on Earth and we should get busy planning a transition to a non-growth-based, truly sustainable existence. To prove what a distraction it is, I will distract myself again this week with another space post. This time, true to the brand, I will do the math on why the infinite resources of space appear to be of questionable use to our human enterprise.

Part of my motivation comes from the bruised, and bruising comments in reaction to the Why Not Space? post. The faith is strong that technologies are already in hand and that we just need NASA to get out of the way so the commercial bounty of the sky will open up and we’ll finally be off to the races. I myself refrained from ruling out such a future, but the mere suggestion that we may fail to expand into space was clearly considered by many to be ridiculous—as if such a fate is predestined: as sure as the sun will tomorrow. Sociological impulses tugged at my physicist bones, tempting me to study exactly how such an unshakable faith has been implanted in so many obviously smart people. For these folks, the arc of the future is as sure as the historical progression from the Dark Ages until now. Wait? Was there something before the Dark Ages? Something grand? Alas, my history fails me.

Leaving the sociology aside—but before we get busy with the math—I’ll share the story that during the comment firestorm, an individual contacted me from NASA headquarters (not to revoke my funding, thankfully), offering thoughtful perspectives on space policy. The part I can’t shake is the statement that it takes decades of serious research to answer two simple questions: “Can humans live and work in space for the long term?” and “Can an economically viable activity be found in space?” Opinions aside, these are open questions, and have been for some time. We have no proof—or even firm expectation—that either is practical or possible.

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Around the time of the final U.S. Space Shuttle flight, a NASA official was asked in a radio interview to explain what was left to inspire young kids about space. The answer was that mining asteroids and the Moon offered a new grand challenge to inspire our kidlets. Granted, space mining probably is a bit more inspiring than off-shore drilling or coal mining as a career choice. It’s got space in it. But are we really serious about getting materials from other bodies within the solar system?

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