The other treasure that impacts bring to the Moon is rocks from elsewhere. The impacts that deliver meteorites from the Earth to Mars deliver far more of them to the surface of the Moon; and there will be meteorites from Mars, too, and from Venus. Random bits of all the inner planets are scattered over the lunar plains—mostly buried a bit below the surface, perhaps, thanks to the slow but ceaseless churn of new impacts. But some will almost certainly still be identifiable, with scrutiny.
In a beautifully titled paper from 2003, three planetary scientists suggested that this made the Moon “Earth’s Attic”: no one knows quite what is stored up there, and a lot of it is probably junk, but there is more of it than you might think, and it is older, too. There might be valuable oddities. There might be precious heirlooms.
On the valuable oddity front, there might be 30kg of rock from Venus on every 100km2 of the lunar surface. Finding them would be a heroic task. But then, getting samples of Venus is always going to be a heroic task, what with its surface being at a temperature of 440°C and smothered in an atmosphere 100 times thicker than the Earth’s. It is hard enough to land there; only two Soviet probes have done so, and they lasted no more than a few hours. Landing, picking up some rocks and getting back into orbit—which is almost as hard to do from the surface of Venus as it is from the surface of the Earth—is a challenge beyond today’s technology. And even if it were possible, it would only bring back bits of today’s fairly young, lava-covered crust. The crust from billions of years ago—back before the advent of that crushing atmosphere, back when Venus might even have been an ocean—would be unreachable.* But there may be some of it on the Moon, because most of the transfer of rocks between the inner planets took place in the heavy-hitting Hadean.
However much of Venus has ended up on the Moon, there will be a lot more of the Earth there. On the same 100km2 patch where scientists might, if lucky, find 30kg of rock from Venus, the Earth’s Attic theorists would expect 20 tonnes of earthrock. Again, most of it would be from the Hadean—a period the record of which on Earth itself is all but non-existent, thanks to the planet’s constant recycling of its rocks. In early 2019, geologists in Houston announced that they had identified what they took to be one such fragment in a breccia brought back by Apollo 14.
It is not just the practices of Earthly geology that have been exported into the solar system through planetary science, nor its subdivisions of time, be they Hadean or Anthropocene. Its oldest, dearest and rarest subject matter sits in the sky above, not the ground below. James Nasmyth was wrong about the Moon preserving the look of the early volcanic Earth, but he was right in seeing it as home to the vestiges of creation.
If there are findable rocks anywhere in the universe which contain traces of the earliest life on Earth, the odds are that they are to be found on the Moon.
* Holocene means “wholly recent”; the Quaternary is stuck with the name it got as the fourth part of a chronological system no one uses anymore; Cenozoic means “the age of recent life”; and Phanerozoic means “the age of visible life”—ie, fossils big enough to be recognised without recourse to microscopy.
† That, at least, is where the International Commission on Stratigraphy puts it. Geologists in China and in Russia both argue for alternative sites and continue to use them as reference points. But all three are within a few million years of one another.
* Though the name Theia is widely used for the junior partner in the giant impact—Theia was the mother of Selene, the moon goddess, in Greek mythology—I should note that the name Tellus for the senior partner is not all that widely used. A lot of people just call it the Earth, or proto-Earth. But I think having a separate name helps, and Tellus is the one that the Chaotian quartet uses, so I have followed suit.
* This is not true.
† Hugh Lofting made use of Darwin’s fission idea in his book “Doctor Dolittle in the Moon” (1928), which may have been the first Moon narrative I ever read. As well as talking with the lunar animals—and plants—Dr Dolittle meets a caveman who has been living on the Moon since it was part of the Earth, rather as Cyrano de Bergerac meets the prophet Elijah when he visits the Moon’s pre-lapsarian Eden. Lofting’s sequel, “Doctor Dolittle and the Secret Lake”, was probably my first exposure to the fiction of global cataclysm, though it is possible that honour should go to Tove Jansson’s “Comet in Moominland”.
* The rock which Dave Scott and James Irwin, the Moonwalkers of Apollo 15, were so excited about here was a lump of near pure plagioclase; it has since been dubbed “the Genesis rock”.
* Measurements of oxygen-isotope ratios in rocks from Venus would be very handy; if they are Earth-like, then Theia could have been, too, and Mars is just an outlier. But getting rocks back from Venus is no easy task, and if there are meteorites from Venus on the Earth, they have yet to be identified.
* The process is also known as “endolithic panspermia”—panspermia being the idea, dating back a century or so, that life spread across the galaxy in the form of spores, and endolithic meaning that in this case it did so inside rocks. I think my term, transpermia, is better in that it brings out the idea that this is life moving specifically from one place to another rather than being broadcast to everywhere.
* This possibility of a clement early Venus is, as it happens, a subject particularly associated with David Grinspoon.
TRAJECTORIES
THE CHALLENGE IN REACHING IT IS NOT MERELY, OR EVEN MAINLY, one of distance. It is one of speed. Spacecraft, like planets and moons, are constantly falling, their trajectories shaped by the gravitational field of the Sun and of nearby smaller bodies. To get from one trajectory to another is a matter of changing velocity in the right direction by the right amount. The change in velocity required to get from one orbit to another is known as “delta-v”. To get from the surface of the Earth to the surface of the Moon requires a delta-v of about 15 kilometres a second (km/s).
This needs to be applied in stages: every time a spacecraft changes its trajectory, it needs an extra dose of delta-v. The biggest requirement is the first. To get into a low orbit around the Earth requires a spacecraft to get up to a speed of 7.7km/s or so. In practice, to overcome various kinds of drag, you need about 9km/s.
From a near-circular orbit around the Earth the spacecraft then needs to get into a highly elliptical one, in which the perigee is still close to the Earth but the apogee is up by the Moon. That requires about 3km/s of delta-v. Once up at the Moon, the spacecraft then needs to change trajectory again to go into orbit around it. That requires another 1km/s.
If, once in lunar orbit, the spacecraft is to land, it needs another 2km/s to lose its orbital velocity and end up stationary on the surface.
Thanks to the lack of an atmosphere, you can get very close to the surface without that final step. Spacecraft have orbited the Moon at altitudes of 30km or so, and made one-off passes closer than that. Orbiting the Moon, though, raises other problems. The mass of the Moon’s crust is not equally distributed, and concentrations of mass in the maria act like phantom reefs, running low-flying satellites aground from a distance unless their orbits are carefully designed so that the perturbing influences cancel each other out. Orbits higher than about 1,200km, on the other hand, are destabilized by the pull of the Earth.
If orbiting the Moon can be hard, coming back is easy. As a spacecraft goes out to the Moon, the Earth’s gravity is pulling it back: it is, in effect, going uphill. On the return trip, as soon as it escapes the Moon’s much weaker pull, the Earth’s gravity does the rest. At the end of the fall you can get delta-v for free by turning your incoming velocity into the scalding heat of re-entry. It took Neil Armstrong and Buzz Aldrin less than 3km/s to get back to the Pacific from Tranquility Base, even allowing for a slight detour to pick up Mike Collins. As one of the characters in “Destination Moon” points out, a rocket as small and primitive as the V-2 would have been able to make the trip.
This demonstrates the way in which what matters in space is d
elta-v, not distance. To get from the Moon to the Earth requires only about a fifth of the delta-v that is needed to make the same journey in the opposite direction.
Another demonstration of this proposition is that the amount of delta-v it takes to get a spacecraft to the surface of the Moon can also get it to destinations much farther afield. “Near-Earth asteroids” (NEA) are only near inasmuch as they have orbits that very occasionally bring them moderately close to the Earth. At any given time a typical NEA will be 100 or 200 times as far away as the Moon. But in terms of delta-v, quite a lot of these asteroids are just as easy to reach as the Moon is; it is just a matter of getting on to the right trajectory and waiting. Indeed, it takes no more delta-v to reach the little moons of Mars than it does the great Moon of Earth (the surface of Mars is another matter). This means the Moon’s closeness to the Earth does not necessarily make it the most obvious place to go if one is keen on exploiting extraterrestrial resources.
Not all orbits have to be around a physical object. When a smaller object orbits a larger one, their gravitational fields combine to create a few places in empty space around which a spacecraft can orbit with minimum fuss. These are called “Lagrangian points” after Joseph-Louis Lagrange, the physicist who first systematically studied them. For the Earth and the Moon, there are two Lagrangian points at the same distance from the Earth as the Moon, one 60° ahead of it in its orbit, one 60° behind. These are called the L4 and L5 points. Two other points sit on the line between the Earth and the Moon. The L1 point sits a bit less than 60,000km above Sinus Medii in the middle of the Moon’s nearside—that is, at a distance equivalent to about 30 times the radius of the Moon, or 15% of the way to the Earth. The L2 point is a little more than 60,000km above the middle of the farside; it sits almost directly above a crater named for Yuri Lipsky, the Soviet scientist who used data from Luna 3 to produce the first maps of the whole Moon. A spacecraft in a “halo orbit” around the L2 point, one that is perpendicular to the line joining Earth and Moon, can enjoy a view of the farside while also seeing the Earth over the limb of the Moon.
Uses for the opportunities provided by all of these Lagrangian points may eventually be found. Orbits around L2 already have. When China’s Chang’e 4 landed in South Pole-Aitken in January 2019, it was able to receive instructions and return data only by means of Queqiao, a relay satellite in a halo orbit able to maintain radio links to the Earth and the lunar far side.
Queqiao means “bridge of magpies”. Zhi Nu was a maiden from the sky who wove brocade out of the clouds for her father, the Jade Emperor. She fell in love with Niu Lang, a cowherd, who climbed into the sky to be with her. The emperor, disapproving of this, decreed that the lovers should live on opposite sides of the Milky Way, an impassable obstacle. But once a year, on the seventh day of the seventh month, magpies form a bridge across the barrier, allowing the two to commune. The tears of the briefly reunited lovers bring rain to the Earth.
- V -
REASONS
IT WAS WONDERFUL: PARENTS CAME TO PULL US TOWARDS THE televisions, which like the Moon were black and white and really, mostly, grey. Or they took us outside and pointed up at the sky. Or both. Sky Moon and screen Moon both there and both theirs and both ours, too; some large, strange change was being shared. Something grown-up that applied to children, too, and to what adults wish children to be.
Not all parents; not all of those of us then children. Not, for everyone, the most important thing or even a thing at all. For some an alienation, or a waste. But for many that encounter with history stayed bright, and for some it went deep. The excited adult pointing up promised a larger world that was yet to come, a world of the sky beyond the screen and the space in between for us to grow into.
We did not know that only 15 Saturn Vs had been built or that there were only nine left. We did not know that the last three Apollo missions were to be cancelled (“I just don’t want to take the risk of a possible goof off,” said President Nixon). We did not know that the USSR, having failed to build a successful Saturn-class super booster of its own, had no plans to put its own people on the Moon, or anywhere beyond low-Earth orbit, or that no one else was in a position to plan at all. We did not appreciate that, for all the excitement of 1969, the American public had never been overwhelmingly keen on Moon missions and that other priorities were looming larger, with price tags of their own. The Moonworld was there for everyone, a future history which, our parents seemed to promise us, would grow up as we did.
And then it didn’t.
It is not that nothing happened. NASA built its space shuttles, and flew more than 100 missions with them, losing two orbiters and 14 souls in the process. America and Russia, in post–Cold War partnership with the European Space Agency, the Japanese and the Canadians, built an ambitious international space station, called it the International Space Station and have kept crews on it continuously for 18 years at the time of writing. For most of that time, rovers trundled across the deserts of Mars, geologizing as they went and the Cassini orbiter danced among Saturn’s moons and rings. Communication satellites became a big business; space became vital to commerce, and to the military.
But it was never enough for the people sometimes called the “orphans of Apollo”—the children of the late 1960s for whom space remains an inspiration and a disappointment. The fact that Apollo had not seemed to come to anything may have disappointed many, but most of them don’t regularly, or ever, give it a thought. For some, though, the curtailment of the space programme felt, as they came of age, like something between a bereavement and a betrayal. These were the people who asked, plaintively, “If we can put a man on the Moon… why can’t we put a man on the Moon?” Or, some would add, a woman. They followed the space programme; they became scientists and engineers so they could work in it; they taught kids about it. Some of it inspired them, some of it thrilled them. None of it satisfied them.
They thought—they still think—that human progress and a human presence beyond the Earth were indistinguishable and good for everyone. They felt the desire to “put a dent in the universe” that Steve Jobs spoke of. But they had the unsettling knowledge that the dent they wanted to make had already once been made, and that the panel beaters of history had, with little fuss, removed more or less every sign of it.
They were, in an odd way, a mirror image of the curiously large group of people who denied that the landings had taken place in the first place, saying instead that they had been faked. Of course they hadn’t been; of course they did. But did not the lack of a subsequent Space Age suggest, in a way, that they might as well not have done? And didn’t that make them fakery of another kind? A dream that does not come true is not necessarily a lie. But, as Bruce Springsteen suggests in “The River”, it could be something worse.
The dissonant loss of the future that their childhood and their beliefs told them was inevitable spurred them on. It also wore them down. All of them felt some bitterness, some resentment, some anger; some of them felt all three and more. They were angry at NASA, at Congress, at the military-industrial complex, at everyone responsible for the future in space coming so far short of what it should have been. They were angry at fools who would not listen, at cynics who listened only to scoff, even at other true believers who didn’t adhere to quite the right version of the creed. At everyone who just didn’t get it.
But if anyone was not getting it, it was the orphans. To a child, all new things are beginnings. Apollo had been an end: what the Space Race had built to, not what it would be built on. It retired what few plausible motives there had ever been for going to the Moon, both in America and everywhere else, and left behind no new reasons with which to replace them. America had successfully signalled to the world its technological pre-eminence. And though science had not been the primary purpose of the programme, advocacy by Shoemaker and others, and the enthusiasm that they engendered for their cause in the astronauts themselves, had made sure that the basic science had been done. What the Moon was made
of; how it had come to be; what has shaped it since: all were basically answered.
Sure, there were types of lunar terrain that had not been walked on. The Earth-free sky of the farside had not been seen. Rocks produced by post-maria volcanism had yet to be sampled. But these were second-order concerns. Science had Mars to land its probes on, Jupiter and Saturn and their moons to fly past, Mercury to visit for the first time. It was headed out, not back—and it could do without astronauts. Space could be a frontier of the mind without the carefully wrapped still-fragile body.
Reaching for the Moon. Crying for the Moon. Walking on the Moon. Drawing down the Moon. They had all been images of fantasy, or frustration. And now they were again.
THE HERO OF ROBERT HEINLEIN’S “THE MAN WHO SOLD THE Moon” (1950), Delos D. Harriman, is a tycoon determined to set the first Moon rocket on its way even if it costs him his whole fortune—and a lot of other people theirs as well.
—Delos, why don’t you give up? You’ve been singing this tune for years. Maybe some day men will get to the Moon, though I doubt it. In any case, you and I will never live to see it.
—We won’t see it if we sit on our fat behinds and don’t do anything to make it happen. But we can make it happen.
As Harriman wheedles business partners, politicians and financiers into supporting his mania, he runs through a broad gamut of funding schemes and rationales. Some were the very ones which would be used by Apollo; in the decades that followed, all would be tried again and found wanting.
The Moon Page 17