The ever reliable Dennis Overbye gives us a look at the Earth’s fate in his most recent story for the New York Times. Citing the work of Klaus-Peter Schroeder (University of Guanajuato, Mexico) and Robert Connon Smith (University of Sussex), Overbye describes our planet’s eventual engulfment by a red giant Sun. Earlier studies had questioned whether the Earth might survive this phase, but Smith and Schroeder say no. Their calculations show a red giant Sun 256 times as wide as today’s star, and fully 2730 times more luminous. And it will swallow the Earth.
I was interested to see Overbye’s reference to a 2001 paper that, in the spirit of speculative jeu d’esprit familiar in good science fiction, looks at a way to save the planet. But first, let’s run through where our star is heading. Burning through its hydrogen on the main sequence, the Sun should keep getting hotter and larger. Figure 1.1 billion years until you reach the point where it is 11 percent brighter than today, creating a greenhouse effect that devastates the biosphere. By the time you’re 3.5 billion years out, the Sun is forty percent more luminous than today and any surviving vestiges of life as we know it should be gone.
But ponder this: We are at present less than halfway through the main sequence life of the Sun. In six billion years, even though the Sun’s luminosity should be a factor of 2.2 greater than its current value, a planet just 1.5 AU out would receive about the same amount of solar energy that Earth now receives. Thus the notion of increasing the radius of the Earth’s orbit, which Don Korycansky and our friend Greg Laughlin (both at UC-Santa Cruz), working with Fred Adams (University of Michigan), describe. Do this right and the lifespan of the surface biosphere gets a five billlion year reprieve. Here’s the basic idea, as drawn from the paper:
An attractive scenario for gradually increasing the Earth’s orbital radius is to successively deﬂect a large object or objects from the outer regions of the solar system (the Oort Cloud or the Kuiper Belt) onto trajectories which pass close to the Earth. By analogy to the gravity-assisted ﬂight paths employed by spacecraft directed to outer solar-system targets… the close passage of such an object to the Earth can result in a decrease in the orbital energy of the object and a concomitant increase of the Earth’s orbital energy.
A science fiction scenario par excellence! And one that recalls Stanley Schmidt’s Sins of the Fathers, which I read long ago (1973-74) when it ran as an Analog serial. There the idea was to move a threatened Earth (the galactic core has exploded) with the help of ingenious alien technologies to a new and safer home. A more recent example of world moving is Robert Metzger’s Cusp (2005), where mysterious alien technologies are activated, the Singularity arrives and stars and planets fly (not to mention ideas galore). I had the good fortune to discuss Cusp with Metzger over lunch in Chapel Hill a couple of years back — if you’re into hard SF that doesn’t let up, be sure to read him.
But back to the paper under discussion. Just how would we move the Earth? The authors work out a typical mass for large Edgeworth/Kuiper Belt objects (1022 grams) and calculate that some 106 passages involving a ‘cumulative flyby mass’ of about 1.5 Earth masses would do the trick, moving the Earth to 1.5 AU. An average of one pass every 6000 years over the remaining lifetime of the Sun ought to do the job, which doesn’t sound as onerous as one might have expected.
And, of course, we have an Edgeworth/Kuiper Belt stuffed with objects larger than 100 kilometers in diameter. Here we’re on speculative ground, but the authors figure the belt should hold as many as 105 such bodies, with a total of perhaps 10 percent of the Earth’s mass. And then there’s the Oort Cloud, thought to hold 1011 objects totaling thirty or more Earth masses. Small trajectory changes would bring many an Oort object into an Earth crossing orbit (Laughlin makes sure that Overbye understands, in the Times article, that he’s not advocating such a dangerous move — one mistake takes out our planet — but simply sketching out the boundaries of the possible).
What’s happening in the rescue scheme is that repeated gravity assists in effect transfer orbital energy from Jupiter to the Earth. A close pass by the Earth by an object in a highly elliptical orbit transfers energy from the object to the Earth. Outbound, the object crosses Jupiter’s orbit, timed to encounter the planet and pick up the energy lost to Earth. The paper spells out the procedure in details that point to a long-term and ‘almost alarmingly feasible’ scenario, one that would use technologies that, although beyond our current capabilities, are by no means beyond the powers of a more advanced civilization. The long-term result seems promising:
Due to the acceleration of the Sun’s luminosity increase, the encounters must be more frequent as the Sun approaches the end of its main-sequence life. In order to use the same secondary body for many encounters, modest adjustments in its orbit are necessary. However, by scheduling the secondary body to encounter additional planets (e.g., Jupiter and/or Saturn) in addition to the primary Earth encounter, the energy requirements for orbital adjustment at the object’s aphelion can be substantially reduced. In particular, the energy consumed by such course corrections is not likely to dominate the energy budget.
And note this interesting fact: The energy required to move the Earth is modest compared to that needed for interstellar travel. Working out the numbers, the team finds that the scheme is actually highly efficient when compared to interstellar migration and compares favorably with various terraforming projects that have been examined in the past. Usefully for more contemporary concerns, the basic methods might also be utilised to move hazardous asteroids, or to set up delivery mechanisms for useful Edgeworth/Kuiper Belt materials whose resources could be exploited.
The paper is Korycansky, Laughlin and Adams, “Astronomical engineering: a strategy for modifying planetary orbits,” Astrophysics and Space Science 275 (2001), pp. 349-366 (abstract). It’s one you don’t want to miss.