Preliminary Interstellar Missions: Prelude to the Stars

 

Astronomers want to harness [the Sun’s] spacetime-warping gravity as a lens to image the surface of exoplanets in astonishing detail.

This would be the next major step, possibly the biggest step in the 21st century for exoplanet exploration.

Using the Sun as a Cosmic Telescope

“The Sun's gravity can be used like a lens to focus light allowing us to get a high quality image of an exoplanet. The best we've got are tiny fuzzy dots, and those are for gas giants larger than Jupiter. With this method, solar gravitational lensing, we could analyze the exoplanet's atmosphere for signs of life and habitability.”\(^{[1]})

The first truly interstellar mission would likely not be to another star system; it is likely that the first interstellar mission will be to send a telescope to a very distant region of interstellar space where it would use the Sun as a giant lens to take pictures of distant exoplanets and to analyze the chemical compositions of those exoplanets atmospheres. The area of space which is contained within the volume in between two imaginary spheres with radii of 1,000 and 550 astronomical units (AUs), respectively, with the Sun as their central points is a region of space which lies well beyond the solar system and within interstellar space. Light emitted by distant stars and exoplanets would get bent around the Sun (due to the Sun curving the space around it) and converge at a focal point inside of that volume.

3E58320900000578-0-image-a-2_1489702334064[1].jpg

This magnification caused by the bending of those distant celestial bodies would be immense and, indeed, it wouldn't be an overstatement to say that this would revolutionize our understanding and the way in which we study exoplanets and the precursors of exobiology. Understanding how powerful such a telescope would be is best understood by analogy. The Hubble space telescope is the most powerful telescope ever built by human beings; it allows us to take photographs of Mars which allow us to discern features on that world which are 20-30 kilometers across.

But even just a modest telescope which relies on using the Sun as a gravitational lens would allow us to discern features on an exoplanet 20 lightyears away with the same level of detail which satellites in Earth-orbit can obtain of the Earth. You could peer into an aliens backyard, so to speak. According to a recent study published at Cornell University, such a super-telescope could make out details a few kilometers across on an exoplanet 30 parsecs (~100 light years) away.\(^{[2]}\) In other words, this telescope could take photographs of an exoplanet 100 lightyears away with a higher resolution than the Hubble space telescope can take of Mars.

“This Hubble Space Telescope image of Mars shows details 20 to 30 miles across. A solar gravitational lens telescope would get even sharper views of exoplanets up to 100 light years away.”\(^{[4]}\) (STScI/AURA), J. Bell (ASU), and M. Wolff)
Source URL: https://www.airspacemag.com/daily-planet/ultimate-space-telescope-would-use-sun-lens-180962499/


Searching for Extraterrestrial Life

Such a telescope would allow us to study the heliosphere and the interstellar medium which would allow us to further access the feasibility of interstellar travel. It would allow us to use parallax to measure the precise position of every star in the Milky Way galaxy; and those stars for which we already know their positions, such a telescope would allow us to obtain more accurate position measurements. And perhaps best of all, this telescope would allow us to perform spectroscopy on distant exoplanets to determine their chemical compositions. There are many exoplanets out there which could potentially harbor life. The three exoplanets within the habitable zone of the star TRAPPIST-1 located 39.5 lightyears away are such examples. With this telescope, we could search for chemicals in those atmospheres which would not likely be present without biological processes.

If you extend a straight line from such a telescope and through the Sun, then (roughly speaking) you could only "look at" star system which, more are less, are on that straight line. This means that such a telescope would only be able to peer into a very tiny patch of sky in its search for biosignatures on exoplanets in distant star system. But by building a sufficiently large swarm of such telescopes which surround the Sun and have the source of power necessary to move to various points within the region of space in between two spheres (centered around the Sun) of radii 550 AU and 1,000 AU, then such a system of telescopes could analyze every portion of the sky in unprecedented detail. The primary purpose of these telescopes would be to look for life by using spectroscopy (see article) to analyze the chemical compositions of those atmospheres. But, you might ask, which particular kinds of elements would signal the existence of life? Put more simply, what should we be looking for? The answer is, we're trying to find particular combinations of molecules (and in certain quantities) which would be exceedingly difficult to be detectable, produced, and continually replenished without life. Such combinations of molecules are called disequilibrium mixtures.

The combination of certain molecules and the absence of certain molecules in an exoplanet’s atmosphere is only likely if life is inhabited on that world: such combinations are called disequilibrium mixtures.

As the light emitted from a star passes through an exoplanet’s atmosphere, the molecules in that exoplanet’s atmosphere absorb only certain wavelengths. Given which wavelengths were absorbed, we can determine whether or not disequilibrium mixtures are present in that exoplanet’s atmosphere and, consequently, assess the likelihood that extraterrestrial life inhabits that world.

NASA/Aaron Gronsta

It is likely that the first disequilibrium mixture which we would look for is the combination of large amounts of oxygen and ozone in an exoplanet's atmosphere. And, something which we do not want to find much of is carbon monoxide. If we used such telescopes to analyze the chemical compositions of other worlds beyond the solar system and if we found the disequilibrium mixture, oxygen plus ozone minus carbon monoxide, then it would be very likely that that world harbors life. Scientists know of no other way besides through biological processes that large amounts of oxygen can be produced. So even if we only found large amounts of oxygen present in an exoplanet's atmosphere, that already would be a strong signal for life. We would also search for the disequilibrium mixture methane plus carbon dioxide minus carbon monoxide. If we found the presence of such a disequilibrium mixture, then this would be a clear signal that life could possibility exist on that world.

The stromatolites shown in the photograph above are fossils which were produced by cyanobacteria 3.5 billion years in Shark Bay, Australia. Cyanobacteria - a simple, single-celled lifeform - produced large amounts of oxygen via photosynthesis causing the first major oxygenation even in Earth’s history.  Photograph taken by Paul Harrison.

The stromatolites shown in the photograph above are fossils which were produced by cyanobacteria 3.5 billion years in Shark Bay, Australia. Cyanobacteria - a simple, single-celled lifeform - produced large amounts of oxygen via photosynthesis causing the first major oxygenation even in Earth’s history. Photograph taken by Paul Harrison.

The history of life on Earth is market by two major oxidization events: the first was the result of large colonies of cyanobacteria releasing large amounts of oxygen into Earth's atmosphere via photosynthesis about 3.5 billion years ago; and the second occurred shortly after Snowball Earth\(^{[5]}\), a period of unprecedented successive ice ages, which shortly thereafter lead to the emergence of the first large, complex, multicellular lifeforms which comprised the Ediacran biota. As we discussed previously, the presence of large amounts of oxygen would be necessary for large, multi-cellular creatures to perform metabolism, to produce energy and therefore to survive. This is why the Ediacran biota (the first large multi-cellular organisms which preceded the Cambrian explosion) did not emerge until shortly after the percentage of oxygen in Earths's atmosphere spiked to roughly 20% during the second great oxygenation event\(^{[5]}\) which proceded Snowball Earth.

Diagram of the evolution of atmospheric oxygen concentration during the history of the planet. © Ido, Pierre Sans-Jofre

Diagram of the evolution of atmospheric oxygen concentration during the history of the planet. © Ido, Pierre Sans-Jofre

Out of the 4.6 billion years that the Earth has existed, it hasn't been until the last roughly 600 million years that this enormous amount of oxygen has been present in the Earth's atmosphere.\(^{[5]}\) This suggests that oxygenation events as dramatic as the second oxygenation event take a very long time to occur and are very unlikely. Out of the percentage of exoplanets out there, we do not expect to find that a high percentage of them will have a percentage of oxygen that is tantamount and similar to the amount which was present roughly 600 million years ago. To the contrary, however, the disequilibrium mixture methane plus carbon dioxide minus carbon monoxide can be produced by the simplest and most primitive forms of life. According to the history of life on this planet, such organisms are not too difficult to evolve and, given an Earth-like planet which orbits a Sun-like star, there's a pretty good chance that such organisms would have had evolved on that planet.

We need to look for fairly abundant methane and carbon dioxide on a world that has liquid water at its surface, and find an absence of carbon monoxide. Our study shows that this combination would be a compelling sign of life. What’s exciting is that our suggestion is doable, and may lead to the historic discovery of an extraterrestrial biosphere in the not-too-distant future.
— Dr. David Catling

According to the astrobiologist and planetary scientist David Catling, "We need to look for fairly abundant methane and carbon dioxide on a world that has liquid water at its surface, and find an absence of carbon monoxide.” He also said that, “Our study shows that this combination would be a compelling sign of life. What’s exciting is that our suggestion is doable, and may lead to the historic discovery of an extraterrestrial biosphere in the not-too-distant future.” I would like to extend on his remark by noting that, according to the Drake equation and data obtained by the Kepler space telescope, provided that we used a swarm of satellites distributed in roughly the shape of a sphere (with a radius of roughly 550 AUs surround the Sun), then the probability of discovering an exoplanet with "simple life" (by using spectroscopy to find the right kinds of disequilibrium mixtures in an exoplanet's atmosphere) would be close to 100%.

The image above is a map of a small fraction of nearby stars in our stellar neighborhood. It is a map of 130 stars located within 50 lightyears of us which are visible with the naked eye. This map shows only a tiny percent of that stars within 50 lightyears away from the Sun. There are in fact 1,800 known stars within this volume of space. To eliminate the threat of nearby supernovae, our descendants will one day visit all of these stars, either directly in which case their species actually arrives at the star, or indirectly by sending spacecraft and robots to those stars.

The image above is a map of a small fraction of nearby stars in our stellar neighborhood. It is a map of 130 stars located within 50 lightyears of us which are visible with the naked eye. This map shows only a tiny percent of that stars within 50 lightyears away from the Sun. There are in fact 1,800 known stars within this volume of space. To eliminate the threat of nearby supernovae, our descendants will one day visit all of these stars, either directly in which case their species actually arrives at the star, or indirectly by sending spacecraft and robots to those stars.

 
Welcome to the Universe: An Astrophysical Tour   is a  popular science  book by  Neil deGrasse Tyson , Michael A. Strauss, and  J. Richard Gott , based on an introductory  astrophysics  course they co-taught at  Princeton University . The book was published by the  Princeton University Press  on September 20, 2016.\(^{[6]}\)

Welcome to the Universe: An Astrophysical Tour is a popular science book by Neil deGrasse Tyson, Michael A. Strauss, and J. Richard Gott, based on an introductory astrophysics course they co-taught at Princeton University. The book was published by the Princeton University Press on September 20, 2016.\(^{[6]}\)

In a previous article which was based largely off of a chapter from the book An Astrophysical Tour of the Universe, we used the Drake equation and data obtained by the Kepler space telescope to calculate that there are roughly 1.2 billion Earth-like planets which orbit Sun-like stars in the Milky Way galaxy. This number is essentially an extrapolation of what the average number of such habitable planets should be given the number and percentage of habitable planets we have already observed. We also calculated that it is reasonable to assume that within a sphere of roughly 40 lightyears which surrounds any point in the Milky Way galaxy, there should be on average something like 6 habitable exoplanets within that sphere. Indeed, we have already found a decent number of Earth-like planets within this sphere including Ross-128, some of the TRAPPIST-1 planets, and Proxima B just to name a few.

We argued that since simple forms of life were fairly easy to evolve on Earth and, relatively speaking, didn't take that long too evolve, we expect that the same will be true on other Earth-like planets which orbit Sun-like stars. And we concluded that, while despite multi-cellular life and especially intelligent life would be very difficult to evolve, the probability of simple, single-celled microbes evolving on habitable exoplanets is probably close to one. So if we used a swarm of telescopes at about 550 AUs away from the Sun and used the Sun as a gravitational lens to focus light which passed through the atmospheres of distant exoplanets and if we passed that light through spectroscopes (equipped to each telescope), then according to data obtained by the Kepler telescope the odds are very likely that we'll find an exoplanet with disequilibrium mixtures (i.e. methane plus carbon dioxide minus carbon monoxide) which signal the existence of simple life. Incidentally, it is possible (though unlikely) that we'd find an exoplanet with an atmosphere whose composition has large amounts of oxygen (with a percentage similar to that present during the second oxygenation event); this would suggests that large, multi-cellular organisms might have evolved on that exoplanet.

James Webb Space Telescope

 

Thus, such telescopes would be immensely useful as precursor missions which would precede the first interstellar missions to other star systems. These telescopes would give us a good idea of which exoplanets look promising and seem like good candidates for extraterrestrial life, and which do not. If we find exoplanets which do seem very promising based on the chemical compositions of their atmospheres, then we'd make interstellar missions to those worlds a priority. In our time, we will see some tremendous advancements being made towards achieving these endeavors. For example, the James Webb telescope is scheduled to launch in a few years and this telescope will be purposed with the task (among others) of performing spectroscopy to analyze the chemical compositions of distant exoplanets; it will be on the hunt for exoplanets with particular kinds of disequilibrium mixtures of the kind which we discussed earlier. Also, since even as far back as the 1980s NASA has conceptualized interstellar spacecraft equipped with telescopes which would go to interstellar space to use the Sun as a gravitational lens. And, more recently, we have seen some pretty serious proposals to send small spacecraft on fly-by missions to the stars including Project Starshot and Project Genesis which will likely send the first interstellar spacecraft to other star systems sometime within the 22nd century, or perhaps even earlier.

I mention all of this to make it clear that what we are discussing are not events which will take place centuries or millennia from now. The discovery of simple forms of single-celled extraterrestrial life using advanced telescopes and space probes flying by distant stars and exoplanets are events which will likely take place within the lifetime of our generation, and of our children and our grandchildren.

Starwisp

Following those preliminary interstellar missions would likely be an actual journey to the stars by robotic emissaries. One very elegant scheme would involve using ideas developed by the science fiction author, science writer and inventor Arthur C. Clarke and the scientist Robert Forward. According to Clarke, we could excavate resources from the planet Mercury using robots and launch those materials as payloads into space by building a mass driver on Mercury. Using that material, we could manufacture an immense solar power station (also called a Clarke station) perhaps at one of the Sun-Mercury Lagrange points. Sunlight pressure on the solar sail of the Clarke station could be used to move that satellite to a different orbit. By tilting the solar sail at a slight angle such that the trailing edge of the sail is closer to the Sun than the leading edge, sunlight pressure would cause the sail to have a component of acceleration opposite to that sail's direction of motion. This would cause the tangential speed of the Clarke station to decrease; as this occurs, the Clarke station would spiral inward towards the Sun. Using this technique, that station could be moved to a location that is 0.1 astronomical units (AUs) away from the Sun where it would orbit around the Sun; that is to say, the Clarke station would be one tenth as far from the Sun as the Earth is. Solar panels on the Clarke station would convert the Sun's solar energy into electricity; this would be used to power a device which would use that electricity to power an ultra-powerful laser beam or a microwave beam. That beam would be concentrated on a sail attached to an interstellar probe such as a Starshot probe (which would be propelled using laser light) or a Starwisp probe (which would be propelled using a microwave beam). (Now, using this scheme, we could use Murcery's resources to manufacture myriad Clarke stations which orbit the Sun in order to build a Dyson swarm like a Shkadov thruster, star lifter, or a stellalaser, but this is something which we'll discuss in more detail later on.)

microwave-sail-starwisp-n.jpg

After such fly-by missions are accomplished, Genesis probes would be the first interstellar probes to actually land on exoplanets and moons within other star systems. As we'll discuss in greater detail later on, the purposes of such probes would be to seed transiently-habitable exoplanets and exomoons in the galaxy with life. Project Genesis calls for one of the boldest and awe-inspiring endeavors in all of human history: to seed the Milky Way galaxy with life.

In the next article, we’ll elaborate on the use of lasers (or masers) to propel small interstellar probes to other star system and we’ll also discuss humans living in comets and using comets as spaceships to set sail for the stars. And three weeks from the release of the article you are reading right now, we’ll examine in detail the prospect of Genesis probes seeding the galaxy with life and the notion of moving the Earth’s biosphere to other exoplanets in the galaxy.


This article is licensed under a CC BY-NC-SA 4.0 license.

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