From the dawn of civilization, humans have dreamed of exploring the cosmos. To date, we have launched over 60 successful missions to the Moon (including six that landed on the Moon with humans), 17 successful missions to Mars, 13 missions to the outer solar system, and five that have left the solar system.
However, many have been concerned lately that the glory days of space exploration are behind us. The Apollo missions ended 44 years ago, and still we have not returned to the Moon. Our current Mars missions are only modestly more sophisticated than earlier missions. And futuristic dreams of humans traveling to the planets and to the cosmos have remained decades if not centuries away.
But within the past year or so, this situation seems to be changing. Perhaps it has been inspired by a string of highly successful Hollywood movies, including Gravity, Interstellar, The Martian and Star Wars: The Force Awakens. Perhaps it stems in part from the surprising success of private firms such as SpaceX and Blue Origin. Or perhaps it is simply the insatiable curiosity and wanderlust that is so deeply ingrained into our species via evolution.
Yuri Milner's plan to explore Alpha Centauri
Arguably the most daring plan to date is the Breakthrough Starshot project that was announced on 12 April 2016 by Russian billionaire Yuri Milner, with backing from physicist Stephen Hawking and Facebook founder Mark Zuckerberg.
Milner proposes to send a fleet of "nanocraft" to explore Alpha Centauri and its planets -- thousands of credit-card-sized spacecraft (to increase the chances that at least some will survive the journey), quickly accelerated to 20 percent of the speed of light by giant light sails powered by laser beams from a kilometer-square array of earth-bound lasers.
Upon arrival in the Alpha Centauri system in 20 years or so, the nanocraft will send back photos and other data via laser beams, which will arrive at Earth four years later. Needless to say, this vision presents daunting technical hurdles, including:
- Fabricating diode lasers, megapixel cameras, computer processors and batteries for the nanocraft, together weighing less than one gram and able to survive 20 years of exposure to interstellar dust and cosmic rays.
- Maintaining integrity of the light sail while it and its nanocraft are being accelerated by lasers.
- Producing sufficient laser power and maintaining the focus of the laser array.
- Detecting the images and data that are sent back to Earth.
For additional details, see the Scientific American report.
An entirely different, but comparably ambitious, proposal to study extraterrestrial civilizations is to use the Sun as a gravitational lens. SETI pioneer Frank Drake, among others, proposes sending spacecraft outside the solar system to the focal point of the Sun's gravitational field, which, by principles of General Relativity, can then see enormously magnified images and even microwave transmissions coming from a distant star system.
Renewed interest in humans on Mars
An equally significant development is the resurgence in interest for humans not only to visit Mars but also to take up residence and ultimately form an independent colony.
The Mars Society observes that a round-trip journey to Mars is possible by manufacturing fuel for the return trip in situ on Mars (otherwise transporting fuel to Mars for the return trip is 90 percent of the outbound payload). In particular, they note that CO2 extracted from Martian atmosphere and H2 produced from Martian ice by electrolysis can be combined to form fuel by the exothermic reaction 3 CO2 + 6 H2 → CH4 + 2 CO + 4 H2O. A fully fueled and tested lift-off vehicle could be ready and waiting on Mars before the astronauts leave Earth.
Mars One, an organization founded in 2012, proposes to send humans to Mars by 2027 and establish a permanent colony there, to be funded in part by a reality TV show. Over 200,000 persons responded to their 2013 call for interest; this list has now been narrowed down to 100.
Tesla and SpaceX founder Elon Musk has also been formulating plans for a Mars colony, which he has promised to announce soon. His project reportedly will be known as the Mars Colonial Transporter, to be powered by a large version of the Raptor rocket engine, specifically designed for the exploration and colonization of Mars.
Advanced propulsion technology
Looking a bit further to the future, advanced propulsion systems will be required if more than a handful of people are to reach Mars or beyond. NASA has been exploring several concepts in this direction, including:
- Ion propulsion: A high-energy electron collides with a xenon atom, releasing electrons, and the charged atom is then discharged at high speed (up to 150,000 kph).
- High-power electric propulsion: This is like ion propulsion, except that the xenon ions are produced by a combination of microwave and magnetic fields, using a process called electron cyclotron resonance.
- Fusion-driven rocket: A fusion energy source releases its energy directly into the propellant, without converting to electricity; the propellant is rapidly heated and accelerated to high exhaust velocity (roughly 100,000 kph) with no physical interaction with the spacecraft, thus avoiding deterioration.
Space travel and Fermi's paradox
These developments have clear implications for Fermi's paradox, that decades-old unsolved conundrum of why, given that an extraterrestrial civilization could explore the Milky Way in a million years or so (an eyeblink in cosmic time), do we not see evidence of even a single society?
Numerous scientists have examined Fermi's paradox in detail and, as we wrote earlier, have proposed various explanations, such as:
- They exist, but are too far away.
- They are under strict orders not to disclose their existence.
- They exist, but have lost interest in communication and exploration.
- They are calling, but we do not recognize the signal.
- Civilizations like us invariably self-destruct.
- We are alone, at least within the Milky Way if not beyond.
All of these explanations have very reasonable rejoinders. Items 2 and 3 (and several other similar proposed explanations) fall prey to a diversity argument -- in a vast galactic ensemble it is hardly credible that every individual in every civilization forever lacks interest in communication and exploration, nor is it credible that some galactic society ban is absolutely 100 percent effective (note that once a signal has been sent, it cannot be called back by any known law of physics). Item 4 does not seem credible, since it is very reasonable to assume that at least some communications are being sent to planets such as Earth in a form that we could readily recognize, and, as before, it is not credible that a ban on such targeted communication could be absolutely 100 percent effective. Item No. 6 (we are alone) seems incredible in light of the thousands of recently discovered extrasolar planets, many in the habitable zone.
With regards to Item 1 (they exist, but are too far away), it is clear that the many exciting new developments in space exploration seriously draw into question the presumed technical impossibility of exploring the cosmos. For example, a fleet of "von Neumann probes" could travel to distant stars, make additional copies of themselves (using the latest software beamed from the home planet) and launch to yet more distant stars. Analyses of this scheme show the entire Milky Way could be explored in a one million years or so. And keep in mind that any other society is, almost certainly, many thousands or millions of years more advanced, so cost and distance cannot be insuperable obstacles.
These developments also draw Item 5 into question (civilizations like us invariably self-destruct). After all, we have survived 200 years of technological adolescence and have not yet destroyed ourselves. And if any of the current exploration and colonization plans work out, then the long-term survival of our species will be immune to possible calamities on Earth. Within a decade, we will become a multi-planet species, and within a century we very likely will be a multi-solar-system species.
So what is the answer to Fermi's paradox? Good question! We humans don't know.
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