Category Archives: Space Travel

Exoplanets Galore!

The year was 2001 – The first of the new millennium. It was also the year astronomers reached a historic milestone: They had catalogued more planets orbiting distant stars than the eight we knew about in our own solar system!

The first of these was discovered in 1988: A Jupiter-sized world orbiting the star Gamma Cephi A at a distance of 45 light years from our sun. Since then, well over 3,600 ‘exo’planets have been discovered using many different detection methods both on the ground and in space. We are truly living in a very different age than we experienced only a few short decades ago. We no longer have to speculate whether other planetary systems exist across the universe. We now have the technology and a growing catalog of examples to prove our solar system is not alone in the universe!

One of these detection methods is the Transit Method. As a planet passes in front of its star as viewed from Earth, the brightness of the star dims by an amount equal to the ratio of the planet’s circular area to the star’s circular area. For example, a Jupiter-sized planet has a diameter of 143,000 km while a sun-like star has a diameter of 1.4 million km, so the ratio of their areas is 1/100. As this exoplanet transits the disk of its star as viewed from Earth, the brightness of the star will dim by 1%. By measuring the time between successive transit ‘dips’ you can deduce the orbit period of the planet. From the orbit period and the mass of the star you also obtain the distance between the star and the planet. Once you have this information, you can estimate the surface temperature of the planet and its average density (rocky planet or gas giant). In the example below, the orbit period is 4.887 days and from the amount of dimming ( 0.7%) the ratio of their areas is 1/142, and planet’s diameter is 1/12 the diameter of its star.

To take advantage of the transit method on a large scale, NASA launched the Kepler Observatory in 2009. Using a super-sensitive digital camera, it took a picture of the same star field in the constellation Cygnus every minute and captured brightness information for over 150,000 stars similar to our sun. Over the course of its survey, now in its eighth year, it has discovered 4,496 exoplanet candidates of which 2,335 have been verified by independent study. These statistics now show that virtually all stars in the sky have at least one planet in orbit around them, and about 1 in 20 have Earth-sized worlds!

What was entirely unexpected was the diversity of exoplanet types, and orbits about their parent stars. Our solar system offered as templates the small rocky Earth-sized worlds of the inner solar system, the two gas giants (Jupiter and Saturn) and the two ice giants (Uranus and Neptune), but among the thousands of worlds discovered so far, a vaster array of possibilities have been found.

There are super-Earths over three times as large as our world apparently fully covered by deep oceans (Kepler 22b). There are hot-Jupiters that orbit their stars in a matter of hours, and swelter at temperatures of thousands of degrees (51 Pegasi b). Some planets have ‘water cycles’ in which silicate rock evaporates from their surfaces, condenses in clouds, and then rains droplets of lava back to the surface (COROT-7b, Alpha Centauri Bb, Kepler 10b). The cloud-shrouded super-Earth GJ 1214b is so hot that its clouds could be made of zinc sulfide and potassium chloride! One exoplanet is so close to its star that it is literally evaporating before our eyes, leaving a huge comet-like tail of gas in its orbital wake (HD 221458b).

The thousands of exoplanets detected by the transit method are especially intriguing. From Earth, as the exoplanet passes in front of its star, some of the starlight passes through the exoplanet’s atmosphere. Various gases in the atmosphere absorb selected wavelengths of this light and leave their unique fingerprints behind for distant astronomers using spectroscopes to study. So far among the dozen or so worlds examined, atmospheres rich in water vapor, carbon dioxide, methane, sulfur and other common molecules have already been found. This figure shows the spectrum of the exoplanet Wasp-19b and signs of methane (CH4) and hydrogen cyanide (HCN) – a lethal mixture for humans to breathe! Future ground and space-based observatories will be able to detect the molecules in thousands of exoplanet atmospheres, but this is no idle scientific pursuit. Living systems on a planetary scale sculpt their atmospheres by creating free molecular oxygen, so if we ever detect an exoplanet atmosphere rich in oxygen, it will be a major discovery of life in the cosmos beyond our own Earth.

Another aspect of the search for Earth-like worlds is to find exoplanets about as large as Earth that orbit their stars in what is called the Habitable Zone (HZ). In this range of orbital distances from the exoplanet’s star, the surface temperatures would allow liquid water to exist. In our solar system, this zone is located between the orbits of Venus and Mars, and Earth is smack in the middle of it. Currently, the various surveys have identified 13 exoplanets that are not only Earth-sized but are within their star’s HZ. The closest of these is GJ 273b at a distance of 12 light years! But there are complicating factors to finding an exact twin to our Earth upon which we might also hope to discover a living biosphere.

If a planet in the HZ has too much carbon dioxide – a potent greenhouse gas – its temperature would be more Venus-like and it would be a desert world or worse. If the exoplanet had significant traces of common gases such as hydrogen cyanide or methane, it would be unbreathable for humans though some extremophile bacteria on Earth thrive in such environments.

Another thing to think about is that an Earth-like world could be in orbit around a Jupiter-sized exoplanet in its star’s HZ. Even though we do not directly see the Earth-sized world there could be several of them as satellite worlds of larger exoplanets. The Jupiter-sized exoplanet WASP-12b appears to have an ‘exomoon’ with several times the mass of Earth. We know of hundreds of Jupiter-sized exoplanets orbiting in the habitable zones of their stars. We can no longer discount the possibility that their exomoons may be Earth-like though currently undetectable.

So far we have had to detect and study exoplanets indirectly from transit and spectroscopic data, but in a growing number of cases we can actually directly see these worlds as they spin around their stars. In 2008, the bright star Fomalhaut was observed by the Hubble Space Telescope and revealed a small planet among the debris of its dense disk of gas and dust. In another instance, the young star HR 8799 was observed by the Keck Observatory in Hawaii and revealed four massive planets (HR8799 b, c, d and e) in orbit. This is only the beginning of direct-imaging studies as we enter a new era of exoplanet studies.

What will the future bring? The spectacular success of the NASA Kepler mission is just the beginning of new missions to come that will discover thousands of new exoplanets. The NASA, Transiting Exoplanet Survey Satellite (TESS) will be launched in 2018 and will perform a Kepler-like survey of the 500,000 brightest stars in the sky. This survey will double the number of known exoplanets and include hundreds of additional Earth-sized candidates. At the same time, the Webb Space Telescope will be launched and among its many research programs will be ones to study the atmospheres of known exoplanets and Earth-sized candidates.

Check back here on June 18 for my next blog!

Space Power!

On Earth we can deploy a 164-ton wind turbine to generate 1.5 megawatts of electricity, but in the also energy-hungry environment of space travel, far more efficient energy-per-mass systems are a must. The choices for such systems are not unlimited in the vacuum of space!

OK…this is a rather obscure topic, but as I discussed in my previous blog, in order to create space propulsion systems that can get us to Mars in a few days, or Pluto in a week, we need some major improvements in how we generate power in space.

I am going to focus my attention on ion propulsion, because it is far less controversial than any of the more efficient nuclear rocket designs. Although nuclear rocket technology is pretty well worked out theoretically and in engineering designs since the 1960s,   there is simply no political will to deploy this technology in the next 50 years due to enormous public concerns. The concerns are not entirely unfounded. The highest-efficiency and least massive fission power plants would use near-weapons grade uranium or plutonium fuel, making them look like atomic bombs to some skeptics!

Both fission and fusion propulsion have a lot in common with ordinary chemical propulsion. They heat a propellant up to very high temperatures and direct the exhaust flow, mechanically, out the back of the engine using tapered  ‘combustion chambers’ that resemble chemical rockets. The high temperatures insure that the isotropic speeds of the particles are many km/sec, but the flow has to be shaped by the engine nozzle design to leave the ship in one direction. The melting temperature of a fission reactor is about 4,500 K so the maximum speed of the ejected thermal gas (hydrogen) passing through its core is about 10 km/sec.

Ion engines are dramatically different. They guide ionized particles out the back of the engine using one or more acceleration grids. The particles are electrostatically guided and accelerated literally one at a time, so that instead of flowing all over the place in the rocket chamber, they start out life already ‘collimated’ to flow in only one direction at super-thermal speeds. For instance, the Dawn spacecraft ejected Zenon particles at a speed of 25 km/sec. If you had a high-temperature xenon gas with particles at that same speed, the temperature of this gas would be 4 million Celsius! Well above the melting point of the ion engine!

We are well into the design of high-thrust ion engines, and have already deployed several of these. The Dawn spacecraft launched in 2007 has visited asteroid Vesta (2011) and dwarf planet Ceres (2015) using a 10 kilowatt ion engine system with 937 pounds of xenon propellant, and achieved a record-breaking speed change of 10 kilometers/sec. It delivered about 0.09 Newtons of thrust over 2,000 days of continuous operation. Compare this with the millions of Newtons of thrust delivered by the Saturn V in a few minutes.

Under laboratory conditions, newer ion engine designs are constantly being developed and tested. The NASA NEXT program in 2010 demonstrated over 5.5 years of continuous operation for a 7 kilowatt ion engine. It used 862 kg of xenon and produced a thrust of 3.5 Newtons, some 30 times better than the Dawn technology.

Theoretically, an extensive research study on the design of megawatt ion engines by David Fearn presented at the Space Power Symposium of the 56th International Astronautical Congress in 2005 gave some typical characteristics for engines at this power level. The conclusion was that these kinds of ion engines pose no particular design challenges and can achieve exhaust speeds that exceed 100 km/sec. As a specific example, an array of nine thrusters using xenon propellant would deliver a thrust of 120 Newtons and consume 7.4 megawatts. A relatively small array of thrusters can also achieve exhaust speeds of 1,500 km/sec using lower-mass hydrogen propellants.

Ion propulsion requires megawatts of energy in order to produce enough continuous thrust to get us to the high speeds and thrusts we need for truly fast interplanetary travel.

The bottom line for ion propulsion is the total electrical power that is available to accelerate the propellant ions. Very high efficiency solar panels that convert more than 75% of the sunlight into electricity work very well near Earth orbit (300 watts/kg), but produce only 10 watts/kg near Jupiter, and 0.3 watts/kg near Pluto. That means the future of fast space travel via ion propulsion spanning our solar system requires some kind of non-solar-electric, fission reactor system (500 watts/kg) to produce the electricity. The history of using reactors in space though trivial from an engineering standpoint, is a politically complex one because of the prevailing fear that a launch mishap will result in a dirty bomb or even a Hiroshima-like event in the minds of the general public and Congress.

The Soviet Union has been launching nuclear reactors into space for decades in its Kosmos series of satellites. Early in 1992, the idea of purchasing a Russian-designed and fabricated space reactor power system and integrating it with a US designed satellite went from fiction to reality with the purchase of the first two Topaz II reactors by the Strategic Defense Initiative Organization (now the Ballistic Missile Defense Organization (BMDO). SDIO also requested that the Applied Physics Laboratory in Laurel, MD propose a mission and design a satellite in which the Topaz II could be used as the power source. Even so, the Topaz II reactor had a mass of 1,000 kg and produced 10 kilowatts for an efficiency of 10 watts/kg. Due to funding reduction within the SDIO, the Topaz II flight program was postponed indefinitely at the end of Fiscal Year 1993.

Similarly, cancellation was the eventual fate of the US SP-100 reactor program. This program was started in 1983 by NASA, the US Department of Energy and other agencies. It developed a 4000 kg, 100 kilowatt reactor ( efficiency = 25 watts/kg) with heat pipes transporting the heat to thermionic converters.

Proposed SP-100 reactor ca 1980  (Image credit: NASA/DoE/DARPA)

Believe it or not, small nuclear fission reactors are becoming very popular as portable ‘batteries’ for running remote communities of up to 70,000 people. The Hyperion Hydride Reactor is not much larger than a hot tub, is totally sealed and self-operating, has no moving parts and, beyond refueling, requires no maintenance of any sort.

Hyperion, Uranium Hydride Reactor (Credit:Hyperion, Inc)

According to the Hyperion Energy Company the Gen4 reactor has a mass of about 100-tons and is designed to deliver 25 megawatts electricity for a 10-year lifetime, without refueling. The efficiency for such a system is 250 watts/kg! Of course you cannot just slap one of  these Bad Boys onto a rocket ship to provide the electricity for the ion engines, but this technology already proves that fission reactors can be made very small and deliver quite the electrical wallop, and do so in places where solar panels are not practical.

Some of the advanced photo-electric system being developed by NASA and NASA contractors are based on the solar energy technology used in the NASA Deep Space 1 mission and the Naval Research Laboratory’s TacSat 4 reconnaissance satellite, and are based on ‘stretched lens array’ lens concentrators for sunlight that amplify the sunlight by up to 8 times (called eight-sun systems). The solar arrays are also flexible and can be rolled out like a curtain. The technology promises to reach efficiency levels of 1000 watts/kg, and less than $50/watt, compared to the 100 w/kg and $400/watt of current ‘one sun’ systems that do not use lens concentrators. A 350 kW solar-electric ion engine system is a suggested propulsion for a 70 ton crewed mission to Mars. With the most efficient stretched lens array solar arrays currently under design, a 350 kW system would have a mass of only 350 kg and cost about $18 million. The very cool thing about this is that improvements in solar panel technology not only directly benefit space power systems for inner solar system travel, but lead to immediate consumer applications in Green Energy!  Imagine covering your roof with a 1-square-meter high efficiency panel rather than your entire roof with an unsightly  lower-efficiency system!

So to really zip around the solar system and avoid the medical problems of prolonged voyages, we really need more work on compact power plant design that is politically realistic. Once we solve THAT problem, even Pluto will be a week’s journey away!

Check back here on Monday, April 24 for my next topic!

That was then, this is now!

Back in the 1960s when I began my interest in astronomy, the best pictures we had of the nine planets were out of focus black and white photos. I am astonished how far we have come since then and decided to devote this blog to a gallery of the best pictures I could find of our solar system neighbors! First, let’s have look at the older photos.

First we have Mercury, which is never very far from pour sun and a very challenging telescopic object.

Above is what mars looked like! Then we have Jupiter and Saturn shown below.

Among the hardest and most mysterious objects were Uranus shown here. I will not show a nearly identical telescopic view of Uranus.

Finally we come to Pluto, which has always been a star-like object for most of the 20th century.

These blurry but intriguing images were the best we could do for most of the 20th century, yet they were enough to encourage generations of children to become astronomers and passionately explore space. The features of mercury were mere blotches of differing shaded of gray. Uranus, Neptune were slightly resolvable to reveal faint details, and distant Pluto remained completely star-like and unresolved, yet we knew it was its own world many thousands of kilometers across. Mars continued to reveal its tantalizing blotchy features that came and went with the seasons along with the ebb and flow of its two polar ice caps. Jupiter was a banded world with its Great Red Spot, but the details of these atmospheric bands was completely hidden in the optical smearing of our own atmosphere. Saturn possessed some large bands, and its majestic ring system could be seen in rough detail but never resolved into its many components. As for the various moons of these distant worlds, they were blurry disks or star-like spots and never revealed their details.

The advent of the Space Program in the 1960s, and the steady investment in spacecraft to ‘fly by’ these planets led to progressively higher and higher resolution images starting with Mariner 4 in 1965 and its historic encounter with Mars, revealing a cratered, moonlike landscape. The Pioneer spacecraft in the early 1970s gave us stunning images of Jupiter, followed by the Voyager spacecraft encounters with the outer planets and their moons. Magellan orbited Venus and with its radar system mapped the surface to show a dynamic and volcanic surface that is permanently hidden beneath impenetrable clouds. Finally in 2015, the New Horizons spacecraft gave us the first clear images of distant Pluto. Meanwhile, many return trips to our own moon have mapped its surface to 2-meter resolution, while the MESSSENGER spacecraft imaged the surface of Mercury and mapped its many extreme geological features. Even water ice has been detected on mercury and the moon to slacken the thirst of future explorers.

For many of the planets, we have extreme close up images too!
Jupiter’s south pole from the Juno spacecraft shows a bewildering field of tremendous hurricanes each almost as large as Earth, swirling about aimlessly in a nearly motionless atmosphere.

Pluto details a few hundred meters across. Can you come up with at least ten questions you would like answers for about what you are seeing?

Here is one of thousands of typical views from the Martian surface. Check out the rocks strewn across the field. Some are dark and pumice-like while others are white and granite-looking. ‘Cats and dogs living together’. What’s going on here?

The Venera 13 image shown below from the surface of Venus is unique and extremely puzzling from a surface that is supposed to be hotter than molten lead.

We also have images from a multitude of moons, asteroids and comets!
The Lunar Reconnaissance Orbiter gave us 2-meter resolution images of the entire lunar surface allowing us to revisit the Apollo landing sites once more:

The dramatic canyons and rubble fields of a comet were brought into extreme focus by the Rosetta mission

Even Saturn’s moon Titan has been explored to reveal its extensive liquid nitrogen tributaries

This bewildering avalanche of detail has utterly transformed how we view these worlds and the kinds of questions we can now explore. If you compare what we knew about Pluto before 2015 when it was little more than a peculiar ‘star in the sky’, to the full-color detailed orb we now see, you can imagine how science progresses by leaps and bounds through the simple technique of merely seeing the object more clearly. It used to be fashionable to speculate about Pluto when all we knew was its size, mass and density and they it had a thin atmosphere. But now we are delightfully challenged to understand this world as the dynamic place that it is with mountains of ice, continent-sized glaciers, and nitrogen snow. And of course, the mere application of improved resolution now lets us explore the entire surface of our moon with the same clarity as an astronaut hovering over its surface from a height of a few dozen feet!

We Old-Timers have had a wonderful run in understanding our solar system as we transitioned from murky details to crystal clarity. All of the easy low-hanging fruit of theory building and testing over the last century has been accomplished for the most-part. Now the ever more challenging work of getting the details straight begins, and will last for another century at least. When you can tele-robotically explore planetary and asteroidal surfaces, or perform on-the-spot microscopic assays of minerals, what incredible new questions will emerge? Is there life below the surface of Europa? Why does Mars belch forth methane gas in the summer? Can the water deposits on the moon be mined? Is Pluto’s moon Charon responsible for the tidal heating of an otherwise inert Pluto?
One can only wonder!

Check back here on Tuesday, April 18 for my next topic!

Hohmann’s Tyrany

It really is a shame. When all you have is a hammer, everything else looks like a nail. This also applies to our current, international space programs.

We have been using chemical rockets for centuries, but since the advent of V2s and the modern space age, these brute-force and cheap work horses have been the main propulsion technology we use to go just about everywhere in the solar system. But this amounts to thinking that one technology can span all of our needs, and the trillions of cubic miles that encompass interplanetary space.

We pay a huge price for this belief.

Chemical rockets have their place in space travel. They are fantastic ways of delivering HUGE thrusts quickly; the method par excellance for getting us off this planet and paying the admission ticket to space.  No other known propulsion technology is as cheap, simple, and technologically elegant as chemical propulsion in this setting.  Applying this same technology to interplanetary travel beyond the moon is quite another thing, and sets in motion an escalating series of difficult problems.

Every interplanetary spacecraft launched so far to travel to each of the planets in our solar system works on the exact same principle. Give the spacecraft a HUGE boost to get it off the launch pad, and with enough velocity to reach the distant planet, then cut the engines off after a few minutes so the spacecraft can literally coast the whole way. With a few more ‘Delta-V’ changes, this is called the minimum –energy trajectory or for rocket scientists the Hohmann Transfer orbit. It is designed to get you there, not in the shortest time, but using the least amount of energy. In propulsion, energy is money. We use souped-up Atlas rockets at a few hundred million dollars a pop to launch space craft to the outer planets. We don’t use  even larger and expensive Saturn V rockets that deliver even more energy for a dramatically-shorter ride.

If you bank on taking the slow-boat to Mars rather than a more energetic ride, this leads to all sorts of problems. The biggest of these is that the inexpensive 220-day journeys let humans build up all sorts of nasty medical problems that short 2-week trips would completely eliminate. In fact, the entire edifice of the $150 billion International Space Station is there to explore the extended human stays in space that are demanded by Hohmann Transfer orbits and chemical propulsion. We pay a costly price to keep using cheap chemical rockets that deliver long stays in space, and cause major problems that are expensive to patch-up afterwards. The entire investment in the ISS could have been eliminated if we focused on getting the travel times in space down to a few weeks.

You do not need Star Trek warp technology to do this!

Since the 1960s, NASA engineers and academic ‘think tanks’ have designed nuclear rocket engines and ion rocket engines, both show enormous promise in breaking the hegemony of chemical transportation. The NASA nuclear rocket program began in the e arly-1960s and built several operational prototypes, but the program was abandoned in the late 1960s because nuclear rockets were extremely messy, heavy, and had a nasty habit of slowly vaporizing the nuclear reactor and blowing it out the rocket engine!  Yet, Wernher  Von Braun designed a Mars expedition for the 1970s in which several,  heavy 100-ton nuclear motors would be placed in orbit by a Saturn V and then incorporated into an set of three interplanetary transports. This program was canceled when the Apollo program was ended and there was no longer a conventional need for the massive Saturn V rockets. But ion rockets continued to be developed and today several of these have already been used on interplanetary spacecraft like Deep Space 1 and Dawn. The plans for humans on Mars in 2030s rely on ion rocket propulsion powered by massive solar panels.

Unlike chemical rockets, which limit spacecraft speeds to a few kilometers/sec, ion rockets can be developed with speeds up to several thousand km/sec. All that they need is more thrust, and to get that they need low-mass power plants in the gigawatt range. ‘Rocket scientists’ gauge engine designs based on their Specific Impulse, which is the exhaust speed divided by the acceleration of gravity on Earth. Chemical rockets can only provide SIs of 300 seconds, but ion engine designs can reach 30,000 seconds or more! With these engine designs, you can travel to Mars in SIX DAYS, and a jaunt to Pluto can take a neat 2 months! Under these conditions, most of the problems and hazards of prolonged human travel in space are eliminated.

But instead of putting our money into perfecting these engine designs, we keep building chemical rockets and investing billions of dollars trying to keep our long-term passengers alive.

Go figure!!!

Check back here on Friday, March 17 for a new blog!

 

Interstellar Travel?

Interstellar travel revolves around the answers to three major questions:

1) Where will we go?

2) What will we do when we get there?

3) How will it benefit folks back on Earth?

Far beyond the issue of whether interstellar travel is technologically possible is the very practical issue of answering these questions long before we turn the first screw in the hardware.

A few of the thousands of stars within 50 light years of Earth.(Credit: Atlas of the Universe)

In science fiction, finding new destinations is usually handled by either manned or unmanned expeditionary forces. Not surprisingly, the hazardous experiences of the manned expeditions are usually the exciting core of the story itself. When we create this technology ourselves, the first  trips are usually to a popular nearby star like Alpha Centauri (e.g Babylon 5). When we are co-opting alien technology, we often have to go to where the aliens previously had outposts often hundreds or thousands of light years away ( e.g Contact or Stargate). In any event, most stories assume that the travelers can select multiple destinations and hop-scotch their way around the local universe within a single human lifetime to find a habitable planet. Apparently, once you have convinced politicians to fund the first trip, the incremental cost of additional trips is very small!

The whole idea of interstellar travel was created by fiction writers, but because it has many elements of good science in it, it is a very persuasive idea. It is an idea located smack in the middle of the ‘gray area’ between fantasy and reality, and this is what goads people on to try to imagine ways to make it a reality. One thing we do know is that it will be an expensive venture requiring an investment at the level of many percent of an entire planet’s GDP.

By some estimates, the first interstellar voyage will cost many trillions of dollars, require decades to construct, and involve tens to hundreds of passengers and explorers. Assuming a project of this scope can even be sold to the bulk of humanity that will be left behind to pay the bills, how will the destination be selected? Will we just point the ship towards any star and commit these resources to a random journey and outcome, or will we know a LOT about where we are going before the fuel is loaded? Most of us will agree that the latter case for such an expensive ‘one of’ mission is more likely. By the way, let’s not talk about the human Manifest Destiny to explore the unknown. Even Christopher Columbus knew his destination in detail (India!) and traveled within a very benign biosphere, free breathable atmosphere and comfortable gravity to get there in a few months.

So…where will we go?

Contrary to popular ideas, we will know our destination in great detail long before we leave our solar system. We will know whether the star has any planets, and we will know it has at least one planet in its habitable zone (HZ) where temperature would allow liquid water to exist. We will know if the planet has an atmosphere or not. We will know its mass and size, and perhaps more importantly, whether the planet has a biosphere. We will not invest perhaps trillions of dollars to study a barren Mars or Venus-like planet. All of these issues will be worked out by astronomical remote-sensing research at far lower cost than traveling there. If a nearby star does not have detectable planets, we will most certainly NOT mount a trillion-dollar mission to just ‘go and see’!

The 10 nearest stars are: Proxima Centauri (4.24 lys), Alpha Centauri (4.36), Barnards Star (5.96), Luhman 16 (6.59), Wolf 359 (7.78), Lalande 21185 (8.29), Sirius (8.59), Luyten 726-8 (8.72), Ross 154 (9.68) and Ross 248 (10.32). This takes us out to a distance of just over 10 light years from Earth. The prospects for an interesting world to visit are not good.

Proxima Centauri has one recently-detected Earth-sized planet orbiting inside its HZ, making it a Venus-like world of no interest. Alpha Centauri B has one unverified Earth-sized planet, but not in the star’s liquid-water HZ. It orbits ten times closer than Mercury. There are no planets larger than Neptune orbiting this star closer than our planet Jupiter. Barnards Star has a no known planets, but a Jupiter-sized planet inside the orbit of Mars is exluded, so this is still a viable star for future searches for terrestrial planets in the star’s HZ. Luhman 16 is a binary system whose members orbit each other every 25 years at a distance of 3 AU. A possible companion orbits one of these stars every month at a distance closer than Mercury. As for the stars Wolf 359, Lalande 21185, Sirius, Luyten 726-8, Ross 154 and Ross 248, there have been searches for Jupiter-sized companions around these  stars, but none have ever been claimed.

So, our nearest stars within 10 light years are pretty bleak as destinations for expensive missions. There is no solid evidence for Earth-sized planets orbiting within the HZs of any of them. These would not be plausible targets because there is so little return on the high cost of getting there, even though that cost in terms of travel time is the smallest of all stars in our neighborhood. This also sets a scale for the technology required. It is not enough to visit our nearest star, but we have to trudge 2 to 3 times farther before we can find better destinations.

Better Destinations.

Let’s take a bigger step. Out to a distance of 16 light years there are 56 normal stars, which include some promising candidate targets.

Epsilon Eridani (10.52 ly) has one known giant planet outside its HZ. It also has two asteroid belts: one at about three times Earth’s distance from our sun (3 AU) and one at about 20 AU. No one would ever risk a priceless mission by sending it to a sparse planetary system with deadly asteroid belts and no HZ candidates!

Groombridge 34 (11.62 ly) –The only suspected planet has a mass of more than five Earths. No mission would be sent to such a planet for which an atmosphere would probably be crushingly dense and probably Jupiter-like even if it was in its HZ.

Epsilon Indi (11.82 ly) – has a possible Jupiter-sized planet with a period of more than 20 years. No known smaller planets.

Artist rendering of the planets Tau Ceti e and f (Credit: PHL @ UPR Arecibo)

Tau Ceti (11.88 ly) probably has five planets between two and six times Earth’s mass, and with periods from 14 to 640 days. Planet Tau Ceti f is colder than Mars and is at the outer limit to the star’s HZ. Its atmosphere might be dense enough for greenhouse heating, so the world might be habitable after all. But this is guesswork not certainty.

Kapteyn’s Star (12.77 ly) – It has two planets, Kapteyn b and Kapteyn c, that are 5 to 8 times the mass of Earth. Kapteyn b has a period of 120 days and is a potentially habitable planet estimated to be 11 billion years old. Again, a massive planet whose surface you could never visit, so what is the point of the interstellar expedition?

Gliese 876 (15.2 ly) has four planets. All have more than 6 times the mass of Earth and orbit closer than the planet Mercury. Gliese 876 c is a giant planet like Jupiter in the star’s habitable zone. Would you bet the entire mission that 876c has habitable ‘Galilean moons’ like our Jupiter? This would be an unacceptable shot in the dark, though a tantalizing one.

So, out to 15 light years we have some interesting prospects but no confirmed Earth-sized planet in its star’s HZ whose surface you could actually visit. We also have no solid data on the atmospheres of any of these worlds. None of these candidates seem worth investing the resources of a trillion-dollar mission to reach and study. We can study them all from Earth at far less cost.

Best Destinations.

If we take an even bigger step and consider stars closer than 50 light years we have a sample of potentially 2000 stars but not all of them have been discovered and cataloged. About 130 are bright enough to be seen with the naked eye. The majority are dim and cool red dwarf stars, which are still good candidates for planetary systems. In this sample we encounter among the known planetary candidates several that would be intriguing targets:

61 Virginis (11.41 ly) – It has three planets with masses between 5 and 25 times our Earth, crowded inside the orbit of Venus. The asteroidal debris disk has at least 10 times as many comets as our solar system. There are no detected planets more massive than Saturn within 6 AU. An Earth-mass planet in the star’s habitable zone remains a possiblity, but the asteroid belts make this an unacceptable high risk target.

Gliese 667 planets (Credit: ESO )

Gliese 667 (23.2 ly) –As many as seven planets may orbit this star, but have not been confirmed. All have masses between that of Earth and Uranus. All but one are huddled inside the orbit of Mercury. Planets c and d are in the star’s HZ and are at least 3 times the mass of Earth. Their hypothetical moons may be habitable.
55 Cancri (40.3 ly)- All five planets orbiting this star are more than five times the mass of Earth. Only 55 Cancri e is located at the inner edge of the star’s HZ and its hypothetical moons could be habitable. More planets are possible within the stable zone between 0.9 to 3.8 AU if their orbits are circular. This is a system we still need to study.

HD 69830 (40.7 ly) has a debris disk produced by an asteroid belt twenty times more massive than that in our own solar system. Three detected planets have masses between 10 to 18 times that of Earth. The debris disk makes this a high-risk prospect even if there are habitable moons.

HD 40307 (41.8 ly) Five of the six planets orbit very close to the star inside the orbit of Mercury. The fifth planet orbits at a distance similar to Venus and is in the system’s habitable zone. The planets range in mass from three to ten times Earth. Again, is a planet in its HZ with a mass too great for a direct human visit a good candidate? I don’t think so.

Upsilon Andromedae (44.25 ly) The two outer planets are in orbits more elliptical than any of the planets in the Solar System. Upsilon Andromedae d is in the system’s habitable zone, has three times the mass of Jupiter, with huge temperature swings. Its hypothetical moons may be habitable.

47 Ursa Majoris (45.9 ly) The only known planet 47 Ursae Majoris b is more than twice the mass of Jupiter and orbits between Mars and Jupiter. The inner part of the habitable zone could host a terrestrial planet in a stable orbit. None yet detected.

There are still many more stars in this sample to detect, catalog and study so it is possible that a Goldilocks Planet could be found eventually. But we are now looking at destinations more than 20 light years away at a minimum. This will considerably increase the cost and duration of any interstellar mission by factors of five to ten times a simple jaunt to Alpha Centauri.

Other issues.

Would you really consider a planet with two to five times Earth’s gravity to be a candidate? Who would want to live under that crushing weight? Many of the candidates we have found so far are massive Earth’s that few colonists would consider standing upon. Their surfaces are also technologically expensive to get to and leave. But perhaps these worlds might have moons with more comfortable gravities? There is always hope, but will that be enough to risk a multi-trillion-dollar mission?

There is also the issue of atmosphere. None of the candidate planets we have discussed transit their stars, so we cannot detect their atmospheres and figure out if they have atmospheres and if their  trace gases would be  lethal. The perfect destination worth the expense of a trip would have a breathable atmosphere with oxygen. Since free oxygen is only produced by living systems, our target planet would have a biosphere. We can only hope that as the surveys of the nearby stars continue, we will find one of these. But statistics suggests we will have to search much farther than 50 light years and a few thousand stars before we encounter one. That makes the interstellar voyage even more costly, not by factors of five and ten, but potentially hundreds of times. But if the trip were to a world with a known biosphere, THAT might be worth the effort, but possibly nothing less than this would be worth the cost, the risk, and the scientific return.

So the bottom line is that the only interstellar destination worth the expense is either one in which colonists can live comfortably on the planet with a lethal atmosphere, hermetically sealed under a dome, or a similar planet with a breathable oxygen atmosphere and a biosphere. Statistically, we will find far more examples of the first kind of target than the second. But in the majority of the cases, we will not be able to detect the atmosphere of an Earth-sized world in its habitable zone before we start the trip, and will have to ‘guess’ whether it even has an atmosphere at all!

The enormous cost of an interstellar trip to a target tens or even hundreds of light years away will preclude any guess work about what we will find when we get there. Consider this: Investing $100 billion to travel to Mars, a low-risk planet we thoroughly understand in detail, is still considered a political pipe dream even with existing technology! What would we call a trip that costs perhaps 100 times as much?

For more on this topic, have a look at my book ‘Interstellar Travel:An astronomer’s guide’. Available at Amazon.com.

 

Return here for my next blog posting on Friday, February   3

The Rush to Mars

Even at the start of NASA’s space program in 1958, the target of our efforts was not the moon but Mars. The head of NASA, Werner von Braun was obsessed with Mars, and his Mars Project book published in 1948, was the blueprint for how to do this, which he revised in 1969. He saw the development of the Saturn V launch vehicle as the means to this end.

A major effort running in parallel with Kennedy’s moon program was the development of nuclear rocket technology. This would be the means for getting to Mars in the proposed expedition launch in November, 1981. The program was abandoned in 1972 when President Nixon unceremoniously canceled the Apollo Program and stopped the production of Saturn Vs. That immediately put the kibosh on any nuclear propulsion efforts because the required fission reactors were far too heavy to be lifted into space by any other means. He resigned from NASA once he realized that his dream would never be realized, and died five years later.

Flash forward to 2004 when President George Bush announced his Space Exploration Initiative to include a manned trip to Mars by the 2030s. There would be manned trips to the moon by 2015 to test out technologies relevant to the Mars trip and to learn how to live there for extended periods of time.

By 2008, the lunar portion of this effort was canceled, however the development of the Orion capsule and what is now called the Space Launch System were the legacies of this program still in place and expected to be operational by ca 2019. The rest of the Initiative is now called NASA’s Journey to Mars, and lays out a detailed plan for astronauts learning how to work farther and farther from Earth in self-sustaining habitats, leading to a visit to Mars in the 2030s. Meanwhile, the International Space Station has been greatly extended in life to the mid-2020s so we can finally get a handle on how to live and work in space and solve the many medical issues that still plague this environment.

However, NASA’s systematic approach is not the only one in progress today.

The entire foundation of Elon Musk’s Space-X company is to build and make commercially profitable successively larger launch vehicles leading to the Interplanetary Transport System which will bring 100 colonists at a time to the surface of Mars in about 80 days starting around 2026. Space-X is even partnering with NASA for a sample return mission called Red Dragon in ca 2018. Meanwhile, a competing program called Mars One (see picture above) proposes a crew of four people to land in 2032 with additional crew delivered every two years. . This will be a one-way do-or-die colony, and loss of life is expected. Mars One consists of two entities: the not-for-profit Mars One Foundation, and the for-profit company Mars One Ventures with CEO Bas Lansdorp at the corporate helm.

But wait a minute, what about all the non-tech issues like astronaut health and generating sustainable food supplies? Astronauts have been living in the International Space Station for decades in shifts, and many issues have been identified that we would be hard pressed to solve in only ten more years. NASA’s go-slow approach may be the only one consistent with not sending astronauts to a premature death on mars, with all the political and social ramifications that implies.

The dilemma is that slow trips to Mars, like the 240-day trips advocated by NASA’s plan exacerbate health effects from prolonged weightlessness including bone loss, failing eyesight, muscle atrophy and immune system weakening. These effects are almost eliminated by much shorter trips such as the 80-day target by Space-X. In fact, the entire $100 billion International Space Station raison de etra is to study long term space effects during these long transits. This existential reason for ISS would have been eliminated had a similar investment been made in ion or nuclear propulsion systems that reduced the travel time to a month or less!

Ironically, Werner von Braun knew about this as long ago as 1969, but his insights were dismissed for political reasons that led directly to our confinement to low Earth orbit for the next 50 years!

Check back here on Sunday, January 22 for the next installment!

Space Travel via Ions

For 60 years, NASA has used chemical rockets to send its astronauts into space, and to get its spacecraft from planet to planet. The huge million-pound thrusts sustained for only a few minutes were enough to do the job, mainly to break the tyranny of Earth’s gravity and get tons of payload into space. This also means that Mars is over 200 days away from Earth and Pluto is nearly 10 years. The problem: rockets use propellant (called reaction mass) which can only be ejected at speeds of a few kilometers per second. To make interplanetary travel a lot zippier, and to reduce its harmful effects on passenger health, we have to use rocket engines that eject mass at far-higher speeds.

In the 1960s when I was but a wee lad, it was the hey-day of chemical rockets leading up to the massive Saturn V, but I also knew about other technologies being investigated like nuclear rockets and ion engines. Both were the stuff of science fiction, and in fact I read science fiction stories that were based upon these futuristic technologies. But I bided my time and dreamed that in a few decades after Apollo, we would be traveling to the asteroid belt and beyond on day trips with these exotic rocket engines pushing us along.

Well…I am not writing this blog from a passenger ship orbiting Saturn, but the modern-day reality 50 years later is still pretty exciting. Nuclear rockets have been tested and found workable but too risky and heavy to launch.  Ion engines, however, have definitely come into their own!

Most geosynchronous satellites use small ‘stationkeeping’ ion thrusters to keep them in their proper orbit slots, and NASA has sent several spacecraft like Deep Space-1, Dawn powered by ion engines to rendezvous with asteroids. Japan’s Hayabusha spacecraft also used ion engines as did ESA’s Beppi-Colombo and the LISA Pathfinder. These engines eject charged, xenon atoms ( ions) to speeds as high as 200,000 mph (90 km/sec), but the thrust is so low it takes a long time for spacecraft to build up to kilometer/sec speeds. The Dawn spacecraft, for example, took 2000 days to get to 10 km/sec although it only used a few hundred pounds of xenon!

But on the drawing boards even more powerful engines are being developed. Although chemical rockets can produce millions of pounds of thrust for a few minutes at a time, ion engines produce thrusts measured in ounces for thousands of days at a time. In space, a little goes a long way. Let’s do the math!

The Deep Space I engines use 2,300 watts of electrical energy, and produced F= 92 milliNewtons of thrust, which is only 1/3 of an ounce! The spacecraft has a mass of m= 486 kg, so from Newton’s famous ‘F=ma’  we get an acceleration of a= 0.2 millimeters/sec/sec. It takes about 60 days to get to 1 kilometer/sec speeds. The Dawn mission, launched in 2007 has now visited asteroid Vesta (2011) and dwarf planet Ceres (2015) using a 10 kilowatt ion engine system with 937 pounds of xenon, and achieved a record-breaking speed change of 10 kilometers/sec, some 2.5 times greater than the Deep Space-1 spacecraft.

The thing that limits the thrust of the xenon-based ion engines is the electrical energy available. Currently, kilowatt engines are the rage because spacecraft can only use small solar panels to generate the electricity. But NASA is not standing still on this.

OLYMPUS DIGITAL CAMERA

The NEXIS ion engine was developed by NASA’s  Jet Propulsion Laboratory, and this photograph was taken when the engine’s power consumption was 27 kW, with a thrust of 0.5 Newtons (about 2 ounces).

An extensive research study on the design of megawatt ion engines by the late David Fearn was presented at the Space Power Symposium of the 56th International Astronautical Congress in 2005. The conclusion was that these megawatt ion engines pose no particular design challenges and can achieve exhaust speeds that exceed 10 km/second. Among the largest ion engines that have actually been tested so far is a 5 megawatt engine developed in 1984 by the Culham Laboratory. With a beam energy of 80 kV, the exhaust speed is a whopping  4000 km/second, and the thrust was 2.4 Newtons (0.5 pounds).

All we have to do is come up with efficient means of generating megawatts of power to get at truly enormous exhaust speeds. Currently the only ideas are high-efficiency solar panels and small fission reactors. If you use a small nuclear reactor that delivers 1 gigawatt of electricity, you can get thrusts as high as 500 Newtons (100 pounds). What this means is that a 1 ton spacecraft would accelerate at 0.5 meters/sec/sec and reach Mars in about a week!  Meanwhile, NASA plans to use some type of 200 kilowatt, ion engine design with solar panels to transport cargo and humans to Mars in the 2030s. Test runs will also be carried out with the Asteroid Redirect Mission ca 2021 also using a smaller 50 kilowatt solar-electric design.

So we are literally at the cusp of seeing a whole new technology for interplanetary travel being deployed. If you want to read more about the future of interplanetary travel, have a look at my book ‘Interplanetary Travel: An astronomer’s guide’, which covers destinations, exploration and propulsion technology, available at Amazon.com.

Stay tuned!

Check back here on Wednesday, January 11 for the next installment!

Image credits:

Ion engine schematic  http://plasmalab.aero.upm.es/~plasmalab/information/Research/ElectricPropulsion.html

Star Destroyer

http://s1187.photobucket.com/user/ringa52577/media/SD-2.jpg.html

To Pluto in 30 days!

OK…While everyone else is worrying how to get to Mars, let’s take a really big step and figure out how to get to Pluto….in a month!

The biggest challenge for humans is surviving the long-term rigours of space hazards, but all that is nearly eliminated if we keep our travel times down to a few weeks.

Historically, NASA spacecraft such as the Pioneer, Voyager and New Horizons missions have taken many years to get as far away from Earth as Pluto. The New Horizons mission was the fastest and most direct of these. Its Atlas V launch vehicle gave it an initial speed of 58,000 km/hr. With a brief gravity assist by Jupiter, its speed was boosted to 72,000 km/hour, and the 1000-pound spacecraft made it to Pluto in 9.5 years. We will have to do a LOT better than that if we want to get there in 1 month!

The arithmetic of the journey is quite simple: Good old speed = distance / time. But if we gain a huge speed to make the trip, we have to lose this speed to arrive at Pluto and enter orbit. The best strategy is to accelerate for the first half, then turn the spacecraft around and decelerate for the second half of the trip. The closest distance of Pluto to Earth is about 4.2 billion kilometers (2.7 billion miles). That means that for 15 days and 2.1 billion kilometers, you are traveling at an average speed of 5.8 million kilometers per hour!

Astronomers like to use kilometers/second as a speed unit, so this becomes about 1,600 km/sec. By comparison, the New Horizons speed was 20 km/sec. Other fast things in our solar system include the orbit speed of Mercury around the sun (57 km/s), the average solar wind speed (400 km/s) and a solar coronal mass ejection event (3,000 km/s).

If our spacecraft was generating a constant thrust by running its engines all the time, it would be creating a uniform acceleration from minute to minute. We can calculate how much this is using the simple formula distance = ½ acceleration x Time-squared. With distance as 2.1 billion km and time as 15 days we get 0.00062 km/sec/sec or 0.62 meters/sec/sec. Earth’s gravity is 9.8 meters/sec/sec so we will be feeling an ‘artificial gravity’ of about 0.06 Gs….hardly enough to feel, so you will still be essentially weightless the whole journey!

If the rocket is squirting fuel (reaction mass) out its engines to produce the thrust, we can estimate that this speed has to be about 1,600 km/sec. Rocket engines are compared in terms of their Specific Impulse (SI), which is the exhaust speed divided by the acceleration of gravity on Earth’s surface, so if the exhaust speed is 1,600 km/sec, then the SI = 160,000 seconds. For chemical rockets like the Saturn V, SI=250 seconds!

What technology do we need to get to these speeds and specific impulses?

The most promising technology we have today is the ion rocket engine, which has SIs in the range of 2,000 to 30,000 seconds .The largest ion engine designs include the VASIMR engine; a proposed 200 megawatt, nuclear-electric ion engine design that could conceivably get us to Mars in 39 days. Ion engines are limited by the electrical power used to accelerate the ions (currently in the kilowatt-range but gigawatts are possible if you use nuclear power plants), and the mass of the ions themselves (currently xenon atoms).

Other designs propose riding the solar wind using solar sails, however although this works on the outward-bound leg of the trip, it is very difficult to return to the inner solar system! The familiar technique of ‘tacking into the wind’ will not work because for sailboats it relies on movement through manipulating pressure changes behind the sail, while solar wind pressure changes are nearly zero. Laser propulsion systems have also been considered, but the power requirements often compete with the total electrical power generated by a large faction of the world for payloads with appreciable mass.

So, some version of ion propulsion with gigawatt power plants (fission or fusion) may do the trick. Because the SIs are so very large, the amount of fuel will be a small fraction of the payload mass, and these ships may look very much like those fantastic ships we often see in science fiction after-all!

Oh…by the way, the same technology that would get you to Pluto in 30 days would get you to Mars in 9 days and the Moon in 5 minutes.

Now, wouldn’t THAT be cool?

If you want to see some more ideas about interplanetary travel, have a look at my book ‘Interplanetary Travel:An astronomer’s guide’ available at amazon.com.

Check back here on Monday, January 2 for the next installment!

Selling Ice to Eskimos

Looking beyond our first journeys to Mars in the 2030s, and perhaps setting up outposts there in the 2040s, a frequently-mentioned plan for commercialization of space often brings up the prospects of interplanetary mining. A bit of careful thought can define the prospects and successes for such a venture if we are willing to confront them honestly.

The biggest challenge is that the inner solar system out to the asteroid belt is vastly different than the outer solar system from Jupiter to the distant Kuiper Belt. It is as though they occupy two completely separate universes, and for all intents and purposes, they do!

The inner solar system is all about rocky materials, either on accessible planetary surfaces and their moons, or in the form of asteroids like this photo of asteroid Vesta. We have studied a representative sample of them and they are rich in metals, silicates and carbon-water compounds. Lots of fantastic raw materials here for creating habitats, building high-tech industries, and synthesizing food.

Humans tend to ‘follow the water’ and we know that the polar regions of Mercury and the Moon have water-ice locked away in permanently shadowed craters under the regolith. Mars is filthy rich with water-ice, which forms the permanent core of its polar caps, and probably exists below the surface in the ancient ocean basins of the Northern Hemisphere. Many asteroids in the outer belt are also rich in water, as are the occasional cometary bodies that pass through our neighborhood dozens of times a year.

The inner solar system is also compressed in space. Typical closest distances between its four planets can be about 30 million miles, so the technological requirements for interplanetary travel are not so bad. Over the decades, we have launched about 50 spacecraft to inner solar system destinations for a modest sum of money and rocketry skill.

The outer solar system is quite another matter.

Just to get there we have to travel over 500 million miles to reach Jupiter…ten times the distance to Mars when closest to Earth. The distances between destinations in the outer solar system are close to one billion miles! We have sent ten spacecraft to study these destinations. You cannot land on any of the planets there, only their moons. Even so, many of these moons (e.g those near Jupiter) are inaccessible to humans due the intense radiation belts of their planets.

The most difficult truth to deal with in the outer solar system is the quality of the resources we will find there. It is quite clear from astronomical studies and spacecraft visits that the easiest accessible resources are various forms of water and methane ice. What little rocky material there is, is typically buried under hundreds of kilometers of ice, like Saturn’s moon Enceladus shown here, or at the cores of the massive planets. The concept of mining in the outer solar system is one of recovering ice, which has limited utility for fabricating habitats or being used as fuel and reaction mass.

The lack of commercializable resources in the outer solar system is the biggest impediment to developing future ‘colonization’ plans for creating permanent, self-sustaining outposts there. This is dramatically different than what we encounter in the inner solar system where minable resources are plentiful, and water is far less costly to access than in the outer solar system.

Astronomically speaking, we will have much to occupy ourselves in developing the inner solar system for human access and commercialization, but there is a big caveat. Mined resources cannot be brought back to Earth no matter how desirable the gold, platinum and diamonds might be that are uncovered. The overhead costs to mine and ship these desirable resources is so high that they will never be able to compete with similar resources mined on Earth. Like they say about Las Vegas, ‘what is mined in space, stays in space’. Whatever resources we mine will be utilized to serve the needs of habitats on Mars and elsewhere, where the mining costs are just part of the high-cost bill for having humans in space in the first place.

The good news, however, is that the outer solar system will be the playground for scientific research, and who knows, perhaps even tourism. The same commercial pressures that will drive rocket system technology to get us to Mars in 150 days, will force these trips to take months, then weeks, then days. Once we can get to Mars in a week or less, we can get to Pluto in a handful of months, not the current ten-year journeys. Like so many other historical situations, scientific research and tourism became viable goals for travel as partners to the political or commercial competition to get to India in the 1500s, the Moon in the 1960s…or Mars in the 2000s.

In the grand scheme of things, we have all the time in the world to make this happen!

For more about this, have a look at my book ‘Interplanetary Travel:An Astronomer’s Guide’, for details about resources, rocket technology, and how to keep humans alive, based upon the best current ideas in astronomy, engineering, psychology and space medicine. Available at Amazon.com

Check back here on Friday, December 30 for the next installment!