A Star Sculpture on Hubble’s Anniversary

The Eagle NebulaHard to believe that it was fully fifteen years ago today that the Hubble Space Telescope was placed into orbit from the Space Shuttle Discovery. Hubble’s list of achievements has been outstanding, from detecting proto-galaxies whose light was emitted less than a billion years after the Big Bang to providing data that helped astronomers confirm the age of the universe, now calculated at some 13.7 billion years. And don’t forget the extraordinary moments closer to home, such as the space telescope’s views of comet Shoemaker-Levy 9, the famous ‘string of pearls,’ hitting Jupiter in 1994. Hubble’s 700,000 images have provided views up to ten times sharper than any previous telescope could offer.

The image above, a part of the Eagle Nebula, shows a tower of cold gas and dust being shaped by the light of hot new stars. It was taken with Hubble’s Advanced Camera for Surveys (ACS), providing a picture so sharp that, at full resolution, the image could be blown up to the size of a billboard without losing detail.

Image: Stars in the Eagle Nebula are born in clouds of cold hydrogen gas that reside in chaotic neighbourhoods, where energy from young stars sculpts fantasy-like landscapes in the gas. The tower may be a giant incubator for those newborn stars. A torrent of ultraviolet light from a band of massive, hot, young stars [off the top of the image] is eroding the pillar. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

And ponder this: the soaring tower of gas in the photograph is roughly 9.5 light years in size, or a little more than twice the distance between the Sun and the Alpha Centauri triple star system. One thing the universe is constantly adjusting is our own sense of perspective!

Virtual Reality Over a Galactic Network

“The prospect of distributing realistic simulations of alien environments throughout the galaxy sheds light on ‘Fermi’s question,’ named after the physicist Enrico Fermi, who is said to have inquired of intelligent extraterrestrials, ‘Where are they?’ The point of Fermi’s question, much elaborated by later thinkers, is that a technically advanced civilization could set up colonies on the planets of nearby stars, which in turn could colonize other star systems, until their race had populated the entire galaxy. Since they are not here, the argument concludes, perforce they are not anywhere, and we are alone in the galaxy.

“Interstellar colonization, however, is arduous and expensive by just about any imaginable standard. It can hardly be justified in terms of population pressure or a need for raw materials: Our Sun, for instance, has enough energy, and the solar system enough space, to accomodate the most vigorous forseeable expansion of our species for many millions of years into the future, and money spent on development within the solar system would reap us many times more rewards than would money spent ferrying people to another star. Unless the sun were about to explode and we had to get out, the only
evident rationale for interstellar colonization by us or anybody else would be curiosity — to give some members of a species the experience of standing on the soil of a nonsolar planet. But VR does much the same thing, and does it more democratically. An automated probe, dispatched by a living species or by an interstellar network to an uninhabited planet, could send back simulations that let everybody ‘be there.’

“The reason aliens are not here, then, need not be because they do not exist. It may simply be that they are content with sims, and feel no more compulsion to travel to distant planets in person than a viewer watching a television documentary about Borneo feels compelled to pack a bag and fly to Borneo. A few might make the long trek to another star, just as a few New Englanders may elect to visit Borneo, but their occasional voyages need not add up to anything like a wave of colonists flooding the galaxy.”

— Timothy Ferris, from The Mind’s Sky: Human Intelligence in a Cosmic Context (New York: Bantam Books, 1992), pp. 52-53.

Asteroid Belt Around a Sun-like Star?

Finding solar systems similar to our own is a continuing quest for planet hunters. Now a star called HD69830, some 41 light years from Earth, has been found to contain a thick band of warm dust that may be an asteroid belt. Although planets have yet to be detected around the star, the find is exciting because HD69830 is similar to the Sun in age and size. And just as Jupiter seems to provide an outer limit to our own asteroid belt, there are suspicions that the asteroid belt around this star may be contained by the gravitational influence of a gas giant planet. If such planets exist around HD69830, they’ll likely be spotted by future planet-hunting missions like SIM, the Space Interferometry Mission, which is scheduled for a 2011 launch.

Asteroid belt viewsBut exoplanetary systems continue to confound our expectations. The newly discovered belt is not only 25 times as dense as our own, it’s also much closer to its star. Imagine a thick belt of primordial debris located inside the orbit of Venus, providing an unforgetable celestial panorama in the night sky of any planet nearby. Its light would be 1,000 times greater than the ‘zodiacal light’ thrown off by our own asteroid belt’s dust.

Image: This artist’s concept illustrates what the night sky might look like from a hypothetical alien planet in a star system with an asteroid belt 25 times as massive as the one in our own solar system (alien system above, ours below). In our solar system, anybody observing the skies on a moonless night far from city lights can see the sunlight that is scattered by dust in our asteroid belt. Called zodiacal light and sometimes the “false dawn,” this light appears as a dim band stretching up from the horizon when the Sun is about to rise or set. The light is faint enough that the disk of our Milky Way galaxy remains the most prominent feature in the sky. (The Milky Way disk is shown perpendicular to the zodiacal light in both pictures.) Credit: NASA/JPL-Caltech/R. Hurt (SSC).

The alien asteroid belt was found with the help of the Spitzer Space Telescope, a space-based infrared observatory. Dr. Charles Beichman of the California Institute of Technology led the team that made the discovery; its findings will be discussed in an upcoming paper in the Astrophysical Journal. Beichman’s group surveyed 85 Sun-like stars but found only a single asteroid belt, a thick band of dust likely augmented by frequent asteroid collisions.

“Because this belt has more asteroids than ours, collisions are larger and more frequent, which is why Spitzer could detect the belt,” said Dr. George Rieke, University of Arizona, Tucson, co-author of the paper. “Our present-day solar system is a quieter place, with impacts of the scale that killed the dinosaurs occurring only every 100 million years or so.”

But is this an asteroid belt or, perhaps, an enormous comet that became trapped in the inner planetary system and is slowly coming apart? Only future studies with Spitzer and other telescopes will provide a definitive answer, but the cometary hypothesis seems to be a long shot. You can read more in this Spitzer press release.

Near Term Technologies II: SailBeam

Centauri Dreams first ran across Jordin Kare’s remarkable SailBeam concept in a report called “High-Acceleration Micro-Scale Laser Sails for Interstellar Propulsion” that the astrophysicist prepared for NASA’s Institute for Advanced Concepts (NIAC). The idea seemed outrageously simple: if you accelerate vast numbers of tiny sails rather than one enormous one, you can bring the same amount of mass to high speeds with a less complex optical system. Using dielectric rather than metal sails, you can accelerate the sails much closer to their power source. The stream of microsails then becomes a source of propulsion as it is vaporized into plasma behind a departing starship.

Dana Andrews, who has worked with Kare on magsail concepts, notes that a SailBeam Boosted Magsail (SBBM) solves a key problem of particle beam propulsion. A neutral particle beam will disperse as it travels, but a stream of low-mass microsails is not limited by such diffraction. Andrews’ MagOrion concept explored some of the same territory, driving a magsail starship by the plasma pulses of small nuclear explosions. But ionizing a stream of incoming microsails should work as well, provided you find an efficient way to do it.

We looked at Andrews’ recent paper “Interstellar Propulsion Opportunities Using Near-Term Technologies” yesterday. In its final sections, Andrews analyzes SailBeam concepts and discusses three ways to ionize the incoming sails. Lasers could be mounted on the starship that would vaporize the sails as they approached. Particle beams could perform the same function, a method Andrews sees as more efficient than lasers. Finally, the microsails could simply impact against a small mass of solid, gas or plasma placed directly behind the starship. This is the least effective method. As Andrews writes:

This approach has the advantage of requiring little complex vehicle hardware, but the disadvantage that the vehicle must supply mass to intercept each sail, at least some of which is lost. The effective specific impulse of the vehicle propulsion is thus no longer infinite, and unless the impact mass lost is comparable to or less than the sail mass, the maximum vehicle velocity will be a fraction of the sail velocity.

All told, SailBeam concepts are cost effective because of their lower average power requirement and the simplicity of the SailBeam accelerator, which Andrews believes could be built with near-term optics. For manned missions, SailBeam would not require — as a Particle Beam Boosted Magsail would — a string of particle accelerators along an ‘interstellar runway’ to achieve adequate acceleration at a slow enough pace to avoid killing the crew.

Andrews advocates a Life Cycle Cost analysis for each kind of mission he discusses to determine which offers the lowest cost per spacecraft. And consider this exotic possibility: an interstellar vehicle could be launched as a number of small payloads at high acceleration. These could then be linked up during the first year of coast to form a larger vehicle that would use magnetic sail braking upon arrival at its destination star system. Andrews calls this the ‘wagon train’ approach to interstellar transportation. It seems a promising concept for interstellar robotic probes.

In summary, the laser-propelled lightsail or the slightly more efficient SBBM alternate, while not as efficient as the PBBM [Particle Beam Boosted Magsail], could well be the system of choice if the price of solar panels continues to drop over the next 30 years. Either way, we can watch the cost of space-based energy fall and predict the time when interstellar exploration becomes affordable.

The bottom line is that interstellar travel looks expensive, but possible. Therefore, the Fermi paradox is still a paradox. It is possible that we are alone in the galaxy, or that no civilization has acquired the necessary technologies, but the former is more likely than the latter.

The Andrews paper is “Interstellar Propulsion Opportunities Using Near-Term Technologies,” in Acta Astronautica Vol. 55 (2004), pp. 443-451. Jordin Kare’s report on SailBeam concepts is “High-Acceleration Micro-Scale Laser Sails for Interstellar Propulsion,” Final Report, NIAC Research Grant #07600-070, revised February 15, 2002. And be aware of the classic paper by C.E. Singer, “Interstellar Propulsion Using a Pellet Stream for Momentum Transfer,” Journal of the British Interplanetary Society 33 (March 1980): 107–115.

Gerald Nordley has done fascinating work on the concept of self-steering pellet propulsion. I’ll discuss his ideas in an upcoming entry.

Interstellar Flight Using Near-Term Technologies

In a paper called “Interstellar Propulsion Opportunities Using Near-Term Technologies,” Dana G. Andrews (Andrews Space, Seattle) sees two criteria for an interstellar mission. First, it must return results within the lifetime of its principal investigator or the average colonist. Second, it must find cost-effective ways to generate energy and to convert raw energy into directed momentum. These are steep requirements — we’re talking 15 to 20 percent of the speed of light, or 45,000 to 60,000 kilometers per second.

Centauri Dreams questions the first criterion, but the need for efficient and effective propulsion is true no matter how fast a mission we manage to design. Robert Forward’s massive Fresnel lenses (1000 kilometers in diameter 15 AU from the laser source) are one of those ‘small problems in engineering’ that the irrepressible Forward managed to concoct while not violating known physics. And Andrews points to a lightsail alternative — building solar-pumped or electrically-powered lasers in the million gigawatt range, allowing a spacecraft to be accelerated to thirty percent of the speed of light within a fraction of a light year with a more achievable 50 kilometer lens.

Beamed power conceptBut Andrews sees a better alternative in a particle beam boosted magsail (PBBM) that substitutes a neutral plasma beam for the laser and uses a magnetic sail instead of a lightsail. The benefits are enormous: a six order of magnitude reduction in the power required during acceleration, with comparable savings in the cost and complexity of the craft itself. Moreover, Andrews’ PBBM gets two to three orders of magnitude increased force on the sail for the same beam power, and eliminates the need for a separate deceleration system, since the magnetic sail can provide needed braking.

Image: In this artist’s conception, a plasma station (lower left) applies a magnetized beam of ionized plasma to a spacecraft bound for Jupiter. Could a fleet of such stations provide enough acceleration to drive a probe to the stars? Credit: John Carscadden, University of Washington.

A magsail uses a magnetic field to accelerate or decelerate by interacting with the plasma found in the solar wind and the interstellar medium. Geoffrey Landis, whose revised paper on the subject was discussed here two days ago, was the first to suggest particle beam acceleration of a magnetic sail back in 1989. Andrews, who has written key papers on magnetic sails with Robert Zubrin, provides in this new study a system description of a possible magsail for plasma beam use. Here is his description of the magsail in the deceleration phase as it approaches its destination star:

The Magsail…makes an excellent brake for an interstellar spacecraft traveling at fractions of the speed of light. A magnetic field moving at relativistic speeds ionizes the interstellar medium and then deflects the resulting plasma, creating drag, which decelerates the spacecraft. The ability to slow down spacecraft from interstellar to interplanetary velocities without the expenditure of rocket propellant results in a dramatic lowering of the total mission mass…

As for the acceleration phase, the physicist sees large interplanetary freighters in the manner of Robert Heinlein’s ‘torchships’ that can provide plasma beams along the flight path — he is assuming that a single neutrally charged beam will disperse after 1 AU, making booster beams necessary for some payloads. So useful is the magsail that it could also be used in a hybrid mission combining lightsail acceleration with magsail deceleration upon arrival at the target star.

Andrews’ “Interstellar Propulsion Opportunities Using Near-Term Technologies” appears in Acta Astronautica Vol. 55 (2004), pp. 443-451. More on this paper tomorrow, in a discussion of the exotic SailBeam concept. And be aware of Andrews and Zubrin’s earlier papers “Magnetic Sails and Interstellar Travel,” IAF-88-553 (1988), and “Progress in Magnetic Sails,” AIAA Paper #90-2367, 1990. The original Geoffrey Landis paper is “Optics and Materials Considerations for a Laser-Propelled Lightsail” (IAA-89-664, 1989), now revised to include Robert Winglee’s M2P2 magsail concepts.