Paul Gilster | Centauri Dreams

Experimenting on an Interstellar Sail

by Paul Gilster | Feb 27, 2025 | Sail Concepts | 1 Comment

The idea of beaming a propulsive force to a sail in space is now sixty years old, if we take Robert Forward’s first publications on it into account. The gigantic mass ratios necessary to build a rocket that could reach interstellar distances were the driver of Forward’s imagination, as he realized in 1962 that the only way to make an interstellar spacecraft was to separate the energy source and the reaction mass from the vehicle. Robert Bussard knew that as well, which is why in more or less the same timeframe we got his paper on the interstellar ramjet. It would scoop up hydrogen between the stars and subject it to fusion. But the Bussard ramjet had to light fusion onboard, whereas a sail propelled by a laser beam – a lightsail – operated without a heavy engine. The idea worked on paper but demanded a laser of sufficient size (Forward calculated over 10 kilometers) to make it a concept for the far future. His solution demanded very large lasers in close solar orbits, and thus an...

Ernst Öpik and the Interstellar Idea

by Paul Gilster | Feb 25, 2025 | Culture and Society | 12 Comments

Some names seem to carry a certain magic, at least when we’re young. I think back to all the hours I used to haunt St. Louis-area libraries when I was growing up. I would go to the astronomy section and start checking out books until over time I had read the bulk of what was available there. Fred Hoyle’s name was magic because he wrote The Black Cloud, one of the first science fiction novels I ever read. So naturally I followed his work on how stars produce elements and on the steady state theory with great interest. Willy Ley’s name was magic because he worked with Chesley Bonestell (another magic name) producing The Conquest of Space in 1949, and then the fabulous Rockets, Missiles, and Space Travel in 1957, a truly energizing read. Not to mention the fact that he had a science column in what I thought at the time was the best of the science fiction magazines, the ever-engaging Galaxy. It still stuns me that Ley died less than a month before Apollo 11 landed on the Moon. My list...

SETI’s Hard Steps (and How to Resolve Them)

by Paul Gilster | Feb 21, 2025 | Astrobiology and SETI | 25 Comments

The idea of life achieving a series of plateaus, each of which is a long and perilous slog, has serious implications for SETI. It was Brandon Carter, now at the Laboratoire Univers et Théories in Meudon, France, who proposed the notion of such ‘hard steps’ back in the early 1980s. Follow-up work by a number of authors, especially Frank Tipler and John Barrow (The Anthropic Cosmological Principle) has refined the concept and added to the steps Carter conceived. Since then, the idea that life might take a substantial amount of the lifetime of a star to emerge has bedeviled those who want to see a universe filled with technological civilizations. Each ‘hard step’ is unlikely in itself, and our existence depends upon our planet’s having achieved all of them. Carter was motivated by the timing of our emergence, which we can round off at 4.6 billion years after the formation of our planet. He reasoned that the upper limit for habitability at Earth’s surface is on the order of 5.6 billion...

Pandora: Exoplanet Atmospheres via Smallsat

by Paul Gilster | Feb 19, 2025 | Exoplanetary Science | 4 Comments

I’ve been digging into NASA’s Small Spacecraft Strategic Plan out of continuing interest in missions that take advantage of miniaturization to do things once consigned to large-scale craft. And I was intrigued to learn about the small spacecraft deployed on Apollo 15 and 16, two units developed by TRW in a series called Particles and Fields Subsatellites. Each weighed 35 kilograms and was powered by six solar panels and rechargeable batteries. The midget satellites were deployed from the Apollo Command and Service Module via a spring-loaded container giving the units a four foot-per-second velocity. Apollo 15’s operated for six months before an electronics failure ended the venture. The Apollo 16 subsatellite crashed on the lunar surface 34 days into its mission after completing 424 orbits. Here I thought I knew Apollo history backwards and forwards and I had never run into anything about these craft. It turns out that smallsats – usually cited as spacecraft with weight up to 180...

A Three-Dimensional Look at an Exoplanet Atmosphere

by Paul Gilster | Feb 18, 2025 | Exoplanetary Science | 3 Comments

Some 900 light years away in the constellation Puppis, the planet WASP-121b is proving an interesting test case as we probe ever deeper into exoplanetary atmospheres. As has been the case with so many early atmosphere studies, WASP-121b, also known as Tylos, is a hot-Jupiter, with a year lasting about thirty Earth hours, in a vise-like tidal lock that leaves one side always facing the star, the other away. What we gain in two new studies of this world is an unprecedented map of the atmosphere’s structure. At stake here is a 3D look into what goes on as differing air flows move from one side of the planet to the other. A jet stream moves material around its equator, but there is a separate flow at lower altitudes that pumps gas from the hottest regions to the dark side. “This kind of climate has never been seen before on any planet,” says Julia Victoria Seidel (European Southern Observatory), lead author of a paper that appears today in Nature. Seidel points out that we have nothing...

What Would Surprise You?

by Paul Gilster | Feb 14, 2025 | Exoplanetary Science | 37 Comments

Someone asked me the other day what it would take to surprise me. In other words, given the deluge of data coming in from all kinds of observatories, what one bit of news would set me back on my heels? That took some reflection. Would it surprise me, my interlocutor persisted, if SETI fails to find another civilization in my lifetime? The answer to that is no, because I approach SETI without expectations. My guess is that intelligence in the universe is rare, but it’s only a hunch. How could it be anything else? So no, continuing silence via SETI does not surprise me. And while a confirmed signal would be fascinating news, I can’t say it would truly surprise me either. I can work out scenarios where civilizations much older than ours do become known. Some surprises, of course, are bigger than others. Volcanoes on Io were a surprise back in the Voyager days, and geysers on Enceladus were not exactly expected, but I’m talking here about an all but metaphysical surprise. And I think I...

Pondering Life in an Alien Ocean

by Paul Gilster | Feb 11, 2025 | Outer Solar System | 18 Comments

No one ever said Europa Clipper would be able to detect life beneath the ice, but as we look at the first imagery from the spacecraft’s star-tracking cameras, it’s helpful to keep the scope of the mission in mind. We’re after some critical information here, such as the thickness of the ice shell, the interactions between shell and underlying ocean, the composition of that ocean. All of these should give us a better idea of whether this tiny world really can be a home for life. Image: This mosaic of a star field was made from three images captured Dec. 4, 2024, by star tracker cameras aboard NASA's Europa Clipper spacecraft. The pair of star trackers (formally known as the stellar reference units) captured and transmitted Europa Clipper's first imagery of space. The picture, composed of three shots, shows tiny pinpricks of light from stars 150 to 300 light-years away. The starfield represents only about 0.1% of the full sky around the spacecraft, but by mapping the stars in just that...

Putting AI to Work on Technosignatures

by Paul Gilster | Feb 7, 2025 | Astrobiology and SETI | 8 Comments

As a quick follow-up to yesterday’s article on quantifying technosignature data, I want to mention the SETI Institute’s invitation for applicants to the Davie Postdoctoral Fellowship in Artificial Intelligence for Astronomy. The Institute’s Vishal Gajjar and his collaborators both in the US and at IIT Tirupati in India will be working with the chosen candidate to focus on neural networks optimized for processing image data, so-called ‘CNN architectures’ that can uncover unusual signals in massive datasets. “Machine learning is transforming the way we search for exoplanets, allowing us to uncover hidden patterns in vast datasets,” says Gajjar. “This fellowship will accelerate the development of advanced AI tools to detect not just conventional planets, but also exotic and unconventional transit signatures including potential technosignatures.” As AI matures, the exploration of datasets is a critical matter as these results from missions like TESS and Kepler are packed with both...

Quantifying the Chances of a Technosignature

by Paul Gilster | Feb 5, 2025 | Astrobiology and SETI | 16 Comments

It’s one thing to talk about technology as we humans know it, but applying it to hypothetical extraterrestrials is another matter. We have to paint with a broad brush here. Thus Jason Wright’s explanation of technosignatures as conceived by SETI scientists. The Penn State astronomer and astrophysicist defines technology in that context as “the physical manifestations of deliberate engineering.” That’s saying that a technology produces something that is in principle observable. Whether or not our current detection methods are adequate to the task is another matter. Image: Artist’s concept of an interesting radio signal from galactic center. But the spectrum of possible technosignature detections is broad indeed, extending far beyond radio. Credit: UCLA SETI Group/Yuri Beletsky, Carnegie Las Campanas Observatory. A technosignature need not be the sign of industrial or scientific activity. Consider: In a new paper in The Astronomical Journal, Sofia Sheikh (SETI Institute) and colleagues...

A Fast Radio Burst in a Dead Elliptical Galaxy

by Paul Gilster | Feb 3, 2025 | Deep Sky Astronomy & Telescopes | 3 Comments

Work is healing, so let’s get back to it. I’m enthralled with what we’re discovering as we steadily build our catalog of fast radio bursts (FRB), close to 100 of which have now been associated with a galaxy. These are transient radio pulses of short duration (down to a fraction of a millisecond, though some last several seconds), the first being found in 2007 by Duncan Lorimer, an astronomer at West Virginia University. Sometimes FRBs repeat, although many do not, and one is known to repeat on a regular basis. What kind of astrophysical processes might be driving such a phenomenon? The leading candidate appears to be supernovae in a state of core collapse, producing vast amounts of energy as stars more massive than the Sun end their lives. Out of such catastrophic events a type of neutron star called a magnetar may be produced, its powerful magnetic field pumping out X-ray and gamma ray radiation. Young, massive stars and regions of active star formation are implicated under this...

Centauri Dreams to Resume Soon

by Paul Gilster | Jan 27, 2025 | Administrative | 45 Comments

I'd like to thank all of you who wrote comments and emails about the recent pause in Centauri Dreams. My beautiful wife Eloise passed away on January 17. It was as peaceful a death as can be imagined, and I am so pleased to say that she was able to stay at home until the end. As she had battled Alzheimer's for eleven gallant years, death was simply a bridge that now had to be crossed. As she did with everything else in her life, she did it with class. This is to let you know that I will be getting Centauri Dreams back into action again in about three weeks. When I began the site in 2004, my primary goal was to teach myself as much as I could about the topics we address by writing about them, which is how I've always tended to learn things. I've always welcomed comments that informed me, caught my errors and extended the discussion into new realms. No one could work with a better audience than the readers I've been privileged to address, and for this I am profoundly grateful.

A Necessary Break

by Paul Gilster | Jan 16, 2025 | Uncategorized | 71 Comments

It's time to write a post I've been dreading to write for several years now. Some of my readers already know that my wife has been ill with Alzheimer's for eleven years, and I've kept her at home and have been her caregiver all the way. We are now in the final stages, it appears, and her story is about to end. I will need to give her all my caring and attention through this process, as I'm sure you'll understand. And while I have no intention of shutting down Centauri Dreams, I do have to pause now to devote everything I have to her. Please bear with me and with a bit of time and healing, I will be active once again.

Recalibrating ‘Hot Jupiter’ Migration

by Paul Gilster | Jan 15, 2025 | Exoplanetary Science | 10 Comments

What catches your eye in this description of an exoplanetary system? Start with a ‘hot Jupiter,’ with a radius 0.87 times that of our Jupiter and an orbit of 7.1 days. This is WASP-132b, confirmed in 2016, and first discovered through the labors of the Wide-Angle Search for Planets program. Subsequent confirmation came through the CORALIE spectrograph installed on the Euler telescope at the European Southern Observatory’s La Silla site. This world orbits a K-class star 403 light years out in Lupus. The CORALIE measurements gave hints of another giant planet in a long period orbit. The system came still further into focus in 2021, when observations from TESS (Transiting Exoplanet Survey Satellite) showed a transiting super-Earth with a diameter of 1.8 Earth radii in a tight orbit of 1.01 days. The mass of the planet, as measured by the HARPS spectrograph at La Silla, is six times that of Earth. So we have both a hot-Jupiter and a super-Earth hugging the star, along with an outer gas...

Planet Population around Orange Dwarfs

by Paul Gilster | Jan 14, 2025 | Exoplanetary Science | 10 Comments

Last Friday’s post on K-dwarfs as home to what researchers have taken to calling ‘superhabitable’ worlds has caught the eye of Dave Moore, a long-time Centauri Dreams correspondent and author. Readers will recall his deep dives into habitability concepts in such essays as The “Habitability" of Worlds and Super Earths/Hycean Worlds, not to mention his work on SETI (see If Loud Aliens Explain Human Earliness, Quiet Aliens Are Also Rare). Dave sent this in as a comment but I asked him to post it at the top because it so directly addresses the topic of habitability prospects around K-dwarfs, based on a quick survey of known planetary systems. It's a back of the envelope overview, but one that implies habitable planets around stars like these may be more difficult to find than we think. by Dave Moore To see whether K dwarfs made a good target for habitable planets, I decided to look into the prevalence and type of planets around K dwarfs and got carried away looking at the specs for 500...

Superhabitability around K-class Stars

by Paul Gilster | Jan 10, 2025 | Exoplanetary Science | 27 Comments

We think of Earth as our standard for habitability, and thus the goal of finding an ‘Earth 2.0’ is to identify living worlds like ours orbiting similar Sun-like stars. But maybe Earth isn’t the best standard. Are there ways planets can be more habitable than our own, and if so where would we find them? That’s the tantalizing question posed in a paper by Iva Vilović (Technische Universität Berlin), René Heller (Max-Planck-Institut für Sonnensystemforschung) and colleagues in Germany and India. Heller has previously worked this issue in a significant paper with John Armstrong (citation below); see as well The Best of All Possible Worlds, which ran here in 2020. The term for the kind of world we are looking for is ‘superhabitable,’ and the aim of this study is to extend the discussion of K-class stars as hosts by modeling the atmospheres we may find on planets there. While much attention has focused on M-class red dwarfs, the high degree of flare activity coupled with long pre-main...

A ‘Manhole Cover’ Beyond the Solar System?

by Paul Gilster | Jan 7, 2025 | Culture and Society | 28 Comments

Let’s start the year with a look back in time to 1957, a time when nuclear bombs were being tested underground for the first time at the Nevada test site some 105 kilometers northwest of Las Vegas. If this seems an unusual place to launch a discussion on interstellar matters, consider the story of an object that some argue became the fastest manmade artifact in history, an object moving so fast that it would have passed the orbit of Pluto four years after ‘launch,’ in the days of Yuri Gagarin and Project Mercury. I’m bringing it up because the tale of the nuclear test known as Plumbob Pascal B is again active on the Internet, and it’s a rousing tale. Operation Plumbob involved a series of 29 nuclear tests that fed the development of missile warheads both intercontinental and intermediate. The history of such underground nuclear testing would make for an interesting book and indeed it has, in the form of Caging the Dragon (Defense Nuclear Agency, 1995), by one James Carothers. But...

Interstellar Reprise: Voyager to a Star

by Paul Gilster | Dec 24, 2024 | Culture and Society | 53 Comments

As I write, Voyager 1 is almost 166 AU from the Sun, moving at 17 kilometers per second. With its Voyager 2 counterpart, the mission represents the first spacecraft to operate in interstellar space, continuing to send data with the help of skilled juggling of onboard systems not deemed essential. Despite communications glitches, the mission continues, and it seems a good time to reprise a piece I wrote on the future of these doughty explorers back in 2015. Is there still time to do something new with the two probes once the demise of their plutonium power sources makes further communications impossible? The idea is hardly mine, and goes back to the Sagan era, as the article below explains. It's also a notion that is purely symbolic, and for those immune to symbolism (the more practical-minded among us) it may seem trivial. But those with a poet's eye may see the value of an act that can offer a futuristic finish to a mission that passed all expectations and will inspire generations...

An Oddity in the Small Magellanic Cloud

by Paul Gilster | Dec 20, 2024 | Deep Sky Astronomy & Telescopes | 3 Comments

Let’s make a quick return to the Magellanics after our recent look at WOH G64, a dying star imaged in the Large Magellanic Cloud (see Close-up of an Extragalactic Star). These satellite galaxies of the Milky Way have long proven useful in helping astronomers study the gravitational interactions that shape them, leading to further understanding of galactic structure. But today I want to focus on the star-forming cluster NGC 346, which presents us with something of a conundrum. Located in the Small Magellanic Cloud some 200,000 light years away, the cluster is massive and particularly lacking in the heavier elements beyond hydrogen and helium. Intensively studied by the Hubble Space Telescope in the mid-2000s, it has become a proxy for much more distant galaxies in the ancient universe, where metals were harder to find. Why, then, did the Hubble data show that while stars in NGC 346 were between 20 and 30 million years old, they were accompanied by planet-forming disks? A new study...

Charting the Diaspora: Human Migration Outward

by Paul Gilster | Dec 18, 2024 | Culture and Society | 40 Comments

It’s not often that I highlight the work of anthropologists on Centauri Dreams. But it’s telling that the need to do that is increasing as we continue to populate the Solar System with human artifacts, which are after all the province of this discipline. I’ve often wondered about the fate of the Apollo landing sites, originally propelled to do so by Steven Howe’s novel Honor Bound Honor Born and conversations with the author ranging from lunar settlement to antimatter. Long affiliated with Los Alamos National Laboratory, Howe is deeply involved in antimatter research through his work as CEO and co-founder of Hbar Technologies. In my conversations with Howe almost twenty years ago while writing my original Centauri Dreams book, he was asking what would happen if commercial interests decided to exploit historical sites from the early days of space exploration. The question is still pertinent. Imagine Armstrong and Aldrin’s Eagle subjected to near-future tourists prying off souvenirs or...

Redefining the Galactic Habitable Zone

by Paul Gilster | Dec 11, 2024 | Astrobiology and SETI | 21 Comments

To understand the Solar System’s past and to tighten our parameters for SETI searches, we need to consider habitability not only as a planetary and stellar phenomenon but a galactic one as well. The Milky Way is a highly differentiated place, its core jammed with older stars and Sagittarius A*, which is almost certainly a supermassive black hole. The gorgeous spiral arms spawn new stars while the globular clusters in the halo house ancient clusters. Where in all this is life most likely to form? And perhaps more to the point, in what ways do stars and their associated planets migrate in the galactic disk? Our Sun raises the issue by virtue of the fact that its metallicity, as measured by the ratio of iron to hydrogen (Fe/H), is higher than nearby stars that are of a similar age. In a new paper from Junichi Baba (Kagoshima University) and colleagues at the National Observatory of Japan and Kobe University, the authors offer this as evidence that the Solar System formed closer to the...

« Older Entries

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).

Now Reading

On Comments

If you'd like to submit a comment for possible publication on Centauri Dreams, I will be glad to consider it. The primary criterion is that comments contribute meaningfully to the debate. Among other criteria for selection: Comments must be on topic, directly related to the post in question, must use appropriate language, and must not be abusive to others. Civility counts. In addition, a valid email address is required for a comment to be considered. Centauri Dreams is emphatically not a soapbox for political or religious views submitted by individuals or organizations. A long form of the policy can be viewed on the Administrative page. The short form is this: If your comment is not on topic and respectful to others, I'm probably not going to run it.

Follow by E-Mail

Advanced Propulsion Research