by Paul Gilster | Oct 17, 2016 | Antimatter |
An antimatter probe to a nearby star? The idea holds enormous appeal, given the colossal energies obtained when normal matter annihilates in contact with its antimatter equivalent. But as we’ve seen through the years on Centauri Dreams, such energies are all but impossible to engineer. Antimatter production is infinitesimal, the by-product of accelerators designed with a much different agenda. Moreover, antimatter storage is hellishly difficult, so that maintaining large quantities in a stable condition requires multiple breakthroughs.
All of which is why I became interested in the work Gerald Jackson and Steve Howe were doing at Hbar Technologies. Howe, in fact, became a key source when I put together the original book from which this site grew. This was back in 2002-2003, and I was captivated with the idea of what could be called an ‘antimatter sail.’ The idea, now part of a new Kickstarter campaign being launched by Jackson and Howe, is to work with mere milligrams of antimatter, allowing antiprotons to be released from the spacecraft into a uranium-enriched, five-meter sail.
Reacting with the uranium, the antimatter produces fission fragments that create what could be considered a nuclear-stimulated ablation blowing off the carbon-fiber sail. As to the reaction itself, Jackson and Howe would use a sheet of depleted uranium U-238 with a carbon coating on its back side. Here’s how the result is described in the Kickstarter material now online:
When antiprotons… drift onto the front surface, their negative electrical charge allows them to act like an orbiting electron, but with different quantum numbers that allow the antiprotons to cascade down into the ground orbital state. At this point it annihilates with a proton or neutron in the nucleus. This annihilation event causes the depleted uranium nucleus to fission with a probability approaching 100%, most of the time yielding two back-to-back fission daughters.
Now we get into a serious kick for the spacecraft:
A fission daughter travelling away from the sail at a kinetic energy of 1 MeV/amu has a speed of approximately 13,800 km/sec, or 4.6% of the speed of light. The other fission daughter is absorbed by the sail, depositing its momentum into the sail and causing the sail (and the rest of the ship) to accelerate.
The concept relies, as Jackson said in a recent email, on using antimatter as a spark plug rather than as a fuel, converting the energy from proton-antiproton annihilations into propulsion.
Image: The original antimatter probe concept. Credit: Gerald Jackson/Hbar Technologies.
The current work grows out of a 2002 grant from NASA’s Institute for Advanced Concepts but the plan is to develop the idea far beyond the Kuiper Belt mission Jackson and Howe initially envisioned. Going interstellar would take not milligrams but tens of grams of antimatter, far beyond today’s infinitesimal production levels. In fact, while the Fermi National Accelerator laboratory has been able to produce no more than 2 nanograms of antimatter per year, even that is high compared to CERN’s output (the only current source), which is 100 times smaller.
Even so, interest in antimatter remains high because of its specific energy — two orders of magnitude larger than fusion and ten orders of magnitude larger than chemical reactions — making further research highly desirable. If the fission reaction the antimatter produces within the sail is viable, we will be able to demonstrate a way to harness those energies, with implications for deep space exploration and the possibility of interstellar journeys.
The original NIAC work led to a sail 5-meters in diameter, with a 15-micron thick carbon layer and a uranium coating 293 microns thick. Interestingly, the study showed that the sail had sufficient area to remove any need for active cooling of the surface. Indeed, the steady-state temperature of the sail would be 570? Celsius, below the melting point of uranium.
Image: A cloud of anti-hydrogen drifts towards the uranium-infused sail. CREDIT: Hbar Technologies, LLC/Elizabeth Lagana.
The work was based around a 10 kg instrument payload to be delivered to 250 AU within 10 years. Turning to interstellar possibilities, Breakthrough Starshot has been talking about reaching 20 percent of lightspeed with a beamed laser array pushing small sails. Jackson and Howe now seek roughly 5 percent of c, making for a mission of less than a century to reach Proxima Centauri, where we already know an interesting planet awaits.
But here’s a significant difference: Unlike Breakthrough Starshot’s flyby assumptions, the antimatter sail mission concept is built around decelerating and attaining orbit around the target star. In the absence of magsail braking against Proxima’s stellar wind, this would presumably also involve antimatter, braking with the same methods to allow for long-term scientific investigation, thus avoiding the observational challenges of a probe pushing past a small and probably tidally-locked planet at 20 percent of lightspeed.
Here’s how Jackson describes deceleration in his recent email:
Our project considers deceleration and orbit about the destination star a mission requirement. There are serious implications for spacecraft velocity when the requirement of deceleration at the destination is imposed. Either drag or some other mechanism needs to be invoked at the destination, or enough extra fuel must be accelerated in order to accomplish a comparable deceleration. Because the rocket equation equates probe velocity with mass utilization, a staged spacecraft architecture is envisioned wherein a more massive booster accelerates the spacecraft and a smaller second stage decelerates into the destination solar system.
The discovery of Proxima b, that interesting planet evidently in the habitable zone around the nearest star, continues to energize the interstellar community. The Kickstarter campaign, just underway and with a goal of $200,000, hopes to upgrade earlier antimatter sail ideas into the interstellar realm. Tomorrow I want to say a few more things about the antimatter sail and the issues the Kickstarter campaign will address as it expands the original work.
by Paul Gilster | Oct 14, 2016 | Deep Sky Astronomy & Telescopes |
I had thought while the power was out this past week that I would like to write about cosmology when it came back. That’s because there’s nothing like a prolonged power outage to adjust your perspective. The big picture beckons. In my case, it was thinking about how trivial being out of power was compared to those who had lost so much more in the wake of the recent hurricane.
So thinking about the cosmos became my shelter from the storm. I appreciated the emails from so many of you, but aside from a major chunk of tree that landed on the roof, we did just fine. In fact, it was deeply moving to see people from the neighborhood — some I knew, some I only recognized — turn up to get up on the roof and move that tree. I’m always reminded to do more for the people around me when I see something like this, and apprehensive that my resolution to do so all too often gets put aside as normal life returns.
The Universe We Can See
Reading by candlelight really is wonderful, and I ask myself why I don’t do it more often. There is something about that soft, flickering light on a well-printed page. And I found reading about cosmology by candlelight was especially pleasing, a way of connecting to a past way of life that had its own conceptions about the cosmos. One thing I read was my notes from work just received when the power went, a paper on galactic structure I fortunately printed just in time.
The conclusion of this one is straightforward: There are at least 10 times as many galaxies in the observable universe as previously thought. Christopher Conselice (University of Nottingham, UK) led an international team that produced this result. I was reading about it in a room full of candles and thinking about room after room of candles stretching out into infinity.
That homely thought weds the prosaic and the vast, sort of the way Douglas Adams did in his famous line on the size of the universe in The Hitchhiker’s Guide to the Galaxy:
“Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.”
I’m sure you’re familiar with the line, but it still raises a chuckle. And this team’s findings cause yet another shift in perspective. Thus Conselice:
“It boggles the mind that over 90% of the galaxies in the Universe have yet to be studied. Who knows what interesting properties we will find when we observe these galaxies with the next generation of telescopes.”
Image: The GOODS South Field (Great Observatories Origins Deep Survey). GOODS draws on observations from the Spitzer, Hubble and Chandra observatories as well as the European Space Agency’s Herschel and XMM-Newton, along with ground-based facilities, to survey the distant universe across the electromagnetic spectrum. Credit: GOODS/Conselice et al.
Before this, we relied on such measures as the Hubble Deep Field images from the 1990s, which helped us arrive at an estimate of between 100 and 200 billion galaxies in the observable universe. Conselice and team studied the matter using Hubble imagery as well as other published data to create a 3D view, estimating the number of galaxies at various times in the history of the cosmos. Their mathematical models pointed to the result, that we have not been able to see most of the galaxies that are out there in our cosmological horizon.
Moreover, the work offers an interesting perspective on galaxies throughout time, one showing how their numbers have changed as the universe evolved. The paper explains that the total number density of galaxies in the universe declines with time:
[The] total number density… declines by a factor of 10 within the first 2 Gyr of the universe’s history, and a further reduction at later times. This decline may further level off between redshifts of z = 1 and z = 2. The star formation rate during this time is also very high for all galaxies, which should in principle bring galaxies which were below our stellar mass limit into our sample at later times. This would naturally increase the number of galaxies over time, but we see the opposite. This is likely due to merging and/or accretion of galaxies when they fall into clusters which are later destroyed through tidal effects, as no other method can reduce the number of galaxies above a given mass threshold.
So what is known as the ‘top-down formation of structure’ in the universe is supported by this work. Most of the galaxies in the first few billion years of the universe were similar in mass to the satellite galaxies we see surrounding the Milky Way. Galactic mergers reduce the total number. And although Conselice and colleagues show us that there are so many galaxies that almost every point in the sky contains part of one, most of these galaxies are invisible not just to the human eye but to our best telescopes.
If that rings a bell, it’s because we’re homing in on Olbers’ paradox, the idea that the darkness of the night sky is in contradiction to the assumption of an infinite universe filled with stars (based on the work of the German astronomer Heinrich Wilhelm Matthias Olbers, 1758-1840). Why is the night sky as dark as it is? I won’t get into the details on dust and gas absorption of light that the paper offers, but will simply quote the finding from the Conselice paper:
It would… appear that the solution to the strict interpretation of Olbers’ paradox, as an optical light detection problem, is a combination of nearly all possible solutions – redshifting effects, the finite age and size of the universe, and through absorption.
The paper is Conselice et al., “The Evolution of Galaxy Number Density at z < 8 and its Implications,” to be published in the Astrophysical Journal. Preprint here.
by Paul Gilster | Oct 11, 2016 | Administrative |
Hurricane Matthew’s effects continue to be felt in the form of flooding, power outages and downed trees. I’m now told not to expect power for 4-6 days. The situation obviously impacts my ability to post here. I’ll try to keep up with comment moderation when possible. Will get things back to normal whenever the lights come back on.
by Paul Gilster | Oct 7, 2016 | Uncategorized |
Have a look at the spiral of pinwheeling dust that can be seen around the young star Elias 2-27. We’re looking at gravitational perturbations in a protoplanetary disk that, as this National Radio Astronomy Observatory news release says, mimic the vast arms we expect in a spiral galaxy. But here we’re looking at a process with implications for planet formation, one that draws on data from the Atacama Large Millimeter/submillimeter Array (ALMA). This is the first time a spiral density wave has been detected in a protoplanetary disk’s planet formation areas.
Image: ALMA peered into the Ophiuchus star-forming region to study the protoplanetary disk around the young star Elias 2-27. Astronomers discovered a striking spiral pattern in the disk. This feature is the product of density waves – gravitational perturbations in the disk. Credit: L. Pérez (MPIfR), B. Saxton (NRAO/AUI/NSF), ALMA (ESO/NAOJ/NRAO), NASA/JPL Caltech/WISE Team.
Some 450 light years from Earth in the Ophiuchus star-forming region, Elias 2-27 is about half the mass of the Sun, though its protoplanetary disk is massive. Although the young star (about a million years old, according to current estimates) is shrouded by the molecular cloud from which it grew, ALMA was able to peer into the mid-plane of the disk to identify the spiral density waves. The spiral arms extend as much as 10 billion kilometers away from the host star.
All this catches the eye because while we can account for star formation from the collapse of gas and dust under the influence of gravity, we need a mechanism to keep enough material from falling into the protostar to ensure that it doesn’t spin up enough to shred itself. The protostellar disk projects angular momentum outward, and is where we can expect planets to form. But standard core accretion models have problems explaining the formation of planets 20 to 30 AU out, where the disk may not be dense enough to allow the process to be efficient.
Gravitational instabilities in the outer disk, however, can produce the kind of dense spiral arms we see here, with new material being pushed out into regions far from the star and collapsing under its own gravity to begin planet formation. Andrea Isella (Rice University) explains:
“We don’t completely understand how planets form, but we suspect there are two ways: Either small particles stick together until they form something like the Earth or Mars, or accreting gas forms a planet like Saturn or Jupiter. But this process works only very close to the star, within a few astronomical units (roughly the distance from the Sun to the Earth), because that’s where all the material is, and it has to have enough density… If a disk is massive enough to be gravitationally unstable, a spiral will form naturally.”
Thus the spiral arms of Elias 2-27 may be the manifestation of an instability that gives birth to a particular kind of exoplanet. Near to the star, the ALMA observations found a flattened dust disk that extends out beyond 30 AU, followed by a narrow band of sharply diminished dust that may indicate a planet in formation. The spiral arms extend outward from the edge of this gap in the disk. Lead author Laura Pérez (Max Planck Institute for Radio Astronomy) notes that an upcoming program will use ALMA data to home in on similar protoplanetary disks as we try to find out whether Elias 2-27’s spiral density waves actually do reveal planet(s) in formation.
The paper is Pérez et al., “Spiral density waves in a young protoplanetary disk,” Science Vol. 353, Issue 6307 (30 September 2016), pp. 1519-1521 (abstract). A Rice University news release is also available.
by Paul Gilster | Oct 6, 2016 | Exoplanetary Science |
Spotting planets a long way from their stars is no easy proposition when you’re using radial velocity methods. The idea is to track the minute movement of the star as it is affected by an orbiting planet, which shows up as a Doppler shift in the data. What we’re actually seeing is the star and planet orbiting the center of gravity, an indirect method of detection that observes not the planet itself but the effects of the planet as it produces this variation in radial velocity.
The first exoplanets were detected this way, and the method has continued to produce new discoveries. But as a planet’s distance from its star increases, radial velocity becomes tricky to use. Now observation times become extended as the planet completes its longer orbit. We face the same issue with the transit method, which charts the drop in brightness as a planet moves across the face of its star as seen from Earth. Here, too, planets in distant orbits around their star are hard to detect because of the lengthy period of time between individual transits.
Because of these issues, we have little data on the occurrence rate of planets in wide orbits, and that is a problem for analyzing planet formation theories like core accretion, gravitational disk instability and planet migration. The long-term radial velocity data of a host star with a companion object beyond 10 AU shows an almost linear trend over a short observing period. Detection of such a trend is not in itself enough to identify the source of the RV signal.
Image: Periodic change in the radial velocity of the primary star shown left is the strong signature of a planet. When the companion object is far way (farther than 10 AU), the orbital period is so long that the radial velocity shows a linear change, as shown in the right panel. (Credit: NAOJ).
Researchers from the Tokyo Institute of Technology have been trying to solve this problem by combining radial velocity methods with direct imaging. With the latter, the planet detection actually becomes easier as the distance between star and planet widens, because the light of the companion object is not swamped by proximity to the host star. The Tokyo team, using an instrument at Okayama Astrophysical Observatory (NAOJ) has targeted stars 1.5 to 5 times as massive as the Sun, looking not just for long period planets but also companion stars.
A long-term radial velocity trend flags such objects, but it’s the follow-up with direct imaging (in this case using the HiCIAO imager at the Subaru Telescope) that has paid off in an analysis of six intermediate-mass giant stars. All six of these stars showed a long-term radial velocity trend. The question thus becomes, is the source of the trend a companion star or a planet?
HiCIAO is a coronagraph that masks the light of the primary star to allow detection of fainter objects. Three of the observed stars — ? Hydra, HD 5608, and HS 109272 — show companion stars in this analysis, while around three others — ? Draconis, 18 Delphinus, and HD 14067 — companion objects more massive than 60 Jupiter masses can be excluded. The latter are considered candidates for hosting brown dwarfs, while 18 Delphinus is the most likely prospect for hosting a high-mass planet (~ 10-50 AU) that is below the current detection limit.
Image: Objects in the yellow circle were detected by the current study. White circles or round squares show the position of the primary star. The primary was masked during the observation to help the detection of the fainter objects nearby. (Credit: NAOJ)
Combining radial velocity with direct imaging has allowed the team to confirm that the companion objects they found were the cause of the long-term trend in the RV data. On a broader level, these methods may help us understand the distribution of planets around stars more massive than the Sun, as the paper on this work notes:
At Okayama Astrophysical Observatory (OAO), an RV survey targeting intermediate-mass giants (1.5–5 M?) has been conducted for over a decade (e.g., Sato et al. 2003). Sato et al. (2008) found that there is a difference between orbits of planets around intermediate-mass stars and around lower-mass FGK stars. Most planets around intermediate-mass stars have a semi-major axis larger than 0.6 AU, while FGK stars have shorter-period planets. Hence, it was suggested that the orbital distribution of exoplanets around intermediate-mass stars is different from that around solar-type stars.
Is the suggestion valid, or simply the result of our limited datasets? The paper continues:
In addition, the OAO survey detected long-term RV trends in several targets, which indicates the presence of distant companions around them… Identifying the companions that generate the RV trend can improve our knowledge of exoplanet populations for intermediate-mass stars, which are not well understood compared to solar-type stars.
This is not the only study that has combined direct imaging and radial velocity trends. In fact, a project called TRENDS (TaRgetting bENchmark objects with Doppler Spectroscopy), led by Justin Crepp (Notre Dame) has detected three low-mass stellar companions using these methods, along with a white dwarf and a brown dwarf companion, working with data on host stars ranging from F-class down to M-dwarfs. Clearly, direct imaging can be a benefit as we try to work out the source of RV trends that point to the existence of a distant companion.
The paper is Ryu et al., “High-contrast Imaging of Intermediate-mass Giants with Long-term Radial Velocity Trends,” Astrophysical Journal Vol. 825, No. 2 (12 July 2016). Abstract / preprint.
