Centauri Dreams
Imagining and Planning Interstellar Exploration
Spacecraft and Their Messages
Just over 8300 people have now signed the petition supporting the New Horizons Message Initiative. The approach of the 10,000 figure reminds me to jog those who haven’t to stop by the site to sign the petition. For those not yet aware of the NHMI, the idea is to upload a crowdsourced package of images and data to the New Horizons spacecraft once it has completed its science mission at Pluto/Charon and any Kuiper Belt Object within range.
Jon Lomberg’s team calls the NHMI a ‘Voyager Golden Record 2.0,’ a worthy goal indeed, and I’ll also mention that the names of the first 10,000 signing the petition will be uploaded along with the images and data. For me, one of the most interesting aspects of the initiative will be to see how the crowdsourcing project works to determine both the form and the content of the message. New Horizons’ principal investigator Alan Stern has signed off on the idea, saying “I think it will inspire and engage people to think about SETI and New Horizons in new ways.”
While we work on developing this self-portrait of our species, it’s interesting to see the new ‘Messages to Bennu!’ campaign that’s developing through the OSIRIS-REx mission, in conjunction with The Planetary Society. OSIRIS-REx stands for — get ready for it — Origins-Spectral Interpretation Resource Identification Security Regolith Explorer. It’s a robotic mission, to be launched in 2016, that will spend more than two years at Bennu, a 500 meter carbonaceous asteroid. A surface sample will then be returned to Earth in 2023.
Image: When the OSIRIS-REx asteroid arrives at asteroid Bennu, it will study the asteroid from a distance before swooping down and grabbing a sample. On board the spacecraft will be the names of everybody participating in the “Messages to Bennu!” campaign. Credit: NASA/GSFC/UA.
The ‘messaging’ side of the mission involves putting a microchip with the names of people who have submitted them to The Planetary Society aboard the vehicle. You can sign up to have your name included here. Planetary Society CEO Bill Nye sees the mission of his organization as being ‘to engage the citizens of Earth in space exploration,’ an ongoing campaign that ‘Messages to Bennu!’ incorporates. We can hope that efforts like OSIRIS-REx and the New Horizons Message Initiative help to reawaken an all too lethargic public involvement with space.
The OSIRIS-REx countdown clock actually started on December 9, 2013, looking 999 days ahead to a launch in September of 2016. Principal investigator Dante Lauretta (University of Arizona) clearly likes the mission’s acronym, saying in a UA news release:
“Osiris was formed from pieces scattered across ancient Egypt, where he awoke as the bringer of life and ruler of the underworld. Our spacecraft has a similar story — it will be consist of components fabricated in locations around the world, that once together, will allow us to connect with a near-Earth object that is an accessible remnant from the formation of our solar system.”
As to Bennu, the target asteroid, it is a near-Earth object whose orbit is completed every 436 days, bringing it close to the Earth every six years. The object is considered a B-type asteroid, a subgrouping of the dark, carbonaceous C-type asteroids. These objects are useful for study because they have undergone little processing since the time of their formation. In addition to in situ studies and the sample return, OSIRIS-REx will also help us refine Bennu’s orbit by studying the Yarkovsky effect — the thermal force on the object — constraining the specific properties of the asteroid that make this effect a factor in its future trajectory. That’s useful information to have as we study near-Earth objects and potentially Earth-crossing orbits.
‘Cluster Planets’: What They Tell Us
2500 light years from Earth in the constellation of Cancer lies Messier 67, an open star cluster that is now known to be home to at least three planets. The new worlds, found using the HARPS spectrograph on the European Southern Observatory’s 3.6-meter instrument at La Silla, come as the result of an observation program covering 88 selected stars in the cluster over a period of six years. The finding is noteworthy because we have so few known planets in star clusters of any kind. Moreover, one of these planets orbits a truly Sun-like star.
Image: This wide-field image of the sky around the old open star cluster Messier 67 was created from images forming part of the Digitized Sky Survey 2. The cluster appears as a rich grouping of stars at the centre of the picture. Credit: ESO/Digitized Sky Survey 2 / Acknowledgement: Davide De Martin.
I’m cautious about calling anything ‘Sun-like’ given how loosely that term has been used over the years, but ESO astronomers say the cluster star YBP1194 fits the bill: It has a similar mass, and shows both chemical abundances and temperatures very close to Sol’s. Of the three discovered worlds, two orbit G-class stars similar to the Sun (the other is YBP1514), while the third orbits the red giant S364. The first two have roughly one-third the mass of Jupiter, orbiting their host stars in seven and five days respectively, while the third, more massive than Jupiter, orbits the red giant in 122 days.
Because most stars are thought to emerge from clusters, the small number of planets found in them has been a puzzle, spurring the recent work, which was led by Anna Brucalassi (Max Planck Institute for Extraterrestrial Physics). Says Brucalassi:
“In the Messier 67 star cluster the stars are all about the same age and composition as the Sun. This makes it a perfect laboratory to study how many planets form in such a crowded environment, and whether they form mostly around more massive or less massive stars.”
Messier 67 contains about 500 stars and is an open cluster, a stellar grouping that has emerged from a single gas and dust cloud in the relatively recent past. Such clusters are normally found in the spiral arms of galaxies like ours. Globular clusters, on the other hand, are the much larger, spherical collections of stars that orbit around the center of the galaxy. Although a handful of planets have been found in open clusters (Messier 44 and NGC 6811 are other examples), no planets have yet turned up in the far more ancient globular clusters.
The lack of detected planets in open and globular clusters has been under discussion for some time now. From the paper:
To explain the dichotomy between field and cluster stars, it has been suggested that the cluster environment might have a significant impact on the disk-mass distribution. Eisner et al. (2008), studying disks around stars in the Orion Nebula Cluster (ONC), proposed that most of these stars do not possess sufficient mass in the disk to form Jupiter-mass planets or to support an eventual inward migration.
Brucalassi’s work, however, leads in a different direction. The paper continues:
van Saders & Gaudi (2011), in contrast, found no evidence in support of a fundamental difference in the short-period planet population between clusters and field stars, and attributed the non-detection of planets in transit surveys to the inadequate number of stars surveyed. This seems to be confirmed by the recent results.
Planets in open star clusters, in other words, are likely to be as common as those around isolated stars, a finding that draws not just from Brucalassi and team’s work but also from a number of recent observations discussed in the paper. The researchers continue to study M67 to examine the mass and chemical makeup of stars with and without planets.
The paper is Brucalassi et al., “Three planetary companions around M67 stars,” accepted for publication in Astronomy & Astrophysics. See also Pasquini et al., “Search for giant planets in M67 I. Overview” (preprint). And take note of Henry Cordova’s “The SETI Potential of Open Star Clusters,” which ran all the way back in 1995 in Vol. 1, No. 4 of SETIQuest, an early and prescient contribution.
Electric Sails: Fast Probe to Uranus
For years now Pekka Janhunen has been working on his concept of an electric sail with the same intensity that Claudio Maccone has brought to the gravitational focus mission called FOCAL. Both men are engaging advocates of their ideas, and having just had a good conversation with Dr. Maccone (by phone, unfortunately, as I’ve been down with the flu), I was pleased to see Dr. Janhunen’s electric sail pop up again in online discussions. It turns out that the physicist has been envisioning a sail mission to an unusual target.
Let’s talk a bit about the mission an electric sail enables. This is a solar wind-rider, taking advantage not of the momentum imparted by photons from the Sun but the stream of charged particles pushing from the Sun out to the heliopause (thereby blowing out the bubble’ in the interstellar medium we call the heliosphere). As Janhunen (Finnish Meteorological Institute) has designed it, the electric sail taps the Coulomb interaction in which particles are attracted or repulsed by an electric charge. The rotational motion of the spacecraft would allow the deployment of perhaps 100 tethers, thin wires that would be subsequently charged by an electron gun with the beam sent out along the spin axis.
Image: The electric sail is a space propulsion concept that uses the momentum of the solar wind to produce thrust. Credit: Alexandre Szames.
The electron gun keeps the spacecraft and tethers charged, with the electric field of the tethers extending tens of meters into the surrounding solar wind plasma — as the solar wind ‘blows,’ it pushes up against thin tethers that act, because of their charge, as wide surfaces against which the wind can push. The sail uses the attraction or repulsion of particles caused by the electric charge to ride the wind, the positively charged solar wind protons repelled by the positive voltage they meet in the charged tethers.
One disadvantage that electric sails bring to the mix, as opposed to solar sails like IKAROS, is that the solar wind is much weaker — Janhunen’s figures have it 5000 times weaker — than solar photon pressure at Earth’s distance from the Sun. This has come up before in comments here and it’s worth quoting Janhunen on the matter, from a site he maintains on electric sails:
The solar wind dynamic pressure varies but is on average about 2 nPa at Earth distance from the Sun… Due to the very large effective area and very low weight per unit length of a thin metal wire, the electric sail is still efficient, however. A 20-km long electric sail wire weighs only a few hundred grams and fits in a small reel, but when opened in space and connected to the spacecraft’s electron gun, it can produce several square kilometre effective solar wind sail area which is capable of extracting about 10 millinewton force from the solar wind.
Computer simulations using tethers up to 20 kilometers in length have yielded speeds of 100 kilometers per second, a nice step up from the 17 kps of Voyager 1, and enough to get a payload into the nearby interstellar medium in fifteen years. Or, as Janhunen describes in the recent paper on a Uranus atmospheric probe, an electric sail could reach the 7th planet in six years. Janhunen sees such a probe as equally applicable for a Titan mission and, indeed, missions to Neptune and Saturn itself, but notice that none of these are conceived as orbiter missions. A significant amount of chemical propellant is needed for orbital insertion unless we were to try aerocapture, but the problem with the latter is that it is at a much lower technical readiness level.
A demonstrator electric sail mission, then, is designed to keep costs down and reach its destination as fast as possible, with the interesting spin that, because we’re in need of no gravitational assists, the Uranus probe will have no launch window constraints. As defined in the paper on this work, the probe would consist of three modules stacked together: The electric sail module, a carrier module and an entry module. The entry module would be composed of the atmospheric probe and a heat-shield.
At approximately Saturn’s distance from the Sun, the electric sail module would be jettisoned and the carrier module used to adjust the trajectory as needed with small chemical thrusters (50 kg of propellant budgeted for here). And then the fun begins:
About 13 million km (8 days) before Uranus, the carrier module detaches itself from the entry module and makes a ~ 0.15 km/s transverse burn so that it passes by the planet at ~ 105 km distance, safely outside the ring system. Also a slowing down burn of the carrier module may be needed to optimise the link geometry during flyby.
Now events happen quickly. The entry module, protected by its heat shield, enters the atmosphere. A parachute is deployed and the heat shield drops away, with the probe now drifting down through the atmosphere of Uranus (think Huygens descending through Titan’s clouds), making measurements and transmitting data to the high gain antenna on the carrier module.
Thus we get atmospheric measurements of Uranus similar to what the Galileo probe was able to deliver at Jupiter, measuring the chemical and isotopic composition of the atmosphere. A successful mission builds the case for a series of such probes to Neptune, Saturn and Titan. Thus far Jupiter is the only giant planet whose atmosphere has been probed directly, and a second Jupiter probe using a similar instrument package would allow further useful comparisons. Our planet formation models, which predict chemical and isotope composition of the giant planet atmospheres, can thus be supplemented by in situ data.
Not to mention that we would learn much about flying and navigating an electric sail during the testing and implementation of the Uranus mission. The paper is Janhunen et al., “Fast E-sail Uranus entry probe mission,” submitted to the Meudon Uranus workshop (Sept 16-18, 2013) special issue of Planetary and Space Science (preprint).
Cloudy Encounter at the Core
The supermassive black hole at the center of our galaxy comes to Centauri Dreams‘ attention every now and then, most recently on Friday, when we talked about its role in creating hypervelocity stars. At least some of these stars that are moving at speeds above galactic escape velocity may have been flung outward when a binary pair approached the black hole too closely, with one star being captured by it while the other was given its boost toward the intergalactic deeps.
At a mass of some four million Suns, Sagittarius A* (pronounced ‘Sagittarius A-star’) is relatively quiet, but we can study it through its interactions. And if scientists at the University of Michigan are right, those interactions are about to get a lot more interesting. A gas cloud some three times the mass of the Earth, dubbed G2 when it was found by German astronomers in 2011, is moving toward the black hole, which is 25,000 light years away near the constellations of Sagittarius and Scorpius.
What’s so unusual about this is the time-frame. We’re used to thinking in million-year increments at least when discussing astronomical events, but G2 was expected to encounter Sagittarius A* late last year. The event hasn’t occurred yet but astronomers think it will be a matter of only a few months before it happens. Exactly what happens next isn’t clear, says Jon Miller (University of Michigan), who along with colleague Nathalie Degenaar has been making daily images of the gas cloud’s approach using NASA’s orbiting Swift telescope.
“I would be delighted if Sagittarius A* suddenly became 10,000 times brighter,” Miller adds. “However it is possible that it will not react much—like a horse that won’t drink when led to water. If Sagittarius A* consumes some of G2, we can learn about black holes accreting at low levels—sneaking midnight snacks. It is potentially a unique window into how most black holes in the present-day universe accrete.”
Image: The galactic center as imaged by the Swift X-ray Telescope. This image is a montage of all data obtained in the monitoring program from 2006-2013. Credit: Nathalie Degenaar.
We have much to learn about the feeding habits of black holes. The Milky Way’s black hole isn’t nearly as bright as those in some galaxies. While we can’t see black holes directly because no light can escape from within, we can see the evidence of material falling into them, and it would be useful to know why some black holes consume matter at a slower pace than others. The X-ray wavelengths that Swift studies should give us our best data on the upcoming black hole encounter. A sudden spike in X-ray brightness would presumably mark the event, and the researchers will post the images online.
In studying black hole behavior, we’re also looking at key information about how galaxies live out their lives. After all, these objects are consuming matter and radically affecting the region around the very heart of the galaxy. “The way they do that influences the evolution of the entire galaxy—how stars are formed, how the galaxy grows, how it interacts with other galaxies,” says Nathalie Degenaar. Those of us of a certain age can delight in the recollection of Fred Hoyle’s 1957 novel The Black Cloud, in which a gas cloud approaching the Solar System turns out to be a bit more than astronomers had bargained for. Don’t miss this classic if you haven’t read it yet — you should have plenty of time to finish it before the G2 event.
Stars at Galactic Escape Velocity
How do you boost the velocity of a star up to 540 kilometers per second? Somewhere in that region, with a generous error range on either side, is the speed it would take to escape the galaxy if you left from our Solar System’s current position. Here on Centauri Dreams we often discuss exotic technologies that could propel future vehicles, but it’s hard to imagine mechanisms that would drive natural objects out of the galaxy at such speeds. Even so, there are ways, as explained by Vanderbilt University’s Kelly Holley-Bockelmann:
“It’s very hard to kick a star out of the galaxy. The most commonly accepted mechanism for doing so involves interacting with the supermassive black hole at the galactic core. That means when you trace the star back to its birthplace, it comes from the center of our galaxy.”
The mechanism works like this: A binary pair of stars moving a bit too close to the massive black hole at the center of the Milky Way loses one star to the black hole while flinging the other outward at high velocity. When you calculate that the black hole has a mass equal to some four million Suns, this works: Stars can indeed be accelerated to galactic escape velocity, and so far a number of blue hypervelocity stars have been found that could be explained this way.
But Holley-Bockelmann and grad student Lauren Palladino have run into something that casts doubt on this explanation, or at least makes us wonder about other methods for making stars travel this fast. Calculating stellar orbits with data from the Sloan Digital Sky Survey, the duo have found about twenty stars the size of the Sun that appear to be hypervelocity stars. Moreover, these are stars whose composition mirrors normal disk stars, leading the researchers to believe they were not formed in the galaxy’s central bulge or its halo.
Image: Top and side views of the Milky Way galaxy show the location of four of the new class of hypervelocity stars. These are sun-like stars that are moving at speeds of more than a million miles per hour relative to the galaxy: fast enough to escape its gravitational grasp. The general directions from which the stars have come are shown by the colored bands. (Graphic design by Julie Turner, Vanderbilt University. Top view courtesy of the National Aeronautics and Space Administration. Side view courtesy of the European Southern Observatory).
Holley-Bockelmann and Palladino are working on possible causes for the movement of these stars, including interaction with globular clusters, dwarf galaxies or even supernovae in the galactic disk. The list of possibilities is surprisingly long, as noted in the paper on this work (internal references omitted for brevity):
While the SMBH [supermassive black hole] at the Galactic center remains the most promising culprit in generating HSVs [hypervelocity stars], other hypervelocity ejection scenarios are possible, such as a close encounter of a single star with a binary black hole… In this case, the star gains energy from the binary black hole and is flung out of the Galaxy while the orbit of the black hole binary shrinks…Another alternative hypervelocity ejection model involves the disruption of a stellar binary in the Galactic disk; here a supernova explosion in the more massive component can accelerate the companion to hypervelocities…
We may know more soon, for the paper points out that a nearby supernova should have contaminated the spectrum of a hypervelocity star. As they delve into these and other possibilities, the researchers are also expanding their search for hypervelocity stars to a larger sample within the Sloan data to include all spectral types.
The findings were announced at the meeting of the American Astronomical Society in Washington this week. The paper is Palladino et al., “Hypervelocity Star Candidates in the SEGUE G and K Dwarf Sample,” The Astrophysical Journal Vol. 780, No. 1 (2014), with abstract and preprint available.
Stormy Outlook for Brown Dwarfs
“Weather on Other Worlds” is an observation program that uses the Spitzer Space Telescope to study brown dwarfs. So far 44 brown dwarfs have fallen under its purview as scientists try to get a read on the conditions found on these ‘failed stars,’ which are too cool to sustain hydrogen fusion at their core. The variation in brightness between cloud-free and cloudy regions on the brown dwarf gives us information about what researchers interpret as torrential storms, and it turns out that half of the brown dwarfs investigated show these variations.
Given the chance nature of their orientation, this implies that most, if not all, brown dwarfs are wracked by high winds and violent lightning. The image below could have come off the cover of a 1950’s copy of Astounding, though there it would have illustrated one of Poul Anderson’s tales with Jupiter as a violent backdrop (“Call Me Joe” comes to mind). Brown dwarfs are, of course, a much more recent find, and in many ways a far more fascinating one.
Image: This artist’s concept shows what the weather might look like on cool star-like bodies known as brown dwarfs. These giant balls of gas start out life like stars, but lack the mass to sustain nuclear fusion at their cores, and instead, fade and cool with time. Credit: NASA/JPL-Caltech/University of Western Ontario/Stony Brook University.
Storms like these inevitably suggest Jupiter’s Great Red Spot, too, but we want to be careful with analogies considering how much we still have to learn about brown dwarfs themselves. What we can say is this: Brown dwarfs are too hot for water rain, leading most researchers to conclude that any storms associated with them are made up of hot sand, molten iron or salts.
The idea that brown dwarfs have turbulent weather is not surprising, but it is interesting to learn that such storms are evidently commonplace on them. Even more interesting is what the Spitzer work has revealed about brown dwarf rotation. Some of the Spitzer measurements found rotation periods much slower than any previously measured. Up to this point the assumption had been that brown dwarfs began rotating quickly shortly after they formed, a rotation that did not slow down as the objects aged. Aren Heinze (Stony Brook University) had this to say:
“We don’t yet know why these particular brown dwarfs spin so slowly, but several interesting possibilities exist. A brown dwarf that rotates slowly may have formed in an unusual way — or it may even have been slowed down by the gravity of a yet-undiscovered planet in a close orbit around it.”
Whatever the case, brown dwarfs do seem to be opening a window into weather systems in exotic places, systems that can be studied and characterized by their variations in brightness. Heinze presented this work at the 223rd annual meeting of the American Astronomical Society in Washington for principal investigator Stanimir Metchev (University of Western Ontario).
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