by Paul Gilster | Sep 11, 2020 | Outer Solar System |
When we looked earlier this week at the Solaris mission, a concept designed to study the Sun’s polar regions, I commented on another early concept called the Auroral Reconstruction CubeSwarm (ARCS). The mission intrigued me because it consisted of CubeSats in swarm formation, working together with numerous ground observatories, to study the Earth’s auroras. The paradigm of miniaturization, low cost and creative design surfaces yet again in Janus, a proposal out of the University of Colorado at Boulder and Lockheed Martin that would involve twin spacecraft studying twin targets, the binary asteroids 1996 FG3 and 1991 VH.
Daniel Scheeres (CU-Boulder) is principal investigator for Janus, the plan being for the university to handle the analysis of data and images from the mission, with Lockheed Martin building and operating the two spacecraft. It should be a familiar role for both entities, as Lockheed Martin supports operations for OSIRIS-REx at asteroid Bennu, while Scheeres leads the radio science team for that mission. Each Janus spacecraft is roughly the size of a suitcase, a carry-on at that, echoing the theme of keeping spacecraft small and straightforward.
Lockheed Martin’s Josh Wood is project manager for Janus:
“We see an advantage to be able to shrink our spacecraft. With technology advancements, we can now explore our solar system and address important science questions with smaller spacecraft… We see this evolution to smaller and more capable spacecraft being a key market in the future for scientific missions. Now, we want to execute and show that we can do it.”
Lowering costs and preparation time by using off-the-shelf components is all part of the same parameter space that supports the movement toward CubeSats and so-called SmallSats. The mission will be part of NASA’s SIMPLEx program, which focuses on small spacecraft and satellites, and is projected to cost less than $55 million.
The twin Janus spacecraft have a long journey ahead, with a gravity assist at Earth following an initial solar orbit and a subsequent trajectory that takes them beyond the orbit of Mars. The craft are to use VACCO MiPS (Micro-Propulsion System), a low-cost, cold gas propulsion option designed for CubeSats consisting of five thrusters for pitch, yaw, roll and delta-v.
Malin Space Science Systems is to provide the instrument suite including visible and infrared cameras, with power delivered by three deployable solar arrays and batteries. Launch is to be in 2022, with the twin craft lofted as secondary payloads on a Falcon-Heavy (Block 5) in the same launch that will carry the Psyche and EscaPADE missions.
From a short summary presented at the 51st Lunar and Planetary Science Conference (2020):
Janus science will combine flyby observations of the target binary asteroids with ground-based observations, enabling the high resolution imaging and thermal data to be placed into a global context and leveraging all available data to construct an accurate topographical and morphological model of these bodies. Based on these measurements, the formation and evolutionary implications for small rubble pile asteroids will be studied.
Image: Rendering of the orbital pattern of the binary asteroid 1999 KW4. We have much to learn about binary asteroids. (Credit: NASA/JPL).
Binary asteroids have yet to be studied up close, but this is a configuration that represents about 15 percent of the asteroids in the Solar System. Says Scheeres:
“We think that binary asteroids form when you have a single asteroid that gets spun up so fast that the whole thing splits in two and goes through this crazy dance… Once we see them up close, there will be a lot of questions we can answer, but these will raise new questions as well. We think Janus will motivate additional missions to binary asteroids.”
by Paul Gilster | Sep 10, 2020 | Asteroid and Comet Deflection |
OSIRIS-REx, the little spacecraft with the big acronym (standing for Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer) has been on station for a year and a half at asteroid Bennu, monitoring the unexpected activity that distinguishes the object. Particle ejection from the surface is the subject of a revised special issue of the Journal of Geophysical Research: Planets collecting 10 papers on the matter.
Specifically, the spacecraft has found that particles of rock mostly of pebble-size are being ejected repeatedly — one or two per day — from the asteroid’s surface, some of them escaping into space, some moving into a temporary orbit, with the rest falling back onto the surface. Just days after entering orbit on December 31 of 2018, OSIRIS-REx began to spot the activity, which the introduction to the special issue refers to as “ongoing mass shedding” involving millimeter- to centimeter-scale particles. What we have on our hands here is an asteroid that is active.
Dante Lauretta (University of Arizona) is OSIRIS-REx principal investigator:
“We thought that Bennu’s boulder-covered surface was the wild card discovery at the asteroid, but these particle events definitely surprised us. We’ve spent the last year investigating Bennu’s active surface, and it’s provided us with a remarkable opportunity to expand our knowledge of how active asteroids behave.”
Image: This mosaic image of asteroid Bennu is composed of 12 images collected on Dec. 2, 2018, by the OSIRIS-REx spacecraft’s PolyCam instrument from a range of 24 kilometers. Credit: NASA/Goddard/University of Arizona.
Bennu is another example of the blurring of the distinction between comets and asteroids, the former composed of ice, rock and dust and subject to heating by the Sun during the approach to perihelion that causes the formation of the familiar tail as vapor and dust is carried into space. The rock and dust characteristic of asteroids wouldn’t seem to lend itself to such activity, but the papers here point to thermal fracturing that is the result of repeated heating and cooling as the object rotates as the likely driver. Also implicated: Impacts from meteoroids hitting the surface. From a paper on thermal fatigue by Jamie Molaro (Planetary Science Institute) and colleagues:
If thermal fatigue indeed plays a role in Bennu’s activity, this has broad implications for our understanding of active asteroids and the asteroid population as a whole. Previous works have hypothesized that thermal fracture processes may generate activity on active asteroids with small perihelion distances, such as (3200) Phaethon (Jewitt & Li, 2010). Our results support this hypothesis. With a diurnal temperature variation of hundreds of degrees, Phaethon’s surface is likely to be subject to thermal shock processes, with fatigue operating at depth to weaken and prepare the rock for disruption. However, the fact that thermal fatigue alone may be capable of generating activity suggests that there may be many more active asteroids than are currently known, likely including many in near?Earth space. With less energetic activity, a lack of tails or comae would make such bodies hard to identify from ground?based observations, and previous missions to visit asteroids up close lacked the capability to detect ejection events like those observed on Bennu.
Image: Occurrence of particle ejection events (circles) in the orbit of Bennu (orange). Ejection events of 2 to 19 particles are denoted by small gray circles; events of 20 or more particles are denoted by large black circles. The phases of the OSIRIS?REx mission that included dedicated particle monitoring (Orbitals A–C) are indicated with blue hatches. The Sun (yellow) and the orbits of Mercury (green), Venus (light blue), Earth (dark blue), and Mars (red) are also shown for reference. Credit: Hergenrother et al. (2020).
Mission scientists have been able to use the unexpected stream of particles as a probe of the asteroid’s gravitational field, for many of them were found to be orbiting much closer than the spacecraft itself, their trajectories sensitive to the irregular gravity of the object. The precision of the measurement is actually higher than would have been possible solely through the instruments aboard OSIRIS-REx. Chesley calls them “an unexpected gift for gravity science at Bennu since they allowed us to see tiny variations in the asteroid’s gravity field that we would not have known about otherwise.”
Remember that this is a sample return mission, with a touchdown planned for October 20 to gather surface material, some of which may contain particles that were ejected and have fallen back to the surface. We can look forward to the spacecraft’s return in September of 2023.
The papers on Bennu are gathered in “Exploration of the Activity of Asteroid (101955) Bennu,” a special edition of the Journal of Geophysical Research: Planets first published in April of 2020 and now updated as of 19 August. The paper quoted above is Molaro et al., “Thermal Fatigue as a Driving Mechanism for Activity on Asteroid Bennu,” JGR Planets 21 July 2020 (abstract).
by Paul Gilster | Sep 9, 2020 | Astrobiology and SETI |
As it is considered a precursor installation, the Murchison Widefield Array (MWA) in Western Australia doesn’t get the press that its proposed successor, the Square Kilometer Array (SKA) regularly receives. That’s to be expected, given the scope of the SKA, which will involve telescopes in both Australia and South Africa. 14 member countries are developing a project that is to reach over a square kilometer of collecting area, containing thousands of dishes and up to a million low-frequency antennas. If it is built, SKA’s angular resolution and survey speed will allow surveys thousands of times faster than those now being conducted.
But the Murchison precursor is alive and well, working the 70-300 MHz range and mapping the radio sky. Established by a consortium of universities — MIT, Swinburne, Curtin and Australian National University — the telescope is located on a site selected by these universities and managed by Curtin University. CSIRO, Australia’s national science agency, would later take over management of the site, which also now houses their SKA precursor instrument, ASKAP.
The MWA may be 50 times less sensitive than the SKA, but it has been put to work in areas ranging from the heliosphere to neutral hydrogen emission from the early universe. Its remit also includes several SETI studies, the latest being a search in the area of the constellation Vela (originally part of the larger Argo Navis constellation). The International Centre for Radio Astronomy Research calls this latest survey “the deepest and broadest search at low frequencies for alien technologies.” The results are now in the books, as reported in Publications of the Astronomical Society of Australia.
CSIRO astronomers Chenoa Tremblay and Steven Tingay (Curtin University) used the telescope’s wide field of view to observe millions of stars simultaneously. No technosignatures were detected, leading Tingay to observe:
“As Douglas Adams noted in The Hitchhikers Guide to the Galaxy, ‘space is big, really big’. And even though this was a really big study, the amount of space we looked at was the equivalent of trying to find something in the Earth’s oceans but only searching a volume of water equivalent to a large backyard swimming pool. Since we can’t really assume how possible alien civilisations might utilise technology, we need to search in many different ways. Using radio telescopes, we can explore an eight-dimensional search space. Although there is a long way to go in the search for extraterrestrial intelligence, telescopes such as the MWA will continue to push the limits—we have to keep looking.”
The scientists observed the sky in Vela for 17 hours, and point to the capabilities of the coming SKA, which if completed could be available late in this decade. SKA would extend the SETI search into billions of star systems, and would be capable of detecting what Tingay calls “Earth-like radio signals” from relatively nearby planetary systems, meaning, I assume, leakage radiation as opposed to directed signals designed to be interstellar. The current work follows earlier MWA surveys, one toward galactic center, the other in the anti-center direction.
From the paper:
Overall, our MWA surveys show the rapid progress that can currently be made in SETI at radio frequencies, using wide field and sensitive facilities, but also show that SETI surveys have a long way to go. The continued use of the MWA, and the future similar use of the SKA at much higher sensitivities, offers a mechanism to make significant cuts into the haystack fraction of Wright et al. (2018), while maintaining a primary focus on astrophysical investigations, making excellent commensal use of these large-scale facilities.
Image: Dipole antennas of the Murchison Widefield Array (MWA) radio telescope in Western Australia. Credit: Dragonfly Media.
On the matter of ‘haystack fractions,’ the phrase comes from a 2018 paper from Jason Wright (Penn State) and co-authors (two students in a graduate SETI course taught by Wright), which attempts to calculate the total fraction of the ‘haystack’ that has been searched to date, producing a result not far off what Jill Tarter calculated back in 2010: “…our current search completeness is extremely low, akin to having searched something like a large hot tub or small swimming pool’s worth of water out of all of Earth’s oceans.” Tarter’s comparison was to a drinking glass’s worth of seawater, so we’re in the same range.
Although I cited it in an earlier article (see Into the Cosmic Haystack) this quotation from the Wright et al. paper bears repeating:
We should be careful, however, not to let this result swing the pendulum of public perceptions of SETI too far the other way by suggesting that the SETI haystack is so large that we can never hope to find a needle. The whole haystack need only be searched if one needs to prove that there are zero needles—because technological life might spread through the Galaxy and/or technological species might arise independently in many places, we might expect there to be a great number of needles to be found. Also, our haystack definition included vast swaths of interstellar space where we have no particular reason to expect to find transmitters; humanity’s completeness to subsets of this haystack—for instance, for continuous, permanent transmissions from nearby stars—is many orders of magnitude higher.
Noting that “the dream of ‘all-sky, all the time” high bandwidth coverage is still worth pursuing, and singling out the Tingay et al surveys at the MWA in particular, Wright and colleagues say that surveys with large bandwidth, long exposures, repeat visits and good sensitivity allow for searches that are orders of magnitude faster than surveys without these qualities. Indeed, the three MWA low frequency surveys, because of their very wide field and sensitivity, dominate the haystack search volume, and as Wright notes, they did this in only a few hours of searching. The paper also notes how rapidly Breakthrough Listen is cutting into this search space, and that was before the most recent Breakthrough results were announced (see SETI: Going Deep with the Data Search).
Image: A 20-second exposure showing the Milky Way overhead the AAVS station. Credit: Michael Goh and ICRAR/Curtin.
The paper is Tremblay and Tingay, “A SETI Survey of the Vela Region using the Murchison Widefield Array: Orders of Magnitude Expansion in Search Space”, Publications of the Astronomical Society of Australia September 8th, 2020 (abstract). The Wright paper is “How Much SETI Has Been Done? Finding Needles in the n-Dimensional Cosmic Haystack,” Astronomical Journal Vol. 156, No. 6 (14 November 2018). Abstract / Preprint.
by Paul Gilster | Sep 8, 2020 | Missions |
I can think of more than one way to get a good look at the Sun’s polar regions. After all, we’ve done it before, through the Ulysses spacecraft, which passed over the Sun’s north and south poles in 1994-1995. A gravity assist at Jupiter was the key to the mission, allowing Ulysses to arc out of the ecliptic and inward to the Sun. But Ulysses lacked the kind of remote-sensing instruments we’d like to use to compile an extensive dataset on the polar magnetic field and, as Don Hassler (SwRI) adds, “the surface/sub-surface flows” we might find in the polar regions. It’s good to see a mission designed for that purpose.
For Hassler is principal investigator on a concept that has just been approved for further study by NASA, with the haunting name Solaris. I say ‘haunting’ because it’s hard for this Stanislaw Lem reader to forget the novel of the same name, published in 1961, that explores the implications of a vast intelligence on a planet far from Earth. I realize this has been done as a film more than once and I’ve seen the films, but I leave their analysis to Centauri Dreams film critic Larry Klaes, who would know how to do justice to them. For me, the written word is the medium of choice, and this is a novel I intend to read again.
Anyway, the proposed Solaris mission is one of five science investigations just approved by NASA as part of the agency’s Medium-Class Explorer (MIDEX) program, with $1.25 million allocated for a nine-month contract for what are known as Phase A concept design studies and analyses to develop the concept. If it flies, Solaris would launch in 2025, like Ulysses using a gravity assist at Jupiter to sling it out of the ecliptic plane, flying over the solar poles at 75 degrees latitude. You might think of the surprises Cassini found at Saturn’s poles, and that odd hexagon at the north pole is still the subject of various competing hypotheses. Will we find something just as odd at the Sun? Hassler notes that we’ll at least get a good look:
“Solaris will spend more than three months over each pole of the Sun, obtaining the first continuous, high-latitude, months-long studies of the Sun’s polar regions. With focused science and a simple, elegant mission design, Solaris will also provide enabling observations for space weather research, such as the first polar views of coronal mass ejections, energetic events that spew highly magnetized plasma from the solar corona into space, causing radio and magnetic disturbances on the Earth.”
And. he adds, “It’s sure to stimulate future research through new unanticipated discoveries.”
Unexpected findings have characterized our explorations of the Solar System from the beginning, with Io and Triton being two outstanding examples, so let’s see what those solar polar regions look like. We’ll keep an eye on the progress of Solaris as it makes its way through the process. The concept calls for a Doppler magnetograph to study the polar magnetic fields and subsurface flows, along with an extreme ultraviolet instrument for polar imaging and a white light coronagraph to examine the solar corona from this perspective.
Image; Southwest Research Institute is developing the concept for a mission to study the Sun’s poles, one of the last unseen places in the solar system. This proposed solar polar NASA mission is designed to revolutionize our understanding of the Sun by addressing fundamental questions that can only be answered from a polar vantage point. Credit: Courtesy of Southwest Research Institute.
And keep an eye on another mission funded for similar Phase A concept design study, the Auroral Reconstruction CubeSwarm (ARCS), in the hands of principal investigator Kristina Lynch (Dartmouth University), a mission that would, like Solaris, be managed by SwRI. For me, the chief interest of ARCS is in its plan for 32 CubeSats and 32 ground-based observatories to work together, in this case on a study of the mechanisms driving Earth’s auroras. CubeSat designs of growing complexity are low-cost ways to fly ever more interesting missions, and the emerging notion of ‘swarm’ missions should turn out to be productive in areas as diverse as planetary imaging and extrasolar planet detection.
Meanwhile, I’m always glad to see continuing interest in missions to the Sun, given our need to understand the issues involved in close solar approaches for potential ‘sundiver’ missions deep into the gravity well for maximum acceleration to targets in the outer system. I’ll also mention Solar Cruiser as a fascinating sail design that could enable study of the Sun’s high latitudes using non-Keplerian orbits enabled by the momentum of solar photons. Principal investigator Les Johnson (MSFC) sees this as an outstanding opportunity to demonstrate the capabilities of large sails as we explore the nearest star in the cosmos.
For more on Solar Cruiser, see Heliophysics with Interstellar Implications. See also Johnson’s analysis “The Solar Cruiser Mission Concept — Enabling New Vistas for Heliophysics,” Bulletin of the American Astronomical Society, Vol. 52, No. 3 (June, 2020). Abstract.
by Paul Gilster | Sep 4, 2020 | Exoplanetary Science |
This morning we have two interesting and complementary studies of GW Orionis to look at, both analyzing what is apparently a planet-forming disk with multiple, misaligned rings around this triple star system some 1300 light years from the Sun. In the more recent of the two, Stefan Kraus (University of Exeter) and colleagues used data from both the Atacama Large Millimeter/submillimeter Array (ALMA) and the European Observatory’s Very Large Telescope (VLT) in detecting warm gas at the inner edge of the misaligned ring, which has broken away from the larger disc, and scattered light from the warped disk surface.
So what could be going on at GW Orionis? What the images reveal is an evolving young system much different from our own. Consider: The inner stars GW Ori A and GW Ori B orbit each other at a separation of a scant 1 AU, while the third star, GW Ori C, orbits the inner stars at a distance of roughly 8 AU, the latter in an orbit that is not aligned with the plane of the inner duo. In our Solar System, we’re used to planets that move in roughly the same plane around the Sun. Here we see a deformed protoplanetary disk that may produce an utterly different result.
Image: Representation of the disc structure and stellar orbit of the GW Orionis triple system, as derived from the ALMA and VLT observations by Kraus et al. Orange rings are the (misaligned) rings seen by ALMA. The transparent surfaces correspond to the lower-density dust filaments that connect the rings and that dominate the emission in scattered light. Credit: Kraus et al., 2020; NRAO/AUI/NSF.
What an intriguing place to study planet formation, and to ponder scenarios as the system evolves. The ALMA data reveal three separate rings with different orientations, located at 46, 185 and 340 AU from the barycenter of the system. The inner ring is misaligned in relation not only to the outer two rings but also to the three stars. Says Kraus:
“In our images, we see the shadow of the inner ring on the outer disk. At the same time, ALMA allowed us to measure the precise shape of the ring that casts the shadow. Combining this information allows us to derive the 3-dimensional orientation of the misaligned ring and of the warped disk surface.”
Image: New observations of GW Orionis, a triple star system with a peculiar inner region, revealed that this object has a warped planet-forming disk with a misaligned ring. The image on the right is from the SPHERE instrument on the European Southern Observatory’s Very Large Telescope, which allowed astronomers to see, for the first time, the shadows this ring casts on the rest of the disk. This helped the researchers figure out the 3D shape of the ring and the overall disk. The left panel shows an artistic impression of the disk’s inner region, including the ring, which is based on the 3D shape reconstructed by the team. Credit: ESO/L. Calçada, Exeter/Kraus et al.
Planets could well emerge here, with the research indicating that the inner ring contains about 30 Earth masses of dust. What’s intriguing is that any planets forming within this inner ring will orbit in a highly oblique fashion at wide separation from the star. Bear in mind that it’s now believed that more than half of the stars in the galaxy are born with one or more companion stars, making for the prospect of a large population of planets on highly inclined, distant orbits.
Kraus and team have been examining GW Orionis for over 11 years, mapping the gravitational interactions at work among the three stars over a full orbital period. It is clear that the stellar orbits are misaligned from each other and from the disk. We’re getting confirmation here through both observations and computer simulations that a theoretical ‘disk tearing effect’ is in play, one that emerges out of the gravitational pull of the three stars and causes the disk to break into separate rings. Observation of the shadow that the inner ring casts upon the rest of the disk was useful in calculating the shape of the ring and the overall disk structure. Moreover, the shape of the inner ring matches predictions of precisely how gravitational interactions would tear the original disk.
Image; ALMA and the SPHERE instrument on ESO’s Very Large Telescope have imaged GW Orionis, a triple star system with a peculiar inner region. Unlike the flat planet-forming discs we see around many stars, GW Orionis features a warped disc, deformed by the movements of the three stars at its centre. The ALMA image (left) shows the disc’s ringed structure, with the innermost ring separated from the rest of the disc. The SPHERE observations (right), repeated here for comparison, allowed astronomers to see for the first time the shadow of this innermost ring on the rest of the disc, which made it possible for them to reconstruct its warped shape. Credit: ALMA (ESO/NAOJ/NRAO), ESO/Exeter/Kraus et al.
The second team, led by Jiaqing Bi (University of Victoria, Canada), likewise used data from ALMA to observe the same disk misalignment, publishing their paper in May. This work confirms that the inner ring is misaligned relative to the outer ring and the three stars, with the outer ring being the largest yet observed in disks of this kind. Both teams used computer simulations to examine causes for the misalignment, with the Bi team suggesting a possibility that does not arise in the Klaus paper. This is team member Nienke van der Marel (University of Victoria):
“Our simulations show that the gravitational pull from the triple stars alone cannot explain the observed large misalignment. We think that the presence of a planet between these rings is needed to explain why the disk was torn apart. This planet has likely carved a dust gap and broken the disk at the location of the current inner and outer rings.”
Such a planet would be the first ever observed to orbit three stars. Moreover, it’s clear from the example of GW Orionis that stellar groupings like this can play havoc with the shape of a protoplanetary disk, doubtless producing worlds in highly inclined orbits around multiple stars. “We predict that many planets on oblique, wide-separation orbits will be discovered in future planet imaging campaigns,” says Kraus co-author and Exeter colleague Alexander Kreplin.
The paper is Kraus et al., “A triple star system with a misaligned and warped circumstellar disk shaped by disk tearing” Science Vol. 369, Issue 6508 (4 September 2020), pp. 1233-1238 (abstract). The Bi paper is “GW Ori: Interactions Between a Triple Star System and Its Circumtriple Disk in Action,” Astrophysical Journal Letters Vol. 895, No. 1 (21 May 2020), Abstract.
