by Paul Gilster | Jan 29, 2020 | Asteroid and Comet Deflection |
Asteroids are objects of obvious scientific interest, not only for their intrinsic properties but also our need to understand how we can change their motion in space in case one looks like it will come dangerously close to Earth in the future. OSIRIS-REx is extracting all kinds of valuable data from asteroid 101955 Bennu, but we should also keep in mind that Bennu itself is a potentially hazardous object, with a small chance (1-in-2700, according to current estimates) of striking the Earth between 2175 and 2199. Thus the second ‘S’ in OSIRIS, which stands for ‘security’, and is all about measuring the factors that affect the object’s trajectory.
When we get samples from Bennu, we’ll have a better idea about the asteroid’s chemistry and morphology, useful for understanding the early Solar System as well as assessing how hazardous such an object is. But we need to know more, which is where NASA’s Double Asteroid Redirection Test (DART) mission comes in. Here the purpose is planetary defense from the start, for DART will demonstrate how a kinetic impactor can change the motion of an asteroid in space. The target is a binary near-Earth asteroid called (65803) Didymos.
Image: Simulated image of the Didymos system, derived from photometric lightcurve and radar data. The primary body is about 780 meters in diameter and the moonlet is approximately 160 meters in size. They are separated by just over a kilometer. The primary body rotates once every 2.26 hours while the tidally locked moonlet revolves about the primary once every 11.9 hours. Almost one sixth of the known near-Earth asteroid (NEA) population are binary or multiple-body systems. Credit: Naidu et al., AIDA Workshop, 2016.
DART will carry an imaging instrument called DRACO (Didymos Reconnaissance & Asteroid Camera for OpNav), which is based on the now familiar LORRI high-resolution imager that flew on New Horizons, and will use roll-out solar arrays (each 8.6 meters by 2.3 meters) and a NEXT-C ion engine for propulsion. The plan is simplicity itself: DART will crash into the Didymos moonlet at 6.6 kilometers per second, which should change the moonlet’s orbital speed around the main body by a fraction of one percent, and the orbital period by several minutes.
Flying with DART will be LICIA, the Light Italian CubeSat for Imaging of Asteroid, which will observe the impact ejecta in the early phase of crater formation following the impact. The dynamic changes DART’s impact produces will be measured partly by what LICIA learns about the fallback ejecta on both asteroids and the subsequent Hera observations of unweathered fresh material on the two objects. NASA describes LICIA this way:
The LICIA Cube is a 6U CubeSat provided by the Italian Space Agency. It will be carried along with DART to Didymos and released approximately 2 days before the DART impact. LICIA Cube will perform a separation maneuver to follow behind DART and return images of the impact, the ejecta plume, and the resultant crater as it flies by. It will also image the opposite hemisphere from the impact. LICIA Cube is 3-axis stabilized and has a propulsion capability of 56 m/s. The onboard imager has a 7.6 cm aperture, F/5.2 telescope, and an IFOV of 2.9 arcsec/pixel.
Image: Two different views of the DART spacecraft. The DRACO (Didymos Reconnaissance & Asteroid Camera for OpNav) imaging instrument is based on the LORRI high-resolution imager from New Horizons. The left view also shows the Radial Line Slot Array (RLSA) antenna with the ROSAs (Roll-Out Solar Arrays) rolled up. The view on the right shows a clearer view of the NEXT-C ion engine. Credit: NASA.
The European Space Agency’s Hera mission — powered by solar arrays with a hydrazine propulsion system — is to be the follow-up, making a post-impact survey using high-resolution visual, laser and radio science to map what will be the smallest asteroid yet visited by spacecraft. The DART collision is scheduled for 2022, with immediate results probably hidden by an expected dust cloud. Hera will investigate the asteroid impact crater and surrounding surface in 2026, allowing scientists to refine their numerical models of the impact process. All of this works toward building a deflection technique for planetary defense.
Earth-based observations of the Didymos system gathered during its close approach in February-May 2017 were analyzed at a workshop in Prague in 2018, allowing constraints to be placed on the strength of the YORP effect, which results from uneven heating on the surface that can alter the object’s spin state. Further observations will tighten these constraints, making the effects of the DART impact easier to separate from the pre-impact state of the system.
A key Hera role in all this will be to measure Didymos’ mass, which will help scientists calculate the efficiency of the impact momentum transfer once we’ve also measured the change in the small moon’s orbital period. Thus the two missions will result in accurate modeling of the response to the impact as well as the likely internal structure of the asteroid. Hera will be carrying two six-unit cubesats of its own to provide spectral measurements of the surface of both asteroids, with a Cubesat called APEX (Asteroid Prospection Explorer) actually landing on one of them. A second CubeSat (Juventas) will measure the gravitational field and internal structure of the small moon, doing a low-frequency radar survey of the asteroid interior.
The DART launch window opens in late July of 2021, with launch aboard a SpaceX Falcon 9, with intercept of the Didymos moonlet in late September of 2022, when the system is about 11 million kilometers from Earth. Earth-based telescopes and planetary radar will be able to measure the effects of the impact to back up the findings of the spacecraft on the scene. The results should be small but highly useful in giving us data on how impacts affect asteroids of this size, with the added benefit of enhancing international cooperation on a matter of global importance.
by Paul Gilster | Jan 28, 2020 | Asteroid and Comet Deflection |
The latest operations of the OSIRIS-REx spacecraft at asteroid Bennu remind me how powerful a wave we’ve unleashed in the coupling of robotics and ever more capable spacecraft components. We’re not exactly at the stage of ‘routine’ asteroid missions, but Hayabusa2 and OSIRIS-REx when seen in the context of upcoming missions like NASA’s DART experiment and the European Space Agency’s Hera are part of our renaissance of this class of object, with results beneficial to science but also practically useful in terms of future impact mitigation. More on DART and Hera tomorrow.
Small objects have plenty to say about our future in space, and I haven’t even mentioned Lucy, which will be studying multiple Jupiter trojans, or the Psyche mission targeting what may be the exposed core of a planetary embryo, or for that matter, the remarkably successful Dawn, which unlocked so many mysteries at Vesta and Ceres. It goes without saying that having an operational spacecraft in the Kuiper Belt is likewise a sign that we are getting pretty good at doing robotic exploration even as we continue to wrestle with the human role on the Moon and Mars.
OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) has now completed a 620 meter flyover of the site called ‘Nightingale’ on its target asteroid as part of the mission’s analysis of the primary sample collection site. To make this happen, the spacecraft left a 1.2 kilometer home orbit and performed an 11-hour transit over the asteroid, accumulating data about the 16-meter wide sample site and returning to safe orbit.
Image: During the OSIRIS-REx Reconnaissance B flyover of primary sample collection site Nightingale, the spacecraft left its safe-home orbit to pass over the sample site at an altitude of 0.4 miles (620 m). The pass, which took 11 hours, gave the spacecraft’s onboard instruments the opportunity to take the closest-ever science observations of the sample site. Credit: NASA/Goddard/University of Arizona.
The spacecraft has been compiling a Natural Feature Tracking image catalog by way of mapping the tiniest details among the boulders and craters of the landing site. The OSIRIS-REx team is also studying observations from the spacecraft’s Thermal Emissions Spectrometer (OTES), the OSIRIS-REx Visual and InfraRed Spectrometer (OVIRS), the OSIRIS-REx Laser Altimeter (OLA), and the MapCam color imager.
So there’s a lot happening at Bennu, including an upcoming flyover, scheduled for February 11, of the backup sample site, which has been given the name ‘Osprey.’ Further flybys in March (Nightingale) and May (Osprey) will take OSIRIS-REx even closer to the surface as the spacecraft goes into Reconnaissance C phase, operating at 250 meters. Assuming all goes well, a multi-hour sampling maneuver will begin in August, using the spacecraft’s robotic arm and sampler head to make contact with the asteroid. We will eventually gain between 60 and 2000 grams of surface material, with return to Earth scheduled for September of 2023.
Image: This artist’s concept shows OSIRIS-REx contacting asteroid Bennu with its sample return instrument. Credit: NASA’s Goddard Space Flight Center.
by Paul Gilster | Jan 22, 2020 | Asteroid and Comet Deflection |
The oldest preserved impact structure on Earth appears to be at Yarrabubba in Western Australia, where a magnetic anomaly about 20 kilometers in diameter has been interpreted to be a remnant of an original impact crater 70 kilometers across. Here, what had been an approximate age of 2.65 to 1.075 billion years has now been constrained to 2.229 billion years, making Yarrabubba 200 million years older than the next oldest impact.
A team led by Timmons Erickson (Curtin University) analyzed the minerals zircon and monazite at the site. Their sample showed shock recrystallization (in the form of so-called neoblasts) from an asteroid strike, the analysis of which allowed them to pin down the structure’s age. A paper just out in Nature Communications reports on the team’s use of uranium-lead (U–Pb) dating to investigate the age of the shock features and impact melt.
A global climate change may have occurred as a result of this impact, perhaps one with consequences for so-called ‘snowball Earth,’ a hypothesis of almost planet-wide ice cover in one or more periods before 650 million years ago. This impact conceivably ended the ice era.
Image: This is Figure 1 from the paper. Caption: Composite aeromagnetic anomaly map of the Yarrabubba impact structure within the Yilgarn Craton, Western Australia, showing the locations of key outcrops and samples used in this study. The image combines the total magnetic intensity (TMI, cool to warm colours) with the second vertical derivative of the total magnetic intensity (2VD, greyscale) data. The demagnetised anomaly centred on the outcrops of the Barlangi granophyre is considered to be the eroded remnant of the central uplift domain, which forms the basis of the crater diameter of 70?km. Prominent, narrow linear anomalies that cross-cut the demagnetised zone with broadly east-west orientations are mafic dykes that post-date the impact structure. Credit: Erickson et al.
The result is intriguing because the Yarrabubba crater was made at a time when rocks on many continents provide evidence of glacial conditions, and oceans were becoming more oxygenated. Thus co-author Nicholas Timms (Curtin University):
“The age of the Yarrabubba impact matches the demise of a series of ancient glaciations. After the impact, glacial deposits are absent in the rock record for 400 million years. This twist of fate suggests that the large meteorite impact may have influenced global climate.
“Numerical modelling further supports the connection between the effects of large impacts into ice and global climate change. Calculations indicated that an impact into an ice-covered continent could have sent half a trillion tons of water vapour – an important greenhouse gas – into the atmosphere. This finding raises the question whether this impact may have tipped the scales enough to end glacial conditions.”
An end to a period of snowball Earth? Perhaps so. Certainly the work is a reminder of how much we have to learn about crater structures on our own planet, and how much we need to acquire precise ages for them. The impact’s effects, assuming a continental ice sheet, would have been powerful. The team’s numerical models of a 70-kilometer impact crater driven into a granitic target with overlying ice sheet (modeled at from 2 to 5 kilometers in thickness) show the almost instantaneous vaporization of huge amounts of ice. From the paper:
The vapourised ice corresponds to between 9?×?1013 and 2?×?1014?kg of water vapour being jetted into the upper atmosphere within moments of the impact… Impact-generated water vapour in the lower atmosphere would have condensed and rapidly precipitated as rain and snow with no significant long-term climate effects, or could have even triggered widespread glacial conditions via cloud albedo effects during interglacial periods. However, ejection of high-altitude water vapour has potential for greenhouse radiative forcing, depending critically on atmospheric residence time.
Image: Zircon crystal used to date the Yarrabubba impact. Credit: Erickson et al.
The authors are quick to note how difficult it is to model the impact’s effects due to our uncertainties about the composition of the Earth’s atmosphere in the period in question. But they note that the atmosphere would have contained only a fraction of current levels of oxygen, making it likely that huge amounts of H2O vapor released instantaneously into the atmosphere would have had global ramifications.
While the ‘snowball Earth’ hypothesis dominates coverage of this paper, I think the broader significance is in the nature of ongoing research. We can inspect the impact record on surfaces like the Moon due to the lack of atmospheric erosion, but on Earth we face the latter as well as the obscuring effects of tectonics. Until we have a better record of terrestrial impacts, we won’t understand the links between impacts and changes to global climate. At least we have the notable exception of the Chicxulub crater in the Gulf of Mexico’s Yucatán Peninsula, a feature under intense study.
Going back much farther in time, however, we’re looking at poorly constrained and ambiguous impact evidence, a problem that this paper addresses for a period that was hitherto lacking in impact analysis at this level of detail. More broadly, we’re reminded as well of the shaping effect of bombardment as young terrestrial planets evolve, and should always be mindful of the contingencies forced upon nascent worlds by system debris.
The paper is Erickson et al., “Precise radiometric age establishes Yarrabubba, Western Australia, as Earth’s oldest recognised meteorite impact structure,” Nature Communications 11, article no: 300, published online 21 January 2020. Full text.
by Paul Gilster | Dec 24, 2019 | Asteroid and Comet Deflection |
The holiday season seems an appropriate time to thank not only my Centauri Dreams readers for their continued high level of discussion in these pages, but also the army of citizen scientists who are out there working on everything from exoplanet detection to asteroid mapping. We saw recently how valuable the work of amateurs like Thiam-Guan Tan can be in confirming a possible exoplanet, while projects like the Habitable Exoplanet Hunting Project continue coming online to push the boundaries of what amateur equipment can do.
Now comes word of the signal contribution made to OSIRIS-REx and its mission to asteroid Bennu. You’ll recall that when the spacecraft arrived at the asteroid, the surface was found to be far more littered with rocks and boulders than anyone had foreseen. Finding a spot for landing and retrieving samples would be no easy task, but it was made substantially more manageable by a team of 3,500 people using their PCs to join in analysis and characterization of the asteroid surface.
These volunteers measured boulders and marked craters, eventually tallying over 14 million annotations of features on Bennu’s emerging global map. Behind all this work was CosmoQuest, a project based at the Planetary Science Institute in Tucson, Arizona.
“It is amazing that more than 3,500 citizen scientists participated in CosmoQuest’s project to map Bennu and help mission scientists find the best place for OSIRIS-REx to collect a sample,” said Pamela L. Gay, Senior Scientist and Senior Education and Communication Specialist at PSI. “This kind of a volunteer effort makes it easier to find safe places to sample and scientifically interesting places to explore.”
Image: This image shows sample site Nightingale, OSIRIS-REx’s primary sample collection site on asteroid Bennu. The image is overlaid with a graphic of the OSIRIS-REx spacecraft to illustrate the scale of the site. Credit: NASA/Goddard/University of Arizona.
During the four month period needed to complete the mapping, some volunteers marked more than 500 images (the average was closer to 10), with each image taking up to 45 minutes to complete. It seems worthwhile to list the usernames of those with the greatest number of contributions: MikeCassidy, Nilium, bc2callhome, zathras, joed, dpi209, and pattyg. PSI’s CosmoQuest team will continue working with the Bennu science team to generate science drawn from the mapping data now that the initial site selection has been performed.
If you missed out on the Bennu mapping but would like to get involved, CosmoQuest intends to be launching new citizen science projects some time in 2020, so keep an eye on the site. A good New Year’s resolution might be to get involved in one or more of the many sites catering not just to amateur astronomers but interested laypeople willing to devote time to image analysis. Have a look, for example, at Zooniverse’s list of projects on physics to get an idea of the range. It’s clear that space missions draw real value out of the kind of citizen participation that, not so many years ago, was limited to watching images on a television. Actually joining in efforts that can assist a mission or discover new worlds through its data is no longer a novelty.
Image: All 3,640 names of the Bennu Mappers are superimposed on this Global Mosaic of the Bennu Asteroid that was acquired by the OSIRIS-REx Mission (the image has to be blown up several times to actually see the names). Credit: Created using sources images from NASA/Goddard/University of Arizona.
Let me wish all of you a wonderful holiday and an energized return to work when the season ends. Working on behalf of ideas one believes in is a high vocation. Let’s continue to focus in 2020 on pushing the seemingly intractable problem of interstellar flight forward with new ideas and clarifications of the old.
by Paul Gilster | Sep 12, 2019 | Asteroid and Comet Deflection |
What appears to be an interstellar comet is heading into the Solar System, with perihelion likely on December 10 of this year, a date that could change as orbital parameters continue to be firmed up. The natural comparison is with ‘Oumuamua, first discovered two years ago and now well on its way out of the system. But the object first labeled gb00234 and now carrying the provisional name C/2019 Q4 (Borisov), while clearly on a hyberbolic orbit, has been found before perihelion and should be visible for a much a longer period of observation and orbital calculation.
Image: Observations suggest that comet C/2019 Q4 (Borisov) may be from outside the Solar System. A hyperbolic solution for the object first labeled gb00234 passes between Mars and Jupiter. (Green=gb00234; Blue=Neptune). Credit: Tony873004 – Own work, CC BY-SA 4.0.
A professional optician and astronomer named Gennady Borisov at the Crimean Astrophysical Observatory (near the Crimean city of Bakhchysarai, on the Crimean peninsula) discovered the object on August 30, 2019 when it was at a distance of approximately 3 AU. This gentleman builds his own instruments, as physicist and radio astronomer Marshall Eubanks (VLBI) noted in a tweet this morning, replying to writer and journalist Corey Powell.
The Minor Planet Center (Smithsonian Astrophysical Observatory) now offers a Minor Planet Electronic Circular (MPEC) on C/2019 Q4 (Borisov) that notes the cometary nature of the object has been confirmed by ‘numerous observers.’ It also includes the news that the object will be visible for a good while. The appearance of the MPEC indicates sufficient data have been accumulated to make the call on the hyperbolic nature of the orbit.
Based on the available observations, the orbit solution for this object has converged to the hyperbolic elements shown below, which would indicate an interstellar origin. A number of other orbit computers have reached similar conclusions, initially D. Farnocchia (JPL), W. Gray, and D. Tholen (UoH). Further observations are clearly very desirable, as all currently-available observations have been obtained at small solar elongations and low elevations. Absent an unexpected fading or disintegration, this object should be observable for at least a year.
Image: C/2019 Q4 (Borisov), in the center of the image. Note what appears to be a short tail extending from the coma. Credit: Gennady Borisov.
What is striking here is the eccentricity, which at 3.08 (a figure worked out from 145 observations to this point) is clearly hyperbolic. Unlike ‘Oumuamua, we have a clear tail and a distinct coma, strengthening the object’s identification as a comet. Like ‘Oumuamua, its hyperbolic orbit is one that is unbound to the Sun, thus making this an interstellar object whose spectra will be examined with great interest. The object’s provisional designation will likely change, as did ‘Oumuamua’s, to something like object 2I/2019, making it the second in the category of interstellar objects, but clearly not the last as we tune up our methods.
Based on the orbital solutions now in play, C/2019 Q4 (Borisov) should close to within about 1.8 AU of the Sun before beginning its exit from the Solar System, allowing observatories all over the world to study it. While ‘Oumuamua presented puzzles that are still being resolved, the cometary nature of this object is becoming established and we will have plenty of time to study it. Remember, too, that the Large Synoptic Survey Telescope (LSST) team anticipates first light in 2020. Its planned surveys are likely to turn up interstellar objects in large numbers.
Particular thanks to Jean Schneider (Observatoire de Paris) for early thoughts on C/2019 Q4 (Borisov), as well as numerous readers who checked in via email.
by Paul Gilster | Jun 27, 2019 | Asteroid and Comet Deflection |
We have two stories with good news on the asteroid impact front this morning. The first, out of the University of Hawaii’s Institute for Astronomy, is the announcement of the detection of a small asteroid prior to its entering the Earth’s atmosphere. That many not sound unusual, but this is the first time an object could be detected in time to move people away from a impact site, even though asteroid 2019 MO was only about 4 meters across and burned up in the atmosphere. The key is warning time, and here that time would have been half a day.
An impactor like the 20-meter object that exploded over Chelyabinsk, Russia in 2013 could, with these same methods, be detected by the ATLAS facility at Maunaloa (Hawaii) several days before impact. ATLAS is made up of two telescopes, one on Hawai?i Island, the other 160 kilometers away at Haleakal?, Maui, providing whole-sky scans every two nights. About 100 asteroids larger than 30 meters in diameter are discovered by the facility every year.
In the case of 2019 MO, the ATLAS Maunaloa site picked up the object on the morning of June 22 when it was about 500,000 kilometers from Earth. Meanwhile, the Pan-STARRS 2 survey telescope at Haleakal? had imaged the same part of the sky about two hours earlier than ATLAS, revealing the incoming asteroid. The combination of data from the two sites allowed the object’s entry path to be refined, showing a likely impact that matched later Nexrad weather data in Puerto Rico, showing an entry over the ocean about 380 kilometers south of San Juan.
Image: A map of the predicted trajectory and final impact location for asteroid 2019MO. The predicted path is based on observations from the University of Hawai?i’s ATLAS and Pan-STARRS survey telescopes. Credit: Larry Denneau (IfA/ATLAS), Brooks Bays (SOEST).
So we’re getting some lead time on impactors even at this small scale, an indication that surprises from larger objects will be less likely in the future. Likewise cheering is news from research at NASA Ames which has just appeared in a special issue of Icarus, growing out of a workshop sponsored at Ames by the NASA Planetary Defense Coordination Office. Its theme: A new look at Tunguska, the 1908 impact that wreaked havoc in Siberia.
500,000 acres of uninhabited forest were flattened by the event, which was seen by few but left enough damage to alert scientists that this had not been a volcanic explosion or a mining accident, even though the first serious investigations didn’t occur until the 1920s. Today we look back at Tunguska as a kind of signature event, says Ames scientist David Morrison:
“Tunguska is the largest cosmic impact witnessed by modern humans. It also is characteristic of the sort of impact we are likely to have to protect against in the future.”
And the good news is that computer modeling discussed in the new papers shows that the interval between impacts like Tunguska is not, as has been previously estimated, on a timescale of centuries but rather millennia. 50 million combinations of asteroid and entry properties were analyzed by the computer models deployed here, some of them trying to match atmospheric pressure waves with the seismic signals recorded on the ground at the time, while others homed in on the kind of event that could produce the Tunguska treefall pattern and soil burn distribution. Four different computer modeling codes arrived at similar conclusions.
Thus we tighten our understanding of how rocks break apart in the atmosphere. The results show a stony Tunguska impactor between 50 and 80 meters in diameter that entered the atmosphere at about 15 kilometers per second, producing the equivalent of a 10 to 30 megaton explosion at between 9.5 and 14.5 kilometers altitude. The researchers used the latest estimates of the asteroid population to make the calculation of millennia between such impacts, correcting the earlier estimate that had been based on a smaller impactor.
Image: Trees flattened by the intense shock wave created in the atmosphere as the space rock exploded above Tunguska on June 30, 1908. The photograph was taken by the Soviet Academy of Science 1929 expedition led by Leonid Kulik. 500,000 acres, the size of a large metropolitan city, were flattened. Flattening trees requires an immense shock wave. Credit: Wikimedia Commons.
The new Tunguska work also draws upon extensive video observations and maps of the Chelyabinsk event. The advances in computer models that result show that the Chelyabinsk impactor was likely a stony asteroid that broke up about 24 kilometers above the ground, producing a shock wave like that of a 550 kiloton explosion. Chelyabinsk-class objects are thought to impact the Earth every 10 to 100 years, so learning how to provide sufficient warning, as the ATLAS and Pan-STARRS work shows us is now possible, can potentially save lives.
“Because there are so few observed cases, a lot of uncertainty remains about how large asteroids break up in the atmosphere and how much damage they could cause on the ground,” says Lorien Wheeler, a researcher from Ames, working on NASA’s Asteroid Threat Assessment Project. “However, recent advancements in computational models, along with analyses of the Chelyabinsk and other meteor events, are helping to improve our understanding of these factors so that we can better evaluate potential asteroid threats in the future.”
The seven papers on Tunguska and computational modeling of impact events growing out of the Ames workshop appear in a special issue of Icarus that can be found here.
