Tag Archives: #asteroids

New Study Finds Evidence of the Origin of Metal-Rich Near-Earth Asteroids (Planetary Science)

Little is known about the population of metal-rich Near-Earth Asteroids (NEAs), their number, origin, and relationship with meteorites found on Earth. A new paper by Planetary Science Institute Associate Research Scientist Juan Sanchez explains how near-infrared spectroscopic data of two NEAs reveals new information about the composition and physical properties of these bodies. 

“We find that both NEAs are composed of mostly metal and a small fraction of silicate minerals, similar to mesosiderites, a rare type of stony-iron meteorites found on Earth,” said Sanchez, lead author of the paper “Physical Characterization of Metal-rich Near-Earth Asteroids 6178 (1986 DA) and 2016 ED85” that appears in Planetary Science Journal. PSI Laboratory Technician Neil Pearson is also an author.

“Analysis of their orbits allows us to trace their origin to a region in the outer asteroid belt where the largest metal-rich asteroids reside,” Sanchez said. The asteroid belt is located between the orbits of Mars and Jupiter. 

“According to some studies, there are more than 60 parent bodies represented among iron meteorites found on Earth; however, those parent bodies have not been identified so far. There are also stony-iron meteorites and metal-rich carbonaceous chondrites whose origin is unknown,” Sanchez said. “Because NEAs represent a direct link between meteorites found on Earth and their parent bodies throughout the Solar System, the identification of metal-rich NEAs get us closer to determining the specific origin of the meteorites that derive from them.” 

The larger NEA, (1986 DA), is shown to be primarily metal by using radar data from an earlier study. Metal has a much higher radar reflectivity than rocky bodies composed of silicate minerals. The team’s new near-infrared spectra of 1986 DA confirmed that the asteroid surface is a mixture of about 85% metal and 15% pyroxene, a rock-forming silicate mineral found in igneous and metamorphic rocks. 

 For the other NEA, 2016 ED85, there is no radar data available, but Sanchez finds that its near-infrared spectrum is almost identical to the spectrum of 1986 DA and other metal-rich asteroids, suggesting that this object has a similar composition. 

The paper’s findings are based on observations from the NASA Infrared Telescope Facility on the island of Hawaii. The work was funded by the NASA Near-Earth Object Observations Program, which also funds the NASA Infrared Telescope Facility.

asteroid 1986 DA
An artist’s impression of a close flyby of the metal-rich near-Earth asteroid 1986 DA. Astronomers using the NASA Infrared Telescope Facility have confirmed that the asteroid is made of 85% metal. Credit: Addy Graham/University of Arizona.

Featured image: Photograph of stony-iron meteorite called mesosiderite showing iron-nickel metal mixed with silicate rocky material. Two metal-rich near-Earth asteroids observed by Planetary Science Institute astronomer Juan Sanchez are thought to be made of this rare class of meteorite. Credit: University of Arizona.


Provided by PSI

SwRI Team Zeroes In On Source Of the Impactor That Wiped Out the Dinosaurs (Planetary Science)

The impactor believed to have wiped out the dinosaurs and other life forms on Earth some 66 million years ago likely came from the outer half of the main asteroid belt, a region previously thought to produce few impactors. Researchers from Southwest Research Institute have shown that the processes that deliver large asteroids to Earth from that region occur at least 10 times more frequently than previously thought and that the composition of these bodies match what we know of the dinosaur-killing impactor. 

The SwRI team — including Dr. David Nesvorný, Dr. William Bottke and Dr. Simone Marchi — combined computer models of asteroid evolution with observations of known asteroids to investigate the frequency of so-called Chicxulub events. Over 66 million years ago, a body estimated to be 6 miles across hit in what is now Mexico’s Yucatan peninsula and formed Chicxulub crater, which is over 90 miles across. This massive blast triggered a mass extinction event that ended the reign of the dinosaurs. Over the last several decades, much has been learned about the Chicxulub event, but every advance has led to new questions.

“Two critical ones still unanswered are: ‘What was the source of the impactor?’ and ‘How often did such impact events occur on Earth in the past?’” Bottke said.

To probe the Chicxulub impact, geologists have previously examined 66-million-year-old rock samples found on land and within drill cores. The results indicate the impactor was similar to the carbonaceous chondrite class of meteorites, some of the most pristine materials in the solar system. Curiously, while carbonaceous chondrites are common among the many mile-wide bodies that approach the Earth, none today are close to the sizes needed to produce the Chicxulub impact with any kind of reasonable probability.

“We decided to look for where the siblings of the Chicxulub impactor might be hiding,” said Nesvorný, lead author of a paper describing the research.

“To explain their absence, several past groups have simulated large asteroid and comet breakups in the inner solar system, looking at surges of impacts on Earth with the largest one producing Chicxulub crater,” said Bottke, one of the paper’s co-authors. “While many of these models had interesting properties, none provided a satisfying match to what we know about asteroids and comets. It seemed like we were still missing something important.” 

To solve this problem, the team used computer models that track how objects escape the main asteroid belt, a zone of small bodies located between the orbits of Mars and Jupiter. Over eons, thermal forces allow these objects to drift into dynamical “escape hatches” where the gravitational kicks of the planets can push them into orbits nearing Earth. Using NASA’s Pleaides Supercomputer, the team followed 130,000 model asteroids evolving in this slow, steady manner for hundreds of millions of years. Particular attention was given to asteroids located in the outer half of the asteroid belt, the part that is furthest from the Sun. To their surprise, they found that 6-mile-wide asteroids from this region strike the Earth at least 10 times more often than previously calculated.

“This result is intriguing not only because the outer half of the asteroid belt is home to large numbers of carbonaceous chondrite impactors, but also because the team’s simulations can, for the first time, reproduce the orbits of large asteroids on the verge of approaching Earth,” said co-author Marchi. “Our explanation for the source of the Chicxulub impactor fits in beautifully with what we already know about how asteroids evolve.”

Overall, the team found that 6-mile-wide asteroids hit the Earth once every 250 million years on average, a timescale that yields reasonable odds that the Chicxulub crater occurred 66 million years ago. Moreover, nearly half of impacts were from carbonaceous chondrites, a good match with what is known about the Chicxulub impactor.

“This work will help us better understand the nature of the Chicxulub impact, while also telling us where other large impactors from Earth’s deep past might have originated,” Nesvorný said.

The journal Icarus is publishing a paper about this research, “Dark Primitive Asteroids Account for a Large Share of K/Pg-Scale Impacts on the Earth” (Volume 368, 1 November 2021, 114621, Elsevier publications).

A link to the published paper can be found here: https://doi.org/10.1016/j.icarus.2021.114621 , while a preprint is available here: https://arxiv.org/abs/2107.03458.

Featured image: An SwRI team modeled evolutionary processes in the main asteroid belt and discovered that impactors such as the one that ended the reign of the dinosaurs are most likely from the outer half of the main asteroid belt. The team also discovered that delivery processes from that region occur 10 times more often than previously thought. © Courtesy of SwRI/Don Davis


Provided by Southwest Research Institute

What Is The Probability And Consequences Of Collision Of Primordial Black Holes With Earth? (Cosmology)

Sohrab Rahvar investigated the scenario of collision of primordial black holes (PBHs) with earth. He showed that this collision has different consequences as heating the interior of the earth through dynamical friction and accretion processes. The findings of this study recently appeared in Arxiv.

Primordial black holes are a hypothetical type of black hole that formed soon after the Big Bang. In the early universe, high densities and heterogeneous conditions could have led sufficiently dense regions to undergo gravitational collapse, forming black holes. They are plausible candidates of dark matter.

Now, by assuming that PBHs fill the dark content of the Milky Way Galaxy in the Galactic halo and dark disk, Sohrab Rahvar calculated the probability of collision of the PBHs with Earth. He showed that the black holes with the mass M < 1015 gr have the chance of more than one impact per billion years with Earth and the rate of black holes that could be trapped in the interior of the earth is almost zero. But, how can we say so confidently that, there are no primordial black holes trapped inside the earth?

“Well, since the velocity of black holes in the halo and the dark disk is high, the dissipation process inside the earth is not effective to decelerate the black holes and black holes do not sink inside the earth where in this case black holes could heat the interior of the earth and finally swallow the whole mass of earth. The number of this event for M ≃ 1015 gr is about 10¯11 Gyr¯1. This calculation assures that probability of primordial black holes being trapped inside the earth is almost zero.”

Another important point to be noted is that, those black holes that cross the earth, heat the interior of the earth through various mechanisms like through dynamical friction which can generate heat in the range of 105–108 J and through accretion process by about 1015 J.

Moreover, he calculated the energy released by a black hole collision with earth and compared it with the impact of asteroids on the earth. He have shown that the accretion process by the black holes is the dominant process for energy release. The amount of energy from this collision is comparable with a kilometer size asteroid where it happens four orders of magnitude more frequently than a black hole collision.

Finally, he concluded that the likelihood of the dangerous impact of primordial black holes with the earth is very low.


Reference: Sohrab Rahvar, “Possibility of Primordial black holes Collision with Earth and the Consequences”, Arxiv, pp. 1-5, 2021.
arXiv:2107.11139


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Astronomers Discovered A New Class Of Super-Slow Rotating Asteroids (Planetary Science)

A team of international astronomers reported the discovery of a new class of super-slow rotating asteroids, having rotational period more than 1000 hours, in data extracted from the Asteroid Terrestrial-impact Last Alert System (ATLAS) and Zwicky Transient Facility (ZTF) all-sky surveys. Their study recently appeared in Arxiv.

Many asteroid rotation periods have already been extracted from the ATLAS photometry data set. In, previous study Erasmus and colleagues reported the shape-models and rotation periods of ∼2750 asteroids of which a few had periods around 800-900 hours and the longest period found was 1236 hours. Later, they published two more papers in which they reported the colours and rotation periods of >1000 main-belt family members and ∼ 40 Jupiter Trojans, respectively, of which the longest period found out of those two studies was 165 hours.

Now, Erasmus and colleagues presented photometry for a smaller number of asteroids (39), some observed for over 5 years in the ATLAS and ZTF surveys. However, each of these 39 asteroids has a rotation period greater than 1000 hours i.e. 42 days (min: 1013 hours; median: 2390 hours; max: 4812 hours), meaning that they have discovered a class of “super slow” rotating asteroids that has never been seen before.

Of the 39 rotation periods they reported, 32 of these objects have periods longer than any previously reported unambiguous rotation periods currently in the Asteroid Light Curve Database. In their sample, 7 objects have a rotation period > 4000 hours and the longest period they reported is 4812 hours (∼ 200 days).

“We do not observe any correlation between taxonomy, albedo, or orbital properties with super-slow rotating status.”

The most plausible mechanism for the creation of these very slow rotators is if their rotations were slowed by YORP spin-down. Super-slow rotating asteroids may be common, with at least 0.4% of the main-belt asteroid population with a size range between 2 and 20 km in diameter rotating with periods longer than 1000 hours.

Figure 1. The collisional (solid) and YORP (dashed) timescales for an object of a given size at a range of rotation periods. The red, blue and green lines indicate the timescales for an object with diameter 5, 15 and 30 km respectively © Erasmus et al.

“The forthcoming LSST will increase the number of known asteroids by a factor of five in a uniform and sparse survey that will be ideal for extending this work to more and much smaller asteroids.”

— concluded authors of the study.

Note:

1) YORP effect: In the YORP effect the body’s shape has a more effective role than albedo in altering the spin rate. For small asteroids (< 10 km), YORP can cause measurable changes in rotation rate. The effect can even speed up the rotation leading to disintegration.


Reference: N. Erasmus, D. Kramer, A. McNeill, D. E. Trilling, P. Janse van Rensburg, G. T. van Belle, J. L. Tonry, L. Denneau, A. Heinze, H. J. Weiland, “Discovery of Super-Slow Rotating Asteroids with ATLAS and ZTF photometry”, Arxiv, pp. 1-12, 2021. https://arxiv.org/abs/2106.16066


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Astronomers Discovered Two Extremely Red Main-belt Asteroids (Planetary Science)

A team of international astronomers discovered two extremely red main-belt asteroids: 203 Pompeja and 269 Justitia. These were identified from combined visible and near-infrared spectroscopic observations collected at the IRTF and SAO observatories. Their study recently appeared in Arxiv.

The majority of asteroids in the Solar System are found in the main asteroid belt (MBA). This is located between the orbits of Mars and Jupiter, with the greatest concentration of asteroids between 2.12 and 3.3 AU. Early in the history of the solar system, the gravity of newly formed Jupiter brought an end to the formation of planetary bodies in this region and caused the small bodies to collide with one another, fragmenting them into the asteroids we observe today. But, not all MBAs experienced catastrophic destruction. Some are larger than 110 km in diameter and still exists, means these objects can be regarded as the last remains of the original population of planetesimals that initially populated the inner solar system. Moreover, if such MBA’s escaped catastrophic destruction, their orbital elements have not been substantially altered which means, those orbital elements maintained their state at the end of migration state of solar system.

Now, a team of international astronomers performed their observations on the 3.0-m NASA Infra-red Telescope Facility (IRTF) on Mt. Mauna Kea, Hawaii, USA and at the 1.0- m Seoul National University Astronomical Observatory (SAO), Republic of Korea. They also recorded asteroidal spectroscopic data by two different instruments mounted on these telescope and discovered two extremely red main-belt asteroids: 203 Pompeja and 269 Justita.

The first extremely red asteroid called, “203 Pompeja” was discovered by chance in the visible to near-infrared wavelength range during spectroscopic survey. It has diameter if 111.3 km and an albedo of 0.045. It is located in the middle main-belt.

While, the second very red asteroid called “269 Justitia” had already been discovered and have a diameter of 54.4 km and an albedo of 0.080. It is also located in the middle main-belt.

Figure 1. Spectra of the very red spectral slope MBAs 203 Pompeja and 269 Justitia and other dark (low-albedo) objects in the visible and near-infrared region. Comparison of the very red asteroids with: top left panel: typical spectral types of dark asteroids from the Bus-DeMeo classification scheme, top right: typical Hildas and Jovian Trojans, bottom left: meteorites with a very red spectral slope, bottom right: dark outer Solar System objects spectrally similar to the very red asteroids © Hasegawa et al.

They also found that, these two asteroids have a redder spectral slope than any other D-type body, which are the reddest objects in the asteroid belt, and similar to RR and IR-class objects found in the outer Solar System among trans-Neptunian objects (TNO’s) and Centaurs.

In addition, their spectroscopic results suggested the presence of complex organic materials on the surface layer of these asteroids, implying that they could have formed in the vicinity of Neptune and been transplanted to the main belt region during a phase of planetary migration.

Finally, 203 Pompeia is the only very red asteroid known so far among the ∼250 bodies with diameter larger than 110 km (i.e. presumably structurally intact) found in the asteroid belt.

“These discoveries add another piece of evidence that the main asteroid belt hosts a population of bodies that were formed in the outskirt of the Solar System.”, concluded authors of the study.


To know more: Read on The Astrophysical Journal Letters the article ” Discovery of two TNO-like bodies in the asteroid belt “, by Sunao Hasegawa, Michael Marsset, Francesca E. DeMeo, Schelte J. Bus, Jooyeon Geem, Masateru Ishiguro, Myungshin Im, Daisuke Kuroda and Pierre Vernazza


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What Would Happen If Dark Asteroid Travels Through A Star? (Cosmology)

Anirban Das and colleagues in their recent paper showed that, when dark asteroids travels through a star, it produces shock waves, which quickly propagate to the stellar surface, where it is released in the form of a transient optical, UV or X-ray emission. They also suggested how we can search and detect such signature. Their study recently appeared in Arxiv.

If you read our articles everyday, you may came across several studies which demonstrated that light dark matter (DM) particles can capture or produce inside stars or compact objects and can change their properties like mass, orbital period, luminosity etc. However, DM could also be in the form of objects of macroscopic mass and size. Such objects are hard to detect because of their rarity. According to several studies heavy DM asteroids can pass through earth but we haven’t detected one yet since the advent of human civilization. Now, Anirban Das and colleagues suggested that dark asteroids in the mass range of 10¯20–10¯11 can pass through stars. Thus, we must look them in the stars.

Figure 1. Depiction of the phases of shock propagation (left). A cylindrical blast wave solution is matched to an N-wave (right) when the shock become weak. The N-wave is propagated along acoustic rays, and becomes strong and deposits its energy near the surface. © Anirban Das et al.

They point out that, because dark asteroids move supersonically in stars, dissipation through any non-gravitational interaction will generate shock waves. This allows the dissipated energy to quickly propagate to the stellar surface, where it is released in the form of a transient, thermal ultraviolet (UV) emission.

“Crucially, such events are correlated with the local DM density, but uncorrelated with the underlying activity of the star.”

We can detect such events without requiring a dedicated search with the help of next-generation survey telescopes. While, in a dense globular cluster, such events occur far more often than flare backgrounds, so, an existing UV telescopes could find them by monitoring regions of high DM density.

“At the opposite end of the mass range, impacts on the Sun are expected to occur annually for mass of dark matter, MDM ≲ 10¯19M, and would be energetic enough to be easily detected by solar observatories.”

“It would be interesting to see if the resolution of these instruments permits such impacts to be distinguished from solar flares. In many of these cases, it may be possible to find impact events in a reanalysis of archival data.”, concluded authors of the study.


Reference: Anirban Das, Sebastian A. R. Ellis, Philip C. Schuster, Kevin Zhou, “Stellar Shocks From Dark Matter”, Arxiv, pp. 1-13, 2021. https://arxiv.org/abs/2106.09033


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Astronomers Presented “THOR” For Main Belt And Kuiper Belt Object Searches (Astronomy)

The number of Solar System minor planet discoveries is growing rapidly thanks to the continuation of present day surveys such as Pan-STARRS and the Catalina Sky Survey, and upcoming surveys such as the Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) and NEOCam, recently renamed to NEO Surveyor. In about a decade, the number of known objects will grow from the currently known 1 million to about 6 million minor planets. Such an increase in discoveries will enable a higher resolution look into the dynamical evolution of our Solar System. However, identifying minor planets in survey images and linking their detections into orbits continues to be a challenging problem. First, linking asteroid detections across multiple nights is difficult due to the sheer number of possible linkages, made even more challenging by the presence of false positives. Second, the motion of the observer makes the linking problem non-linear as minor planets will exhibit higher order motion on the topocentric sky over the course of weeks. Finally, once potential linkages have been established, they need to be confirmed as possible orbits using computationally expensive orbit determination software. For example, the Vera C. Rubin Observatory estimates it will discover nearly six million Main Belt asteroids that will be observed hundreds of times over the course of its ten year survey. Naively attempting to link hundreds of millions of asteroid detections over a ten year period is not computationally feasible.

To make the linking problem more computationally tractable, surveys which aim to discover minor planets focus on constructing “tracklets”: two dimensional sky-plane motion vectors consisting of two or more detections spaced typically 20-90 minutes apart that constrain the direction and rate of motion of potential moving objects. Tracklets are constructed to reduce the number of possible linkages that could be formed by providing information on plausible direction and skyplane angular velocity. They are then linked into inter-night linkages known as “tracks”: sky-plane paths of motion containing several tracklets spanning up to ∼ 15 nights, typically modelled with low-order polynomials. In the case of the LSST, for a moving object to be discoverable it must be observed at least twice a night on at least three unique nights within the 15 day window to go through the trackletto-track creation process. In part due to the relative motion of the observer and the rate of motion of moving objects, both tracklets and tracks can exhibit high residuals relative to the fitted low-order polynomial, requiring relaxed fitting tolerances that can in turn lead to the creation of many spurious candidate linkages. Orbit determination (OD) algorithms are therefore required to run on each candidate track so spurious linkages can be identified and be removed.

The Zwicky Transient Facility (ZTF), an optical timedomain survey scanning the entire northern hemisphere of sky at a rate of more than 3700 deg² hr−¹ , can be seen as a precursor to the LSST. ZTF uses the ZTF Moving Object Discovery Engine (ZMODE) algorithm. Instead of linking tracklets directly into tracks, ZMODE first attempts to build a “stringlet”. A stringlet forms an intermediate step between tracklets and tracks which allows for the linking of pairs of detections across nights before tracks are built. This approach was designed to accommodate ZTF’s cadence during its main survey, where the cadence is frequently too sparse to form short intra-night tracklets.

Recently, Holman and colleagues has shown promising results by shifting the reference frame for linking detections to the heliocenter. By assuming a heliocentric distance and its rate of change, cleverly fitting tracklets for the remaining unknown parameters in inertial space, then propagating the resulting “arrows” to a common epoch, arrows corresponding to the same minor planets will form clusters. These clusters can then be extracted and subsequently validated by orbit fitting. As a testament to the effectiveness of HelioLinC, some 200,000 new minor planet orbits were recovered from the Minor Planet Center’s Isolated Tracklet File (ITF).

Common in all of these approaches is the requirement to build tracklets, which in turn requires a telescope to perform multiple revisits to the same field in a night, then more revisits a few nights later, and so on. For a survey that cannot cover the entire visible sky twice per night, this leads to up to a factor of two reduction of the nightly surveyed area. For a survey such as the LSST, which aims to balance four different science drivers, requiring such a cadence decreases the overall ease by which the other science drivers can be accommodated. It is therefore prudent to investigate whether linking algorithms that are cadence independent can be constructed and whether such algorithms can perform as good or better than the current methods. An algorithm that does not demand a high revisit cadence could increase the efficiency of future surveys, as well as help multi-science missions such as the LSST.

Now, Moeyens and colleagues presented one such cadence- and observer-independent linking algorithm: “Tracklet-less Heliocentric Orbit Recovery” (THOR). Rather than shifting the origin to the heliocenter like Holman and colleagues, they choose to shift the linking frame of reference to a series of dynamically selected heliocentric “test orbits”. The main insight is that transforming detections into the frame of a test orbit linearizes the motion of all objects in a relatively thick bundle of orbits near the test orbit (in phase space), which can then be picked out with line-detection algorithms such as the Hough transform. This provides a path to scanning an otherwise voluminous 6D phase space with a finite number of test orbits and at feasible computational cost.

“By sparsely covering regions of interest in the phase space with “test orbits”, transforming nearby observations over a few nights into the co-rotating frame of the test orbit at each epoch, and then performing a generalized Hough transform on the transformed detections followed by orbit determination (OD) filtering, candidate clusters of observations belonging to the same objects can be recovered at moderate computational cost and little to no constraints on cadence.”

They validated the effectiveness of this approach by running on simulations as well as on real data from the Zwicky Transient Facility (ZTF). They have shown that, THOR can link Main Belt asteroids and more distant populations at high completeness and at moderate computational cost. On two weeks of simulated data, THOR recovered 91.3% of the 18,332 that were ideally findable, whereas a tracklet-based linking algorithm would have recovered none.

On two weeks of ZTF data, THOR linked 97.2% of the 21,542 objects with at least five detections (a factor of ∼ 2 recovery increase over MOPS and a factor of ∼ 1.5 increase over ZMODE). THOR recovered orbits for 97.4% of objects beyond 1.7 au, with 98.4% of objects recovered beyond 2.5 au.

Figure 1. In the top panel, simulated detections of real orbits on the first night of a simulated survey are plotted in grey. The survey consists of 16 ten deg² fields visited once every other night over a 14-night window. The location of the test orbit on the first night is shown as a black plus sign. The red circle outlines the cell of gathered detections which are plotted in blue. In the bottom panel, the test orbit is propagated to all possible times in the survey (the remaining six possible exposures) with a cell of observations gathered at each predicted location and epoch. The simulated detections of the subsequent six visits are plotted in grey in addition to those from the first night. The gathered detections are plotted in blue as in the top panel. The black line tracks the sky-plane motion of the test orbit, with its location on each line plotted as black plus signs. This figure was generated using plots simulations.ipynb. © Moeyens et al.

Furthermore, by comparing the 2018 sample to the catalog of orbits as presently known (April 2021), they showed that the lower limit on purity of THOR sub-missions to the MPC would be 97.7% and – assuming all candidates shown in Figure 1 above, are confirmed as real – possibly as high as 100%. This, in combination with its capability to discover objects regardless of cadence or observer, renders it immediately useful for Main Belt and Kuiper Belt Objects (KBO) searches on survey data and archival datasets.

According to authors, while THOR can be applied to running surveys, application to archival datasets – such as ZTF, CSS, or PanSTARRS – is interesting as well. The most straightforward way to do this would be to run a sliding ∼two week window over the dataset, from beginning to the end, running THOR at each window instance. For each window run, orbits could be computed for the discovered objects and projected to the past (and future) for discovery of additional observations (and improvement of orbital solutions). This is the approach they themselves plan to take with the ZTF archival data.

Assuming the 11 objects discovered here are representative of remaining undiscovered objects in ZTF, this search would likely yield on order of 1, 000 asteroids. The discovery potential with deeper archival datasets (e.g., DES data or the DECam archive) is likely to be significantly larger.

The THOR package and demo Jupyter notebooks are open source and available at this https URL.

Featured image: The observations of the 11 discovered candidates are plotted in red, with the sky-plane motion of their best-fit orbits plotted as lines. 10 of the objects show MBA-like best fit orbit solutions. The remaining object has a hyperbolic orbit solution and corresponds to recovery observations of the hyperbolic comet C/2018 U1. The “wiggles” apparent in some of the best-fit orbit lines are due to the motion of the observer (topocentric motion). This figure was generated using plots ztf.ipynb. © Moeyens et al.


For more:

Joachim Moeyens, Mario Juric, Jes Ford, Dino Bektesevic, Andrew J. Connolly, Siegfried Eggl, Željko Ivezić, R. Lynne Jones, J. Bryce Kalmbach, Hayden Smotherman, “THOR: An Algorithm for Cadence-Independent Asteroid Discovery”, Arxiv, pp. 1-22, 2021. https://arxiv.org/abs/2105.01056


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‘The Line is Getting Fuzzier’: Asteroids and Comets May be More Similar Than We Think (Planetary Science)

As anyone who has ever tried to clean a home knows, ridding yourself of dust is a Sisyphean effort. No surface stays free of it for long. It turns out that space is somewhat similar. Space is filled with interplanetary dust, which the Earth constantly collects as it plods around the sun – in orbit, in the atmosphere, and if it’s large enough, on the ground as micrometeorites.

While specimens may not be large, it turns out such dust particles are reforming scientists’ conception of asteroids and comets and are enough to reconstruct entire scenes in the history of the solar system.

Asteroids and comets are primitive bodies left over from early in solar system formation, so the more we can know about their composition, the more we know about where they formed. Those asteroids that formed in the same neighbourhood as comets tend to be closer in composition to them.

Trying to break down the asteroid-comet continuum and categorise how similar asteroids could be to comets is what Dr Pierre Beck is doing in the SOLARYS project at France’s University of Grenoble Alpes.

There are about a million asteroids registered officially and there should be many more, he explains.

‘Traditionally, these objects have been thought of as the most primitive in the solar system. You can look at the ingredients and see what was there, how they were accreted and how they were formed a long time ago.’

Similar primordial material that formed Earth or Mars has experienced geological activity and been fundamentally changed by conditions like heat, pressure and erosion.

‘The most primitive objects therefore don’t come to Earth in the form of rocks, but in the form of dust,’ he said. ‘While the expected (amount) of meteorites that come to Earth in a year may be 5-6 tonnes – for dust it is 40,000 tonnes.’

Using samples of interplanetary dust collected from high in our stratosphere and micrometeorites from pristine locations like Antarctica, Dr Beck is using a new method of infrared spectroscopy combined with atomic force microscopes to examine their spectra and properties on the micrometre-scale.

Like an archaeologist placing artefacts from a dig site, he can then compare those results to existing data from asteroids in space. ‘When you’re a geologist and you find a rock, you have an outcropping and try see the rock in its context,’ Dr Beck said.

‘In the past we thought asteroids are rocks, comets are icy. But now we see that there are comets that are almost inactive…and there are asteroids that are active.’

— Dr Jessica Agarwal, Technical University of Braunschweig, Germany

Compounds

Using changes in infrared laser light on samples that are just 10-20 micrometres, his team can for the first time pick out silicate minerals and organic compounds without using harsh chemicals that would disturb the material. They also construct bigger models of the samples in the lab to refine what to look for to identify and categorise asteroids and comets with ground-based telescopes.

What they have found in the dust are complex organic polymers, rich in hydrocarbons and elements like nitrogen and oxygen or sometimes deuterium (heavy water).

‘There is a big debate on how these extra-terrestrial organics were formed. One hypothesis is that ice mixtures were irradiated, but in that case different types of ice mixtures should yield different types of organics,’ said Dr Beck.

Studying the chemical composition of these samples should help him to learn more about asteroids’ origins as well as the difference between D-type asteroids, dark and difficult to detect bodies, some with icy interiors, which originate around Jupiter and beyond, and icy comets.

‘If we understand that, it will tell us what the outer solar system is made of and more about the initial stuff that came into the solar system.’

Knowing where certain organic dust types can be found could even help future space probes.

‘You could view some of these asteroids as a fuel source,’ he said. If there are reduced organic compounds, he says, they could be used as a source of energy.

Comets

The presence of such compounds in interplanetary dust is just one thing making scientists wonder if asteroids and comets aren’t necessarily so different after all. Dr Jessica Agarwal at the CASTRA project thinks there may be overlap for other reasons, too.

Using data from the European Space Agency’s Rosetta probe that studied Comet 67P/Churyumov–Gerasimenko and from astronomical telescopes, Dr Agarwal and her team at the Technical University of Braunschweig in Germany looked at how comets and asteroids actively emit material into space.

‘We aim to better understand the processes that lead to changes in the surfaces and interiors of comets and asteroids,’ she said. ‘We also hope to better understand their primitive nature, or how they were 4.5 billion years ago.’

Using data from several instruments onboard Rosetta, Dr Agarwal’s team has been able to model the properties of cometary dust in the environment of Comet 67P. They found that the dust particles could be loose aggregates of micron-sized silicate and sub-micron-sized carbonaceous components.

‘We are also observing huge boulder-size materials coming out from Comet 67P, coming from certain specific places on the surface…a fountain of boulders,’ Dr Agarwal explained.

COSIMA is the in-situ dust analyzing instrument on board space probe Rosetta to comet 67P/Churyumov-Gerasimenko. Image credit – DLR German Aerospace Center, licensed under CC BY 2.0

Active asteroid

Comets are not the only bodies to emit material. Take the case of asteroid 288P. A so-called active asteroid that emits dust, from a distance it looks like a comet with a dusty tail.

‘The weird thing about 288P was that its nucleus looked double…and in the end, I thought, well maybe it’s a binary?’ Dr Agarwal said. ‘We had to wait a couple of years to reobserve it from close up, and then in 2016 we got more Hubble time and really saw that it was two components.’

Their measurements determined that this first-of-its-kind asteroid to be observed is comprised of two similarly-sized pieces, orbiting each other 100 kilometres apart.

‘We found it by chance. We don’t know if there are more systems like it that we don’t see,’ Dr Agarwal said.

They theorise that the asteroids were irradiated by the sun and begin to rotate, splitting in two when they spun too fast to hold together. The distance between the pair may be due to a jet of gas vaporising from the surface that propelled one rock away like a rocket. They are still trying to figure out what causes the tail.

Scientists have long thought that asteroids mainly evolved through collisions, but it’s possible that for smaller asteroids, fast rotation plays just as much of a role.

Their research has revealed a range of active asteroids, from those which have a one-off burst of activity (as if from an impact), to those that emit bursts of dust repeatedly.

‘There is some process happening more or less randomly that triggers the eruption of dust clouds,’ Dr Agarwal said, referring to asteroids which emit the repeated dust bursts. ‘We think maybe it is fast rotation that triggers landslides or something like that.’

The upshot of all of this is that the distinction between comets and asteroids may be more of a spectrum than a hard divide.

‘The line is getting fuzzier. In the past we thought asteroids are rocks, comets are icy. But now we see that there are comets that are almost inactive…and there are asteroids that are active. There is more of a transition between those two populations than we thought in the past,’ Dr Agarwal said.

The research in this article was funded by the EU’s European Research Council. 

Featured image: Using data from several instruments onboard Rosetta, CASTRA’s team has modeled the properties of cometary dust in the environment of Comet 67P. Image credit – ESA/Rosetta/NAVCAM, CC BY-SA IGO 3.0


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How Scientists Are ‘Looking’ Inside Asteroids? (Planetary Science)

Asteroids can pose a threat to life on Earth but are also a valuable source of resources to make fuel or water to aid deep space exploration. Devoid of geological and atmospheric processes, these space rocks provide a window onto the evolution of the solar system. But to really understand their secrets, scientists must know what’s inside them.

Only four spacecraft have ever landed on an asteroid – most recently in October 2020 – but none has peered inside one. Yet understanding the internal structures of these cosmic rocks is crucial for answering key questions about, for example, the origins of our own planet.

‘Asteroids are the only objects in our solar system that are more or less unchanged since the very beginning of the solar system’s formation,’ said Dr Fabio Ferrari, who studies asteroid dynamics at the University of Bern, Switzerland. ‘If we know what’s inside asteroids, we can understand a lot about how planets formed, how everything that we have in our solar system has formed and might evolve in the future.’

Then are also more practical reasons for knowing what’s inside an asteroid, such as mining for materials to facilitate human exploration of other celestial bodies, but also defending against an Earth-bound rock.

NASA’s upcoming Double Asteroid Redirection Test (DART) mission, expected to launch later this year, will crash into the 160m in diameter asteroid moon Dimorphos in 2022, with the aim of changing its orbit. The experiment will demonstrate for the first time whether humans can deflect a potentially dangerous asteroid.

But scientists have only rough ideas about how Dimorphos will respond to the impact as they know very little about both this asteroid moon, and its parent asteroid, Didymos.

To better address such questions, scientists are investigating how to remotely tell what’s inside an asteroid and discern its type.

Types

There are many types of asteroids. Some are solid blocks of rock, rugged and sturdy, others are conglomerates of pebbles, boulders and sand, products of many orbital collisions, held together only by the power of gravity. There are also rare metallic asteroids, heavy and dense.

‘To deflect the denser monolithic asteroids, you would need a bigger spacecraft, you would need to travel faster,’ said Dr Hannah Susorney, a research fellow in planetary science at the University of Bristol, the UK. ‘The asteroids that are just bags of material – we call them rubble piles – can, on the other hand, blow apart into thousands of pieces. Those pieces could by themselves become dangerous.’

Dr Susorney is exploring what surface features of an asteroid can reveal about the structure of its interior as part of a project called EROS.  

This information could be useful for future space mining companies who would want to know as much as possible about a promising asteroid before investing into a costly prospecting mission as well as knowing more about potential threats.

‘There are thousands of near-Earth asteroids, those whose trajectories could one day intersect with that of the Earth,’ she said. ‘We have only visited a handful of them. We know close to nothing about the vast majority.’

During the fourth ever landing on an asteroid, Bennu was mapped thanks to a mosaic of images collected by NASA’s OSIRIS-REx spacecraft. Peering inside an asteroid is the next crucial step. Image credit – NASA/Goddard/University of Arizona

Topography

Dr Susorney is trying to create detailed topography models of two of the most well-studied asteroids – Itokawa (the target of the 2005 Japanese Hayabusa 1 mission) and Eros (mapped in detail by the NEAR Shoemaker space probe in the late 1990s).

‘The surface topography can actually tell us a lot,’ Dr Susorney said. ‘If you have a rubble pile asteroid, such as Itokawa, which is essentially just a bag of fluff, you cannot expect very steep slopes there. Sand cannot be held up into an infinite slope unless it’s supported. A solid cliff can. The rocky monolithic asteroids, such as Eros, do tend to have much more pronounced topographical features, much deeper and steeper craters.’

Susorney wants to take the high-resolution models derived from spacecraft data and find parameters in them that could then be used in the much lower resolution asteroid shape models created from ground-based radar observations.

‘The difference in the resolution is quite substantial,’ she admits. ‘Tens to hundreds of metres in the high-res spacecraft models and kilometres from ground-based radar measurements. But we have found that, for example, the slope distribution gives us a hint. How much of the asteroid is flat and how much is steep?’

Coloured topographical maps from Dr Susorney show Eros (left), a rocky monolithic asteroid, as having steeper craters than Itokawa (right), a rubble pile asteroid. Image credit – Hannah Susorney

Dr Ferrari is working with the team preparing the DART mission. As part of a project called GRAINS, he developed a tool that enables modelling of the interior of Dimorphos, the impact target, as well as other rubble pile asteroids.

‘We expect that Dimorphos is a rubble pile because we think that it formed from matter ejected by the main asteroid, Didymos, when it was spinning very fast,’ Dr Ferrari said. ‘This ejected matter then re-accreted and formed the moon. But we have no observations of its interior.’

An aerospace engineer by education, Dr Ferrari borrowed a solution for the asteroid problem from the engineering world, from a discipline called granular dynamics.

‘On Earth, this technique can be used to study problems such as sand piling or various industrial processes involving small particles,’ Dr Ferrari said. ‘It’s a numerical tool that allows us to model the interaction between the different particles (components) – in our case, the various boulders and pebbles inside the asteroid.’

‘Asteroids are the only objects in our solar system that are more or less unchanged since the very beginning of the solar system’s formation.’

— Dr Fabio Ferrari, University of Bern, Switzerland

Rubble pile

The researchers are modelling various shapes and sizes, various compositions of the boulders and pebbles, the gravitational interactions and the friction between them. They can run thousands of such simulations and then compare them with surface data about known asteroids to understand rubble pile asteroids’ behaviour and make-up.

‘We can look at the external shape, study various features on the surface, and compare that with our simulations,’ Dr Ferrari said. ‘For example, some asteroids have a prominent equatorial bulge,’ he says, referring to the thickening around the equator that can appear as a result of the asteroid spinning.

In the simulations, the bulge might appear more prominent for some internal structures than others.

For the first time, Dr Ferrari added, the tool can work with non-spherical elements, which considerably improves accuracy.

‘Spheres behave very differently from angular objects,’ he said.

The model suggests that in the case of Dimorphos, the DART impact will create a crater and throw up a lot of material from the asteroid’s surface. But there are still many questions, particularly the size of the crater, according to Dr Ferrari.

‘The crater might be as small as ten metres but also as wide as a hundred metres, taking up half the size of the asteroid. We don’t really know,’ said Dr Ferrari. ‘Rubble piles are tricky. Because they are so loose, they might as well just absorb the impact.’

No matter what happens on Dimorphos, the experiment will provide a treasure trove of data for refining future simulations and models. We can see whether the asteroid behaves as we expected and learn how to make more accurate predictions for future missions that lives on Earth may very well depend on.

The solar system’s asteroid belt contains C-type asteroids, which likely consist of clay and silicate rocks, M-type, which are composed mainly of metallic iron, and S-type, which are formed of silicate materials and nickel-iron. Image credit – Horizon

The research in this article was funded by the EU.

Featured image: The shape of asteroids such as 243 Ida can reveal information about what they’re made of, which can, in turn, tell us more about the formation of the solar system. Image credit – NASA/JPL/USGS


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