Posted on by Jon Voisey

Trojans May Yet Rain Down

It would be an interesting survey to catalog the initial reactions readers have to “Trojans”. Do you think first of wooden horses, or do asteroids spring to mind? Given the context of this website, I’d hope it’s the latter. If so, you’re thinking along the right lines. But how much do you really know about astronomical Trojans?

While most frequently used to discuss the set of objects in Jupiter’s orbital path that lie 60º ahead and behind the planet, orbiting the L4 and L5 Lagrange points, the term can be expanded to include any family of objects orbiting these points of relative stability around any other object. While Jupiter’s Trojan family is known to include over 3,000 objects, other solar system objects have been discovered with families of their own. Even one of Saturn’s moons, Tethys, has objects in its Lagrange points (although in this case, the objects are full moons in their own right: Calypso and Telesto).

In the past decade Neptunian Trojans have been discovered. By the end of this summer, six have been confirmed. Yet despite this small sample, these objects have some unexpected properties and may outnumber the number of asteroids in the main belt by an order of magnitude. However, they aren’t permanent and a paper published in the July issue of the International Journal of Astrobiology suggests that these reservoirs may produce many of the short period comets we see and “contribute a significant fraction of the impact hazard to the Earth.”

The origin of short period comets is an unusual one. While the sources of near Earth asteroids and long period comets have been well established, short period comets parent locations have been harder to pin down. Many have orbits with aphelions in the outer solar system, well past Neptune. This led to the independent prediction of a source of bodies in the far reaches by Edgeworth (1943) and Kuiper (1951). Yet others have aphelions well within the solar system. While some of this could be attributed to loss of energy from close passes to planets, it did not sufficiently account for the full number and astronomers began searching for other sources.

In 2006, J. Horner and N. Evans demonstrated the potential for objects from the outer solar system to be captured by the Jovian planets. In that paper, Horner and Evans considered the longevity of the stability of such captures for Jupiter Trojans. The two found that these objects were stable for billions of years but could eventually leak out. This would provide a storing of potential comets to help account for some of the oddities.

However, the Jupiter population is dynamically “cold” and does not contain a large distribution of velocities that would lead to more rapid shedding. Similarly, Saturn’s Trojan family was not found to be excited and was estimated to have a half life of ~2.5 billion years. One of the oddities of the Neptunian Trojans is that those few discovered thus far have tended to have high inclinations. This indicates that this family may be more dynamically excited, or “hotter” than that of other families, leading to a faster rate of shedding. Even with this realization, the full picture may not yet be clear given that searches for Trojans concentrate on the ecliptic and would likely miss additional members at higher inclinations, thus biasing surveys towards lower inclinations.

To assess the dangers of this excited population, Horner teamed with Patryk Lykawka to simulate the Neptunian Trojan system. From it, they estimated the family had a half life of ~550 million years. Objects leaving this population would then undergo several possible fates. In many cases, they resembled the Centaur class of objects with low eccentricities and with perihelion near Jupiter and aphelion near Neptune. Others picked up energy from other gas giants and were ejected from the solar system, and yet others became short period comets with aphelions near Jupiter.

Given the ability for this the Neptunian Trojans to eject members frequently, the two examined how many of the of short period comets we see may be from these reservoirs. Given the unknown nature of how large these stores are, the authors estimated that they could contribute as little as 3%. But if the populations are as large as some estimates have indicated, they would be sufficient to supply the entire collection of short period comets. Undoubtedly, the truth lies somewhere in between, but should it lie towards the upper end, the Neptunian Trojans could supply us with a new comet every 100 years on average.

Posted on by Nancy Atkinson

ISS Instrument Detects X-ray Nova

Comparison of all-sky images before and after Sept. 25 when the nova was found. Credit: JAXA

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An instrument on board the International Space Station has discovered an X-ray nova. The science team from the Monitor of All-sky X-ray Image (MAXI) instrument on the Exposed Facility of the Japanese Kibo reported a short-lived X-ray nova became visible in the constellation of Ophiuchus on September 25, 2010, and the MAXI team confirmed that it was an uncatalogued X-ray source. Astronomers say the outburst is likely to be from a binary system with a black hole. The nova was named “MAXI J1659-152, in honor of the MAXI instrument.

X-ray novas appear suddenly in the sky and dramatically increases in strength over a period of a few days and then decreases, with an overall lifetime of a few months. Sometimes, these elusive novas have an optical counterpart. Unlike a conventional nova, in which the compact component is a white dwarf, an X-ray nova may be caused by material falling onto a neutron star or a black hole.

ESA’s INTEGRAL gamma-ray observatory also detected hard X-ray emission from the same position, and NASA’s Swift Observatory also was alerted by the flare-up. Following the discovery, many other astronomical observatories around the world have made follow-up observations in X-ray, gamma-ray, visible, infrared, and radio wavelengths. This discovery was led by Prof. Hitoshi Nego, a member of the MAXI team.

Source: JAXA

Posted on by Jon Voisey

Does a “Rock Comet” Generate the Geminids?

Meteor
Geminid meteor shower

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Many annual meteor showers have parent bodies identified. For example, the Perseids are ejecta from the comet, Swift-Tuttle and the Leonids from Tempel-Tuttle. Most known parent bodies are active comets, but one exception is the Geminid meteor shower that peaks in mid December. The parent for this shower is 3200 Phaethon. Observations of this object have shown it to be largely inactive pegging it as either a dead comet or an asteroid. But on June 20, 2009, shortly after perihelion, 3200 Phaethon brightened by over two magnitudes indicating this object may not be as dead as previously considered. A new paper considers the causes of the brightening and concludes that it could be a new mechanism leading to what the authors deem a “rock comet”.

David Jewett and Jing Li of UCLA, the authors of this new paper, consider several potential causes. Due to the size of 3200 Phaethon, they suggest that a collision is unlikely. One clue to the reason for the sudden change in brightness was a close link of a half of a day to a brightening in the solar corona. Given a typical solar wind speed and the distance of 3200 Phaethon at the time, this would put the Geminid parent just at the right range to be feeling the effects of the increase. However, the authors conclude that this cannot be directly responsible by imparting sufficient energy on the surface of the object to cause it to fluoresce due to an insufficient solar wind flux at that distance.

Instead, Jewett and Li consider more indirect explanations. Due to the temperature at 3200 Phaethon’s perihelion (0.14 AU) the presence of ices and other volatile gasses frozen solid and then blasting away as often happens in comets was ruled out as they would have been depleted on earlier orbits. However, the blow from the increased solar wind may have been sufficient to blow off loosely bound dust particles. While this is plausible, the authors note that the amount of mass lost if this were the case would be a paltry 2.5 x 108 kg. While it’s possible that this may have been the cause of this single brightening, this amount of mass loss to the overall stream of particles responsible for the Geminid shower would be insufficient to sustain the stream and similar losses would have to occur ~10 times per orbit of the body. Since this has not been observed, it is unlikely that this event was tied to the production of the meteors. Additionally, it is somewhat unlikely that it could even be the event for this sole case since repeated perihelions would slowly deplete the reservoir of available dust until the body was left with only a bare surface. Unlike active comets which continually free dust to be ejected through sublimation of ice, 3200 Phaethon has no such process. Or does it?

The novel proposition is that this object may have an unusual mechanism by which to continually generate and liberate dust particles of the size of the Geminids. The authors propose that the heating at perihelion causes portions of the rock to decompose. This process is greatly enhanced if the rock has water molecules bonded to it and lab experiments have shown that this can lead to violent fracturing. Such processes, if present, could easily lead to the production of new dust particles that would be liberated during close approach to the sun. This would make this object a “rock comet” in which the properties of a comet’s dust ejection via gasses would be carried out by rocks.

To confirm this hypothesis, future observations would be needed to search for subsequent brightening at perihelion. Similarly, it should be expected that such a process may make a faint cometary tail with only a dust component that may be visible as well, although the lack of any such detection so far, despite studies looking for cometary tails, casts some doubt on this process.

Posted on by Jon Voisey

Possibility for White Dwarf Pulsars?

AE Aquarii - A possible White Dwarf Pulsar
The white dwarf in the AE Aquarii system is the first star of its type known to give off pulsar-like pulsations that are powered by its rotation and particle acceleration.

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Some satellites get all the glory. While Hubble, Chandra, and Spitzer frequently make headlines with their stunning images, many other space based observatories silently toil away. One of them, known as the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) has been in orbit since 2006, but rarely receives media attention although a stunning discovery has led to the publication of over 300 papers within a single year. A new paper in that onslaught has proposed an interesting new object: pulsars powered by white dwarfs.
PAMELA isn’t a satellite in its own right. It piggybacks on another satellite. Its mission is to observe high energy cosmic rays. Cosmic rays are particles, whether they be protons, electrons, nuclei of entire atoms, or other pieces, that are accelerated to high velocities, often from exotic sources and cosmological distances.

Among the types of particles PAMELA detects is the elusive positron. This anti-particle of the electron is quite rare due to the scarcity of anti-matter in general in our universe. However, much to the surprise of astronomers, in the range of 10 – 100 GeV, PAMELA has reported an abundance of positrons. In even higher ranges (100 GeV – 1 TeV) astronomers have found that there is a rise in both electrons and positrons. The conclusion from this is that something is able to actually create these particles in these energy ranges.

A flurry of papers went to publication to explain this unexpected finding. Explanations ranged from showers of particles created by even higher energy cosmic rays striking the interstellar medium, to the decay of dark matter, to neutron stars, pulsars, supernovae, and gamma ray bursts. Indeed, many events that produce high energies are sufficient to spontaneously produce matter from energy through the process of pair production. However, the range of these ejected particles would be limited. Effects, such as synchrotron and inverse Compton emission would drain their energy over large distances and as such, by the time they reached PAMELA’s detectors would be too low energy to account for the excesses in the observed energy ranges. From this, astronomers are presuming the culprits are in the local universe.

Joining the long list of candidates, a new paper has proposed a mundane object could be responsible for the high energy necessary to create these energetic particles, albeit with an unusual twist. Neutron stars, one of the potential objects formed in a supernova, are known to release large amounts of energies when spinning quickly while creating a strong magnetic field in the form of pulsars, but the authors propose that white dwarfs, the products of the slow death from stars not massive enough to result in a supernova, may be able to do the same thing. The difficulty in creating such a white dwarf pulsar is that, since white dwarfs don’t collapse to such a small size, they don’t “spin up” as much as they conserve angular momentum and shouldn’t have the sufficient angular velocity necessary.

The authors, led by Kazumi Kashiyama at Kyoto University propose that a white dwarf may reach the necessary rotational speed if they undergo a merger or accrete a sufficient amount of mass. This idea is not unheard of since white dwarf mergers and accretion are already implicated in Type Ia Supernovae. The combination of this with the expectation that around 10% of white dwarfs are expected to have magnetic fields of 106 Gauss, the steps necessary to produce a pulsar from a white dwarf seem to be in place. They note that since white dwarfs tend to have weaker magnetic fields, they shed their angular momentum more slowly and would last longer. Although this duration is still far longer than humans can possibly watch, this may indicate that many of the pulsars observed in our own galaxy are white dwarfs.

Next, the authors hope to conclusively identify such a star. The creation of each of these types of pulsars may provide a clue: Since neutron stars form from supernovae, they are surrounded by a shell of gas that contains a shock front from the supernova itself, which is more dense than the interstellar medium in general. As particles pass through this shock front, some of them would be lost. The same would not be said for white dwarfs which formed from a more gentle release and aren’t impeded by the relatively high density area. This shift in energy distributions may be one distinguishing characteristic.

Some stars have even been tentatively proposed as candidates for white dwarf pulsars. AE Aquarii was seen to give off some pulsar-like signals. EUVE J0317-855 is another white dwarf that appears to meet the qualifications, although no signals have been detected from this star. This new class of stars would be able to explain the excess signal in the higher energy range detected by PAMELA and will likely be the target of further observational searches in the future.

Posted on by Nancy Atkinson

New Phenomenon: “Coreshine” Provides Insight into Stellar Births

The molecular cloud CB 244 in the constellation Cepheus, 650 light-years from Earth. Credit: MPIA

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From the Max Planck Institut für Astronomie:

Science is literally in the dark when it comes to the birth of stars, which occurs deep inside clouds of gas and dust: These clouds are completely opaque to ordinary light. Now, a group of astronomers has discovered a new astronomical phenomenon that appears to be common in such clouds, and promises a new window onto the earliest phases of star formation. The phenomenon – light that is scattered by unexpectedly large grains of dust, which the discoverers have termed “coreshine” – probes the dense cores where stars are born. The results are being published in the September 24, 2010 edition of the journal Science.

Stars are formed as the dense core regions of cosmic clouds of gas and dust (“molecular clouds”) collapse under their own gravity. As a result, matter in these regions becomes ever denser and hotter until, finally, nuclear fusion is ignited: a star is born. This is how our own star, the Sun, came into being; the fusion processes are responsible for the Sun’s light, on which life on Earth depends. The dust grains contained in the collapsing clouds are the raw material out of which an interesting by-product of star formation is made: solar systems and Earth-like planets.

What happens during the earliest phases of this collapse is largely unknown. Enter an international team of astronomers led by Laurent Pagani (LERMA, Observatoire de Paris) and Jürgen Steinacker (Max Planck Institute for Astronomy, Heidelberg, Germany), who have discovered a new phenomenon which promises information about the crucial earliest phase of the formation of stars and planets: “coreshine”, the scattering of mid-infrared light (which is ubiquitous in our galaxy) by dust grains inside such dense clouds. The scattered light carries information about the size and density of the dust particles, about the age of the core region, the spatial distribution of the gas, the prehistory of the material that will end up in planets, and about chemical processes in the interior of the cloud.

The molecular cloud CB 244 in the constellation Cepheus, 650 light-years from Earth. Credit: MPIA

The discovery is based on observations with NASA’s SPITZER Space Telescope. As published this February, Steinacker, Pagani and colleagues from Grenoble and Pasadena detected unexpected mid-infrared radiation from the molecular cloud L 183 in the constellation Serpens Cauda (“Head of the snake”), at a distance of 360 light-years. The radiation appeared to originate in the cloud’s dense core. Comparing their measurements with detailed simulations, the astronomers were able to show that they were dealing with light scattered by dust particles with diameters of around 1 micrometer (one millionth of a meter). The follow-up research that is now being published in Science clinched the case: The researchers examined 110 molecular clouds at distances between 300 and 1300 light-years, which had been observed with Spitzer in the course of several survey programs. The analysis showed that the L 183 radiation was more than a fluke. Instead, it revealed that coreshine is a widespread astronomical phenomenon: Roughly half of the cloud cores exhibited coreshine, mid-infrared radiation associated with scattering from dust grains in their densest regions.

The discovery of coreshine suggests a host of follow-on projects – for the SPITZER Space Telescope as well as for the James Webb Space Telescope, which is due to be launched in 2014. The first coreshine observations have yielded promising results: The unexpected presence of larger grains of dust (diameters of around a millionth of a meter) shows that these grains begin their growth even before cloud collapse commences. An observation of particular interest concerns clouds in the Southern constellation Vela, in which no coreshine is present. It is known that this region was disturbed by several stellar (supernova) explosions. Steinacker and his colleagues hypothesize that these explosions have destroyed whatever larger dust grains had been present in this region.

Source: Max Planck

Posted on by Jon Voisey

Electric Resistance May Make Hot Jupiters Puffy

The Sun’s magnetic field

One of the surprises coming from the discoveries of the class of exoplanets known as “Hot Jupiters” is that they are puffed up beyond what would be expected from their temperature alone. The interpretation of these inflated radii is that extra energy must be being deposited in the regions of the atmosphere with large amounts of circulation. This extra energy would be deposited as heat, causing the atmosphere to expand. But from where was this extra energy coming? New research is suggesting that ionized winds passing through magnetic fields may create this process. Continue reading “Electric Resistance May Make Hot Jupiters Puffy”

Posted on by Nancy Atkinson

Watch the Aurora Borealis Live via Webcam

All About Aurora
Aurora Borealis. From Wikimedia Commons.

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If you’ve even seen the Aurora Borealis live, you know how awe-inspiring it can be. But if you live too far south, or aren’t a night owl, there’s now a way for to you see the aurora, via the web, every night. Last night was the world premier of AuroraMAX – an online observatory which began streaming Canada’s northern lights live over the Internet. “Armchair skywatchers everywhere can now discover the wonder of the northern lights live on their home computer screen,” says Canadian Space Agency President Steve MacLean. “We hope that watching the dance of the northern lights will make you curious about the science of the sky and the relationship we have with our own star, the Sun.”

In addition to nightly broadcasts of the aurora, AuroraMAX will help demystify the science behind the phenomenon, offer tips for seeing and photographing auroras, and highlight Canadian research on the Sun-Earth relationship. The website will also include an image gallery with still photos and movies from previous nights.

Auroras occur as charged particles from the Sun collide with gases in Earth’s upper atmosphere. The launch of AuroraMAX coincides with the beginning of aurora season in northern Canada, which generally begins in late August or early September and ends in May. Aurora enthusiasts will be able to follow AuroraMAX through solar maximum, the most active period of the Sun’s 11-year cycle, which should produce more frequent and intense auroras on Earth. Solar maximum is currently expected in 2013.

AuroraMAX is a collaborative public engagement initiative between the CSA, the University of Calgary, the City of Yellowknife and Astronomy North.

You can get updates from AuroraMAX via Twitter.

Posted on by Nancy Atkinson

Astrophysics From the Moon

Lunar New Year

Many astronomers feel that the Moon would be an excellent location for telescopes, — both on the surface and in lunar orbit – and these telescope could help answer some of the most important questions in astronomy and astrophysics today. One proposal calls for a lunar orbiting low frequency antenna that could measure the signatures of the first collapsing structures in the early universe. Dr. Jack Burns from the University of Colorado, Boulder, discussed the idea for the Lunar Cosmology Dipole Explorer (LCODE) at the NASA Lunar Science Institute’s Lunar Forum this summer.

“The Moon in many ways is a truly unique platform from which we can look outward into the cosmos and do some unique astronomical observations,” said Burns, who is also the Director of the NASA/NLSI Lunar University Network for Astrophysics Research (LUNAR).

What makes the Moon so inviting is that the lunar far side is uniquely radio quiet in the inner part of the solar system, as the far side is always facing away from the Earth, and the Moon itself blocks out any interfering man-made signals from radio, TV and satellites.

In this radio quiet zone, astronomers could study the very early universe, back to less than half a billion years after the Big Bang, probing what is called the Dark Ages, before the first stars and galaxies formed.

LCODE would be a satellite orbiting the Moon carrying a single dipole antenna, kind of like your car antenna, Burns said, but it has two ends. “It flies around the Moon and we take data only when we are above the far side, the shielded zone where we are free of radio interference,” said Burns, “and that allows us, because it is so quiet there, to take measurements of these very faint emissions from this very early era in our universe’s history.”

Example of dipole antenna.

The orbiting dipole would allow scientists to look for these signals over the entire sky. If that is successful, the next stage would be to put an array of dipole antennas on the surface, perhaps even about ten thousand antennas, and use it as a radio interferometer that would “allow us to actually get some resolution to do some imaging,” Burns said, “and explore the composition of these structures in the early universe that eventually go on to form stars and galaxies.”

Other proposals for doing radio astronomy from the Moon would be to study the sun at low frequencies, below 10 megahertz. The sun emits very strong low frequency radio waves, and these are related to Coronal Mass Ejections, which produce very high energy particles which can interfere with satellites and could potentially be very harmful to future astronauts traveling in interplanetary space. “We hope to be able to image and to understand how these particles are accelerated,” Burns said.

The other interesting regions of the Moon from which to do astronomy would be the poles in permanently shadowed craters, which are very cold — only about 40 degrees above absolute zero – which would make an excellent site for infrared telescopes which need to be cooled down to very low temperatures.

You can listen to an interview with Jack Burns about LCODE on the 365 Days of Astronomy podcast.

Posted on by Nancy Atkinson

Water on the Moon Could be Bad News for Future Lunar Astronomy

A false colour composite of the distribution of water and hydroxyl molecules over the lunar surface. Credit: ISRO/NASA/JPL-Caltech/Brown Univ./USGS

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The recent discovery of water on the Moon may have a serious impact on future plans for lunar based astronomy. Space scientists from the Chinese Academy of Sciences have calculated that the scattering caused by molecules vaporized in sunlight could heavily distort observations from telescopes mounted on the Moon.

“Last year, scientists discovered a fine dew of water covering the Moon. This water vaporizes in sunlight and is then broken down by ultraviolet radiation, forming hydrogen and hydroxyl molecules. We recalculated the amount of hydroxyl molecules that would be present in the lunar atmosphere and found that it could be two or three orders higher than previously thought,” said Zhao Hua, who presented his team’s results at the European Planetary Science Congress in Rome.
The research has particular implications for the Chinese Lunar lander, Chang’E-3, which is planned to be launched in 2013. An ultraviolet astronomical telescope will be installed on the Chang’E-3 lander, which will operate on the sunlit surface of the Moon, powered by solar panels.

“At certain ultraviolet wavelengths, hydroxyl molecules cause a particular kind of scattering where photons are absorbed and rapidly re-emitted. Our calculations suggest that this scattering will contaminate observations by sunlit telescopes,” said Zhao.

The Moon’s potential as a site for building astronomical observatories has been discussed since the era of the Space Race. Lunar-based telescopes could have several advantages over astronomical telescopes on Earth, including a cloudless sky and low seisimic activity.

The far-side of the Moon could be an ideal site for radio astronomy, being permanently shielded from interference from the Earth. Radio observations would not be affected by the higher hydroxyl levels.

Source: European Planetary Science Conference

Posted on by Steve Nerlich

Astronomy Without A Telescope – Not So Ordinary

The Small and Large Magellanic Clouds - not the kind of things you usually find near large spiral galaxies. Cerro Tololo observatory, Credit: Fred Walker.

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Sorry – a bit of southern sky bias in this one. But it does seem that our favourite down under naked eye objects are even more unique than we might have thought. The two dwarf galaxies, the Large and Small Magellanic Clouds, orbit the Milky Way and have bright star forming regions. It would seem that most satellite galaxies, in orbit around other big galaxies, don’t. And, taking this finding a step further, our galaxy may be one of a declining minority of galaxies still dining on gas-filled dwarf galaxies to maintain a bright and youthful appearance.

We used to think that the Sun was an ordinary, unremarkable star – but these days we should acknowledge that it’s out of statistical mid-range, since the most common stars in the visible universe are red dwarfs. Also, most stars are in binary or larger groups – unlike our apparently solitary one.

The Sun is also fortunately positioned in the Milky Way’s habitable zone – not too close-in to be constantly blasted with gamma rays, but close-in enough for there to be plenty of new star formation to seed the interstellar medium with heavy elements. And the Milky Way itself is starting to look a bit out of the ordinary. It’s quite large as spiral galaxies go, bright with active star formation – and it’s got bright satellites.

The Lambda Cold Dark Matter (CDM) model of large scale structure and galaxy formation has it that galaxy formation is a bottom-up process, with the big galaxies we see today having formed from the accretion of smaller structures – including dwarf galaxies – which themselves may have first formed upon some kind of dark matter scaffolding.

Through this building-up process, spinning spiral galaxies with bright star forming regions should become common place – only dimming if they run out of new gas and dust to feast on, only losing their structure if they collide with another big galaxy – first becoming a ‘train wreck’ irregular galaxy and then probably evolving into an elliptical galaxy.

The  Lambda CDM model suggests that other bright spiral galaxies should also be surrounded by lots of gas-filled satellite galaxies, being slowly draw in to feed their host. Otherwise how is it that these spiral galaxies get so big and bright? But, at least for the moment, that’s not what we are finding – and the Milky Way doesn’t seem to be a ‘typical’ example of what’s out there.

The relative lack of satellites observed around other galaxies could mean the era of rapidly accreting and growing galaxies is coming to a close – a point emphasised by the knowledge that we observe distant galaxies at various stages of their past lives anyway. So the Milky Way may already be a relic of a bygone era – one of the last of the galaxies still growing from the accretion of smaller dwarf galaxies.

Supernova 1987a, which exploded near the Tarantula Nebula of the Large Magellanic Cloud. Credit: Anglo-Australian Observatory.

On the other hand – maybe we just have some very unusual satellites. To a distant observer, the Large MC would have nearly a tenth of the luminosity of the Milky Way and the Small MC nearly a fortieth – we don’t find anything like this around most other galaxies. The Clouds may even represent a binary pair which is also fairly unprecedented in any current sky survey data.

They are thought to have passed close together around 2.5 billion years ago – and it’s possible that this event may have set off an extended period of new star formation. So maybe other galaxies do have lots of satellites – it’s just that they are dim and difficult to observe as they are not engaged in new star formation.

Either way, using our galaxy as a basis for modelling how other galaxies work might not be a good idea – apparently it’s not so ordinary.

Further reading: James, P. A. And Ivory C.F. On the scarcity of Magellanic Cloud-like satellites.