‘Oumuamua: a visitor from the stars

It is easy to forget the size of the Universe. For all its divisions and separations, Earth is not a good model for space: it is just too small. Nowadays, a seasoned traveler may have seen much of our world. Most of it can be reached within a day or two of travel. Of course, there are exceptions, some of which have even been covered on this blog. Bouvet Island is an example. But largely, the Earth has been reduced to a comprehensible size.

Go beyond the Earth, and this changes. Space is in fact remarkably close. By definition, it begins 100 km above the surface. This is not too far, a fact used by organizations such as Virgin Space to devise brief space journeys. The space station is a bit over 200 km above our heads, again a comprehensible distance. (Reaching it is doable, but arriving there is very hard because of its sheer speed. Almost all of the fuel used by the rocket is to get the right velocity.) But now look further. Geostationary satellites are 40,000 km away. Imagine a space elevator transporting you at a leisurely 100 km/hr. You’ll pass the Space Station after an hour or two. Getting to a geostationary satellite will take you more than two weeks! Do pack some sandwiches. You’d like to visit the moon? That will take 5 months in the elevator.

Go further, and it does become advisable to speed up a bit. Mars would take a life time – to be precise, around 70 years. And in all these years, the only things you will have seen through the elevator window are the Space Station, one geostationary satellite, and the Moon – and that was 69 years ago. Instead, we can hitch a ride on New Horizons. It flew past the Moon within 9 hours of launch. Jupiter took a year (versus 500 years in the space elevator), and its eventual destination, Pluto, took 9 years. And New Horizons didn’t stop there. It is now in the Kuiper belt, a region in the solar system beyond Pluto where some of the building blocks of the planets survive. It will pass one of these objects in the Kuiper belt, a small rock with the evocative name Ultima Thule, at 5:30am (UT) on Jan 1. It will be exciting to see the images come back, although that will not be instantaneous. During the fly-by the spacecraft is too busy to transmit, and afterwards it will take days to (many) months to receive all the data – the internet over those distances is not fast.

Thule approaching New Horizons

After the Ultima Thula encounter, New Horizons will aim for the stars, following four older spacecraft which are also leaving the Solar System. Of the five, New Horizons is the third fastest: both the Voyager probes, 40 years old, are going faster, because they picked up more speed on the journey from gravity assists, while the even older Pioneers go slower. All are speedily fleeing the Sun, at a speed of 14 km/s for New Horizons and 17 km/s for Voyager 1. (Interestingly, the Voyager probes are about the same age as the wrecks train carriages used by our local train company. They form two extremes of the technology of the 1970’s.)

But the stars are a different problem. They are so much further. The Voyagers have now moved into interstellar space, meaning that they are outside the reach of the solar wind. New Horizons will eventually do that as well, however we’ll never know when as the transmitter will fail before that time. But a craft that took 40 years to get to interstellar space, will need 40,000 years to get to another star. The solar system is tiny compared to the distance between the stars. We are well and truly isolated, able to look at the stars but never to visit them.

But in 2017, to everyone’s big surprise, we had a visitor from outer space. An asteroid was found with a peculiar orbit, and when the computers had finished their calculations, we knew it wasn’t part of the solar system. It had come from elsewhere and was just passing through – a fly-by visitor coming in from the dark for a brief sojourn in the warmth of our Sun. It almost hit us – the ultimate hit-and-run.


It was found with Pan-STARRS (Panoramic Survey Telescope and Rapid Response System). The telescope is located on Haleakala (one of a number of observatories build on volcanoes, from Mauna Kea to Etna, a curious link between the power of the stars and that of the earth). Pan-STARS is devised specifically to find near-earth asteroids, ones that could be a danger to us: a highly unlikely danger, but the amount of damage a major impact could do makes it important to find any potential culprit. It does that by taking images of much of the sky every night, and looking for objects that are moving between two images. If it is moving, it must be nearby, inside the solar system and the faster it seems to be moving, the closer it is likely to be. The task is to identify them, and from the data calculate the distance and the orbit. If the orbit gets close to Earth, it is added to the danger list.

The Pann-Stars telescope at Haleakala

On October 17, 2017, Pan-STARRS saw an object assumed to be a comet. The first person to spot it in the data was Rob Weryk, from the university of Hawai’i, on October 19. Within hours it was realized that the orbit was a strange one,. Astronomers around the world were notified, and telescopes began turning towards it. The discovery number would normally have been C/2017 U1 (C for comet, 2017 is the year of discovery, U is for the second half of October and ‘1’ means it was the first comet found in that period). But a closer look revealed no evidence of the gas that should be found around comets. It seemed to be a rock rather than a comet, and these are given the designation ‘A’ (for asteroid) rather than ‘C’. A name change was in order, and the object became known as A/2017/U1. A rocky body is more likely to have originated from the inner solar system, while comets come in from the cold. A/2017/U1 clearly came from far. However, sometimes asteroids can get too close to Jupiter, and can be thrown far out at elliptical orbits. It became more complicated when Alan Fitzsimmons (Belfast) took a spectrum and found it to be extremely red, a characteristic of objects found beyond Neptune. It now seemed similar to Ultima Thule. Luckily, this discovery did not require another name change. But what was it doing in the inner solar system?


A/2017/U1 was found to be on an extremely elliptical orbit. A highly elliptical orbit is normal for comets, on the way from the icy outer regions of the system and falling in towards the warm centre where Earth is. But not as extreme as this!

Let’s go back to school. Orbits in the solar system are described by ellipses. We have known this since Kepler, and this law of Kepler lead directly to Newton’s law of gravity. Ellipses are classified by a number that (with a slight lack of imagination) is called the eccentricity, and given the symbol e. A circle (which also is an ellipse) has e=0, and the more elliptical the orbit the higher e: the most extreme ellipse (a parabola) has e=1.

A quick revisit of our mathematics reveals than an ellipse has two foci. The standard way to draw an ellipse is to take two pins and put them into a piece of paper (do check what lies below the paper. Wall paper is ideal especially if still on the wall, but you may want to check with the owner. Saying that it is homework tends to do the trick.) Now take a string and put it around the two pins. Take a pen and use it as the third corner of the string. Move the pen around while keeping the string taut. You’ll draw a beautiful ellipse, a piece of wall art that will be the envy of all the neighbourhood if not the house owner. The mathematical way of describing this is that for each point on the ellipse, the distances to each of the two foci adds up to the same number.

Still with me? The eccentricity is now defined as how close the focus is to the drawn curve. (Strictly, if we say that the furthest point of the ellipse is a distance a from the centre, and the focus a distance f, then the eccentricity is e = f/a.) For a circle, the focus is in the centre, therefore f=0 and e=0. As you move the pins away from each other, the drawn curve becomes more and more elongated. Finally, the focus is on the curve and e=1. You now have a parabola.

Planets in the solar system have orbits with e close to zero, i.e. they are almost circles. Mars is the most deviant, with e=0.09 (Kepler found his law from the movement of Mars). Comets tend to have very high eccentricity: Halley’s comet has e=0.97. And many comets go even higher.

After this high-school interlude, let’s get back to the story. When the orbit of A/2017 U1 had been calculated, people immediately sat up. It was off the scale. The eccentricity was e=1.2. It wasn’t an ellipse any longer! There is one kind of orbit for which this is the case. It is a hyperbola, an orbit of something that isn’t orbiting the Sun but only flies past. An object on a hyperbolic orbit will never come back. That also means it has never been here before. A/2017 U1 wasn’t a member of the solar system – it came from beyond! Even the hyperbole-deniers were quickly convinced: this was our first interstellar visitor. All this happened within the first few days of its discovery – science can go fast at times. The NASA press release came out on October 26, and the news went around the world within minutes.

Hyperbolic orbits are not entirely unknown in the solar system. However, so far we have only seen a few objects leave on one, and in all cases because they were thrown out from the system by an interaction. A few comets came too close to Jupiter, and ended up with eccentricity just above one and were forcefully expelled. And of course, the spacecrafts mentioned above are all on hyperbolic orbits. But we had never seen a comet or other object come in on a hyperbolic orbit.

A/2017 U1 had come into the solar system from the direction of Lyra (home of the bright star Vega, although that is not on its path), passed the Sun inside the orbit of Mercury in September 2017, and was already on the way out when it was discovered. It had come close to Earth on October 14 – close being about 60 times as far as the Moon. As far as protection against impacts. Pann-STARS had not done well, as this projectile was only seen days after it had missed us. But how close was this, compared to emptiness of the space it came from! Even from nearby Pluto, to aim this close to us is like hitting a target 40 meters away within 10 centimeters. But this object came from a 100,000 times further. It is like hitting, from 40 meters away, the heart of a red blood cell. (We normally use the width of a hair for illustrating micron-size thickness, but a hair is 50-100 micron in width, far too large for this kind of accuracy. This of course is just splitting hairs.) We had a close call.

As the visitor moved away from the Sun, it rapidly became fainter. The last HST observation was on 2018 January 2, when it was already twice as far from the Sun as Mars. By now, a year later, it will be further than Jupiter. It will be inside the solar system for years to come, but we will never see it again.

What’s in a name

We needed to find yet another name. There was no system yet for interstellar objects, but the International Astronomical Union quickly came up with one. The letter I was accepted (standing for interstellar), and in view of the scarcity of such objects, the discovery number was put before the I. This left us with a scheme as simple as that used for the first asteroids, and A/2017 U1 now became 1I/2017 U1. (The ‘U1’ is not really needed and even the year of discovery can be left out without causing confusion.) However, the IAU rules allowed the discoverers to propose a proper name as well. This being Hawai’i, they chose the local word for first messenger. Our visitor is now called (in the short version) 1I/ ‘Oumuamua. The glutteral stop can be confusing (or forbidden in some word processors) and is often left out. And so we ended up with the name ‘Oumuamua. It seems unpronounceable – but perhaps that is appropriate in view of its origin. It resonates with the movie ‘Arrival’ of the previous year, where the alien language could be written and read – but not spoken.

Half the telescopes in the world must have been point at it in the weeks following the announcement. The brightness and distance suggested it was around 500 meters across, with a very large uncertainty. But the brightness seemed inconsistent. In November 2017 a light curve was published which showed huge periodic changes, by over a factor of 10. This meant two things: it was rotating, and one part was reflecting much more light. The light curve suggested this was due to the shape. A picture emerged of a highly elongated object, tumbling with a period of 8.67 hours in such a way that sometimes we saw it edge on (making it very faint), sometimes not.

This plot shows how the interstellar asteroid `Oumuamua varied in brightness during three days in October 2017. The large range of brightness — about a factor of ten (2.5 magnitudes) — is due to the very elongated shape. The different coloured dots represent measurements through different filters, covering the visible and near-infrared part of the spectrum. The dotted line shows the light curve expected if `Oumuamua were an ellipsoid with a 1:10 aspect ratio, the deviations from this line are probably due to irregularities in the object’s shape or surface albedo.

The dotted line in the light curve is a model for an ellipsodial shape with aspect ratio of 10:1 (10 times longer than it is wide!). It fits the rough curve but there are many deviations in the light curve, meaning that the surface is either irregular or varies in brightness. No other object is known to be as elongated but the asteroid (1865) Cerberus has a similar light curve and may also have such a shape.

We needed an artist’ impression and an artist was quickly found to make one. The image is still published weekly, even if it is completely made up.

The picture leaves it unclear whether the object looks like a disk or a flattened cigar. The current models favour (but don’t prove) the latter: it appears to look like a cigar where the flattening of the cigar is less than a factor of 1.7.

How does a cigar rotate? The answer is, with great complexity. It tumbles, precesses and nutates all at the same time, and with different periods. The main period (8.67 hours) is end-to-end of the cigar. The long axis also precesses with a period of around 55 hours. And there is more complexity. The best way to describe is saying that it ‘tumbles’.

How large is it? That we have not been able to measure directly, as it has always been too small to resolve with our telescopes. The best estimates we have come from how much sunlight it reflects. That requires an assumption of how dark the surface is. Most asteroids are fairly dark, so people assumed that the surface reflects 5% or so of the incoming light. But the assumption of a dark surface has been challenged. Infrared observations failed to detect the heat signal from ‘Oumuamua, and it should have been seen at this size. The people behind that study suggest that the surface is much brighter (reflective) and that ‘Oumuamua is smaller than thought, possibly as small as 240 by 40 meters.


The speed of ‘Oumuamua is impressive. It reached 44 km/s at closest approach to the Sun, and came in from afar at an original speed of 25 km/s. That is almost twice the speed of the Voyagers. We could not easily accelerate a rocket to those kind of speeds! Doing it to a rock 500 meter long is a different prospect altogether. How did it get so much speed? It turns out, it didn’t. It borrowed it from us.

The Sun is one star among many in the Milky Way. The whole galaxy rotates and the Sun goes around the galaxy every 200 million years or so. But it is not like the orbit of the planets. The stars aren’t on strictly aligned orbits: they move around a bit. You can compare it to falling snow, where every snow flake falls at a slightly different speed. Astronomers define the local velocity by averaging over the nearest stars: the average velocity (and direction) of all the nearby stars is called the Local Standard of Rest. The Sun moves at some 20 km/s with respect to this rest frame. This is a typical speed. It turns out that the speed of ‘Oumuamua, and its direction, is rather close to the standard of rest (it differs by about 10 km/s from this). ‘Oumuamua wasn’t moving fast at all: it was almost stationary, and it was hit by the Sun as it came flying past. The hit-and-run was us, and ‘Oumuamua was the victim, not the perpetrator.

But where had it come from? Velocities close to the local standard of rest are a characteristic of young stars and regions of star formation. It suggests that ‘Oumuamua was expelled by its host star while the star was still young. Models suggests that during the formation of planets, many small objects may be ejected from their systems. Most likely, but unproven, this happened to ‘Oumuamua. The fact that its velocity was so close to that of the standard of rest also suggest the ejection happened at low speed. It makes most sense if it all happened in the outer regions, far from the host star, in the local equivalent of our Kuiper belt. Could ‘Oumuamua be like Ultima Thule?

There is disagreement whether such ejections happens often enough to make the discovery of ‘Oumuamua a predictable accident. ‘Oumuamua was found remarkably close to Earth, but such small, faint objects would be very difficult to see further. The discovery suggests there must be many more of these passers-by which we never knew about. And that in turn suggests that each star system expels a lot of its debris.

But when and where did this happen? Following the trajectory back in time gives a blank. You have to to take into account how much stars themselves have moved over time, and this is not perfectly known. But we have been unable to find any nearby star (within 15 light years) along the trajectory. To be more precise, 6 possible candidates were found, but in all cases the encounters were too remote to be a plausible origin. The conclusion is that either we do not know stellar trajectories well enough, or ‘Oumuamua is older than 5 million years.

That is not surprising. It could very easily be 10 or 100 times as old, in which case the precise origin is well and truly lost in time.

Another study took a different, and more promising approach. These authors looked further, and used less accurate data. They found that of a list of potential origin stars, a number were associated to a group related to the Pleiades, a cluster of young stars with the right velocity. It is speculative, but not impossible that ‘Oumuamua came from a star which itself had escaped from the seven sisters.

The Pleiades, also known as the seven sisters (although the cluster has many more stars than seven).


But this was not the end of the story. The orbit did not perfectly follow the expected Keplerian trajectory. It seemed to be going a little faster than expected, requiring some on-going acceleration. That is not unique: both comets and spacecrafts can show this effect. The additional acceleration is very small but was detectable. It was modeled as a term that decreased with distance to the Sun. The most likely cause is outgassing, where ‘Oumuamua is losing a small amount of water. But this is what comets do. Is ‘Oumuamua a comet after all? That would make sense: because comets exist in the outer regions of their star systems, it is much easier to lose them to interstellar space compared to asteroids.

However, the amount of outgassing is minute. Comets are icy bodies, which begin to evaporate when approaching a star and heating up. Over time, more and more of the surface ice is lost and the organic molecules stay behind. The surface now darkens and the comet declines in activity. Halley’s comet is already far less bright than it was a few thousand years ago. ‘Oumuamua would have escaped from the outer, cold regions of it host star and may never have been close to a star. In that case, why would it be so inactive? A recent paper suggest this is due to exposure by cosmic rays, while in interstellar space. These can produce a mantle of organic material, insulating and hiding the icy centre. However, this raises the question why normal comets don’t show this. The authors suggest that this is due to the small size of ‘Oumuamua, which means nothing inside is far from the surface. It could have lost all of its volatiles.

Clearly, we are left with many unknowns. We know ‘Oumuamua came from the stars, but we do not know where, from which star or even what kind of star, how old it is or what is made of. Was it an asteroid or a comet, or something different? We never got an image. We do not know how common such objects are, although the fact that we found one so close to us suggest there are many of them. After such an amazing discovery, we seem to be left knowing less than we did before.

Final words

There has been a lot of discussion whether ‘Oumuamua could have an artificial origin. The idea of a fly-by intruder goes back to a story by Stanislaw Lem where the invader was ancient and derelict, and was worked out further by Arthur Clarke in Rama. It was a good story. The reality is that rocks, comets and asteroids may not always stay in the star system where they formed. Especially early on, many escape and together leave a debris field in the interstellar space through which we move.

But one day, perhaps, we may ourselves travel out (once we have re-learned our 50-year old technology). ‘Oumuamua shows us a better way for star travel. It didn’t try to achieve light speed. It used speeds within our reach, got into interstellar space, and waited for the Sun to pass by. As it flew past the Sun, it gained a lot of speed: a gravity assist by the Sun ensured that it left us going around 60 km/s with respect to the Local Standard of Rest. No rocket could do this. The secret of traveling to the stars may be to take it slow, and use nearby stars to pick up the needed speeds. It would take millions of years. But the stars have time – they can wait.

Albert, December 2018

257 thoughts on “‘Oumuamua: a visitor from the stars

  1. BBC have an article on Anak with the latest sat imagery. They rather helpfully have an interactive photo that you can slide to compare before and after and it is fair to say that the bulk of the island from 2018 has indeed gone awol!

    • Latest VAA from Darwin VAAC:

      DTG: 20190103/2200Z
      VOLCANO: KRAKATAU 262000
      PSN: S0606 E10525
      SUMMIT ELEV: 813M
      ADVISORY NR: 2019/21
      OBS VA DTG: 03/2200Z
      OBS VA CLD: SFC/FL380 S0612 E10522 – S0611 E10546 – S0552
      E10614 – S0531 E10558 – S0602 E10518 MOV NE 10KT SFC/FL450
      S0620 E10508 – S0659 E10429 – S0624 E10406 – S0603 E10459
      MOV SW 35KT
      FCST VA CLD +6 HR: 04/0400Z SFC/FL380 S0613 E10522 – S0600
      E10515 – S0538 E10536 – S0513 E10619 – S0540 E10634 – S0609
      E10548 SFC/FL450 S0609 E10532 – S0752 E10307 – S0640 E10220
      – S0552 E10411 – S0554 E10527
      FCST VA CLD +12 HR: 04/1000Z SFC/FL380 S0611 E10519 – S0609
      E10547 – S0540 E10635 – S0513 E10619 – S0537 E10535 – S0559
      E10515 SFC/FL450 S0610 E10534 – S0553 E10527 – S0552 E10410
      – S0639 E10219 – S0752 E10307
      FCST VA CLD +18 HR: 04/1600Z SFC/FL380 S0611 E10520 – S0600
      E10515 – S0539 E10534 – S0513 E10619 – S0540 E10634 – S0609
      E10546 SFC/FL450 S0609 E10535 – S0757 E10312 – S0645 E10223
      – S0555 E10413 – S0555 E10527
      NXT ADVISORY: NO LATER THAN 20190104/0100Z”


      Don’t think she has really stopped erupting.

      • The problem is that with the main vent being underwater, the Surtseyan aspect of it messes up the accuracy of the mass ejection rate calculations. FL380 equates to 1081 m³/s DRE, but that is based on the original edifice height and not having a significant water interaction. Specifically, note that the height listed in the VAAC report is 813 meters. All photographic evidence points toward that actually being a bay now.

        • The height is definitely not 813 meters any more. Probably takes some time to have it revised down. FL is however the same no matter summit height. SW advisory was to FL550 for quite some time, but was down to FL070 shortly (1. – 2. jan.) if I remember correctly. Now back to 450.

          Have you made any new calculations as to ejecta, VEI or stratosfheric effect GeoLurking?

          • The problem as GeoLurking said is that the surtseyan activity is adding a lot of water vapor to the plume and as it condenses and the liquid water freezes further up in the atmosphere it releases energy and that is what is driving the convection, or in other words it is just behaving as a local thunderstorm would and the height of the plume is not a good indicator of the eruption rates.

            The tropopause in that area is more or less at the same height the plume had, about 16.5 km. The height of the plume also remained constant for a long time so I think it got stuck at the tropopause with in my opinion no or very small stratospheric injection, and even in that case the content of SO2 should be very low as it is not the typical volcanic plume, it did lack the spectral signature of ash.

            The GVP is currently considering the eruption to be a VEI 1, it has yet to stop so it not definitive and I don’t now if the number can change if reviewed the data but I guess that would be the official VEI for now.

          • Answer to all: thank you for answering. I learn new things every day here. 😉

            I can easily imagine the mix of water and ejecta making it quite hard to estimate the volume. I see wiki has it (already) as a VEI4? (with ? behind 4). Seemingly based on the volcanodiscovery-report on dec. 23. that it “might already be a VEI4”. Seems a bit overrated from my perspective.

            Is it possible though that the material from the collapse (explotion?) from dec. 22. and onwards could qualify as ejecta due to the explotion in whole or partially? The latest number on the missing part of the cone I’ve seen is 0,16-0,18 km3. A great deal of it obviously ended up elsewhere from downward slope/direction, so could part of that be classified as ejecta? On top of what obviously has be ejected I mean?

            It would only take apx. 50% of the missing cone + some ejecta to reach VEI4 (>0,1 km3).

            Any thoughts on that?

          • I am not sure but I think only tephra counts as volume so that the slided materials would not count. Same way that lava flows are not considered or that big mega landslides from oceanic islands do not count as VEI 7 eruptions. Only explosive products I guess. The cone I imagine was most likely not ejected outwards it just slided down into the caldera.

            I guess maybe the new tuff ring comprises the volume?, it is after all the only part that can be really measured.

          • I dont really like that list, it is too simple, and if I was better at coding I would edit it. For one, ontake was less than VEI 1, it was phreatic, and not even very big it was just the fatality count that made it known. Two, it assumes VEI is the only factor in eruption size and that is flawed for a long list of reasons that have been gone over on here enough times before. Three, and leading on from that, 3 of the 5 big events since 2000 were effusive, including the two biggest by volume. Holuhraun and leilani will be rated as VEI slightly-above-0 by the traditional system, so apparently were dwarfed by 30 second steam explosion with no magma.

            Among big eruptions in the 21 century that aren’t on here because of VEI rating, there is hekla 2000 (0.2 km3 with lava), grimsvotn 2004 (0.2 km3 with lava), sierra negra 2005 (0.1 km3), piton de la fournaise 2007 (0.1 km3) alu dalafilla 2008 (0.2 km3?) nyamuragira 2010 and 2011-2012 (0.1 and 0.3 km3), tolbachik 2012-2013 (at least 0.5 km3), bardarbunga 2014 (1.8 km3), wolf 2015 (0.2 km3), kilauea 2018 (1.2 km3) and sierra negra 2018 (at least 0.5 km3). Between 200 and 2018 pu’u o’o also erupted about 4 km3 of lava. All of these are VEI 4 minimum by all definitions if converted to tephra or as solid lava, and all the eruptions here that reach over 0.3 km3 are VEI 5 equivalents. Holuhraun would be almost 6 km3 explosive eruption.

            As can be seen, not a very good list if it misses half the members.

          • “Have you made any new calculation”

            No, for the reasons stated. Additionally, I am unsure of the actual edifice height. “It never was 813 meter? That is the height of Rakata.” – (Albert)

            ☼ → The Mastin et al equation (and the Sparks equation that it is similar to) are dependent on how high above the edifice the plume tops out. The core of the study depends on the amount of heat energy available from a quantity of magma. That’s how the mass ejections rates can be calculated.

            With the large amount of water involved, the errors in the calculation become overwhelming and I have no way available to back out the reduction in SO2 load. In theory, you could get an estimate of the mass percentage of Sulfur based on Tio2 and FeO ratios, but you would need a geochemical analysis of the tephra to get that. With most of it getting tangled up with the water, I doubt much if any is getting to the stratosphere.

            The big issue is that I feel no motivation to calculate something that can not even start with reasonable assumptions.

            From the outset, Anak would have been up into high VEI range already, if it were a pure magma based eruption. All it really was is a stationary pyrocumulus cloud and not an ash plume.

      • Nice to see some realism here. VEI-4 is a massive event. Anak Krakatau was a small volcano.

        • St. Helens was a full-Plinian VEI-5. Anak last week was a VEI-3 intensity that reached VEI-4… big difference

          • Has there even been any ahsfall reported? cause as far as I know no. Someone who has been following the news and reports would know better, I haven’t, but I think only rain was reported, it was raining water and no wonder cause the plume was water. Such an eruption can’t be called a major explosive event.

          • My argument is that any eruption that last 6 days with a plume to 55,000ft non-stop surely has to reach the VEI-4 boundary, even if a lot of the cloud is steam.

          • My argument is that the plume was not a reliable way to measure the size of the event. Visual observation confirmed inttermitent surstseyan explosions driving a plume that was mostly (basically all) made up of steam, as expected from this there haven’t been ashfalls reported, that doesn’t sound to be a VEI 4. Something else that doesn’t seem characteristic of a major explosive event was that the eruption was inttermitent, VEI 4 eruptions achieve high rates by having explosions so close one after the other that it turns into continuous output of lava, I think I have seen that in every VEI 4 eruption on video I got my eyes on. So then why do I think the plume was not reliable? I initially recurred to moist convection, GeoLurking who here had the knowledge to confirm if this was right did say that moist convection can boost the height of a plume by a few km. Now, here we are in a situation where this “boosting” is the greatest it can be, we are not talking about strombolian activity in a moist environment, this is surtseyan activity adding the water content directly into the plume in an already moist environment. Added to this there were other thunderstorms in the area which means the atmosphere was ready to generate updrafts, it just needed a little push from the eruption. The height of the plume might seem exagerated for the average cumulonimbus cloud but this is the place on Earth where tropopause might very well be the highest located allowing these clouds to grow farther up.

          • I’m going to try to reach out to people with more “qualified” opinions. I’ve got no idea which of us is closer to being correct. Hopefully some answers soon!

          • If only the USGS was actually able to work properly, they would have been a good place to ask about it. Shame a man-baby in a certain coloured house is having a tantrum…

          • I expect that a VEI 4 is not possible. The slide was about 0.15 km3 in total. Most of that went into the sea. The explosive part must have been much smaller, as the energy to eject all that was lacking. So the eruption was at most a VEI 3 and may not even have reached that.

  2. And in other news, a tropical storm is heading for Thailand. It is not severe as Pacific cyclones go (sustained winds are 75 km/h) but bad news for all the tourists (and possibly Bangkok airport). Very unusual for this time of the year.

  3. Strangely enough Chris Lintott did ask a question about the formation of Ultima Thule at yesterday’s press conference. Radioactive heat gets a mentions as well!


    ‏Verified account @chrislintott
    17h17 hours ago

    I asked (thanks @daverothery) about why a process that produced two roughly spherical bodies didn’t produce a finally spherical object. Mark Showalter said that a third body may have carried away the momentum (and perhaps escaped entirely). #ultimaflyby
    3 replies . 10 retweets 33 likes

    ‏Verified account @chrislintott

    One of the team came over (didn’t catch name) to say that one possibility is a time delay during which a tiny amount of radioactive decay heats the bodies, giving them more strength before the (very slow) final collision. #ultimaflyby
    11:34 am – 3 Jan 2019

    New conversation
    Emily Lakdawalla
    ‏ @elakdawalla
    17h17 hours ago

    Replying to @chrislintott

    …as my article from yesterday says 😉
    1 reply . 1 retweet 9 likes
    ‏Verified account @chrislintott
    17h17 hours ago

    So it does. I’d forgotten that bit – thanks.
    1 reply . 1 retweet 4 likes

    Lakdawalla article at http://www.planetary.org/blogs/emily-lakdawalla/2019/mu69-baby-comet-contact-binary.html

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