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.
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.
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.
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.
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.
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