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The short, interesting paper Discovery of a Meteor of Interstellar Origin describes a proposed discovery of a second interstellar object. This one hit the Earth, and was discovered not by telescope, but by "data mining" the CNEOS fireball catalog https://cneos.jpl.nasa.gov/fireballs/

It is not hard to find in the table based on the date given in the paper, but the information listed is sparse, there's no ID number or information how this was obtained:

peak brightness UTC lat lon alt(km) v(km/s) vx vy vz 2014-01-08 17:05:34 1.3S 147.6E 18.7 44.8 -3.4 -43.5 -10.3 total radiated kJ calc total imact kt 3.1e10 0.11

Question: With what instrument was this particular fireball observed and recorded, and how was it analyzed and processed into a trajectory and state vector to be entered into this table?

Figure 1. Trajectory of the January 8, 2014 meteor (red), shown intersecting with that of Earth (blue) at the time of impact, ti = 2014-01-08 17:05:34.

Figure 2. Size distribution of interstellar objects based on the detection of 'Oumuamua and of the meteor detected at 2014-01-08 17:05:34 UTC. Red lines indicate the envelope for possible power-law fits (slopes of -1.9 to -3.8), given 95% Poisson distribution confidence intervals on each number density (based on a single detection for each). The range of possible power-law slopes is consistent with that inferred for small bodies in the Kuiper belt, -2.5 to -3 (Kenyon & Bromley 2004).

"US Government sensors is a euphemism for "nuclear weapon warning early warning satellites", which is why we won't know details of the actual sensor: It is a military satellite and secret.

This particular event occurred in the Bismark sea, North of Papua New Guinea, it probably could have been seen from Papua or parts of Indonesia, if the sky was clear (which it rarely is, in the middle of the tropical rain band), but if it was seen, it wasn't reported to the American meteor society (reports from that time)

The satellites are designed to look for rocket launches, and to track the missiles as the climb through the atmosphere to determine the likely target. Thus they are able to get accurate position and velocity vectors of meteors. If you know the position and velocity vector of the meteoroid just prior to impact, it is an exercise in Newtonian gravity to work out the orbit of the meteoroid around the sun. (It is a three body problem as the meteoroid is moving in both the Earth's and the Sun's gravitational field)

It turns out I should have known the answer to this question because about six weeks ago I asked How was the 18 December 2018 Bering Sea Fireball detected and characterized? Was it a serendipity or a weather satellite?.

I'll draw upon my answer to that question here.

Scott Manley just addressed this in his new video 173 Kiloton Explosion Over Bering Sea Was Asteroid Breaking Up a bit after01:00:

But there are sensors all over the world looking for these big blasts because they look a lot like nuclear blasts; they have the same kind of energy, and there are many people who want to know if someone is testing nuclear weapons, so these things get picked up by that same network.

But the nature of this network means that we don't really get the results from it very quickly.

## ‘Oumuamua, the first known interstellar visitor, is just a comet

When astronomers first spotted the celestial object now known as ‘Oumuamua skittering across the sky last October after it had dived around the sun, its elongated trajectory and rapid speed quickly revealed that it came from outside the solar system.

Learning anything else about our first-known interstellar visitor, however—such as whether it was an asteroid or a dim comet—proved far more challenging, as it departed our planetary vicinity as quickly as it arrived. Either classification would have important implications not only for ‘Oumuamua itself, but also for understanding how planetary systems form.

Now a team of researchers monitoring the object on its journey back to the stars say they have the answer: ‘Oumuamua is almost certainly a comet, albeit one fittingly alien from those we find orbiting the sun. Using NASA’s Hubble Space Telescope and other ground-based instruments, the team observed ‘Oumuamua’s changing position across time and plotted its outbound trajectory, finding that, remarkably, it did not follow the path they had anticipated. The result appears in the June 27 edition of Nature.

“When put together, these positions showed that the motion of ‘Oumuamua was slightly different than what we expected,” said lead study author Marco Micheli of the European Space Agency’s SSA–NEO Coordination Center.

‘Oumuamua’s motion, it turns out, was changing ever-so-slightly over time—suggesting some force other than the sun’s steadily diminishing gravitational pull was acting on it.

The strange push was small, about two million times weaker than the pull of gravity on Earth’s surface and about 1,000 times smaller than the effect of the sun’s gravity, Micheli said.

Even so, over time that tiny push made big changes: At the distance of Jupiter, the team’s measurements show, ‘Oumuamua’s position was shifted from expectations by approximately the width of the giant planet. But what was nudging the mysterious visitor?

‘Oumuamua swung around the sun and its closest planets like a giant boomerang from above. Image by NASA/JPL

To find out, Micheli and his colleagues first simulated its journey through the solar system, accounting for gravitational shoves from all eight planets, Pluto, the moon and the largest bodies in the Asteroid Belt. They also investigated other possibilities such as the influence of “radiation pressure” from sunlight, tweaked rotational rates from uneven solar heating of ‘Oumuamua’s surface or potential collisions with other objects that could have affected the visitor’s trajectory. None of these explained the observed changes.

“There are a whole bunch of potential reasons that the comet could get nudged,” said cometary scientist and study co-author Karen Meech, a planetary scientist at the University of Hawai’i at Mnoa. “We systematically ruled them all out. The only one that’s physically plausible that remains is outgassing.”

In other words, ‘Oumuamua’s shift was self-induced, caused by the rocketlike effect of streams of gas shooting out of sunlight-warmed ice at or near its surface. Such a phenomenon is regularly observed in ordinary comets that pass near the sun—just as ‘Oumuamua had.

### A tale of no tail

Understanding what this all means requires a quick history lesson: Our solar system was a violent, chaotic place when it was young. In the first few hundreds of millions of years—an eyeblink in astronomy timescales—a wealth of material is thought to have been kicked out of the solar system as a result of gravitational interactions among the giant planets and other, smaller bodies.

Most of those outcasts were rich in icy, cometlike material from the outer solar system rather than rocky asteroids. If our solar system is typical (and there are as of yet few reasons to believe it is not), then most young planetary systems should suffer from similar violence and the space between the stars should be littered with more ejected comets than asteroids. Comets would thus be the default emissaries from other stars.

But as telescopes around the world turned their attention to ‘Oumuamua, it became clear the visitor showed no signs of cometary activity. It lacked the streaming tail of a comet or any signs of ice and gas emerging after sizzling so close to the sun. Some astronomers suggested the comet had been fried by interstellar radiation, forming a crust of material that shielded the lighter ices beneath from the sun’s evaporative warmth.

So, if Micheli and Meech’s data is sound, why didn’t earlier observations detect any gas—or, for that matter, the associated shift in ‘Oumuamua’s motion? One of the biggest reasons is because the emission of gas—and the resulting change in motion—was very small.

“[The push] was so little that it could not have been seen in our observations,” Micheli said–particularly, according to Meech, given the brevity of ‘Oumuamua’s close encounter with Earth and the object’s inherent dimness. Within a week of discovery ‘Oumuamua had receded so far from our planet, it was 10 times dimmer than when first spotted after a month it was a hundred times darker still. That made some observations difficult, if not impossible, Meech said.

When astronomers study comets, they hunt for cyanide, which when excited by starlight emits a distinct, telltale blue glow readily detectable with advanced telescopes. The compound is stirred into the comet at its formation, a fingerprint of the early planetary system. If ‘Oumuamua streamed cyanide gas, however, it was below the detection limits of current instruments.

That null result suggests ‘Oumuamua must have a cyanide to water ratio at least 15 times lower than our solar system’s most cyanide-depleted comets—further proof the object was truly not born in our solar system.

“It would not surprise me that a different solar system would have a totally different environment, and that we might find cyanide depletion,” said cometary scientist Matthew Knight of the University of Maryland, College Park, who was not part of the study team.

A dearth of small, light-reflecting dust particles on its surface could also account for ‘Oumuamua’s cloaked cometary emission. Knight said the object’s apparent lack of small dust could have come about in two different ways: Either it could have passed by its star multiple times in its home system, in which case stellar winds would eventually blow away its smallest dust particles or the dust could have slowly eroded away from eons of exposure to cosmic radiation during its long sojourn in interstellar space.

He doubts the first explanation, simply because most of the objects ejected from a planetary system are done so early on, before they can be so intensely baked by their stars. Although it is still possible to eject things from the solar system today, a late-breaking outcast would be rare in contrast to the wealth of material ejected in the early years.

Statistically speaking, ‘Oumuamua should not have had enough time to shed its small particles before leaving its home system. The second option—the gradual erosion of dust from cosmic radiation—is the explanation Meech and others find most likely.

‘Oumuamua is long gone, forever faded from the view of even the world’s current best telescopes. But astronomers are now busying themselves preparing for the next visitor. Meech said the completion in the 2020s of several planned next-generation observatories will allow for more detailed examinations of the solar systems’ next interlopers.

“Now that we know from direct experience how interstellar objects behave, we hope that the next time an object like this is discovered we will be able to get even more detailed observations,” Micheli said. “Hopefully the next one may also remain observable for a bit longer, giving us more time to study its motion and its composition in greater detail.”

Left: 'Oumuamua, our solar system’s first known interstellar visitor, appears as an elongated, metallic, asteroid-like object in this artist’s impression. New evidence, however, suggests 'Oumuamua more closely resembles a comet. Photo by M. Kornmesser/ESO

## Interstellar Meteor Likely Struck Earth In 2014, Say Astronomers

A meteor during the peak of the 2009 Leonid Meteor Shower. The photograph shows the meteor, . [+] afterglow, and wake as distinct components.

Using a NASA catalog of documented meteor strikes on Earth’s atmosphere, two Harvard University researchers report the detection of the first interstellar meteor ever observed in our solar system.

The one-meter diameter meteor --- spotted on January 8, 2014, off the coast of Papua New Guinea --- clocked in with an estimated mass of 500 kg. So, it burned up completely while traveling through the atmosphere just north of Manus Island, Abraham Loeb, the paper’s co-author and Chair of Harvard University’s Department of Astronomy, told me.

No meteor fragments were recovered, but Loeb says he’s 95% to 99% percent confident that the object that created this 2014 meteor show originated from outside the solar system.

One of Loeb’s astrophysics students, Harvard University undergraduate Amir Siraj, made the discovery after scouring NASA’s CNEOS (Center for Near-Earth Object (NEO) Studies) catalog to look for small high-velocity impactors. Siraj and Loeb’s analysis is reported in a paper just submitted to The Astrophysical Journal Letters.

“We found that one of these meteors, specifically the one that burned up in the atmosphere [in 2014], had to have been traveling extremely fast in order to hit the Earth at the direction and speed that it did,” Siraj, the paper’s lead author, told me.

Siraj says that with respect to the average velocity of stars in the solar neighborhood, this object was moving extremely fast, at around 60-km per second. Such an object’s high speed, enough to eject it from its own solar system and send it on an interstellar trajectory as a gravitationally-unbound object, can only be produced in the innermost cores of a planetary system, says Loeb, the paper’s co-author.

“One way of explaining this would be to posit that it came from a star in the thick disk of the Milky Way galaxy since thick disk stars have higher velocity dispersions,” said Siraj. However, he says, if so, this would be an unusual origin given that thick disk stars are also quite rare in the solar neighborhood.

Siraj and Loeb say that claims for Earth-striking interstellar meteors date back to at least 1940. But the speeds of such previously-claimed objects tend to hover just above the velocity threshold needed to be of interstellar origin. The difference with their new paper, says Siraj, is that this 2014 meteor can withstand a 45% reduction in its reported speed and still retain a high-enough velocity to dictate an origin from outside our solar system.

Unfortunately, the researchers have little idea from what specific group of stars this particular space rock might have originated. That’s because small errors in calculated impact speed can greatly affect the object’s pre-collision trajectory.

But they estimate 450 million such collisions over Earth's lifetime , meaning such interstellar meteors strike Earth’s atmosphere about once every decade.

What should ground observers be doing to make better use of future such interlopers?

The ability to perform spectroscopy on such objects as they burn up in Earth’s atmosphere would tell astronomers a lot about their composition. As they write in their paper, it’s expected that at least some isotope ratios in objects formed in other solar systems are expected to be markedly different than those found in our own.

Siraj and Loeb even note that precision tracking with the upcoming Large Synoptic Survey Telescope (LSST) might help track meteors of interstellar origin back to their parent systems.

Understanding the composition of such objects would teach us an enormous amount about the chemistry of other planetary systems, say Siraj.

## “Proof?” –‘Oumuamua-Like’ Interstellar Object Struck Earth in 2014

Interstellar meteors may be common, and could potentially help life travel from star to star throughout the Milky Way, according to Harvard astronomer’s Amir Siraj and Avi Loeb who report that they have uncovered possible evidence of an extrasolar object striking the Earth back in 2014 from their study of the Center for Near-Earth Object database. They were searching the data for telltale objects that traveled faster than normal, suggesting that it was likely ejected out of an alien star system.

The first known visitor from interstellar space, a cigar-shaped object named ‘Oumuamua, was detected in 2017. Scientists deduced the origins of the 1,300-foot-long (400 meters) object from its speed and trajectory, which suggests it may have come from another star, or perhaps two.

Avi Loeb, the chair of astronomy at Harvard University, noted that one would expect smaller interstellar visitors would be far more common, with some of them perhaps colliding with Earth often enough to be noticeable.

Loeb made headlines around the globe last fall 2018 when he suggested that the interstellar object known as ‘Oumuamua might an alien spacecraft. ‘Oumuamua was believed to have come from outside of the solar system because its trajectory showed it was not gravitationally bound to the sun—also, it traveled faster than traditional space objects. In this new effort, Loeb and his undergraduate assistant Siraj claim to have found evidence of another object from outside of the solar system.

The Harvard team report that they found three hits, two of which they dismissed because of incomplete data. But the third described a meteor that was believed to be slightly less than a meter wide that had been observed disintegrating in the atmosphere on January 8th, 2014, at a height of 18.7 kilometers near Papua New Guinea. Its speed had been measured by a government sensor at 216,000 km/h. By tracing its trajectory backward, the researchers report that it likely came from outside of our solar system.

Trajectory of the January 8, 2014 meteor (red), shown intersecting with that of Earth (blue) at the time of impact, ti = 2014-01-08 17:05:34. Credit: arXiv:1904.07224 [astro-ph.EP]

If their finding proves valid, it would be the first known instance of an extrasolar object striking the Earth.

Our first documented interstellar visitor, Oumuamua, was discovered Oct. 19 using the Pan-STARRS telescope, which is operated near the summit of Maui’s Haleakala volcano by the Institute for Astronomy at the University of Hawaii as it traveled through the inner solar system, at a distance of about 19 million miles from Earth. An analysis of its trajectory suggests that it came in from a place far beyond the solar system, somewhere in the constellation Lyra, heading towards the constellation Pegasus.

Named Oumuamua, Hawaiian for “Messenger from Afar”, it’s believed to be the first interstellar object observed passing through our solar system. Many tried listening to it for radio signals, to see if they could determine what it was. Was it a shard from an ancient asteroid, a weird comet? Or was it something else?

So The Daily Galaxy emailed Loeb. Here’s what we asked: “We’d like to include a quote (of any length) from you on your thoughts about human implications of the Oumuamua “spacecraft” debate. In short, it seems that we are rooting beyond the science for validation of the spacecraft hypothesis. In the rancorous, tribal environment we’re living through, it appears the human species is yearning for validation of intelligent life beyond our fragile Blue Dot.”

I was very surprised about the reaction of the media to our paper. We did not have a press release. The paper was submitted for publication ten days ago and posted on the online arXiv at the same time. It was reviewed and accepted for publication within a record time of only a few days. I received positive reactions from distinguished astronomers, such as the Astronomer Royal in the UK, Lord Martin Rees.I am glad to see the excitement about the paper, but it was not written for that purpose. We just followed the standard practice of scientific research.

I prefer not to assign probabilities to the nature of Oumuamua, we just need to be practical and collect more data on it or other members of its population. The interpretation of existing and future data is my plan for the future.

It is exciting to live at a time when we have the scientific technology to search for evidence of alien civilizations. The evidence about Oumuamua is not conclusive but interesting. I will be truly excited once we have conclusive evidence.

Oumuamua deviates from a trajectory that is solely dictated by the Sun’s gravity. This could have been the result of cometary outgassing, but there is no evidence for a cometary tail around it. Moreover, comets change the period of their spin and no such change was detected for Oumuamua. The excess acceleration of Oumuamua was detected at mutiple times, ruling out an impulsive kick due to a break up of the object. The only other explanation that comes to mind is the extra force exerted on Oumuamua by sunlight. In order for it to be effective, Oumuamua needs to be less than a millimeter in thickness, like a sail. This led us to suggest that it may be a light-sail produced by an alien civilization.

I welcome other proposals, but I cannot think of another explanation for the peculiar acceleration of Oumuamua.

The response to my paper with my postdoctoral fellow, Shmuel Bialy, has been truly remarkable. We submitted it for publication only a week ago. It was accepted for publication in The Astrophysical Journal Letters merely three business days later. The attention was created by blogs on Centauri Dreams and Universe Today. But by now, Twitter is humming continuously about it.

Our own civilization is currently engaged in developing the light-sail technology. The solar-sail principle was already demonstrated by the Japanese IKAROS projectand is being developed towards the goal of reaching high speeds by the Starshot project of the Breakthrough Prize Foundation, for which I chair the advisory board. It is conceivable that more advanced civilization are using this technology routinely, and this resulted in space debris of the type of Oumuamua.

Looking ahead, we should search for other interstellar objects in the sky. Such a search would resemble my favorite activity with my daughters when we vacation on a beach, namely examining shells swept ashore from the ocean. Not all shells are the same, and similarly only a fraction of the interstellar objects might be technological debris of alien civilizations. But we should examine anything that enters the Solar System from interstellar space in order to infer the true nature of Oumuamua or other objects of its mysterious population.

## Unknown Uncertainties

That is also the view of Paul Chodas, the CNEOS catalog&rsquos manager at NASA&rsquos Jet Propulsion Laboratory. &ldquoWe at CNEOS simply post the fireball data that is reported to us we have no information on the uncertainties,&rdquo he says.

In March of this year, Chodas says, he and other CNEOS staffers flagged 2014&rsquos Papua New Guinea meteor as potentially interstellar based on their own calculations of its orbit&mdashbut did not publish that result due to concerns about the data&rsquos quality. Loeb and Siraj&rsquos &ldquoquite extraordinary&rdquo and &ldquohighly speculative&rdquo claim, he says, &ldquois based on just a few numbers that are likely highly uncertain.&rdquo (In their paper, Loeb and Siraj cite previous work reporting that the CNEOS catalog&rsquos typical uncertainty for the velocity of a meter-sized meteor is less than a kilometer per second&mdashan insignificant offset in the enormous measured speed of their candidate interstellar fireball.)

Asked about uncertainties in the CNEOS fireball catalog, Lindley Johnson, NASA&rsquos &ldquoplanetary defense officer,&rdquo notes that its entries represent the use of data &ldquoin a way it was never, ever originally intended.&rdquo Although initially conceived as a simple list of fireball times, locations and energy levels, more than a decade ago the catalog also began incorporating estimates of speed and directionality for particularly data-rich events, in hopes that researchers could use those projections to track down meteorite debris fields from large fireballs that occurred over land. Soon, particularly bold analysts were using those projections to look back in time, piecing together the potential orbital histories of meteors to link them and any meteorites they produced to certain families of asteroids. That was &ldquoalready stretching the credence in the data beyond anything really scientifically valid,&rdquo Johnson says. &ldquoNow [Loeb and Siraj] want to speculate based on such tenuous data that some could be interstellar objects? That really stretches the credibility past the breaking point for me.&rdquo

Peter Brown, a planetary astronomer and leading meteor expert at Canada&rsquos Western University, says that even though the CNEOS catalog is on average of very high quality, the validity of any single data point&mdashparticularly for smaller meteors&mdashremains questionable. &ldquoStatistically, I think the catalog&rsquos derived orbits and velocities and trajectories are fine,&rdquo he says. &ldquoBut we simply don&rsquot know which ones are good and which ones are bad.&rdquo Furthermore, Brown says, of the thousands of small fireballs previously detected by other, independent surveys using ground-based cameras and radar stations, not one has clearly exhibited a hyperbolic trajectory. &ldquoIf a tenth or a twentieth of a percent of the population was hyperbolic as Loeb and Siraj claim, you&rsquod expect to have a fair number of hyperbolics in the data from ground-based networks&mdashbut we don&rsquot see that.&rdquo

Even so, Brown adds, &ldquoit is a fantastic thing that others are coming from different disciplines and applying their own approaches to this rich data set&hellip. Interstellar meteorites must be hitting Earth&rsquos atmosphere, and fireballs are the natural way to look for them. We just have to find them convincingly, in ways that can&rsquot be dismissed as measurement uncertainties.&rdquo

This, naturally, is part of Loeb and Siraj&rsquos grand plan. The next step in the quest for interstellar meteors, they say, is to ensure that potentially hyperbolic fireballs can be not only detected but also characterized. Observed with the right equipment, a fireball&rsquos light can be broken up into a multicolored spectrum which acts as a &ldquobarcode&rdquo to reveal the object&rsquos chemical composition&mdasha critical clue as to whether or not it formed around our sun.

&ldquoEvery few years we should have one of these hyperbolic meteors,&rdquo Loeb says. &ldquoIf we just ensure observers are flagging fireballs with excess velocities, we should be able to set up spectroscopic surveys to get each one&rsquos spectrum as it burns up in the atmosphere and indeed demonstrate an origin beyond our solar system. Surely this is something worth investing in!&rdquo

## Apparent trajectory of Oumuamua on Earth's sky, past and future

That's actually a pretty neat depiction of decreasing parallax with distance.

Somebody help me out here, I have no idea what's going on.

‘Oumuamua is the first interstellar object observed by humans in our solar system. It was identified in October.

This is where the object would appear when viewed from Earth when viewed at the dates provided at each instance.

I agree and the other comments haven't helped at all. ELI5??

3-dimensional motion mapped onto a surface of a 2-dimensional sphere.

It approached us from above, as we orbit around the sun while looking at it for a couple of years, it looks like an ever widening spiral, it is us moving in circles.
As it recedes below us, same process in reverse.

An interstellar alien race did donuts above the Earth, did a close pass burnout, and are continuing the donuts in another part of the sky. Reckless vermin.

The first I heard of this I immediately thought of the book "Rendezvous with Rama" by Arthur C. Clarke.

If you haven't read it, go do it now.

Rendezvous with Rama by Arthur C Clarke, reproduced in full here. It's a short, captivating, and accessible hard science fiction that postulates on humanity's first contact with aliens in an altogether novel way (no pun intended.)

It is far and away one of my favorite novels. Many years ago I wrote a Reddit comment that I pull back up time and again in conversations like this one, to entice the reader into sharing in the literary experience that is Rendezvous with Rama. I'll paste it again below. It is the opening page of the story and sets the grand stage for the subsequent narrative, embedded with my own contextual annotations for context and flair.

Sooner or later, it was bound to happen. On June 30, 1908, Moscow escaped destruction by three hours and four thousand kilometers--a margin invisibly small by the standards of the Universe. On February 12, 1947, another Russian city had a still narrower escape, when the second great meteorite of the twentieth century detonated less than four hundred kilometers from Vladivostok, with an explosion rivaling that of the newly invented uranium bomb.

In those days there was nothing that men could do to protect themselves against the last random shots in the cosmic bombardment that had once scarred the face of the Moon. The meteorites of 1908 and 1947 had struck uninhibited wilderness but by the end of the twenty-first century there was no region left on Earth that could be safely used for celestial target practice. The human race had spread from pole to pole. And so, inevitably.

At 0946 GMT on the morning of September 11 in the exceptionally beautiful Summer of the year 2077, most of the inhabitants of Europe saw a dazzling fireball appear in the Eastern sky. Within seconds it was brighter than the Sun, and as it moved across the heavens--at first in utter silence--it left behind it a churning column of dust and smoke. Somewhere above Austria it began to disintegrate, producing a series of concussions so violent that more than a million people had their hearing permanently damaged. They were the lucky ones.

Moving at fifty kilometers a second, a thousand tons of rock and metal impacted on the plains of northern Italy, destroying in a few flaming moments the labor of centuries. The cities of Padua and Verona were wiped from the face of the Earth and the last glories of Venice sank forever beneath the sea as the waters of the Adriatic came thundering landward after the hammer blow from space.

Six hundred thousand people died, and the total damage was more than a trillion dollars. But the loss to art, to history, to science--to the whole human race, for the rest of time--was beyond all computation. It was as if a great war had been fought and lost in a single morning and few could draw much pleasure from the fact that, as the dust of destruction slowly settled, for months the whole world witnessed the most splendid dawns and sunsets since Krakatoa.

After the initial shock, mankind reacted with a determination and a unity that no earlier age could have shown. Such a disaster, it was realized, might not occur again for a thousand of years--but it might occur tomorrow. And the next time, the consequences could be even worse.

Very well there would be no next time.

A hundred years earlier, a much poorer world, with far feebler resources, had squandered its wealth attempting to destroy weapons launched, suicidally, by mankind against itself. The effort had never been successful, but the skills acquired then had not been forgotten. Now they could be used for a far nobler purpose, and on an infinitely vaster stage. No meteorite large enough to cause catastrophe would ever again be allowed to breach the defenses of Earth.

So began project SPACEGUARD. Fifty years later--and in a way that none of its designers could ever have anticipated--it justified its existence.

-Rendezvous with Rama, by Arthur C. Clark - If you never have, you owe it to yourself to read this book. At least the first three pages, which may conjure up interesting questions in your mind given this real-life discovery of an interstellar visitor.

## First Encounter

The two teams used techniques that sensed different aspects of the comet, like blind monks touching an elephant.

The first study, in Nature Communications and led by Stefano Bagnulo (Armagh Observatory, UK), measured the light scattered by the coma, the diffuse “atmosphere” of gas and dust around the comet. Photons bouncing off dust grains in the coma will not only change direction (some of them toward Earth), but they’ll also become polarized. The smaller the grains, the more they polarize the light.

This diagram shows unpolarized light scattering off a molecule. In the process, the light becomes polarized, but the amount of polarization depends on the angle it scatters. Observing scattered light from many angles (e.g., as a comet traverses the inner solar system) can reveal the nature of what's doing the scattering.
Harvard Natural Sciences Lecture Demonstrations

Sunlight scatters off molecules in Earth’s atmosphere in the same way, making Earth’s sky blue. In the case of Comet Borisov, the high degree of polarization suggests the dust grains in the coma are similarly tiny, on the sub-micron scale, says team member Ludmilla Kolokolova (University of Maryland). That’s about the same size as the wavelength of visible light.

This finding makes Comet Borisov unlike any other comet in the solar system except one: Comet Hale-Bopp (C/1995 O1). This brilliant comet was visible to the naked eye for a year and a half, a record that jived with scientists’ assessment that Hale-Bopp had only approached the Sun perhaps once before, around 2000 B.C. Years of observations of “The Great Comet of 1997” confirmed that the dust particles suffusing its coma were smaller than around any other observed comet.

What this means for Borisov is that its encounter with our Sun was the first time it came up close to any star, including its own. It must have originated far out from its host before being ejected into interplanetary space, making our Sun the first to quicken its cometary activity.

Unfortunately, further observations to confirm the small-grain scenario were scuttled due to the COVID-19 pandemic, which shuttered Paranal Observatory from March through August 2020. Operations have restarted in limited mode but it was already too late for the observations Bagnulo had planned. “These data would have been very useful to further characterize the dust particles,” Bagnulo says.

## On the hunt

Siraj and co-author Avi Loeb, the chair of Harvard's astronomy department, have been thinking a lot about the best ways to find more interstellar visitors. For example, the duo recently scoured the fireball database compiled by the Jet Propulsion Laboratory's Center for Near-Earth Object Studies (CNEOS), which contains information about hundreds of meteor impacts on Earth over the past three decades.

One of the CNEOS data points stood out, Siraj and Loeb found. The trajectory and blazing speed of a meteor that burned up in the sky near Papua New Guinea on Jan. 8, 2014, indicate that the object came from interstellar space, the researchers reported in a paper this past April.

That paper hasn't made it past peer review yet, largely because the CNEOS database does not report measurement error. Such information is classified it could reveal details about the sensitivity of space-based sensors operated by the U.S. government. Siraj and Loeb were able to constrain the error bars for the January 2014 meteor — a roughly 3-foot-wide (1 meter) object they call CNEOS 2014-01-08 — thanks to two helpful colleagues with the required security clearance. But The Astrophysical Journal Letters, which is assessing the paper, has had difficulty finding a reviewer able to do the same, Siraj said. (Once that paper is published, the number of confirmed interstellar visitors to our solar system will double, to two.)

Now, Siraj and Loeb have applied this same basic idea — using a cosmic body as an interstellar-meteoroid detector — to the moon. In the new study, the pair calculated the detection potential of a telescope in lunar orbit, which would hunt for the streaking motion of incoming meteoroids as well as the bright flashes and craters generated when these bodies slam into the moon's gray dirt.

Siraj and Loeb found that a telescope with an aperture of at least 6.5 feet (2 m), orbiting 62 miles (100 kilometers) above the lunar surface, would likely spot at least one interstellar impact per year. (For comparison, NASA's iconic Hubble Space Telescope has a primary mirror 7.9 feet, or 2.4 m, wide.)

That rate is far from set in stone, the researchers stressed. After all, it's based on an imperfect estimate of the interstellar-object population within our solar system.

"We're dealing with such a small amount of calibration — 'Oumuamua and this other tentatively discovered meteor — so there's a lot of uncertainty there," Siraj said. "But something the size of Hubble should get us in the ballpark where we should see one per year."

As with CNEOS 2014-01-08, these special strikes could be identified based on the speed and trajectory of the impactors, the researchers wrote. And this information would lead to other insights.

"Just these basic measurements would allow us to constrain the 3D velocity, the mass and, most importantly, the density [of the impactors]," Siraj said. "This would allow us to see, for example, if a mini 'Oumuamua struck the lunar surface — we'd be able to figure out, is it icy, is it rocky, et cetera."

Spectroscopic observations of the plume generated by such strikes could also reveal key details of the impactors' composition, he added.

And it wouldn't be a tragedy if the telescope failed to spot any interstellar impacts, Siraj said. This would be a valuable data point in itself, suggesting that the population of interstellar objects in our solar system is much smaller than scientists had thought.

## ‘Oumuamua Interstellar Object Was Not an Alien Spacecraft

On October 19, 2017, astronomers discovered the first known interstellar object to visit our solar system. Early reports of the odd characteristics of “‘Oumuamua” led to speculation that the object might be an alien spacecraft. Now a new analysis by an international team of astronomers co-led by, University of Maryland Associate Research Scientist Matthew Knight, strongly indicates that its origin is purely natural.

When first spotted by the Panoramic Survey Telescope and Rapid Response System 1 telescope located at the University of Hawaii’s Haleakala Observatory, the object defied easy description, simultaneously displaying characteristics of both a comet and an asteroid. Astronomers formally named the object 1I/2017 U1 and with a common name of ‘Oumuamua, which roughly translates to “scout” in Hawaiian.

Researchers from UMD and around the world immediately raced to collect as much data as possible in the few weeks they had to observe the strange visitor before ‘Oumuamua traveled beyond the reach of Earth’s telescopes. The new findings by Knight and colleagues from some 13 institutions and five countries are reported in the July 1, 2019, issue of the journal Nature Astronomy.

“We have never seen anything like ‘Oumuamua in our solar system. It’s really a mystery still,” Knight said. “But our preference is to stick with analogs we know, unless or until we find something unique. The alien spacecraft hypothesis is a fun idea, but our analysis suggests there is a whole host of natural phenomena that could explain it.”

As Knight and his colleagues summarized in their study, ‘Oumuamua is red in color, similar to many small objects observed in our solar system. But that’s where the familiarity ends.

‘Oumuamua likely has an elongated, cigarlike shape and an odd spin pattern—much like a soda bottle laying on the ground, spinning on its side. According to Knight, its motion through our solar system is particularly puzzling. While it appeared to accelerate along its trajectory—a typical feature of comets—astronomers could find no evidence of the gaseous emissions that typically create this acceleration.

“The motion of ‘Oumuamua didn’t simply follow gravity along a parabolic orbit as we would expect from an asteroid,” Knight said. “But visually, it hasn’t ever displayed any of the cometlike characteristics we’d expect. There is no discernable coma—the cloud of ice, dust and gas that surrounds active comets—nor a dust tail or gas jets.”

Knight worked with Alan Fitzsimmons, an astronomer at Queen’s University Belfast in Northern Ireland, to assemble a team of 14 astronomers hailing from the U.S. and Europe. The International Space Science Institute in Bern, Switzerland, served as a virtual home base for the collaboration.

“We put together a strong team of experts in various different areas of work on ‘Oumuamua. This cross-pollination led to the first comprehensive analysis and the best big-picture summary to date of what we know about the object,” Knight explained. “We tend to assume that the physical processes we observe here, close to home, are universal. And we haven’t yet seen anything like ‘Oumuamua in our solar system. This thing is weird and admittedly hard to explain, but that doesn’t exclude other natural phenomena that could explain it.”

The new research paper is primarily an analysis of existing data, including of a December 2017 study of ‘Oumuamua’s shape and spin pattern co-authored by Knight and a team of UMD astronomers. This earlier paper, published in The Astrophysical Journal Letters, relied on data from the Discovery Channel Telescope (DCT) at the Lowell Observatory in Arizona. UMD is a scientific partner of the DCT, along with Boston University, the University of Toledo and Northern Arizona University.

Knight, Fitzsimmons and their colleagues considered a number of mechanisms by which ‘Oumuamua could have escaped from its home system. For example, the object could have been ejected by a gas giant planet orbiting another star. According to theory, Jupiter may have created the Oort cloud—a massive shell of small objects at the outer edge of our solar system—in this way. Some of those objects may have slipped past the influence of the sun’s gravity to become interstellar travelers themselves.

The research team suspects that ‘Oumuamua could be the first of many interstellar visitors. Knight is looking forward to data from the Large Synoptic Survey Telescope (LSST), which is scheduled to be operational in 2022.

“In the next 10 years, we expect to begin seeing more objects like ‘Oumuamua. The LSST will be leaps and bounds beyond any other survey we have in terms of capability to find small interstellar visitors,” Knight said. “We may start seeing a new object every year. That’s when we’ll start to know whether ‘Oumuamua is weird, or common. If we find 10-20 of these things and ‘Oumuamua still looks unusual, we’ll have to reexamine our explanations.”

The research paper, “The Natural History of ‘Oumuamua,” the ‘Oumuamua ISSI Team (Michele Bannister, Asmita Bhandare, Piotr Dybczyński, Alan Fitzsimmons, Aurélie Guilbert-Lepoutre, Robert Jedicke, Matthew Knight, Karen Meech, Andrew McNeill, Susanne Pfalzner, Sean Raymond, Colin Snodgrass, David Trilling and Quanzhi Ye), was published in the journal Nature Astronomy on July 1, 2019.

The December 2017 research paper on the object by the UMD research team was, “On the Rotation Period and Shape of the Hyperbolic Asteroid 1I/‘Oumuamua (2017 U1) from Its Lightcurve,” Matthew Knight, Silvia Protopapa, Michael Kelley, Tony Farnham, James Bauer, Dennis Bodewits, Lori Feaga and Jessica Sunshine, was published in The Astrophysical Journal Letters.

This work was supported by the UK Science and Technology Facilities Council (Award Nos. ST/P0003094/1 and ST/L004569/1), the National Science Foundation (Award Nos. AST1617015 and 1545949), NASA (Award Nos. GO/DD-15405, GO/DD-15447, NAS 5-26555, NNX17AK15G and 80NSSC18K0829), the National Science Centre in Poland (Award No. 2015/17/B/ST9/01790) and the European Research Council (Award No. 802699). The content of this article does not necessarily reflect the views of these organizations.

Looking forward, the Large Synoptic Survey Telescope will be operational by 2020-2022, and make the study of space rocks much easier.

On August 30th, 2019, Gennady Borisov detected another interstellar object that appeared to be something we might call the ʻOumuamua 2.0. As observed by NASA’s Scout system, it appears to have an unusual orbit, and measurements taken by compiling data provided by the Canada-France-Hawaii Telescope classify the object as an interstellar space rock. Its closest approach would be in the month of December.

Assumptions have been made regarding the trajectory and traits of this rock, and it is under constant observation. Whether a cometary tail will appear, or not, in the course of its path is the question. Bumps in speed have also been kept a look-out for, to confirm the interstellar nature of the rock. If there is no out-gassing, no tail, and no explanation regarding its origin, the nature of the ʻOumuamua 2.0 will be confirmed, and will aid in our study of the increasingly frequent detections of interstellar objects.