Comets – astroengine.com

Interstellar Comet Borisov Looks Weirdly Familiar

The gas cyanogen has been detected in 2I/Borisov’s coma—a historic detection of a gas commonly found in regular comets.

Artist’s impression of a cometary nucleus. [ESA]

It’s official, the solar system is playing host to its second confirmed interstellar visitor only two years after the strangely-shaped `Oumuamua was spotted receding into interstellar expanse. While `Oumuamua was historic in that it was the first confirmed interstellar comet to be discovered, according to a new study (which has yet to be peer reviewed), this newest interstellar vagabond is potentially more significant:

“For the first time we are able to accurately measure what an interstellar visitor is made of, and compare it with our own solar system,” said Alan Fitzsimmons of the Astrophysics Research Center, Queen’s University Belfast, in a statement.

So, why are astronomers so excited about 2I/Borisov?

A (Cometary) Star Is Born

In late August, the comet was discovered by Gennady Borisov, an amateur astronomer in Crimea, and initially designated “C/2019 Q4” because, well, it looked like a regular comet. It was only after repeated observations by Borisov, and confirmed by other amateur and professional astronomers, that the object’s path and speed through the solar system could be realized. It turned out to be traveling fast.

Like, really, really fast.

Clocked at a breakneck pace of 93,000 miles per hour (150,000 kilometers per hour), astronomers realized that C/2019 Q4 was a special kind of comet. While it was found to possess the characteristics of a regular comet (it has a faint coma and tail) there is no way that it’s gravitationally bound to our Sun. Its trajectory is hyperbolic. In other words, it didn’t originate in our solar system—it’s an alien visitor.

This simple animation depicts the comets path through our neighborhood; there’s little ambiguity in the fact that it doesn’t intend to hang around for very long:

With only a slight tug by our Sun’s gravity, the interstellar visit will careen out of the solar system in a few months. [NASA/JPL-Caltech]

Last week, these factors all culminated in the International Astronomical Union (IAU) officially classifying C/2019 Q4 as the second unambiguous interstellar comet discovery to date. It was therefore reclassified as “2I/Borisov” (1I/`Oumuamua being the first, of course). It’s thought interstellar junk passes through our solar system all the time, but only two comets (to date) have been confirmed to have an interstellar origin, suggesting our observational techniques are improving.

Now, the really neat thing about 2I/Borisov is that it’s a lot more active than `Oumuamua; the latter produced very little in the way of a discernible coma or tail after its discovery. Borisov, however, is being far more generous, already allowing astronomers to grab a crude spectroscopic snapshot of the gases being vented into space.

A Mysterious Interstellar Time Capsule

After being thwarted by the glare of the Sun on Sept. 13, an international team of astronomers was able to use the William Herschel Telescope on La Palma in the Canary Islands in the morning of Sept. 20 to measure the light that was being scattered off the gases in its tenuous coma. Follow-up spectral analysis by the TRAPPIST-North telescope in Morocco was also used.

Measuring the spectrum of Borisov allows us to understand the chemical composition of the ices that are fizzing into space as they are slightly heated by our Sun’s radiation via a process known as sublimation. And this is profoundly awesome.

To capture the spectra of any comet reveals the chemicals it contained when it formed billions of years ago. In our solar system, comets are considered to be icy time capsules; they formed from the solar nebula when the Sun was a proto-star and the planets were just starting to accrete from the surrounding protoplanetary disk of ancient debris. To see the chemicals contained within the vapor of these fizzing “dirty snowballs” gives us a five-billion-year-old glimpse of what the solar nebula and its system of baby planets would have contained.

2I/Borisov as imaged by the Gemini North Telescope on Hawaii. [GEMINI OBSERVATORY/NSF/AURA/INTERNATIONAL ASTRONOMICAL UNION]

Borisov wasn’t formed in our solar nebula, however, it was formed from the nebula of a distant, unknown star of unknown age. We have little idea as to where or when it originated (though there’s little doubt that astronomers will use data from the European Gaia space telescope to try to figure out a rough estimate, as they did with `Oumuamua).

Surprisingly Familiar

While previous observations of the comet’s nucleus have revealed a reddish tinge that is similar to the long-period comets that originate from our solar system’s Oort Cloud (such as Hale-Bopp and Hyakutake), the new study has been able to identify another familiar trait: its venting gases contain cyanogen. This chemical is a simple, yet toxic molecule containing one carbon atom and a nitrogen atom (CN). Cyanogen is commonly found in regular comets born in the solar system.

The researchers were also able to make an estimate of the ratio of the dust to gas that is being blasted from the comet’s nucleus and, you guessed it, it is roughly in agreement to what you’d expect a regular comet to generate.

All of these findings point to an unexpected conclusion, as the researchers highlight in their paper: “If it were not for its interstellar nature, our current data shows that 2I/Borisov would appear as a rather unremarkable comet in terms of activity and coma composition.” In other words, if it wasn’t for its extreme speed, 2I/Borisev would look like a regular comet from our solar system.

Does this mean all comets from any star system have similar compositions? That doesn’t seem possible, considering we know other stars and their associated nebulae their comets would have formed from contain different chemicals to our own. It could just mean that Comet Borisov was ejected from a nearby star that formed in the same stellar nursery as our Sun five billion years ago and should therefore contain approximately the same chemicals. But for now, it’s too early to say.

Obviously, more work needs to be done and, fortunately, we have time. The comet will reach perihelion (point of closest approach to the Sun) in early December, and astronomers are predicting maximum nucleus activity in December and January before it starts to recede into the interstellar night.

So, watch this space.

Sadly, ‘Oumuamua Isn’t Piloted by Joyriding Aliens

An international team of experts have teamed up to conclude that the interstellar visitor isn’t what we hoped it was.

An artist’s impression of the strangely-elongated interstellar object ‘Oumuamua that zoomed through our solar system in 2017 [ESO/M. Kornmesser]

It probably comes as no surprise that the scientific consensus of ‘Oumuamua’s origins have concluded that it is a natural object, despite how funky and alien spaceship-looking the interstellar visitor at first appeared. According to a new study published today in the journal Nature Astronomy, the findings of 14 international experts have been pooled to categorically say that ‘Oumuamua isn’t an artificial object piloted by an intelligent extraterrestrial species, but instead “has a purely natural origin.”

“The alien spacecraft hypothesis is a fun idea, but our analysis suggests there is a whole host of natural phenomena that could explain it,” said the team’s leader Matthew Knight, from the University of Maryland, in a statement.

This most recent study comes hot on the heels of a fair amount of speculation that the spinning cigar-shaped object, which was detected by the Pan-STARRS1 telescope in Hawaii on Oct. 19, 2017, could be artificial. One of the more vocal advocates of this possibility, Avi Loeb of Harvard University, investigated the idea that ‘Oumuamua may be an interstellar probe that used our sun’s radiation pressure for a boost in velocity as it flew through the inner solar system. While the world’s media loved this concept (as did I), many scientists balked and emphasized the need to take the Occam’s razor approach and instead focus on natural explanations, not aliens. But, as pointed out by Loeb, while more likely explanations existed, considering the most extreme ones is still a part of the scientific process.

“This is how science works,” said Loeb in an interview for The Harvard Gazette late last year. “We make a conjecture … and if someone else advances another explanation, we will compare notes and the next time we see an object of this type we will hopefully be able to tell the difference. That’s the process by which science makes progress.”

Deep down, we all had the sense that the interstellar visitor likely wasn’t aliens (though it did spawn some wonderful debates about mind-boggling interstellar distances, the challenges of visiting other star systems, and why ET would bother popping by for a whistle-stop tour without saying “hi”), but this new study convincingly sounds the death knell for the possibility of aliens taking a joyride through our galactic neighborhood.

The new study is clear, in which the researchers write: “Here we review our knowledge and find that in all cases, the observations are consistent with a purely natural origin for ‘Oumuamua.”

So, what does the study conclude?

The object is most likely an ancient interstellar comet that randomly encountered our solar system after drifting through interstellar space for millions of years. The mechanisms by which ‘Oumuamua was ejected from its star system of birth remains up for debate, but the study’s authors point to the likelihood of a Jupiter-like world that may have gravitationally ejected the object when it strayed too close, helping it achieve escape velocity and a future lost deep in the interstellar expanse—until it encountered our solar system.

Even the behavior of the ancient comet as it traveled through the inner solar system agrees with theoretical predictions. The small boost in velocity as it made close approach to our sun was caused by ices (entombed under ‘Oumuamua’s surface) being heated and vented into space as a vapor (and not aliens hitting the gas). This behavior in comets is well-known, but the problem with ‘Oumuamua is that it exhibited few signs of being a comet—it didn’t develop a tail nor did it develop a coma, two clues of its cometary nature. But this object is different from the comets we know; it has been drifting through the galaxy for eons, perhaps it lost the majority of its ice in previous stellar encounters, or perhaps it contained little in the way of volatiles during its formation. Comets and asteroids also have a lot more in common that the textbooks may tell us, so perhaps it did vent small quantities of vapor to give it a boost, but not enough for astronomers to observe a tail and coma. In short, ‘Oumuamua shares similar traits to other objects that exist in our solar system

“While ‘Oumuamua’s interstellar origin makes it unique, many of its other properties are perfectly consistent with objects in our own solar system,” added Robert Jedicke of the University of Hawai’i’s Institute for Astronomy (IfA) and collaborator in the Nature Astronomy study.

The key thing that makes ‘Oumuamua so captivating, however, is not how it behaved when it entered the solar system and used the sun to change its course, it’s that we know it came from interstellar space, the first of its kind that we’ve ever encountered. Undoubtedly, the solar system has been visited countless times by junk that has been shed by other stars in our galaxy—there’s a lot of stars carrying around a lot of comets and asteroids, after all, they’re probably scattered around the Milky Way like baby’s toys being thrown out of strollers—but this is the first, special interstellar visitor that we’ve only just had the ability to detect.

The best news? There will be more.

Humanity is rapidly advancing through a “golden age” for astronomy and, if these interstellar vagabonds are as common as we now believe, we’re on the verge of detecting many more of them. For example, the Large Synoptic Survey Telescope (LSST), which is being constructed in Chile, is expected to become operational in 2022 and it will be so powerful that astronomers predict at least one ‘Oumuamua-like object will be spotted per year. Once we grasp how often these things turn up, perhaps we’ll be prepared enough to have a robotic spacecraft intercept one to see what these visitors from other stars really look like instead of depending on distant observations.

Of course, this whole episode could be a cautionary tale. Perhaps our advanced alien neighbors disguise their spacecraft to look like passing comets to get a closer look of primitive intelligences such as ourselves.* ‘Oumuamua being identified as an interstellar comet is exactly what they want us to believe…

*This was inspired by a tweet I read this morning, but I forgot who tweeted it and it appears I didn’t “like” it, so it’s since been lost to the twitterverse. Thank you to whomever tweeted it, it formed the seed to this blog!

Our Universe Is a Cosmic Mixologist Looking for the Recipe of Life

Creating the conditions of interstellar space in the lab has led to a sweet discovery

The Egg Nebula, as imaged by Hubble, is a protoplanetary nebula with a young star in its core [NASA/ESA]

What do you get if you combine water with methanol and then bombard the mix with radiation? It turns out that the resulting cocktail is where the building blocks for life are found. But these chemicals aren’t bubbling out of the puddles of primordial goo pooling on some alien planet; the cocktail shaker is the frigid depths of interstellar space and the mixologist is the universe.

As described in a new study published on Tuesday in Nature Communications, a team of NASA scientists took what they knew of interstellar space and recreated it in a laboratory experiment. Interstellar space may not seem like a place where the chemistry of life could gain a foothold, but given enough time and the right ingredients, chemical reactions do happen — albeit very slowly. And if there’s one thing the universe has it’s time, and we’re beginning to understand that the cosmos we reside in could be a vast organic experiment.

“The universe is an organic chemist,” said Scott Sandford, a senior scientist in the NASA Ames Astrophysics and Astrochemistry Laboratory and co-investigator of the study. “It has big beakers and lots of time — and the result is a lot of organic material, some of which is useful to life.”

To see what chemistry might be going on in the void between the stars, the researchers simulated this extreme environment inside a vacuum chamber at Ames that was cooled to near-absolute zero. Inside, they placed an aluminum substance and then added the gaseous mixture of water vapor and methanol, a very common carbon-based molecule that is known to exist throughout our galaxy. Holding the aluminum at such low temperatures caused a frosty layer to form upon it. Then, they irradiated the substance with ultraviolet light — a form of radiation that is abundant in stellar nurseries, for example — and found that some interesting chemical reactions had occurred.

They discovered that a variety of sugar derivatives had formed on the substance — and one of those sugars was 2-deoxyribose. Yes, the same stuff you’d find in deoxyribonucleic acid. That’s the “D” in our DNA.

But this isn’t the first time an essential ingredient for life has been created in the lab while simulating the conditions of interstellar space. In 2009, the same team announced the discovery of uracil in their laboratory experiments — a key component of ribonucleic acid (RNA), which is central to protein synthesis in living systems. Also, in 2016, a French group discovered the formation of ribose, the sugar found in RNA.

“For more than two decades we’ve asked ourselves if the chemistry we find in space can make the kinds of compounds essential to life. So far, we haven’t picked a single broad set of molecules that can’t be produced,” said Sandford in a NASA statement.

Although these are significant discoveries that provide new insights to how and where the most basic ingredients for life may form, it’s a long way from helping us understand whether or not life is common throughout the universe. But it turns out that some of the coldest spaces in the cosmos could also be the most fertile environments for the formation of a range of chemicals that are essential for life on Earth. It’s not such a reach, then, to realize that the protoplanetary disks surrounding young stars will also contain these chemicals and, as planets form, these chemicals become an intrinsic ingredient in young planets, asteroids and comets. Over four billion years ago, when the planets condensed from our baby Sun’s nebulous surroundings, Earth may have formed with just the right abundance of molecules that form the backbone of DNA and RNA to kick-start the genesis of life on our planet. Or those ingredients were delivered here later in the frozen cores of ancient comets and asteroids.

The building blocks of life are probably everywhere, but what “spark” binds these chemicals in such a way that allows life to evolve? This question is probably well beyond our understanding for now, but it seems that if you give our Cosmic Mixologist enough time to concoct all the chemicals for life, life will eventually emerge from the cocktail.

If Aliens Pilot Interstellar Object ‘Oumuamua, They Snubbed Us

The Seti Institute has monitored the object for radio transmissions, just in case it isn’t natural

We humans are a sensitive bunch. We keep pondering the question: “are we alone?” If we consider the answer is a “yes,” we then start having an existential crisis over our place in the universe. But if the answer is a “no,” a can of worms open and we start asking even more questions. “If they’re out there, where are they?” “Isn’t it a bit weird we haven’t heard from our extraterrestrial neighbors?” “Are they just too far away for us to communicate?” and my personal favorite: “Have they consciously decided not to communicate with us because we’re considered not worth communicating with?!” The Fermi Paradox is certainly as paradoxical as they come.

Cue a random object that cruised through our solar system last year. The interstellar visitor zoomed right into our interplanetary neighborhood, used the Sun’s gravity for a cheeky course correction, and then slingshotted itself back out into deep space. The whole thing happened so quickly that astronomers only noticed when the thing was speeding away from us at high speed.

Naturally, we took a hint from science fiction, remembering Arthur C. Clarke’s classic novel “Rendezvous With Rama” — when a huge artificial object appears from interstellar space and a brave team of astronauts are sent to intercept it. Might this interstellar object also be artificial? After all, it has an odd, tumbling shape (like a spinning cigar) and the precision at which it flew past us with the trajectory it did (using the Sun to change its direction and speed of travel) just feels artificial.

So, with the help of the SETI Institute’s Allen Telescope Array (ATA) in California, astronomers decided to take aim at the departing object from
Nov. 23 and Dec. 5, 2017, when it was 170 million miles from Earth. The objective was to listen out for artificial radio transmissions that might reveal any kind of extraterrestrial intelligence. By monitoring frequencies from 1 to 10 GHz (at 100 MHz intervals), the ATA would be able to detect a very low powered onmidirectional transmitter, with a transmitting power as low as 10 Watts — the approximate equivalent to a citizen band radio.

According to the SETI study to be published in the February 2019 issue of Acta Astronautica, no signals were detected. Though this is obviously a blow for working out whether this thing was being actively piloted by some kind of intelligence, it does narrow down the true nature of the object, that has since been named ‘Oumuamua — which, in Hawaiian, roughly means “scout,” or “messenger.”

“We were looking for a signal that would prove that this object incorporates some technology — that it was of artificial origin,” said Gerry Harp, lead author of the study, in a SETI Institute statement. “We didn’t find any such emissions, despite a quite sensitive search. While our observations don’t conclusively rule out a non-natural origin for ‘Oumuamua, they constitute important data in assessing its likely makeup.”

Although this doesn’t prove ‘Oumuamua isn’t an alien spacecraft, it does put limits on the frequencies it could be transmitting on, if it is transmitting. And even if it isn’t transmitting, it doesn’t mean it’s not artificial. Could it be an ancient spacecraft that’s been sailing the interstellar seas for millions or billions of years, long after its intelligent occupants have died? Or long after its artificial intelligence has run out of energy?

Or — and this is the big one — did it zoom through our solar system, aware of our presence, and not bother communicating with us? If that scenario played out, we need to re-open that can o’ worms and try to understand where we stand in the universal ecosystem of competing intelligences. Perhaps we are the cosmic equivalent of an ant colony; our intelligence just isn’t worth the time when compared with the unimaginable alien intelligences that have the technology to send ‘Oumuamuas to probe distant star systems for life.

Alas, it’s probably a case of Occam’s razor, where the simplest explanation is most likely the correct one: ‘Oumuamua is probably a strange-looking asteroid or ancient comet that was randomly shot at us by some distant star system and astronomers were lucky to detect it. But, we still need to ponder the least likely explanations, you just never know…

Exocomets Seen Transiting Kepler’s Stars

If you thought detecting small planets orbiting stars dozens of light-years distant was impressive, imagine trying to “see” individual comets zoom around their star. Well, astronomers have done just that after poring over 201,250 targets in the Kepler dataset.

NASA’s Kepler mission has been taking observational data since 2009, staring unblinkingly at a small area of sky in the direction of the constellation Cygnus until it transitioned into the K2 mission in 2013. In total, the space telescope has discovered over 2,500 confirmed exoplanets (and over 5,000 candidate exoplanets), transforming our understanding of the incredible menagerie of alien worlds in our galaxy. After including discoveries by other observatories, we know of over 3,500 exoplanets that are out there.

kepler-exoplanets — Kepler looks for very slight dips in light as exoplanets pass in front of their stars to detect alien worlds (NASA/JPL-Caltech)

Kepler detects exoplanets by watching out for periodic dips in the brightness of stars in its field of view. Should a slight dip in brightness be detected, it could mean that there’s an exoplanet orbiting in front of its host star—an event known as a “transit.” While these transits can help astronomers learn about the physical size of exoplanets and the period of their orbits, for example, there’s much more information in the transit data than initially meets the eye.

In a new study to be published in the journal Monthly Notices of the Royal Astronomical Society on Feb. 21, a team of researchers are reporting that they have found evidence for individual comets transiting in front of two stars. They detected six individual transits at the star KIC 3542116, which is located approximately 800 light-years from Earth, and one transit at KIC 11084727. Both stars of a similar type (F2V) and are quite bright.

Though other observations have revealed dusty evidence of cometary activity in other star systems before, this is the first time individual comets have been found leaving their own transit signal in Kepler data. And it turns out that their transit fingerprint is a little bit special:

“The transits have a distinct asymmetric shape with a steeper ingress and slower egress that can be ascribed to objects with a trailing dust tail passing over the stellar disk,” the astronomers write in their paper (arXiv preprint). “There are three deeper transits with depths of ≃ 0.1 percent that last for about a day, and three that are several times more shallow and of shorter duration.”

In other words, when compared with the transit of an exoplanet, comet transits appear wonky (or asymmetric). This is because comets possess tails of gas and dust that trail the nucleus; as the comet passes in front of its star, starlight is quickly blocked, but as it drifts by in its orbit, the dusty tail will act as a starlight dimmer, gradually allowing more starlight to be seen by Kepler. An exoplanet—or, indeed, any spherical object without a dusty tail—will create a symmetrical dip in the transit signal. Other possible causes of this unique transit signal (such as starspots and instrumental error) were systematically ruled out. (Interestingly, in a 1999 Astronomy & Astrophysics paper, this asymmetric comet transit signal was predicted by another team of researchers, giving this current work some extra certainty.)

But just because there was evidence of six comet transits at KIC 3542116, it doesn’t mean there were six comets. Some of those transits could have been caused by the same comet, so the researchers have hedged their bets, writing: “We have tentatively postulated that these are due to between 2 and 6 distinct comet-like bodies in the system.”

Using these transit data, the study also takes a stab at how big these comets are and even estimates their orbital velocities. The researchers calculate that these comets have masses that are comparable to Halley’s Comet, the famous short-period comet that orbits the sun every 74-79 years and was last visible from Earth in 1986. For the deeper transits (for KIC 3542116 and the single transit at KIC 11084727), they estimate that the comets causing those transits are travelling at speeds of between 35 to 50 kilometers per second (22 to 31 miles per second). For the shallow, narrow transits at KIC 3542116, the inferred speeds are between 75 to 90 kilometers per second (47 to 56 miles per second).

“From these speeds we can surmise that the corresponding orbital periods are ⪆ 90 days (and most probably, much longer) for the deeper transits, and ⪆ 50 days for the shorter events,” they write.

But the fact that comets were detected at two similar F2V-type stars gives the researchers pause. Is there something special about these stars that means there’s more likelihood of possessing comets? Or is it just chance? Also, the fact that these comet transits were identified by visually analyzing the Kepler datasets suggests that there are likely many more transits hiding in the archived Kepler observations.

One thing’s for sure: this is a mind-blowing discovery that underscores just how valuable exoplanet-hunting missions are for probing the environment around other stars and not just for discovering strange new worlds. I’m excited for what other discoveries are waiting in Kepler transit data and for future exoplanet-hunting missions such as NASA’s Transiting Exoplanet Survey Satellite (TESS) that is scheduled for launch this year.

The Solar System Just Had an Interstellar Visitor. Now It’s Gone

Comet-PanSTARRS-1 — Hello, goodbye interstellar comet. The hyperbolic orbit of Comet C/2017 C1 as plotted by JPL’s Small-Body Database Browser (NASA/JPL-Caltech)

Update: At original time of writing, C/2017 U1 was assumed to be a comet. But Followup observations by the Very Large Telescope in Chile on Oct. 25 found no trace of cometary activity. The object’s name has now been officially changed to A/2017 U1 as it is more likely an interstellar asteroid, not a comet.

Astronomers using the PanSTARRS 1 telescope in Maui may have discovered an alien comet.

Comets and asteroids usually originate from the outermost reaches of the solar system — they’re the ancient rocky, icy debris left over from the formation of the planets 4.6 billion years ago.

However, astronomers have long speculated that comets and asteroids originating from other stars might escape their stars, traverse interstellar distances and occasionally pay our solar system a visit. And looking at C/2017 U1’s extreme hyperbolic trajectory, it looks very likely it’s not from around these parts.

“If further observations confirm the unusual nature of this orbit this object may be the first clear case of an interstellar comet,” said Gareth Williams, associate director of the International Astronomical Union’s Minor Planet Center (MPC). A preliminary study of C/2017 U1 was published earlier today. (Since this statement, followup observations have indicated that the object might be an asteroid and not a comet.)

According to Sky & Telescope, the object entered the solar system at the extreme speed of 16 miles (26 kilometers) per second, meaning that it is capable of traveling a distance of 850 light-years over 10 million years, a comparatively short period in cosmic timescales.

Spotted on Oct. 18 as a very dim 20th magnitude object, astronomers calculated its trajectory and realized that it was departing the solar system after surviving a close encounter with the sun on Sept. 9, coming within 23.4 million miles (0.25 AU). Comets would vaporize at that distance from the sun, but as C/2017 U1’s speed is so extreme, it didn’t have time to heat up.

“It went past the sun really fast and may not have had time to heat up enough to break apart,” said dynamicist Bill Gray. Gray estimates that the comet is approximately 160 meters wide with a surface reflectivity of 10 percent.

But probably the coolest factor about this discovery is the possible origin of C/2017 U1. After calculating the direction at which the comet entered the solar system, it appears to have come from the constellation of Lyra and not so far from the star Vega. For science fiction fans this holds special meaning — that’s the star system where the SETI transmission originated in the Jodie Foster movie Contact.

For more on this neat discovery, check out the Sky & Telescope article.

Spinning Comet Slams its Brakes as It Makes Earth Flyby

comet-spin — Images of Comet 41P/Tuttle-Giacobini-Kresak’s jets as observed by the Discovery Channel Telescope on March 19, 2017 (Schleicher/Lowell Observatory)

Although comets are static lumps of ancient ice for most of their lives, their personalities can rapidly change with a little heat from the sun. Now, astronomers have witnessed just how dynamic comets can be, seeing one dramatically slow its rate of rotation to the point where it may even reverse its spin.

Comets are the leftover detritus of planetary formation that were sprinkled around our sun 4.6 billion years ago. These primordial icy remains collected in the outermost reaches of the solar system and that’s where they stay until they get knocked off their gravitational perches to begin an interplanetary roller coaster ride. Some are unlucky and end up diving straight to a fiery, solar death. But others set up in stable orbits, making regular passes through the inner solar system, dazzling observers with their beautiful tails formed through heating by the sun.

One mile-wide short-period comet is called 41P/Tuttle-Giacobini-Kresak and it’s a slippery celestial object. First discovered in 1858 by U.S. astronomer Horace Parnell Tuttle, it disappeared soon after. But in 1907, French astronomer Michael Giacobini “rediscovered” the comet, only for it to disappear once again. Then, in 1951, Slovak astronomer Ľubor Kresák made the final “discovery” and now astronomers know exactly where to find it and when it will turn up in our night skies.

Its name, Tuttle-Giacobini-Kresak, reflects the wonderful 100-year discovery and rediscovery history of astronomy’s quest to keep tabs on the comet’s whereabouts.

comet-spin-2 — Comet 41P/Tuttle-Giacobini-Kresak as observed on March 22, 2017 (Kees Scherer/Knight Observatory, Tomar)

Now, 41P is the focus of an interesting cometary discovery. Taking 5.4 years to complete an orbit around the sun, 41P came within 13-million miles to Earth earlier this year, the closest it has come to our planet since it was first discovered by Tuttle. So, astronomers at Lowell Observatory, near Flagstaff, Ariz., used the 4.3-meter Discovery Channel Telescope near Happy Jack, the 1.1-meter Hall telescope and the 0.9-meter Robotic telescope on Anderson Mesa, to zoom-in on the interplanetary vagabond to measure its rotational speed.

Comets can be unpredictable beasts. Composed of rock and icy volatiles, when they are slowly heated by the sun as they approach perihelion (the closest point in their orbit to the sun), these ices sublimate (i.e. turn from ice to vapor without melting into a liquid), blasting gas and dust into space.

Over time, these jets are known to have a gradual effect the comet’s trajectory and rotation, but, over an astonishing observation run, Lowell astronomers saw a dramatic change in this comet’s spin. Over a short six-week period, the comet’s rate of rotation slowed from one rotation every 24 hours to once every 48 hours — its rate of rotation had halved. This is the most dramatic change in comet rotation speed ever recorded — and erupting jets from the comet’s surface are what slammed on the brakes.

This was confirmed by observing cyanogen gas, a common molecule found on comets that is composed of one carbon atom and one nitrogen atom, being ejected into space as the comet was being heated by sunlight.

“While we expected to observe cyanogen jets and be able to determine the rotation period, we did not anticipate detecting a change in the rotation period in such a short time interval,” said Lowell astronomer David Schleicher, who led the project, in a statement. “It turned out to be the largest change in the rotational period ever measured, more than a factor of ten greater than found in any other comet.”

For this rapid slowdown to occur, the researchers think that 41P must have a very elongated shape and be of very low density. In this scenario, if the jets are located near the end of its length, enough torque could be applied to cause the slowdown. If this continues, the researchers predict that the direction of rotation may even reverse.

“If future observations can accurately measure the dimensions of the nucleus, then the observed rotation period change would set limits on the comet’s density and internal strength,” added collaborator Matthew Knight. “Such detailed knowledge of a comet is usually only obtained by a dedicated spacecraft mission like the recently completed Rosetta mission to comet 67P/Churyumov-Gerasimenko.”

Heavy Stellar Traffic Sends Dangerous Comets Our Way

New image of comet ISON — Comet C/2012 S1 (ISON) as imaged by TRAPPIST–South national telescope at ESO’s La Silla Observatory in 2013 (TRAPPIST/E. Jehin/ESO)

Sixty-six million years ago Earth underwent a cataclysmic change. Back then, our planet was dominated by dinosaurs, but a mass extinction event hastened the demise of these huge reptiles and paved the way for the mammalian takeover. Though there is some debate as to whether the extinction of the dinosaurs was triggered by an isolated disaster or a series of disasters, one event is clear — Earth was hit by a massive comet or asteroid and its impact had global ramifications.

The leading theory is that a massive comet slammed into our planet, creating the vast Chicxulub Crater buried under the Yucatán Peninsula in Mexico, enshrouding the atmosphere in fine debris, blotting out the sun for years.

Although there is strong evidence of comet impacts on Earth, these deep space vagabonds are notoriously hard to track, let alone predict when or how often they may appear. All we know is that they are out there, there are more than we thought, they are known to hit planets in the solar system and they can wreak damage of apocalyptic proportions.

Now, using fresh observations from the European Space Agency’s Gaia mission, astronomer Coryn Bailer-Jones, who works at the Max Planck Institute for Astronomy in Munich, Germany, has added an interesting component to our understanding of cometary behavior.

Stellar Traffic

Long-period comets are the most mysterious — and troubling — class of comet. They will often appear from nowhere, after falling from their distant gravitational perches, zoom through the inner solar system and disappear once more — often to be never seen again. Or they hit something on their way through. These icy bodies are the pristine left-overs of our solar system’s formation five billion years ago, hurled far beyond the orbits of the planets and into a region called the Oort Cloud.

In the Oort Cloud these ancient masses have remained in relative calm far from the gravitational instabilities close to the sun. But over the eons, countless close approaches by other stars in our galactic neighborhood have occurred, causing very slight gravitational nudges to the Oort Cloud. Astronomers believe that such stellar encounters are responsible for knocking comets from this region, sending them on a roller-coaster ride to the inner solar system.

The Gaia mission is a space telescope tasked with precisely mapping the distribution and motion of stars in our galaxy, so Bailer-Jones has investigated the rate of stellar encounters with our solar system. Using information in Gaia’s first data release (DR1), Bailer-Jones has published the first systematic estimate of stellar encounters — in other words, he’s estimated the flow of stellar traffic in the solar system’s neighborhood. And the traffic was found to be surprisingly heavy.

In his study, to be published in the journal Astronomy & Astrophysics, Bailer-Jones estimates that, on average, between 490 and 600 stars will come within 16.3 light-years (5 parsecs) of our sun and 19-24 of them will come within 3.26 light-years (1 parsec) every million years.

According to a press release, all of these stars will have some gravitational effect on the solar system’s Oort Cloud, though the closest encounters will have a greater influence.

This first Gaia data release is valid for five million years into the past and into the future, but astronomers hope the next data release (DR2) will be able to estimate stellar traffic up to 25 million years into the past and future. To begin studying the stellar traffic that may have been responsible for destabilizing the dinosaur-killing comet that hit Earth 66 million years ago will require a better understanding of the mass distribution of our galaxy (and how it influences the motion of stars) — a long-term goal of the Gaia project.

An Early Warning?

Spinning this idea into the future, could this project be used to act as an early warning system? Or could it be used to predict when and where a long-period comet may appear in the sky?

In short: “No,” Bailer-Jones told Astroengine via email. “Some close stellar encounters will for sure shake up the Oort cloud and fling comets into the inner solar system, but which comets on which orbits get flung in we cannot observe.”

He argues that the probability of comets being gravitationally nudged can be modeled statistically, but this would require a lot of assumptions to be made about the Oort Cloud, a region of space that we know very little about.

Also, the Oort Cloud is located well beyond the sun’s heliosphere and is thought to be between 50,000 and 200,000 AU (astronomical units, where 1 AU is the average distance between the sun and the Earth) away, so it would take a long time for comets to travel from this region, creating a long lag-time between stellar close approach and the comet making an appearance.

“Typically it takes a few million years for a comet to reach the inner solar system,” he added, also pointing out that other factors can complicate calculations, such as Jupiter’s enormous gravity that can deflect the passage of comets, or even fling them back out of the solar system again.

This is a fascinating study that goes to show that gravitational perturbations in the Oort Cloud are far from being rare events. A surprisingly strong flow of stellar traffic will constantly rattle otherwise inert comets, but how many are dislodged and sent on the long journey to the solar system’s core remains a matter for statistics and probability.

Massive, Long-Period Comets Are Way More Common Than We Thought

During the formation of the solar system, when the planets were molten messes and asteroid collisions (or “mudball” collisions, possibly) were commonplace, chunks of icy debris were flung away from the chaos surrounding our messy young star and relegated to a lifetime of solitude in the furthest-most reaches of the sun’s gravitational influence. This debris eventually settled and formed what is known as the Oort Cloud, a mysterious spherical shell of countless mountain-sized objects located nearly 200 billion miles away.

As the Oort Cloud is so distant, and there are no telescopes on Earth (or off-Earth) that can resolve these objects, we can only guess at how many icy lumps are out there lurking in the dark. But should a passing star cause a gravitational wobble in that region, a few of those ancient objects may be knocked off their delicate gravitational perches and they take the plunge back toward the sun, becoming what we humans call “long-period comets.” Only when we see these comets can we get a hint of the population of the Oort Cloud and the nature of long-period comets. But, as many of these deep space vagabonds have orbital periods of hundreds to millions of years, they are notoriously difficult to track.

A long period comet may appear in the sky tomorrow, but it may not return in Earth’s skies until the age of humanity is long gone and intelligent cockroaches roam the planet. It’s hard to keep track of comets with orbital periods longer than our lifespans, let alone the lifespan of our civilization.

So it may not come as a surprise that astronomers have woefully underestimated the number of long-period comets, according to a new study using observations from NASA’s Wide-field Infrared Survey Explorer, or WISE, mission. But not only that, these things are a lot bigger than we thought.

The study, which has been published in The Astrophysical Journal, found that WISE had detected three to five times more long-period comets pass the sun over an eight-month period than expected and revealed that there are seven-times more long-period comets at least 1 kilometer across.

“The number of comets speaks to the amount of material left over from the solar system’s formation,” said lead author James Bauer, of the University of Maryland, College Park, in a NASA statement. “We now know that there are more relatively large chunks of ancient material coming from the Oort Cloud than we thought.”

WISE completed its primary mission in 2011, but has now embarked on a new mission to look out for dim asteroids and comets that stray close to Earth, called NEOWISE (NEO is for “Near-Earth objects”). During its primary mission, WISE was tasked to observe the universe in infrared wavelengths — revealing the otherwise hidden secrets of distant galaxies and the faint glow of mysterious objects traveling through the solar system. Among these objects were a surprising number of long-period comets, objects that WISE was uniquely qualified to characterize.

When comets approach the sun, their ices sublimate, dust is blasted into space and they form their trademark coma (a gaseous “atmosphere”) and tails around their nuclei. These factors obscure the main mass of the comet; astronomers cannot directly see the icy nucleus through the gas and dust — astronomers therefore have a hard time estimating the size of the comet.

comet-dust — To gauge the size of a comet’s nucleus, WISE precisely measures the size of the comet’s coma and subtracts those measurements from dust models to reveal the nucleus’ size (NASA/JPL-Caltech)

But studying WISE’s precision infrared measurements of the comets’ comas, the researchers were able to deduce the actual nuclei sizes by subtracting observational data from theoretical models of the behavior of dust around a comet. In all, 56 long-period comets were studied and compared with observations of 95 “Jupiter family comets” — comets that have short orbital periods around the sun and are gravitationally influenced by Jupiter. This comparison between the two families of comets revealed that long-period comets aren’t only bigger than we expected, these monsters are up to twice the size of Jupiter family comets.

“Our results mean there’s an evolutionary difference between Jupiter family and long-period comets,” Bauer said.

The difference in comet sizes may not come as a surprise — Jupiter family comets have orbital periods less than 20 years and therefore spend much more time being heated by the sun. They lose mass through ice sublimation that, in turn, dislodges dust and other material, ultimately shedding mass. Long-period comets on the other hand are pristine having spend most of their lives in the deep space deep freeze, so they hold onto the material they were born with billions of years ago. Long-period comets are the epitome of primordial.

Naturally, no comet research would be complete without an Existential Reality Check™ and, as you may have guessed, this new research has a dark side.

“Comets travel much faster than asteroids, and some of them are very big,” said co-author Amy Mainzer, principal investigator of the NEOWISE mission at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Studies like this will help us define what kind of hazard long-period comets may pose.”

Alien vs. Comet: Is the SETI “Wow!” Signal Dead? (Astroengine Video)

There’s a new hypothesis about what happened on August 15, 1977, and, sadly, it doesn’t involve aliens — just a photobombing comet. I was surprised about the controversy surrounding Antonio Paris’ research into the possibility of comets generating radio signals at 1420MHz and mimicking the famous “Wow!” signal nearly 40 years ago, so I decided to record Astroengine’s second YouTube video on the topic. Enjoy! And remember to subscribe and like, there’s a lot more to come!