Astronomy is one of humanity’s oldest obsessions, reaching back all the way to prehistoric times. Long before the Scientific Revolution taught us that the Sun is at the center of the Solar System, or modern astronomy revealed the true extend of our galaxy and the Universe, ancient peoples were looking up at the night sky and finding patterns in the stars.
For some time, scholars believed that an understanding of complex astronomical phenomena (like the precession of the equinoxes) did not predate the ancient Greeks. However, researchers from the Universities of Edinburgh and Kent recently revealed findings that show how ancient cave paintings that date back to 40,000 years ago may in fact be astronomical calendars that monitored the equinoxes and kept track of major events.
Having studies countless asteroids in near-Earth space, astronomers have come to understand that the majority of these rocks fall into one of two categories: S-type (grey) and C-type (red). These are defined by the types of materials on their surfaces, with S-type asteroids being primarily composed of silicate rock and C-type asteroids being made up of carbon materials.
However, there is also what are known as blue asteroids, which make up only a fraction of all known Near-Earth Objects (NEO). But when an international team astronomers observed the blue asteroid (3200) Phaeton during a flyby of Earth, they spotted behavior that was more consistent with a blue comet. If true, then Phaeton is of a class of objects that are so rare, they are almost unheard of.
On October 31st, 2015, NASA tracked a strange-looking comet as it made a close flyby of Earth. This asteroid, known as 2015 TB145, was monitored by the multiple observatories and radar installation of the agency’s Deep Space Network. Because of the timing and the skull-like appearance of this asteroid, scientists nicknamed it the “Death Comet”.
Naturally, there was no reason to worry, as the asteroid posed no threat and passed within about 498,900 km (310,000 mi) of Earth. But the timing and the appearance of the comet were nothing if not chilling. And coincidentally enough, the “Death Comet” (aka. “The Great Pumpkin Comet”), will be passing Earth for the second time, this time shortly after Halloween.
A periodic comet may put on a fine show for northern hemisphere viewers over the next few months.
Comet 21/P Giacobini-Zinner is currently a fine binocular comet, shining at +8th magnitude as it cruises across the constellation Cassiopeia. This places it above the horizon for the entire night for observers north of the equator in August, transiting the local meridian at dawn. And unlike most comets that get lost in the Sun’s glare (like the current situation with C/2017 S3 PanSTARRS), we’ll be able to track Comet 21/P Giacobini-Zinner right through perihelion on September 10th.
This is because the comet is on a short period, 6.6 year orbit around the Sun that takes it from an aphelion of 6 Astronomical Units (AU) exterior to Jupiter’s orbit, to a perihelion of 1.038 AU, just 3.3 million miles (5.2 million kilometers) exterior to Earth’s orbit. The 2018 apparition sees the comet pass 0.392 AU (36.5 million miles/58.3 million kilometers) from the Earth on September 11th.
This is the closest passage of the comet near Earth since September 14th, 1946, and won’t be topped until the perihelion passage of September 18th, 2058. Its next cycle of passes to Earth closer than 0.1 AU aren’t until next century in the years 2119 and 2195, respectively.
Discovered by astronomer Michel Giacobini at the Côte d’Azur Observatory in Nice, France on the night of December 20th, 1900 as it was crossing the constellation Aquarius, the 21st periodic comet was recovered two orbits later by Ernest Zinner on October 23rd, 1913 as it passed a series of variable stars near Beta Scuti.
Though the comet generally tops out at +8th magnitude, it has been known to undergo periodic outbursts near perihelion, bringing it up about 3 magnitudes (about 16 times) in brightness. This occurred most notably in 1946.
Comet 21/P Giacobini-Zinner is also the source of the Draconid (sometimes referred to as the Giacobinid) meteors, radiating from the constellation Draco the Dragon on and around October 7th and 8th. Feeble on most years, this shower can produce surprises, such as occurred in 1998, 2005 and most recently in 2011, when a Draconid outburst topped a zenithal hourly rate of 400 meteors per hour, flirting with ‘meteor storm’ status. And while we’re not expecting a meteor storm to accompany the 2018 perihelion passage of Comet 21/P Giacobini-Zinner, you just never know… it’s always worth keeping an eye out on early October mornings for the “Tears of the Dragon,” just in case. Note that the Moon reaches New phase on October 9th, just a few days after the meteor shower’s expected annual peak, a fine time to watch for any unheralded Draconid outbursts.
Prospects for Comet 21P
The comet is visible from the northern hemisphere through the remainder of August and all through September as it glides across Auriga, Taurus and Gemini and visits several well known celestial sights. In fact, it actually transits in front of several deep sky objects, including Messier 37 (Sept 10th), and Messier 35 (Sept 15th).
The comet will be moving at about two degrees per day when it’s nearest to the Earth, on and around September 11th.
We begin to lose the comet, as it heads southward in late October. Still, the comet is over 50 degrees above the eastern horizon at dawn come October 1st as seen from latitude 30 degrees north, having maintained a similar elevation throughout most of September. Not bad at all.
Here are some upcoming dates with destiny for Comet 21/P Giacobini-Zinner:
August 19: Crosses into the constellation Camelopardalis.
August 29: Crosses into the constellation Perseus.
August 30th: Crosses into the constellation Auriga.
September 2: Passes one degree from the bright star Capella.
Sept 7-8: Grouped 2 degrees from the open clusters M36 and M38.
Sept 10: Photo-Op: Skirts very near the open cluster M37. Also reaches perihelion on this date, at magnitude +7.
Sept 11: Passes closest to the Earth, at 0.392 AU distant.
Sept 13: Nicks the corner of the constellation Taurus.
Sept 14th : Enters the constellation Gemini.
Sept 15th: Photo-Op: crosses in front of the open cluster M35.
Sept 16: Crosses the ecliptic southward and near the +3.3 magnitude star Propus (Eta Geminorum).
Sept 17: Crosses into Orion.
Sept 21: Crosses into Gemini.
Sept 23: Crosses into Monoceros.
Sept 24: Passes near the Christmas Tree Cluster, NGC 2264.
Oct 1: Crosses the galactic plane and the celestial equator southward.
Oct 7: Crosses in front of the open cluster M50.
Oct 10: Crosses into Canis Major.
Oct 31st: Passes near the bright star Aludra and may drop below +10th magnitude.
Binoculars are your best friend when you’re looking for comets brighter than +10th magnitude. With a generous field of view, binoculars allow you to sweep a suspect area until the faint fuzzball of a comet snaps into view. I like to ‘ambush’ a comet as it passes near a bright star, and a good time to spot comet 21/P Giacobini-Zinner is coming right up on September 2nd when it passes less than one degree from the bright +0.1 magnitude star Capella.
Don’t miss this year’s fine apparition of Comet 21/P Giacobini-Zinner, coming to a night sky near you.
Comets are one of those great question marks in observational astronomy. Though we can plot their orbits thanks to Newton and Kepler, just how bright they’ll be and whether or not they will fizzle or fade is always a big unknown, especially if they’re a dynamic newcomer from the Oort Cloud just visiting the inner solar system for the first time.
We had just such a surprise from a cosmic visitor over the past few weeks, as comet C/2017 S3 PanSTARRSerupted twice, brightening into binocular visibility. Discovered on December 23rd 2017 during the PanSTARRS survey based on Haleakala, Hawai’i, S3 PanSTARRS is on a long-period, hyperbolic orbit and is most likely a first time visitor to the inner solar system.
S3 PanSTARRS was not only rocked by two new outbursts in quick succession, but seems to have undergone a tail disconnection event just last week, leveling off its brightness at around +8 magnitude and holding. This puts it in the range of binoculars under dark skies, looking like a fuzzy globular that refuses to snap into focus as it currently glides through the constellation of Camelopardalis the Giraffe the dawn sky.
As July closes out, the time to catch sight of Comet S3 PanSTARRS is now, before it’s lost in the Sun’s glare. From latitude 40 degrees north, the comet sits 20 degrees above the northeastern horizon, about an hour before sunrise. By August 7th however, it drops below 10 degrees altitude. From there, the comet begins to circle the Sun as seen from the Earth beginning to favor southern hemisphere observers at dawn, who may be able to track it straight through perihelion on August 16th, if its brightness holds up. From there, northern hemisphere viewers may get a second view at dawn in September, again, if its brightness holds.
You never know when it comes to comets. Here’s a brief rundown of the celestial happenings for comet C/2017 S3 PanSTARRS:
3- Crosses into the constellation Gemini.
4- Passes near the bright star Castor.
5- Passes near the bright star Pollux.
7- Crosses into the constellation Cancer.
7- Passes closest to the Earth, at 0.758 Astronomical Units (AU) distant.
8- Crosses southward over the ecliptic plane.
9- Passes just 4 degrees from the Beehive cluster, M44.
11- Passes 2 degrees from the open cluster M67.
12- Passes 10.5 degrees from Sun (1st apparent close pass as seen from the Earth)
13- Crosses into the constellation Hydra.
15- Reaches maximum brightness: the comet may top +2nd magnitude in mid-August.
16- Reaches perihelion at 0.21 AU from the Sun.
18- Crosses into the constellation Sextans.
30-Crosses into the constellation Leo.
31-Crosses the ecliptic plane northward.
3- passes 4 degrees from the Sun.
25- Crosses into the constellation Coma Berenices.
From there, Comet C/2017 S3 PanSTARRS drops back below 6th magnitude in September, then below 10th magnitude in October as it heads back off into the icy realms of the outer solar system.
Be sure to nab this icy interloper why you can. The quote comet hunter David Levy, “Comets are like cats… they have tails, and they do exactly what they want.”
70,000 years ago, our keen-eyed ancestors may have noticed something in the sky: a red dwarf star that came as close as 1 light year to our Sun. They would’ve missed the red dwarf’s small, dim companion—a brown dwarf—and in any case they would’ve quickly returned to their hunting and gathering. But that star’s visit to our Solar System had an impact astronomers can still see today.
The star in question is called Scholz’s star, after astronomer Ralf-Dieter Scholz, the man who discovered it in 2013. A new study published in the Monthly Notices of the Royal Astronomical Society by astronomers at the Complutense University of Madrid, and at the University of Cambridge, shows the impact Scholz’s star had. Though the star is now almost 20 light years away, its close approach to our Sun changed the orbits of some comets and asteroids in our Solar System.
When it came to our Solar System 70,000 years ago, Scholz’s star entered the Oort Cloud. The Oort Cloud is a reservoir of mostly-icy objects that spans the range from about 0.8 to 3.2 light years from the Sun. Its visit to the Oort Cloud was first explained in a paper in 2015. This new paper follows up on that work, and shows what impact the visit had.
“Using numerical simulations, we have calculated the radiants or positions in the sky from which all these hyperbolic objects seem to come.” – Carlos de la Fuente Marcos, Complutense University of Madrid.
In this new paper, the astronomers studied almost 340 objects in our Solar System with hyperbolic orbits, which are V-shaped rather than elliptical. Their conclusion is that a significant number of these objects had their trajectories shaped by the visit from Scholz’s star. “Using numerical simulations, we have calculated the radiants or positions in the sky from which all these hyperbolic objects seem to come,” explains Carlos de la Fuente Marcos, a co-author of the study now published in Monthly Notices of the Royal Astronomical Society. They found that there’s a cluster of these objects in the direction of the Gemini Constellation.
“In principle,” he adds, “one would expect those positions to be evenly distributed in the sky, particularly if these objects come from the Oort cloud. However, what we find is very different—a statistically significant accumulation of radiants. The pronounced over-density appears projected in the direction of the constellation of Gemini, which fits the close encounter with Scholz’s star.”
There are four ways that objects like those in the study can gain hyperbolic orbits. They might be interstellar, like the asteroid Oumuamua, meaning they gained those orbits from some cause outside our Solar System. Or, they could be natives of our Solar System, originally bound to an elliptical orbit, but cast into a hyperbolic orbit by a close encounter with one of the planets, or the Sun. For objects originally from the Oort Cloud, they could start on a hyperbolic orbit because of interactions with the galactic disc. Finally, again for objects from the Oort Cloud, they could be cast into a hyperbolic orbit by interactions with a passing star. In this study, the passing star is Scholz’s star.
The timing of Scholz’s star’s visit to the Oort Cloud and our Solar System strongly coincides with the data in this study. It’s very unlikely to be coincidental. “It could be a coincidence, but it is unlikely that both location and time are compatible,” says De la Fuente Marcos. In fact, De la Fuente Marcos points out that their simulations suggest that Scholz’s star approached even closer than the 0.6 light-years pointed out in the 2015 study.
The one potentially weak area of this study is pointed out by the authors themselves. As they say in their summary, “…due to their unique nature, the orbital solutions of hyperbolic minor bodies are based on relatively brief arcs of observation and this fact has an impact on their reliability. Out of 339 objects in the sample, 232 have reported uncertainties and 212 have eccentricity with statistical significance.” Translated, it means that some of the computed orbits of individual objects may have errors. But the team expects the overall conclusions of their study to be correct.
The study of minor objects with hyperbolic orbits has heated up since the interstellar asteroid Oumuamua made its visit. This new study successfully connects one population of hyperbolic objects with a pre-historic visit to our Solar System by another star. The team behind the study expects that follow up studies will confirm their results.
Ever since we’ve been able to get closer looks at comets in our Solar System, we’ve noticed something a little puzzling. Rather than being round, they’re mostly elongated or multi-lobed. This is certainly true of Comet 67P/Churyumov-Gerasimenko (67P or Chury for short.) A new paper from an international team coordinated by Patrick Michel at France’s CNRS explains how they form this way.
The European Space Agency (ESA) spacecraft Rosetta visited 67P in 2014, end even placed its lander Philae on the surface. Rosetta spent 17 months orbiting 67P, and at its closest approach, Rosetta was only 10 km (6 mi) from 67P’s surface. Rosetta’s mission ended with its guided impact into 67P’s surface in September, 2016, but the attempt to understand the comet and its brethren didn’t end then.
Though Rosetta’s pictures of 67P are the most detailed comet pictures we have, other spacecraft have visited other comets. And most of those other comets appear elongated or multi-lobed, too. Scientists explain these shapes with a “comet merger theory.” Two comets collide, creating the multi-lobed appearance of comets like 67P. But there’s been a problem with that theory.
In order for comets to merge and come out looking the way they do, they would have to merge very slowly, or else they would explode. They would also have to be very low-density, and be very rich in volatile elements. The “comet merger theory” also says that these types of gentle mergers between comets would have to have happened billions of years ago, in the early days of the Solar System.
The problem with this theory is, how could bodies like 67P have survived for so long? 67P is fragile, and subjected to repeated collisions in its part of the Solar System. How could it have retained its volatiles?
In the new paper, the research team ran a simulation that answers these questions.
The simulation showed that when two comets meet in a destructive collision, only a small portion of their material is pulverized and reduced to dust. On the sides of the comets opposite from the impact point, materials rich in volatiles withstand the collision. They’re still ejected into space, but their relative speed is low enough for them to join together in accretion. This process forms many smaller bodies, which keep clumping up until they form just one, larger body.
The most surprising part of this simulation is that this entire process may only take a few days, or even a few hours. The whole process explains how comets like 67P can keep their low density, and their abundant volatiles. And why they appear multi-lobed.
The simulation also answered another question: how can comets like 67P survive for so long?
The team behind the simulation thinks that the process can take place at speeds of 1 km/second. These speeds are typical in the Kuiper Belt, which is the disc of comets where 67P has its origins. In this belt, collisions between comets are a regular occurrence, which means that 67P didn’t have to form in the early days of the Solar System as previously thought. It could have formed at any time.
The team’s work also explains the surface appearance of 67P and other comets. They often have holes and stratified layers, and these features could have formed during re-accretion, or sometime after its formation.
One final point from the study concerns the composition of comets. One reason they’re a focus of such intense interest is their age. Scientists have always thought of them as ancient objects, and that studying them would allow us to look back into the primordial Solar System.
Though 67P—and other comets—may have formed much more recently than we used to believe, this process shows that there is no significant amount of heating or compaction during the collision. As a result, their original composition from the the early days of the Solar System is retained intact. No matter when 67P formed, it’s still a messenger from the formative days.