Sky & Telescope news
Yes, it was too good to be true. The cosmic "discovery of the century" last March has officially blown up. Or will blow up next week when a new analysis of polarization in the cosmic microwave background is officially released.
The excitement burst onto the world 10 months ago when the BICEP microwave background team, working with three years of data from their detectors at the South Pole, announced that they had found modern cosmology's holy grail: evidence of gravitational waves distorting spacetime during the Big Bang's first trillionth of a trillionth of a trillionth of a second. Such gravitational waves are a key prediction of the cosmic-inflation theory of how the Big Bang happened. Inflation already rested on several impressive pieces of evidence. This was supposed to be the crowning proof that nailed it for good. People were talking Nobel Prize.
But just as our July-issue cover story appeared, problems surfaced. The BICEP team had thought that the patch of sky it examined was free of Milky Way dust that could add signals of its own to the telltale "B-mode" polarization patterns in the microwave background. That assumption of a clean sky was wrong, said members of Europe's Planck microwave-background team. The BICEP group had misinterpreted a preliminary dust map that the Planck researchers had shown at a conference.
The two groups combined forces to try to tease apart the extremely subtle signals involved. Their preliminary report last fall cast the BICEP finding into deeper doubt (see Dust Makes Cosmic Inflation Signal Iffy).
The Planck-BICEP collaboration will issue its final report in a few days. According to a leak on a Planck website (soon taken down but being widely reported on), the groups conclude that the Milky Way's messy dust signal could account for everything that the BICEP team has seen.
That doesn't mean Big Bang gravitational waves don't exist, just that they're below the current level of detection. Any signal from them must have a so-called r value of less than 0.13, compared to the unexpectedly high r of 0.16 to 0.20 that the BICEP team originally reported.
In an NBC News report, cosmologist Sean Carroll sums up the situation:
The bottom line is simply that the current data don't say much about inflation, one way or another. The original BICEP2 detection seems to have been incorrect, but it was at a surprisingly high amplitude, so having it go away just sends us back to where we were a year ago.
There's hope for this pursuit yet. A next-generation project on the drawing boards is intended to measure r to an uncertainty of 0.001, hopefully good enough to separate any real inflationary signal from the foreground contamination.
Following the news leak, Planck has put up this summary on its site: Planck: Gravitational Waves Remain Elusive.
We'll have more to say when the Planck-BICEP collaboration releases the full analysis next week.
Some of the prettiest nighttime sights involve the close pairing of two solar-system bodies, and February features events with the Moon and Jupiter, then Venus and Mars.
Early this month, on February 3rd, you'll see the full Moon and Jupiter rise together in the east just after sunset. They'll be separated by about 5°. Astronomers refer to these close pairings as conjunctions. Although one is near and the other far, both the Moon and Jupiter are positioned on the side of Earth opposite the direction toward the Sun, and as such both appear about as bright as they ever get.
A second and much closer conjunction comes later in the month, when Venus and Mars close to within ½° of each other low in the west after sunset on the 21st. The previous evening they're joined by a razor-thin, 36-hour-old crescent Moon.
Meanwhile, the distinctive hourglass-shaped constellation of Orion is riding high in the south as darkness settles in. In the middle are the three equally spaced stars in his belt; to their upper left is the red-tinged supergiant star Betelgeuse, and to the lower right is icy white Rigel. Orion is followed across the sky by the bright stars Sirius and Procyon, which mark the locations of his two hunting dogs. To Orion's upper right is Taurus, the Bull, anchored by the reddish star Aldebaran.
This is just a sample of the many sky sights that await you on February evenings. To get familiar with the bright stars and planets overhead, download our 6-minute-long stargazing podcast below.
There's no better guide to what's going on in nighttime sky than the February issue of Sky & Telescope magazine.
Citizen scientists are exploring exoplanets’ birthplaces, classifying more than 1 million infrared sources and finding 37 disk candidates (so far) for follow-up study.
Who would Superman be without his tragic past, or Spider-Man without the radioactive arachnid? Origin stories similarly define the character of the hot Jupiters, super-Earths, and ice giants that we see through the exoplanet-hunting telescopes of today and tomorrow.
But our understanding of exoplanet origins remains limited. Their birthplaces, the dusty disks that surround stars, are easy enough to find — astronomers just look for extra infrared emission from the dust. Roughly one in five sunlike stars has a detectable debris disk, and astronomers already know of a few thousand protoplanetary disks (those with dust and gas) and a few hundred debris disks (those with just dust). Only few of these are imaged at high angular resolution to suss out the details. Where disks form, around which stars, and when remain open questions.
Now the astronomers of DiskDetective, a citizen-science project led by Marc Kuchner (NASA Goddard) and the Zooniverse team, are taking a different, Kepler-like approach to exoplanet origins.Taking a Census
Kepler’s goal was to build a huge sample of exoplanets. Individual exoplanets (particularly the Earth-size ones in Earth-like orbits that were its ultimate goal) would be too far away to study in any detail, but detail was never the real aim. The vast database is now yielding statistical answers to the question “how did we get here?”, revealing the fraction of stars that host planets and the fraction of those planets that might be habitable.
DiskDetective is applying the same statistical approach to the stellar disks that ultimately become planetary systems, amassing candidates to answer similar questions about how and when planets form.
DiskDetective uses data from the Wide-field Infrared Survey Explorer (WISE) satellite, which mapped the entire sky’s infrared emission and found hundreds of thousands of infrared sources that could be disks around nearby stars . . . or far-off galaxies, asteroids, interstellar dust clouds, and other extended infrared sources. To pick out the disks, DiskDetective employs citizen scientists (i.e., you or me, or anyone with an internet connection and some spare time) to classify each of 278,000 infrared sources in the WISE catalog.
Try it yourself: Go to DiskDetective.org and you’ll receive a snappy tutorial just a few minutes long on what circumstellar disks look like in WISE, Digitized Sky Survey, and 2MASS data. Then you’re ready to start classifying.
Many find citizen science projects like this one (and others at Zooniverse.org) a relaxing and fulfilling pastime. Kuchner says that, when the project released its first batch of 32,000 sources (20,000 of these in rotation at any one time), some volunteers complained they were seeing repeats. They weren’t wrong: within two months, those volunteers had already classified 20,000 objects each! Roughly 20 “super-users” have done about half the classifying, he says.
All of that work has netted more than 1 million classifications of WISE objects so far, turning up 478 “objects of interest” (possible disks) and 37 strong disk candidates. The strong candidates range in distance from 80 to 3,300 light-years. By 2018 the project is expected to net 1,000+ disks.
As with Kepler, DiskDetective astronomers cannot rely on survey data alone. They are conducting follow-up observations on the candidates found so far to confirm that they really are disks rather than a background galaxy. They also take spectra to confirm the star’s spectral type.
Ultimately, the Hubble Space Telescope, James Clerk Maxwell Telescope, ALMA, and other telescopes may image promising candidates to reveal disk structure, including spiral shapes and disk gaps seen in other sources.
Learn more about citizen science opportunities in Sky & Telescope's March 2014 issue.
Friday, January 30
The waxing gibbous Moon shines to the upper left of Orion in twilight, as shown above. The Moon is straighter above Orion later in the evening. It's near Zeta (ζ) Tauri, the fainter of the two stars that mark the tips of Taurus's long horns.
Algol in Perseus is at its minimum brightness this evening, magnitude 3.4 instead of its usual 2.1, for a couple hours centered on 7:31 p.m. EST.
Saturday, January 31
With a small telescope, you can watch Jupiter's inner moon Io fade away into eclipse by Jupiter's shadow around 10:45 p.m. EST (7:45 p.m. PST). Io will be just barely off Jupiter's western limb when it dwindles into eclipse. That's because we're only 6 days from Jupiter's opposition, with the Sun almost straight behind our backs.
Sunday, February 1
The tiny black shadow of Jupiter's moon Io crosses the face of the planet from 7:57 to 10:14 p.m. Eastern Standard Time, going from celestial east to west. Io itself follows just 7 minutes behind its shadow.
Io then exits the planet's western limb at 10:21 EST — right near where Europa, in the background, is heading toward the limb. The two satellites appear to pass each other extremely closely. Then at 10:31 p.m. EST, Europa disappears into eclipse by Jupiter's shadow barely off the limb. Watch the shell games of these comings and goings!
Meanwhile down in Jupiter's atmosphere, the Great Red Spot crosses Jupiter's central meridian around 9:39 p.m. EST.
For listings of all such events happening on and around Jupiter in February, see the February issue of Sky & Telescope, pages 51–53.
Monday, February 2
The bright Moon is just a day short of full this evening. Look to its right for Procyon (off the chart here), and to its lower left for brighter Jupiter. High above the Moon are Pollux and Castor.
Tuesday, February 3
The full Moon climbs the eastern sky this evening with Jupiter shining a few degrees to its left. They may look paired, but Jupiter is actually 1,600 times farther away. (Which is why it looks small despite being 40 times larger than the Moon in diameter!)
Wednesday, February 4
Jupiter shines above the Moon this evening. Look closer to the Moon's left for Regulus, the Alpha star of Leo.
Thursday, February 5
The waning gibbous Moon rises around 8 p.m., depending on where you live in your time zone. High above it, Jupiter is already shining brightly. Look between them and a touch left for fainter Regulus.
"If I had to choose just one deep-sky object to demonstrate the appeal of binocular astronomy, it would probably be the Pleiades," writes Gary Seronik. The Pleiades are certainly a nice group to become intimate with. For instance, they hold a binocular secret in their center: the 8th-magnitude double star South 437, barely resolvable with 10× glasses. See Gary's column and chart in the February Sky & Telescope, page 45. (There, the large black circle on his chart spans a typical 10× binocular's 5° field of view.)
Friday, February 6
Jupiter is at opposition tonight: opposite the Sun as seen from Earth. So it rises around sunset, shines highest in the south around midnight, and sets at sunrise. Opposition is also just about when an outer planet is nearest to Earth and appears biggest and brightest. So Jupiter is now 45 arcseconds across its equator, its biggest this year. It remains essentially that large in your telescope all February.
Saturday, February 7
Orion strides high in the southeastern sky after dusk, with his three-star belt pointing down toward brilliant Sirius, the Dog Star. The bright trio of Sirius, Betelgeuse high above in Orion's shoulder, and Procyon to their left form the big, equilateral Winter Triangle. Compared to the Summer Triangle, this one is brighter, more shapely, and more colorful.
Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.
This is an outdoor nature hobby; for an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential guide to astronomy. Or download our free Getting Started in Astronomy booklet (which only has bimonthly maps).
Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The standards are the little Pocket Sky Atlas, which shows stars to magnitude 7.6; the larger and deeper Sky Atlas 2000.0 (stars to magnitude 8.5); and once you know your way around, the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.
You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, the bigger Night Sky Observer's Guide by Kepple and Sanner, or the beloved if dated Burnham's Celestial Handbook.
Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically (able to point with better than 0.2° repeatability, which means fairly heavy and expensive). As Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."This Week's Planet Roundup
Mercury is hidden deep in the glow of sunrise.
Venus (magnitude –3.9, in Aquarius) shines in the west-southwest during evening twilight. It's climbing a little higher each week, on its way to a grand apparition as the high "Evening Star" this spring.
Mars (magnitude +1.2, in Aquarius) glimmers weakly above Venus and a bit left. Look carefully; Mars is less than a hundredth as bright! The gap between Venus and Mars diminishes from 10° to 7° this week. They're heading for a close conjunction in late February.
Jupiter (magnitude –2.6, at the Leo-Cancer border) is at opposition this week. It comes into view low in the east-northeast as twilight fades, and by 9 p.m. it's high enough in the east for good telescopic viewing. Down below it is fainter Regulus (magnitude +1.4). Jupiter and Regulus pass highest in the south in the middle of the night.
Saturn (magnitude +0.5, at the head of Scorpius) rises around 2 or 3 a.m. and is well up in the southeast before dawn. Look 1° lower right of it for Beta Scorpii, magnitude 2.5, a showpiece double star for telescopes. Below them by 9° is orange Antares.
Uranus (magnitude 5.8, in Pisces) is still in the southwest right after dusk.
Neptune has slunk away into the afterglow of sunset.
All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America.
Eastern Standard Time (EST) is Universal Time (UT, UTC, or GMT) minus 5 hours.
“This adventure is made possible by generations of searchers strictly adhering to a simple set of rules. Test ideas by experiments and observations. Build on those ideas that pass the test. Reject the ones that fail. Follow the evidence wherever it leads, and question everything. Accept these terms, and the cosmos is yours.”
— Neil deGrasse Tyson, 2014.
Astronomers have potentially confirmed a five-planet exoplanet system around an 11-billion-year-old star in our galaxy. The star, Kepler-444 (a.k.a. HIP 94931 in the constellation Lyra), is a K0V star, meaning it’s a red dwarf slightly cooler than our Sun that’s fusing hydrogen in its core.
Tiago Campante (University of Birmingham, UK, and Aarhus University, Denmark) and colleagues investigated the five repeating planet-like transit signals, which were caught by the Kepler spacecraft over the course of the four years of the craft’s observations. With estimated sizes ranging from 0.4 to 0.74 times Earth, all five should be rocky, but not a one is in the star’s habitable zone: they all orbit the parent star within 0.08 Earth-Sun distances, or less than one-fifth the size of Mercury’s orbit. An optimistic habitable zone would start at 0.47 Earth-Sun distances, nearly six times farther out.
Kepler-444 is not the first star of this age with planetary children: astronomers have also found two planets around Kepler-10 (about 10 billion years old) and two planets around Kapteyn’s star (also around 11 billion years old). Those planets are larger (super-Earth class or a bit bigger). But as the press release from Yale explains, it’s exciting to find exoplanets that formed around the same time as the Milky Way itself was coalescing.
You can read more in the team’s paper, which appears in the February 1st Astrophysical Journal. (Preprint draft here on the arXiv.) There's also a neat animation of the Kepler-444 system (comparison with our solar system begins at 0:35 time stamp).
The post The Gemini Constellation beyond my balconyon the urban area! appeared first on Sky & Telescope.
Research shows that the magnetic fields in the asteroid parent bodies of two meteorites lasted hundreds of millions of years after our solar system’s formation.
Today, active magnetic fields surround Earth, Mercury, all the outer planets, and Jupiter’s moon Ganymede. But a new study in last week’s Nature shows that during the first tens of millions of years in the solar system’s life, numerous small bodies also possessed magnetic fields. Like miniature Earths, these objects had dynamos (the circulation of conducting fluid) in their cores to power their magnetic fields.
The study was conducted on the famous meteorites Imilac and Esquel. Both objects were found in South America; Imilac was discovered in 1822, Esquel in 1951. Both are pallasites, which come from the core-mantle boundary of a once-molten asteroid. Research shows that they acted as “recording devices” for the magnetic fields that once coursed through their parent bodies.How To Read A Meteorite
When the molten conducting fluid inside one of those parent bodies solidifies, it locks in an imprint of the magnetic field that previously existed. Using nanoscale imaging, the international team of researchers led by James Bryson (University of Cambridge) were able to measure these meteorites’ cooling and solidification rates and their magnetic activity. To measure these characteristics, they look at the physical structure of tetrataenite, an iron-nickel alloy, in each meteorite.
The team’s observations show that “asteroid magnetic fields probably were generated a lot like that of the Earth: by motion of iron metallic fluid in a core that is undergoing crystallization to form a solid core,” said planetary scientist Ben Weiss (MIT), who was not involved in the study. Previous paleomagnetic measurements were used to argue for the existence of such cores in small planetary bodies, but “beyond this fundamental inference, we don’t know much about ancient core convection within asteroids,” wrote Bryson on the project’s blog.
Analogous to the way tree rings chronicle droughts and times of plenty, the matrices of tetrataenite within Imilac and Esquel record changes of strength and direction of the magnetic field produced by their parent bodies over time — and the eventual shutoff of the field once the asteroid’s core solidified. These are some of the first observations of how an asteroid’s magnetic field changes in time, notes Weiss.
Pallasite meteorites solidified slowly, on the rate of 2 to 9 Kelvin per million years, which allows for the tetrataenite “islands” to form snapshots of magnetic activity. The researchers saw that these islands exsolved and hardened over time as they cooled, so their sizes serve as rulers to measure the rate of cooling in the parent bodies, and thus the devolving of the magnetic fields.Magnetic Fields Extend Longer Than Previously Thought
While the exact size of Imilac’s and Esquel’s parent bodies are unknown, the researchers modeled the cooling of a 400-km-wide body—a size consistent with small rocky objects existing in the early solar system.
Previous research assumed that convection in these bodies was thermally driven (like a boiling pot of water, which transfers heat with physical motion from the pot’s bottom to the top). However, the data imprinted within the two meteorites shows magnetic activity lasting well beyond what thermally-driven convection could have sustained.
Both the technique used for measuring these fossil fields and the results themselves have implications for planetary science. As Weiss explains, the research shows magnetic fields can be recorded “not just as magnetization (what gives a refrigerator magnet its pull), as is traditionally understood but also in the [physical] structures of minerals themselves.”
Further, since the team has showed that solidification-driven magnetic fields existed in the early solar system, we now have evidence that “a widespread, intense, and long-lived epoch of magnetic activity likely existed among many small bodies,” Bryson notes in the project’s blog. This activity lasted tens to hundreds of millions of years after the solar system’s formation, a timescale that’s an order of magnitude greater than previous theories had assumed.
James F. J. Bryson et al. “Long-lived magnetism from solidification-driven convection on the pallasite parent body.” Nature, Published online 21 January 2015.
Sky & Telescope's year-at-a-glance guide to celestial happenings is a symphony of detailed calculations and clear, elegant design.
If you're an active backyard observer — or just celestially curious — you're always looking for quick, reliable information about "what's up" in the nighttime sky. When is the next new Moon? Can I see Saturn tonight? When will twilight end?
The editors at Sky & Telescope are no different. And while we have scores of detailed references to draw from, one of the handiest is the "Skygazer's Almanac" found in every January issue of the magazine. This single sheet represents a fruitful melding of detailed computations and graphic representation that has been perfected over the years.
A graphical sky almanac has accompanied every January issue of Sky & Telescope since 1942 — that's more than 70 years! The main purpose of this chart is to let you see, at a glance, which planets and constellations are visible, and what the Moon's phase is, on any given date and time of night throughout the year.
There have been three "epochs" of this chart since its inception. From 1942 to 1980, each January issue featured the "Graphic Timetable of the Heavens," prepared annually by the Maryland Academy of Sciences. Then for two years it was called the "Graphic Ephemeris," computed and drafted by Michael Jay Jones (who had done this work for the Maryland Academy before then).
These charts had the same basic content. They showed the hours of night increasing from left to right, while the dates of the year ran from top (January 1st) to bottom (December 31st). Smooth curves provided the times of sunrise and sunset, as well as the end of evening twilight and start of morning twilight. Additional curves give the rise, transit, and set times of bright planets and stars. Moon symbols appeared at the times of its rising or setting each night, while other symbols indicated planetary conjunctions and oppositions.SGA: The Next Generation
As good as they were, both of those charts suffered from the limitations of black-and-white printing. So in 1983 Sky & Telescope introduced its own computer-generated chart, the "Skygazer's Almanac." We introduced some big changes then, as well as other enhancements and improvements over the years:
- Curves for the planets became color-coded for easy recognition.
- Small white dots now mark 5-minute increments horizontally and each day vertically.
- The Moon symbols actually look like its phase each night as it waxes and wanes.
- Symbols now give the dates and best viewing times of annual meteor showers.
- Custom visibility curves are added for predicted bright comets (Halley, Hale-Bopp).
Two key upgrades have really improved the chart's overall utility. First, we omitted the daylight portions of what had been a rectangular graph. This gave the chart a pleasing hourglass shape and, conveniently, freed up space to list key evening and predawn events down the left and right sides, respectively.
Second, we made versions for different latitudes. The original was plotted for those living at latitude 40° north, for use throughout North America and much of Europe. But beginning in 1998, additional charts were created for latitude 50° north (handy for northern Europeans) and 30° south (for use in Australia and the southern parts of Africa and South America).
These charts aren't just used by backyard astronomers. If you visit any major observatory, don't be surprised if there's a "Skygazer's Almanac" hanging on the wall. In fact, some years ago we concluded that the two-page chart that's bound into each January issue could be made much more readable in a poster-size version — so now a 30-by-22-inch full-color wall chart is available as well.
We pack a lot of information into each chart, and each is accompanied by a sheet of instructions that tell you what the symbols represent and how to "read" the events of a given night.
Take a look at the section above, which is shown at roughly the full size of the wall chart. You can see how the times of sunset and the end of twilight come later and later throughout January and February. Moon symbols indicate that it's full on the evening of January 4th and again on February 3rd. An orange curve shows that Mercury made a brief evening appearance in mid-January, and the light blue one shows Venus lingering a bit higher up after sunset each week. Mars sets around 8 p.m. each night; Jupiter rises at that time on January 1st but comes up two hours earlier, around 6 p.m., by the 27th.
Other curves show when Sirius rises, the Pleiades and Orion Nebula transit the north-south meridian, and when Polaris culminates directly over the North Celestial Pole. Clearly, January and February are busy months, celestially speaking!
The more you refer to this chart, the sooner you'll get a feel for the march of planets and constellations — not just during a single night but from week to week during the year. In fact, if you compare this year's chart with those from past years, you'll discover more and more about the clockwork of the heavens. For example, on charts eight years apart, the curves for Venus match almost perfectly — a celestial cycle known to the ancient Maya. On charts 19 years apart, the Moon makes its own encore performance.
Working up the "Skygazer's Almanac" takes a lot of effort — but it's one of the most rewarding projects I do all year. If you've got one, please add a comment below to let us know how you use it and what improvements we might make.
Astrobiologists discuss the search for life beyond Earth — join the conversation!
Courtesy of The Kavli Foundation, Sky & Telescope is featuring an in-depth Q&A with two astrobiologists on the search for extraterrestrial life.
Within 10 years, scientists at NASA and elsewhere are aiming to send life-seeking robots to Mars and to Saturn’s icy moon Enceladus. But first they must decide where to look, what evidence of life to look for and how to detect it.
To answer those questions, they’ve turned to “extremophiles,” microorganisms that inhabit the Earth’s most hostile environments. By stretching the limits of life on this planet, these organisms are improving the odds that faraway places thought to be uninhabitable may harbor it, too.
DiRuggiero and McKay discussed the next missions to search for life in our solar system in The Kavli Foundation roundtable discussion Microbiome and Astrobiology: How to Search for Life on Other Worlds. Now, join the conversation with two prominent astrobiologists who study life at the extremes and how to hunt for it on other worlds.
JOCELYNE DIRUGGIERO, PhD is Associate Research Professor in the Department of Biology at Johns Hopkins University in Baltimore and a member of the University’s Institute for Planets and Life. She studies how microorganisms adapt to extreme environments and what that can teach us about searching for life on other planets.
CHRISTOPHER McKAY, PhD is a senior scientist in the Space Science and Astrobiology Division at NASA Ames Research Center who studies life in Mars-like environments on Earth and plans missions to search for evidence of it.
LINDSAY BORTHWICK (moderator) has been working as a science journalist for more than a decade and holds a Master’s degree in neuroscience.
Has Comet Q2 Lovejoy stoked you to see more of these celestial travelers? We look into the crystal ball to see what's coming in 2015.
How fortunate we are to begin 2015 with a naked eye comet. Many of you have undoubtedly seen Terry Lovejoy's most recent discovery, C/2014 Q2 Lovejoy, either in the flesh or in photos across the Web. Topping out around magnitude +3.8 earlier this month, this icy blue gem still hovers around +4.5 as January draws to a close. Who knows — Q2 may turn out to be the best comet of the year.
Looking back, 2014 was a generous one for bright comets. Bright could mean "visible with the naked eye," but since naked-eye comets are so scarce, we'll choose a slightly different definition which (I hope) comet aficionados will find acceptable. How about anything visible in an ordinary pair of 7 x 50 binoculars from a reasonably dark sky? That would set our limiting magnitude at about +8.
Using this criterion, 2014 presented skywatchers with six bright comets — C/2013 R1 Lovejoy (6th magnitude in January); C/2014 E2 Jacques (7 in August); C/2013 V5 Oukaimeden (+6.5 in September), C/2012 K1 PanSTARRS (+7.5 in October), 15P/Finlay (briefly at +8.7 during an unexpected outburst in December), and of course Q2 Lovejoy (+5.0 in December).
2015 began with a bang with Lovejoy and a second surge from from 15P/Finlay to magnitude +7.5 in mid-January, but we'll soon enter the doldrums as Q2 Lovejoy fades below 6th magnitude sometime next month.
Barring the discovery of a bright newcomer, the new year offers up three bright entries: 88P/Howell, C/2014 Q1 PanSTARRS, and C/2013 US10 Catalina. Let's look at each in turn.
* 88P/Howell — Discovered with the 0.46-m Schmidt telescope at Palomar Observatory on 1981. It reaches perihelion on April 6th, when it could become as bright as 8th magnitude. Northerners need not apply — this comet will be only be visible from the southern hemisphere during the fall season (April, May). More on 88P.
* C/2014 Q1 PanSTARRS — Not only is Hawaii the surfing capital of the world, but it's lately become a hotbed of comet discovery thanks to the Panoramic Survey Telescope & Rapid Response System (PanSTARRS) survey atop Mt. Haleakala, a favorite tourist destination. Created to discover and characterize Earth-approaching asteroids and comets, the automated survey has bagged more than 80 new comets since full-time science operations began in 2010.
Discovered in August 2014, Q1 PanSTARRS will reach perihelion on July 6, 2015, after passing just 0.3 a.u. from the Sun. Expectations are high for it to grow a long, bright tail and possibly crest to magnitude +3 at nightfall during July and early August in the middle of southern winter.
* C/2013 US10 Catalina — Finally, northern folk get their due! US10 was discovered by the Catalina Sky Survey on Halloween 2013. For much of the year, the comet remains the province of southern hemisphere skywatchers. In late July and early August, it reaches magnitude +7 and becomes a south circumpolar object. By late September the comet achieves naked eye visibility (6th magnitude). After perihelion on November 15th perihelion, it surges into view for northern hemisphere skywatchers and peaks at around magnitude +3. As 2015 gives way to 2016, US10 remains bright as it buzzes Arcturus on New Year's night.
One last comet that must be mentioned, even if it's not expected to surpass 10th magnitude, is 67P/Churyumov-Gerasimenko. Thanks to the Rosetta mission, 67P has become the most intensely interesting comet in recent years. After months of close-up photos of the comet's nucleus, amateur astronomers are eager to see "Chury" for themselves. We'll have our chance in late August when it returns to the morning sky in Gemini-Cancer following perihelion on the 13th.
While comets seem sparse this year, there's no telling when a new one will blow out of nowhere and surprise us all. Old, familiar ones sometimes experience bright outbursts as well. Their unpredictability just keeps us coming back for more.
Once the Rosetta spacecraft arrived at Comet 67P/Churyumov-Gerasimenko last August, European scientists used an array of instruments to assess every nook and cranny of the remarkable two-lobed nucleus.
It took the comet-chasing Rosetta spacecraft 10½ years to reach its objective. That's a long time to wait to find out if all the systems and instruments are going to work as planned.
Fortunately, vindication for the European Space Agency and for scores of mission scientists came soon after the spacecraft reached Comet 67P/Churyumov-Gerasimenko last August 6th. In fact, even before the hitchhiking Philae lander made the first-ever soft landing on a comet on November 12th, the mother ship's 11 experiments had scrutinized the stark, two-lobed nucleus from tip to tip — occasionally from as close as 6 miles (10 km). The first two months' observations are summarized in a set of seven articles published last week in Science.
In my view, the most far-reaching discovery by Rosetta to date was actually announced some weeks ago, ahead of the magazine's print edition: Comet 67P's water molecules have a deuterium-to-hydrogen ratio far higher than that in Earth's oceans. Particularly because this is a Jupiter-family comet (JFC), the type that are most likely to have collided with Earth over the eons, this argues that comets contributed very little of our planet's water.
Meanwhile, the spacecraft's basic measurements of this object are interesting in themselves. Far from being an idealized orb of ice and rock, the comet's nucleus has a two-lobed shape that invites wild speculation as to its origin.
In an article describing results from the OSIRIS camera, Holger Sierks (Max Planck Institute) and his team note that the larger lobe is 2.5 miles (4.1 km) long, while the smaller one spans 1.6 miles (2.6 km). They're joined by a narrow waist, which intriguingly has been the source of most of the comet's escaping gas and dust to date. It's not clear whether the comet assembled as two large masses or that its midsection was once much plumper but has gradually eroded away.
Four other details about the nucleus are intriguing. First, it's very dark overall, with an average reflectivity of just 6% — nearly black, much like charcoal. This jibes with the albedos found at Comet 9P/Tempel 1 nearly a decade ago at Comet 103P/Hartley 2 in 2010 by NASA's Deep Impact spacecraft. Ditto for the nucleus of 1P/Halley. There's an unmistakable trend here: when you think "comet," think black.
What has likely happened is that rapidly escaping gas has carried off bits of dust that are not moving as fast, so they eventually settle back onto the surface. The comet's low gravity (less than 1⁄100,000 that on Earth), combined with its 12.4-hour spin, distribute these particles all around to create an even veneer.
Second, based on how strongly it attracts Rosetta, the nucleus must have a mass of about 10 billion metric tons. That's certainly hefty — it's about 25 million times the mass of the International Space Station, for example. But Sierks and his team report that the comet's overall density is just 0.47 g/cm3 — similar to wood. It's not made of wood, of course, but the ice-and-rock interior must be very "fluffy," with a porosity of 70% to 80%.
Third, scans by Rosetta's VIRTIS instrument (short for Visible, Infrared and Thermal Imaging Spectrometer) hasn't found any ice on the surface of 67P. Instead, a team led by Fabrizio Capaccioni (INAF, Italy) reports that the dark exterior is covered with complex, carbon-rich organic molecules. "VIRTIS clearly observed a comet that is different from the other JFCs encountered so far," the groups notes, because other such comets do have exposures of water ice on their surfaces. (The original leader of the VIRTIS team, Angioletta Coradini, never got to see these results; she died of cancer in 2011.)
The VIRTIS data suggest the organic molecules have lots of carbon-hydrogen bonds but few involving nitrogen. The team doesn't speculate as to what these are — but I will! It's possible that compounds are polycyclic aromatic hydrocarbons (PAHs) — much like the stuff coating the dark half of Saturn's moon Iapetus (not to mention what results when you grill a steak too long).
Finally, the surface is Comet 67P/Churyumov-Gerasimenko is a wonderland of weird landforms. The OSIRIS team has subdivided Comet 67P into 19 regions, all named for ancient Egyptian deities, that correspond to five distinct terrain types: smooth, brittle with pits and circular structures, large depressions, dust covered, and consolidated ("rock-like"). For example, Hapi (the god who makes the Nile River flood each year) is the smooth terrain in the neck that, thus far, has dominated the comet's outgassing. Looming above Hapi is a striking cliff face. Gas-spewing pits dot the region called Seth (a violent god associated with storms and disorder), and elsewhere the camera recorded enigmatic mounds, about 10 feet (3 m ) across, that the OSIRIS team calls "goosebumps."
Not a bad scientific haul for two months' work, and many other results (such as water-escape rates and solar-wind interactions) are detailed in the Science papers. And yet the most exciting events are still months away. Comet 67P/Churyumov-Gerasimenko is moving inward toward its August 13th pass through perihelion, by which time the nucleus should be jetting much more vigorously.
Read all about the Rosetta mission in Sky & Telescope's August 2014 issue.
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