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What is "the fall equinox" and how do we know when it happens?
To those who’ve unpacked their winter coats, closed their windows at night, and felt that telltale crispness in the air, it seems that autumn has already started. Astronomically speaking, however, the fall season only comes to the Northern Hemisphere on Tuesday, September 23, 2014 at 02:29 UTC (Monday, September 22 at 10:29 p.m. EDT). At that moment, the Sun passes over the Earth’s equator heading south; this event is called the autumnal equinox.
This seems awfully precise for seasons that gradually flow from one to the next, but the reason we say this event means the “End of Summer” and “Beginning of Fall” is because it is marked by a key moment in Earth’s annual orbit.
The apparent position of the Sun in our sky is further north or further south depending on the time of year due to the globe's axial tilt. Earth's rotational axis does not point straight up and down, like the handle of a perfectly spinning top, but is slanted about 23.5° with respect to our orbit around the Sun.
Another way to think of this is that the plane drawn by Earth's orbit around the Sun (called the ecliptic) is tilted with respect to the planet's equator. From the perspective of Earthlings like us, the Sun follows the ecliptic in its path through the sky throughout the year. Each day the apex of the Sun's arc moves depending on the time of the year. To observers at northern latitudes (e.g., the continental United States), the Sun appears to sneak higher in the sky between late December and late June, only to drop down again from late June through late the next December. The equinox occurs when the Sun is halfway through each journey.
Earth’s axial tilt also produces our seasons. When Earth is on one side of its orbit, the Northern Hemisphere is tipped towards the Sun, receiving more direct solar rays that produce the familiar climes of summer. When Earth is on the opposite side of its orbit half a year later, the Northern Hemisphere is tipped away from the Sun. The slanting solar rays heat the ground less, producing the colder winter season.
The same is true in reverse for the Southern Hemisphere, of course. Christmas is a warm holiday in Sydney, Australia. For those living in equatorial regions, however, there are usually only two recognizable seasons: wet and dry; and the days themselves vary less in length.
Aside from the aforementioned celestial arrangement, several other noteworthy things happen on the equinox date:
- For the Southern Hemisphere, the seasons are reversed so September’s equinox marks the beginning of spring, while March’s equinox signals the start of fall.
- Day and night are nearly the same length; the word “equinox” comes from the Latin aequinoctium meaning “equal night,”according to the Oxford English Dictionary. However a poke around your almanac will show that day and night are not precisely 12 hours each, for two reasons: First, sunrise and sunset are defined as when the Sun’s top edge—not its center—crosses the horizon. Second, Earth’s thick atmosphere distorts the Sun’s apparent position slightly when the Sun sits very low on the horizon.
- The Sun rises due east and sets due west, as seen from every location on Earth—the equinoxes are the only times of the year when this occurs.
- Should you be standing on the equator, the Sun would pass exactly overhead at midday. Should you be at the North Pole, the Sun would skim around the horizon as the months-long polar night begins.
Curious about what's happening in the sky? Check out the 2015 Sky & Telescope Observing Wall Calendar!
NASA's latest interplanetary spacecraft has settled into orbit around the Red Planet. Its year-long atmospheric studies could reveal how and why Mars lost so much of its primordial atmosphere.
When it comes to interplanetary exploration, you've got to trust your hardware. That was the case this evening, when the scientists and engineers for NASA's latest deep-space sortie could do little more than wait anxiously, fingers crossed, at a control center in Littleton, Colorado. Out there, 138 million miles (222 million km) and 12½ light-minutes from Earth — too far away to control directly — the Mars Atmosphere and Volatile Evolution spacecraft (MAVEN) successfully fired a cluster of six engines for 33 minutes and slipped into orbit around the Red Planet.
The spacecraft didn't exactly shout "I'm here" after the 10-month, 442 million-mile cruise that began last November 18th. But Doppler shifts in a weak radio beacon showed that the engines had reduced the approach velocity by about 4,000 feet (1.23 km) per second, slowing the craft enough for the planet's gravity to snare the spacecraft at 10:24 p.m. EDT (2:24 Universal Time on September 22nd). MAVEN had arrived at Mars.
For now, the spacecraft will follow a looping polar orbit that varies from 240 to 27,700 miles (380 to 44,600 km) in altitude. Over the next 6 weeks, the engines will fire again to shrink a 4½-hour-long orbit ranging from 95 to 3,850 miles, and then small thrusters will trim that further to a final, 3½-hour loop.
Unlike NASA's other Martian explorers, which have largely focused on the state of the planet's surface and its geologic evolution, MAVEN will study the Martian atmosphere exclusively. It carries eight instruments, six of which will measure charged particles, electromagnetic fields, and plasma waves in the solar wind as it sweeps past the planet. An imaging ultraviolet spectrograph and a mass spectrometer, both mounted on a steerable platform at the end of a short boom, will assess the upper atmosphere's chemical makeup.What Happened to Mars?
Over the next year, flight controllers will command MAVEN to make five "deep dips", dropping it to altitudes as low as 77 miles (125 km) to sample directly the uppermost wisps of the planet's already tenuous air. These observations hope to answer a longstanding puzzle among planetary scientists. There's ample evidence that, early in its history, the Red Planet had a much denser atmosphere. Rain fell from its sky, and water coursed across its landscape.
But then something happened to the atmosphere: it basically vanished and, with it, the brief era when Mars might have been suitable as an abode for life. Mars quickly became the desolate, frigid world we see today. Researchers led by Bruce Jakosky (University of Colorado), MAVEN's principal investigator, want to know what happened to all that gas (most of it carbon dioxide) and, especially, to the ample water that once existed on the Martian surface.
One leading theory is that the gas escaped irrevocably to space, stripped away by the solar wind rushing past. Here on Earth, our planet's magnetosphere serves as an obstacle to the solar wind, keeping it from interacting directly with our atmosphere. But once Mars lost its global magnetic field, billions of years ago, the upper atmosphere became vulnerable.
MAVEN's spectrometers will attempt to determine if hydrogen atoms, torn from water molecules by ultraviolet sunlight, are escaping to space, and at what rate. "The stripping of gas from the atmosphere to space might have been the driving mechanism for climate change on Mars," Jakosky says.
For now, he and his team will ready the spacecraft to begin observations in early November. Results will not come quickly, he cautions, because it will take months to build up enough measurements to have a clear sense of what's going on — or going away.
However, one early, unexpected, and unprecedented opportunity will come relatively soon, when Comet Siding Spring (C/2013 A1) brushes within 82,000 miles of the Red Planet on October 19th. Because any cometary particles will strike at 35 miles (56 km) per second, there's some concern for the safety of MAVEN and other orbiters circling Mars. They'll be positioned on the back side of the planet during the time of greatest danger.
A few days before and after the comet's closest approach, MAVEN's ultraviolet spectrograph will measure both the abundance of gases within C/2013 A1's coma and also its effects on the Martian upper atmosphere (heating from cometary dust impacts or a temporary increase in water-vapor content). "We should have some pretty spectacular results," Jakosky promises.
Our special issue, "Mars: Mysteries & Marvels of the Red Planet," is loaded with spectacular photos and a must-read for anyone interested in this intriguing neighboring world.
Friday, September 19
In early dawn Saturday morning, Jupiter shines upper left of the waning Moon in the east, as shown at right. How long has it been since you turned your scope on either Jupiter or the maria-covered waning crescent?
Saturday, September 20
In bright twilight, Mercury and fainter Spica are in conjunction 0.6° apart just above the west-southwest horizon. Use binoculars to scan for them about 20 minutes after sunset.
The eclipsing variable star Algol (Beta Persei) should be at its minimum light, magnitude 3.4 instead of its usual 2.1, for a couple of hours centered on 10:55 p.m. EDT.
In early dawn on Sunday the 21st, the waning crescent Moon shines far below Jupiter and closer to the right of Regulus, as shown above.
Sunday, September 21
Aquila's dark secret: If you're blessed with a really dark sky, try finding the big dark nebula known as "Barnard's E" near Altair in Aquila, using Gary Seronik's Binocular Highlight column and chart in the September Sky & Telescope, page 45.
And if you have a sky that dark, also use binoculars to investigate the big, dim North America Nebula and its surroundings near Deneb in Cygnus using the September issue's Deep-Sky Wonders article, page 56.
Monday, September 22
The September equinox comes at 10:29 p.m. on this date EDT (2:29 September 23rd UT). This is when the Sun crosses the equator heading south for the year. Fall begins in the Northern Hemisphere, spring in the Southern Hemisphere. Day and twilight-plus-night are nearly equal in length. The Sun rises and sets almost exactly east and west.
As summer ends, the Sagittarius Teapot is moves west of due south during evening and tips increasingly far over, as if pouring out the last of summer.
Tuesday, September 23
Arcturus is the bright star fairly high due west at nightfall. It's an orange giant 37 light-years away. Off to its right in the northwest is the Big Dipper, most of whose stars are about 80 light-years away.
Algol is at minimum light again for a couple hours centered on 7:44 p.m. EDT.
Wednesday, September 24
Mars is within 4° of Antares (passing north of it) from this evening through the 30th. Mars is just a little brighter and almost the same color as its namesake star; "Antares" is Greek for "anti-Mars."
Thursday, September 25
With the coming of fall, Deneb slowly replaces Vega as the bright star nearest to the zenith just after nightfall (for mid-northern latitudes).
Friday, September 26
As early as 8 or 9 p.m. now look for Fomalhaut, the lonely 1st-magnitude Autumn Star, twinkling on its way up from the southeast horizon. It will be highest due south around 11 or midnight (depending on your location).
Saturday, September 27
Low in the southwest in twilight, Mars and Antares are passing 3° apart this evening and Sunday evening, as shown below. Meanwhile, off to their right, the waxing crescent Moon floats a couple degrees to the lower right of Saturn (for North America).
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 (magnitude 0.0) remains very deep in the sunset. Scan for it with binoculars just above the west-southwest horizon about 20 minutes after sundown. Fainter, twinklier Spica is right nearby. Mercury and Spica appear closest together, 0.6° apart, on Saturday evening the 20th.
Venus (magnitude –3.9) is barely above the horizon due east shortly before sunrise. Bring binoculars.
Mars (magnitude +0.8, in Scorpius) glows low in the southwest at dusk near similarly colored Antares (magnitude 1.0). They'll pass 3° apart on September 27th and 28th.
Jupiter (magnitude –1.9, in Cancer) rises around 3 a.m. and shines brightly in the east before and during dawn. It forms a roughly equilateral triangle with Pollux above it (by about two fists at arm's length) and Procyon to their right. Farther to the right or lower right of Procyon sparkles brighter Sirius.
Saturn (magnitude +0.6, in Libra) is sinking low into the afterglow of sunset. Look for it well to the right of the Mars-Antares pair, and perhaps a little lower depending on your latitude.
Uranus (magnitude 5.7, in Pisces) and Neptune (magnitude 7.8, in Aquarius) are high in the southeast and south, respectively, by 11 p.m. See our Finder charts for Uranus and Neptune online or in the September Sky & Telescope, page 50.
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 Daylight Time (EDT) is Universal Time (UT, UTC, or GMT) minus 4 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 detected a supermassive black hole in the center of a tiny galaxy — where it has no right to be.
Don't be fooled by the small size of the ultracompact dwarf galaxy M60-UCD1 — it harbors a supermassive black hole, according to research published in the September 18th Nature. The new finding makes M60-UCD1 the smallest and least massive galaxy known to contain such a gargantuan black hole and is the first concrete evidence for how ultracompact dwarf galaxies form.
M60-UCD1 is located in the Virgo Cluster, about 54 million light-years from Earth. It is one of the most massive and brightest ultracompact dwarf galaxies, objects that can shove up to 200 million solar masses into a radius 160 light-years or less. They are similar in size to globular clusters but are at least 10 times more massive.
Ultracompact dwarf galaxies have perplexed astronomers since they were discovered over a decade ago. Scientists were uncertain how these tiny galaxies formed. Two competing theories emerged: either they were unusually massive star clusters, or they were the remnants of larger galaxies that had been stripped down to their cores by the gravitational pull of massive neighbors.
The study is the first to provide observational evidence for the stripped-down theory, by revealing the presence of a supermassive black hole. Supermassive black holes have more than a million times the mass of the Sun and are found at the centers of large galaxies, but usually not in dwarf galaxies. But at 21 million times the mass of the sun, M60-UCD1’s black hole is “remarkably massive,” says study coauthor Anil Seth (University of Utah). “It’s about five times more massive than the Milky Way’s black hole, in spite of the fact that this object — this ultracompact dwarf galaxy — is 500 times smaller and a thousand times less massive” than the Milky Way.
The existence of the supermassive black hole indicates that M60-UCD1 was once a larger galaxy, and that many of its stars were later ripped away by interactions with its neighbor, the much larger elliptical galaxy M60. Given the black hole's mass, the original galaxy likely had a central bulge roughly 100 times more massive than the current galaxy's total stellar mass.
“They’ve really, finally, after a decade, got some solid evidence that it’s one model rather than the other, that at least some of them must have been at the center of a galaxy in the past,” says Michael Drinkwater (University of Queensland, Australia), who was not involved in the research.
M60-UCD1’s black hole is particularly impressive given the galaxy’s small size. It makes up a whopping 15 percent of the galaxy's total mass, whereas the black hole in our Milky Way makes up only a fraction of a percent of the total mass of the galaxy. Most galaxies follow the Milky Way’s pattern, although exceptions exist.
Characterizing such a tiny object is a difficult task. The astronomers used the Gemini North 8-meter telescope on Hawaii’s Mauna Kea to discern the motions of stars in the galaxy. They found that the stars in the center of the galaxy were orbiting at roughly 175 kilometers per second (390,000 mph), much faster than expected and indicating the presence of a black hole.
“Immediately, as soon as I saw the stellar motions map, I knew that there was something exciting,” says Seth. “It had a larger black hole than even we had considered as our maximum case.”
The result could have some broader implications, the astronomers say. There are many ultracompact dwarf galaxies — around 50 are known in the nearest galaxy clusters — and these objects have a common peculiarity. Many ultracompact dwarf galaxies are more massive than expected based upon their luminosities — a possible indication of a black hole, but not the only plausible explanation.
However, in the case of M60-UCD1, once the astronomers included the black hole in their calculations, the galaxy’s stellar mass matched what astronomers would expect, given its luminosity. This result, they argue, indicates that the same process is likely also the explanation for the high mass estimates for other ultracompact dwarf galaxies.
Another possible explanation for the unexpectedly large masses is that the average mass of stars in these dwarfs is much higher than in standard galaxies.
“Really, until we go and measure a few more, it’s hard to know the relative importance of the two origins,” says Drinkwater.
And measuring a few more will be the next step in this research. Most of the known ultracompact dwarf galaxies are too faint to study with this method, but the next generation of telescopes should allow astronomers to look for more supermassive black holes in other objects of this type.
The result indicates a new place for scientists to search for black holes. If the astronomers are correct and many ultracompact dwarf galaxies do contain black holes at their centers, then the team predicts that the true number of massive black holes in the local universe may be twice the current estimate.
This is still speculation at this stage, Drinkwater says, but it’s an idea that would be interesting to investigate.
“This certainly opens up that possibility that they’re there, and that’s very exciting,” said Seth.
Reference: A. C. Seth et al. "A supermassive black hole in an ultra-compact
dwarf galaxy." Nature. September 18, 2014.
Explore the marvels of the universe with the best of Sue French's S&T columns, in her book Deep-Sky Wonders.
Ten thousand stars bedazzle the eye on a dark night. Wait, how many?
Go out on a dark night and you'd swear there are thousands of stars in the sky. Too many to count. 10,000 at least. But why guess when someone has already done the counting for you? Astronomer Dorrit Hoffleit of Yale University, well known for her work with variable stars, compiled the Yale Bright Star Catalog decades ago. It tabulates every star visible from Earth to magnitude 6.5, the naked eye limit for most of humanity.
You might be in for a surprise when you read it, though. The total comes to 9,096 stars visible across the entire sky. Both hemispheres. Since we can only see half the celestial sphere at any moment, we necessarily divide that number by two to arrive at 4,548 stars (give or take depending on the season). And that's from the darkest sky you can imagine. I don't know about you, but that number seems paltry to one's impression of an inky night in the backcountry.
At the poles, where the north and south polestars are pinned to the zenith and no stars rise or set, the same ~4,500 stars are visible every single clear night of the year. At northern mid-latitudes, the pole star is halfway up in the northern sky, allowing us to peer deeper into the southern realms of the celestial sphere. During the course of a year from latitude 45° north, we see roughly half again as many stars as we do at a particular time on a given evening. That tallies up to approximately 6,800 stars. Still pretty lean, but apparently enough to convey the impression of an intensely starry sky.
Astronomers use the magnitude scale to measure star and planet brightness. Each magnitude is 2.5 times brighter than the one below it. Altair, in Aquila the Eagle, shines at about magnitude +1 which is 2.5 times brighter than a 2nd magnitude star, which is 2.5 times brighter than a 3rd magnitude star, and so on.
A first magnitude star is 2.5 x 2.5 x 2.5 x 2.5 x 2.5 (about 100) times brighter than a 6th magnitude star.
The bigger the magnitude number, the fainter the star. If an object is really bright, it’s assigned a negative magnitude. Sirius, the brightest star sparkles at magnitude –1.4, Jupiter at –2.5, and Venus tops the planets at –4.4. The Full Moon reaches a magnificent –12.7, bested only by the Sun at –26.7.
While the total number of naked eye stars may seem unimpressive, consider what happens to the sky in and around cities, where most of us live. From the suburbs, the magnitude limit is around +4 for a worldwide total of about 900 stars or half that for your location. If we set the city limit at magnitude +2 (stars similar to the Big Dipper in brightness) we're left with just 70 stars worldwide, or 35 stars visible from say, downtown Chicago or Boston.
No wonder city dwellers are stunned by the night sky when they take their first trip to the country. Stars barely exist for those trapped beneath an ever-present dome of light pollution.
Numbers increase exponentially if we go in the opposite direction as there are far more faint stars than bright. The standard limit for a pair of 50-mm binoculars is 9th magnitude, opening up a vista of some 217,000 stars across the heavens. Impressed? A 3-inch telescope pulls in a treasure-worthy 5.3 million, enough for several lifetimes of viewing pleasure. Dare I go further?
On the very best nights, I can reach 16th magnitude with my 15-inch telescope, or 380 million stars. Well, only half that really, but who's counting?
How many stars can you see with binoculars? Use Gary Seronik's Binocular Highlights to help you find out!
NASA’s Night Sky Network is conducting a new survey in order to better help the amateur astronomy community.
Picture astronomy experienced not from the confining seats of a planetarium, but from a blanket under a sky so dark the Milky Way almost casts shadows. Now picture dozens of professional and amateur astronomers alike inviting you to look at clusters, nebulas, and galaxies through their telescopes, while diving into the secrets of the universe by discussing exoplanets, black holes, and dark energy.
These star parties, thrown by any local astronomy club, are one of the best ways to orient yourself to the night sky and the universe. And club meetings offer further opportunities still, letting you talk to researchers, try out new equipment, and even embark on citizen science adventures.
But how often do events as we’ve described here actually happen? And what are amateurs finding is the most — or least — helpful in their outreach endeavors?
NASA’s Night Sky Network is conducting a new survey to better understand the landscape of educational outreach performed by astronomy clubs. It will then use this data to assess the needs of the amateur astronomy community for the next five years.
We encourage all amateurs to spend some time taking the survey. Whether you’ve been hooked on astronomy since the space race or are entirely new to the field, your feedback is crucial. S&T will also publicize the results after they’re released and will use them as a guide in our own planning.
NASA’s Night Sky Network is a community of more than 400 astronomy clubs across the U.S. that share their time and telescopes with the public. They have held nearly 30,000 events and have inspired over 3 million members.
The survey, which will run until the end of September, is fairly straightforward. It asks questions about your local astronomy club, any astronomy activities you participate in, and any challenges you face in outreach. After the results are in, check back at Sky & Telescope, as we’ll make sure to provide an update.
On November 11th, the European Space Agency's Rosetta spacecraft will dispatch the heavily instrumented Philae lander to an area called "Site J" on one end of Comet 67P/Churyumov–Gerasimenko.
The European Space Agency's Rosetta spacecraft finally arrived at Comet 67P/Churyumov–Gerasimenko a month ago, after a 10-year cruise through interplanetary space. In an ideal world, Comet C-G would have been a nice smooth ball of dust and ice with a big X marking the safest and most scientifically interesting landing site for the craft's Philae lander. Had that been the case, says Rosetta mission manager Fred Janssen, his team would have put the odds of a successful landing at 70% or 75%.
But nature has thrown the project a few curves. Not only is the comet's nucleus complicated — an irregular, double-lobed structure 2½ miles (4 km) long — but it's also much rougher and craggier than expected. Add to that the comet's ahead-of-schedule activity (it's already giving off jets of gas despite being 3.4 astronomical units from the Sun), and all bets are off. At an ESA press briefing earlier today, Janssen declined to offer a revised risk assessment. "No site meets all the engineering criteria," he allowed.
That said, Philae has to set down someplace, and the team has winnowed down an initial set of 10 candidate sites to primary and backup locations. The best location, designated Site J, is on the comet's smaller lobe (think of it as the "head"); the backup, Site C, is on the larger "body." Engineers opted to stay clear of the smooth-textured "neck" between them, because from there it would be difficult for Philae to remain in constant radio contact with the main spacecraft as it orbits the nucleus.
“This was not an easy task," noted Stephan Ulamec, lander manager for the German space agency DLR. "Site J is a mix of flat areas and rough terrain. It’s not a perfectly flat area. There is still risk with high-slope areas.”
The smaller-lobe site won out in part because cameras have already identified two small pits near it that are sources of outgassing. "Site J meets all the criteria to fulfill all the science criteria," explains Jean-Pierre Bibring, the lander's lead scientist. Each of Philae's 10 instruments will be able to operate "at least once to its full capability."
These sites will get another two months of scrutiny from the battery of remote-sensing instruments aboard Rosetta, which will soon move in closer from its current distance of 20 miles (30 km), first to an altitude of 12 miles (20 km) and then to just 6 miles (10 km).
Assuming no "gotchas" emerge, the spacecraft will release Philae on November 11th for its 7-hour-long "fall" the the comet's surface. Once it makes contact at roughly 2 miles per hour (1 meter per second), the card-table-size, 220-pound (100-kg) lander will anchor itself using a mechanical harpoon, then quickly take a 360° panorama and measure the pressure of cometary gas surrounding it.
The comet's nucleus is not a solid ball of frozen water and dirt; instead, the terrain looks very dark, due to carbon-rich organic molecules, and it appears surprisingly rocky and angular — perhaps a consequence of shock compression during impacts. Early estimates suggest the mean density is only 0.4 gram per cubic centimeter, and the surface gravity is only about 1⁄100,000 that on Earth.
Philae's scientific payload includes a drill that will extract samples of the nucleus from depths of at least 9 inches (230 mm), likely deep enough to ensure that the excavated material is pristine and unaltered by the comet's passes near the Sun. (It comes as close as 1.24 a.u., or 115 million miles, every 6.45 years.) Samples will then be fed to three instruments to determine the molecular, elemental, and isotopic composition, augmented by elemental assays by another spectrometer on the surface. An onboard radar and acoustic sounder will probe the the interior structure to depths of several tens of meters. Other instruments will measure the nucleus's magnetic and electrical properties. And there'll be plenty of panoramic images from an onboard camera.
With luck, Philae will last several months, powered initially by onboard batteries and later by small solar-cell panels. Meanwhile, Rosetta will accompany Comet Churyumov–Gerasimenko as it swings through perihelion (August 13, 2015) and beyond. The hope is to get at least 13 months of observations. Perhaps, Bibring muses, the long run of observations will allow mission scientists to determine if Comet C-G got its two-faced personality from two cometary masses that merged or because a single body has eroding into the wild shape seen today.
Get all the details on the Rosetta-Philae mission in the August issue of Sky & Telescope magazine.
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