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A Kuiper Belt object discovered nearly a decade ago turns out to be much darker — and thus larger — than thought. It's also an especially slow spinner.
Read the fine print on any newly discovered solar-system object, and you’ll find its size is often just a guesstimate. That’s because all the finders typically know is the magnitude — but is the object big and dark or small and bright? They have to assume an albedo, or surface reflectivity, in order to estimate the diameter.
Recently a team of planetary astronomers worked to pin down the characteristics of dwarf planet 2007 OR10 to learn not just its intrinsic brightness but its size, shape, and surface properties. Surprisingly, they found that it’s bigger and darker than anyone thought. In fact, 2007 OR10 is the largest as-yet-unnamed world in our solar system and the third largest of the known dwarf planets.
András Pál (Konkoly Observatory, Budapest) led the study, which used data from NASA’s repurposed planet-hunting Kepler space telescope — its mission now known as K2 — along with archival data from the ESA’s infrared-sensing Herschel Space Observatory, whose instruments recorded far-infrared and submillimeter wavelengths (55 to 672 microns). Their results will appear in the Astronomical Journal.
By measuring the heat coming off an object, and then combining this data with its visible brightness, Pál explains, “one can unambiguously obtain the size of the object and the surface albedo as well.”Unlocking 2007 OR10's Dark Secrets
To get an accurate model of the heat coming off 2007 OR10, however, the researchers had to determine the rotational period of the dwarf planet. They did that by looking at small variations in 2007 OR10’s brightness, which K2 measures very precisely, and looking for regular repetitions.
“If you look very accurately at the brightness of an object as it rotates, then you might see certain bright spots, like the big, bright spot we saw on Pluto with New Horizons, the heart-shaped spot,” says Geert Barentsen (NASA Ames Research Center), a researcher with the K2 mission. “Then as the body rotates, we will see the light go up and down every time the bright spot comes around.”
Watch the video below to see Kepler's observations of OR10 (indicated by the arrow) over a period of 19 days in late 2014. (The object's apparent movement against the stars is caused by Kepler's changing position as it orbits the sun, and the diffuse light at right is from Mars, which was near the field of view.)
Konkoly Observatory / László Molnár and András Pál
From the Kepler data, the researchers determined that the best fit to the light curve was a surprisingly slow rotational period of a little bit less than 45 hours, longer than most objects orbiting beyond Neptune.
Meanwhile, the Herschel data showed the team how much sunlight 2007 OR10 absorbs and later radiates as heat. This value, combined with the Kepler data, let them calculate the dwarf planet’s size and how reflective it is.
2007 OR10 turns out to be much larger than thought. A previous estimate using this same technique had suggested a diameter of roughly 795 miles (1,280 km). However, without a handle on the object’s rotation period, past studies were limited in their ability to estimate its overall shape and surface character.
After folding together all these adjustments, the team determined that 2007 OR10 has an albedo of just 9% — half that found in the previous study. The much darker surface implies, in turn, a much bigger diameter, and Pál’s team calculates this to be 955 miles (1,535 km).A New Name for "Snow White"?
This larger size means 2007 OR10 probably has the gravity to retain icy coatings of volatile chemical compounds such as methane (CH4), carbon monoxide (CO), and molecular nitrogen (N2). Herschel’s infrared spectra also reveal a reddish surface consistent with a covering of methane frost, which creates red-tinged hydrocarbon compounds when bombarded by sunlight and cosmic rays.
Astronomers Megan Schwamb, Michael Brown, and David Rabinowitz spotted OR10 in 2007 as part of a search for distant solar-system bodies. They nicknamed it “Snow White,” thinking it was quite a bit smaller and more reflective. Eventually, they’ll have the honor of giving it an official name.
“It will no doubt get a name soon,” now that more is known about this mysterious object, says Barentsen, “and it might be a name that links back to what we have learned in this study.”
The orbiting science laboratory begins a series of marathon passes this week that favor the northern hemisphere.
It's the surefire highlight of any public star party, and you won't find it at the eyepiece.
Folks are always amazed to see the International Space Station pass by overhead. There it is: humanity at its best, cooperating in space. And the good news is, the International Space Station (ISS) will be putting on its best performance of 2016 starting this week, as it reaches a stretch of full illumination throughout the length of its orbit.
This means we're in for multiple visible passes of the ISS worldwide. The ISS leaves the dwindling shadow of the Earth later this week on Thursday, May 26th at 13:14 Universal Time (UT), and doesn't enter it again until more than four days later on May 30th at 19:25 UT. This occurs twice a year within a few weeks of either solstice. The May through July season favors the northern hemisphere, and the November through January season favors the southern hemisphere.
In fact, you may just see the ISS more than once on the same night. The ISS orbits Earth once every 93 minutes at an altitude of 251 miles. For example, the United Kingdom, which sits along latitude 50° to 60°N, can witness four to five passes of the ISS in a single night. A UK satellite hunting friend of mine, David Daniels (@OzoneVibe on Twitter) has suggested we term this phenomenon a FISION (Five ISS Sightings In One Night).
Why the high inclination orbit? The station's 51.65-degree inclination is a direct result of international cooperation and assures that spaceports worldwide have access to the ISS. The core Zarya module for the station was launched from the Baikonur Cosmodrome in Kazakhstan (46°N) on November 20, 1998. Back during the age of the space shuttle, you could tell when a shuttle was headed to the station by its launch azimuth — it would chase after the ISS up the U.S. eastern seaboard.
As a happy result of this high-inclination orbit, the ISS is routinely visible to more than 99% of humanity. The station reaches an elevation of greater than 10° above the horizon between latitudes 63°N and 63°S.
The ISS can shine at magnitude -5 when it passes directly overhead — brighter than Venus. Using 15x45 imaged-stabilized binoculars, I can just make out the beginnings of the structure of the ISS. Depending on its orientation, the station can look like a tiny box or a small glowing Star Wars TIE fighter on a good pass.High Beta Angle Season Begins
NASA calls this time of year "high beta angle season," referring to the steep angle between the station's orbit and the Sun. Though good for groundbased observers, continuous exposure to sunlight (and heat) poses potential problems in space, so NASA typically feathers the giant solar arrays of the station to create artificial shadows and cool the station.
The best way to know when the station is visible is to utilize one of the many tracking websites or apps out there. The Heavens-Above website is a great standby, and now has an app available for Android. Iphone users favor ISS Spotter and ISS Detector, and NASA's Spot the Station offers email alerts. And Sky & Telescope's Satellite Tracker and Transit Tracker tools will provide predictions for not just for upcoming passes, but for upcoming transits as well, such as when the ISS passes over the face of the Moon.
Be patient during your ISS vigil. The station will typically become apparent about 10° above the horizon under clear skies. The station will appear to move slowly at first, picking up speed as it passes directly overhead. This apparent change is an illusion, as the ISS is about 500 miles away when first sighted near the horizon coming at you, but only 250 miles away overhead. And remember, while aircraft generate their own blinking lights, satellites like the ISS are so high overhead, they shine via reflected sunlight that's still passing over the curve of the Earth.
Looking for a challenge? Catching a transit of the ISS across the face of the Sun or Moon is fun to try. To do this, you'll need to know when a given transit occurs from your location at an exact time. This might not happen right in your backyard — the path of an ISS transit over the Earth is only about seven miles wide — so be prepared to travel a bit. And keep in mind, the ISS crosses the 30' span of the Sun or Moon in just over a second. Not only that, but in most instances, it's invisible on approach. (The one exception is an illuminated pass of the Moon.) So timing is everything.
In addition to S&T's transit tool, CALSky also lists upcoming transits of the Sun and Moon by the ISS up to two days out and will generate email alerts for your location. But these predictions are subject to change: the station is periodically boosted using the thrusters of visiting spacecraft to combat atmospheric drag, and it occasionally carries out short-notice Debris Avoidance Maneuvers (DAMs) as well. These maneuvers can shift the prospective path of a transit at the last moment. Likewise, the orbit of the ISS evolves over time, and it generally takes a day or so for updates provided by the U.S. Joint Space Operations Center (JSpOC) to percolate out to all of the various tracking platforms. Advanced satellite trackers generally go to NORAD's Space-Track site to get their orbital elements from the source.
June will also see busy times for the station. First, the next crewed expedition launches from Baikonur on June 24th, followed by a SpaceX Falcon 9 launch of CRS9 on June 27th. Then on July 7th, the Progress 64P resupply vehicle heads towards the ISS. It's worth watching passes of the ISS right around this time, as spacecraft approaching and departing the station will appear as +2 magnitude "stars" following the same path. The most dramatic rendezvouses to watch were back during the U.S. space shuttle program, which ended in 2011.
Be sure to keep an eye on the sky in the coming week, as multiple ISS passes light up the night sky.
The post I took my MEADE Telescope at night in 2013 at 00:05 am & what i got is my masterpiece. appeared first on Sky & Telescope.
The post Mars comparison as it approaches the opposition of 2016 appeared first on Sky & Telescope.
Friday, May 20
• The nearly full Moon looms low in the east-southeast at sunset and shines above Mars as twilight fades. How soon can you pick out Mars? How much later will it be until you can pick out fainter Antares, rising 8½° below Mars?
And what about Saturn, 7½° left of Antares? Saturn and Antares rise at the same time if you're near 35° N latitude (North Carolina, central California). If you're north of there Saturn rises first; south of there, Antares.
And how soon can you spot the second-brightest star in this area? It's Delta Scorpii, now just 1° below Mars. That's less than a finger-width at arm's length.
Saturday, May 21
• Mars is at opposition tonight, opposite the Sun as seen from Earth. It's almost at its closest to Earth for this apparition, though not exactly so until the 30th.
• Full Moon this evening; by coincidence, the Moon and Mars are both at opposition. The Moon forms a rough rectangle with Mars to its right or lower right, Antares farther below it, and Saturn to its lower left, as shown above (seen from North America). Think photo opportunity.
Sunday, May 22
• The Moon now rises in twilight with Saturn about 4° to its right (as seen from North America). Mars and Antares are farther to their right, as shown above.
Monday, May 23
• As spring grows late, the "Spring Star" Arcturus shines very high in the southeast after dark (very high over Mars). The "Summer Star" Vega, equally bright, dominates the sky lower toward the east-northeast.
Arcturus is a type-K1.5 giant and thus shines pale orange-yellow, like a drop of rich ginger ale. Below Mars is Antares, an M1.5 supergiant with a deeper fire color than Arcturus. Mars currently looks much yellower than Antares, at least to my eyes, but that's probably because it's so much brighter. Brightness makes any color look desaturated (more toward white) — an illusion of human color vision and camera chips too.
• Jupiter's Great Red Spot transits Jupiter's central meridian around 10:45 p.m. EDT. It's positioned in excellent view for an hour before and after it transits.
Tuesday, May 24
• Jupiter's moon Europa crosses Jupiter's face tonight from 9:24 p.m. to 12:12 a.m. EDT, followed by its especially tiny black shadow from 11:57 p.m. to 2:40 a.m. EDT. (Subtract 3 hours to get PDT.)
Wednesday, May 25
• The enormous Arch of Spring is sinking ever lower in the west. As twilight fades, its top pair of stars, Pollux and Castor, still stand high in the west-northwest. Look for the Arch's left end, Procyon, about two fists at arm's length lower left of Pollux. Farther to the lower right of Castor is its right end: little Menkalinan and then brilliant Capella.
• Jupiter's Great Red Spot transits Jupiter's central meridian tonight around 12:23 a.m. EDT.
Thursday, May 26
• With summer less than a month away (astronomically speaking), the last star of the Summer Triangle rises above the eastern horizon at the end of twilight. That's Altair, the lower-right corner of the Triangle. Its highest and brightest corner is Vega. The third is Deneb, sparkling less far to Vega's lower left.
Friday, May 27
• Have you been watching the Mars-Antares-Saturn triangle change shape? It's lengthening as Mars moves westward against the stars, away from the head of Scorpius. This will continue until the end of June. Then Mars will start to slingshot back to fly right between Antares and Saturn in late August. Plan to watch this slow summer drama.
• Jupiter's Great Red Spot transits Jupiter's central meridian tonight around 2:02 a.m. EDT; 11:02 p.m. PDT. It's positioned in excellent view for an hour before and after.
Saturday, May 28
• Constellations seem to twist around fast as they pass your zenith, if you're comparing them with the direction "down." Just a week ago the Big Dipper floated horizontally in late twilight an hour after sunset as seen from 40° N latitude. Now it's tilted almost 45° at that time, bowl down. Another two weeks and it will be hanging straight down by its handle at that time.
• Jupiter's Great Red Spot transits Jupiter's central meridian around 9:54 p.m. EDT.
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.
Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The basic standard is the Pocket Sky Atlas (in either the original or new Jumbo Edition), which shows stars to magnitude 7.6.
Next up is the larger and deeper Sky Atlas 2000.0, plotting stars to magnitude 8.5, nearly three times as many. The next up, once you know your way around, is 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, or the bigger Night Sky Observer's Guide by Kepple and Sanner.
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 (meaning heavy and expensive). And 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 and Venus are hidden in the glare of the Sun.
Mars (magnitude –2.0, at the head of Scorpius) reaches opposition Saturday night May 21–22. It's closest to Earth on May 30th. After dark, orange-yellow Mars shines in the southeast almost as brightly as white Jupiter high in the southwest.
Look for Antares about 9° lower left of Mars during evening, and Saturn 7° or 8° to the left of Antares. The Mars-Antares-Saturn triangle stands highest in the south around 1 or 2 a.m.; this is the best time for telescopic viewing of the two planets. By early dawn the triangle is low in the southwest, now with Saturn over Antares.
In a telescope all this week and next, Mars appears 18.4 to 18.6 arcseconds in diameter, its biggest and best until July 2018. See our telescopic guide to Mars, with map, in the April Sky & Telescope, page 48, or the version online. And set our Mars Profiler for your time and date. If you're ambitious and have a big scope, now's the time to try hunting Phobos and Deimos, the two tiny Martian moons, using the June Sky & Telescope, page 48.
Jupiter (magnitude –2.2, in southern Leo) stands high in the south in twilight, then starts declining toward the southwest. See our telescopic guide to Jupiter in the March Sky & Telescope, page 48.
Saturn (magnitude +0.1, in southern) shines lower left of Mars closer to the left of Antares in the evening. The three cross the southern sky through the night, and by dawn they're low in the southwest. Saturn is nearing its own opposition on June 2nd. See our telescopic guide in the June Sky & Telescope, page 48.
Uranus is veiled by the glow of dawn.
Neptune (magnitude 7.9, in Aquarius) is low in the east-southeast before dawn begins.
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
During two Martian years, Curiosity tracks seasonal patterns in local atmosphere, temperature, and maybe even methane.
Last week, Curiosity completed its second Martian year, even though it landed in Gale Crater almost four years ago. Mars is much further from the Sun than Earth is, so the Red Planet takes nearly twice as long to complete one orbit around our star: 687 days to our planet’s 365 days.
For the past two Martian years, Curiosity has been busy collecting rock samples. Those have revealed that this part of Mars was once habitable. But the rover has also been measuring atmospheric pressure, temperature, and methane levels. These measurements show seasonal changes, much like Earth’s yearly rhythm.
Seasons, both on Mars and on Earth, happen because of the planet’s tilt—its rotation axis inclines 25 and 23 degrees from vertical, respectively. Mars also has a longer elliptical orbit compared to Earth, causing longer winters and shorter summers in its southern hemisphere.
Curiosity found that seasonal temperatures at Gale Crater follow a periodic pattern like we see in Los Angeles—though at much lower temperatures, as seen in this graphic chart. Temperatures in LA range from 90 degrees Fahrenheit (32 degrees Celsius) in the summer to 50 F (10 C) in the winter. Curiosity’s Rover Environmental Monitoring Station (REMS) has measured variations in temperatures ranging from 60.5 F (15.9 C) on a summer afternoon to -148 F (-100 C) on a winter night.
The chart also shows seasonal patterns between water content and relative humidity. The air in Gale Crater is very dry, with water content around 40 parts per million (ppm). As a comparison, a typical reading on Earth might be 20,000 ppm. Curiosity found that during the southern hemisphere’s summer, relative humidity—a measure of how close water vapor is to saturating the air—is low. In the winter, even though there is little water content in the air, the temperature drops so much that humidity actually goes up to 70%.
On Earth, we associate humidity with warm environments, like Florida in the summer, but high relative humidity can also happen in the winter. “For a given amount of water vapor, the colder the air gets, the closer it is to saturating, and the relative humidity goes up,” says project scientist Ashwin Vasavada (JPL). Gale’s winter humidity is high enough that frost might form on the ground, but the rover hasn’t found any.
Changing Methane Levels
Another interesting trend from the two-year cycle is the periodic methane levels. During Curiosity’s two Martian years at Gale Crater, it has also been measuring background methane levels using the tunable laser spectrometer in its Sample Analysis at Mars (SAM) suite of instruments. In the rover’s first cycle, it detected a spike in methane to 7 parts per billion by volume (ppbv) in the local atmosphere. This sparked media attention, especially because methane’s existence on Mars was previously a controversial issue.
Methane is the most abundant hydrocarbon in the solar system, but it must be replenished—either by biotic or abiotic activity — because it is easily destroyed by sunlight’s ultraviolet rays. So when Michael J. Mumma (NASA/Goddard Space Flight Center) and Vladimir A. Krasnopolsky (Catholic University of America) found methane on Mars in 2004, scientists met it with skepticism.
Once Curiosity landed, it began using the SAM instruments to analyze the Martian atmosphere and reported an upper limit of just 0.18 ppbv with an uncertainty of ± 0.67ppbv. Because the levels were so low, mission scientists concluded there was no trace of methane. But when the rover detected the 7ppbv spike during its first autumn, it confirmed methane’s presence in the atmosphere, though scientists are still not sure where it came from.
The methane measurements Curiosity has taken over the last two Martian years also show a “background” level that falls mostly between 0.3 and 0.8 ppbv and possibly follows seasonal patterns. The 7 ppbv spike happened for several weeks during the first autumn, so mission scientists checked carefully for a repeat spike during the second autumn. But concentrations stayed low.
“Doing a second year told us right away that the spike was not a seasonal effect,” said Christopher Webster (JPL) of the SAM team in a press release. "It’s apparently an episodic event that we may or may not ever see again.”
The mission continues to monitor methane levels, which appear to be even lower in autumn than in other seasons. If confirmed, the low autumn level could be a delayed reaction to summer’s high ultraviolet radiation.
Technological innovations, and Curiosity’s continuous measurements, have allowed scientists to resolve the methane controversy in less than a decade. Only time will tell what other seasonal patterns Curiosity finds.