An artist’s impression of Orbital Reflector unfurled in space. Credit: Orbital Reflector/Nevada Museum of Art
UPDATE – SpaceX has now set a firm date and time for the Spaceflight SSO-A launch from Vandenberg Air Force Base for Monday, December 3nd at 18:31 Universal Time (UT).
A unique smallsat mission promises to be the latest satellite “brighter than a Full Moon!” in the night sky… or not.
An artist's conception of asteroid 3 Juno. Credit: David Aguilar, Harvard-Smithsonian Center for Astrophysics.
Not all oppositions are created equal. This week’s sky target offers a good case in point, as asteroid 3 Juno reaches its most favorable viewing position for the decade. Continue reading “Catching Asteroid 3 Juno at Its Best”
Comet 46P Wirtanen in Cetus from September, 2018. Image credit and copyright: Hisayoshi Kato.
One. More. Comet. Though the next great ‘Comet of the Century’ has yet to make its appearance in 2018, we’ve had a steady stream of binocular comets this year, including Comets C/2017 S3 PanSTARRS, 38P Stephan-Oterma, and 21P Giacobini-Zinner.
An amazing view. The Barnard loop through Orion (right) Comet 21P Giacobini-Zinner (lower left) and Comet 38P Stephan-Oterma (upper left, see enlargement below). Image credit and copyright: Hisayoshi Kato.
A view along the solar system, using Cosmic Watch.
A view along the Solar System, using Cosmic Watch.
Watching the heavens on a nightly, or even casual basis? The web and modern technology has certainly altered the landscape of modern astronomy, (mostly) for the better. Once, we all huddled around cardboard planispheres, illuminated by red flashlights; now, it’s now a common sight to see illuminated smartphone apps accompanying telescopes at star parties, all waving skyward with virtual planetarium programs guiding users around the night sky.
Comet 21P/Giacobini-Zinner from August 14th. Image credit and copyright: Rolando Ligustri.
Comet 21/P Giacobini-Zinner from August 14th. Image credit and copyright: Rolando Ligustri.
A periodic comet may put on a fine show for northern hemisphere viewers over the next few months.
Comet 21/P Giacobini-Zinner is currently a fine binocular comet, shining at +8th magnitude as it cruises across the constellation Cassiopeia. This places it above the horizon for the entire night for observers north of the equator in August, transiting the local meridian at dawn. And unlike most comets that get lost in the Sun’s glare (like the current situation with C/2017 S3 PanSTARRS), we’ll be able to track Comet 21/P Giacobini-Zinner right through perihelion on September 10th.
The orbit of comet 21P, showing the comet’s position at closest approach. Credit: NASA-JPL.
This is because the comet is on a short period, 6.6 year orbit around the Sun that takes it from an aphelion of 6 Astronomical Units (AU) exterior to Jupiter’s orbit, to a perihelion of 1.038 AU, just 3.3 million miles (5.2 million kilometers) exterior to Earth’s orbit. The 2018 apparition sees the comet pass 0.392 AU (36.5 million miles/58.3 million kilometers) from the Earth on September 11th.
This is the closest passage of the comet near Earth since September 14th, 1946, and won’t be topped until the perihelion passage of September 18th, 2058. Its next cycle of passes to Earth closer than 0.1 AU aren’t until next century in the years 2119 and 2195, respectively.
Comet 21/P from August 10th. Image credit and copyright: John Purvis.
Discovered by astronomer Michel Giacobini at the Côte d’Azur Observatory in Nice, France on the night of December 20th, 1900 as it was crossing the constellation Aquarius, the 21st periodic comet was recovered two orbits later by Ernest Zinner on October 23rd, 1913 as it passed a series of variable stars near Beta Scuti.
Though the comet generally tops out at +8th magnitude, it has been known to undergo periodic outbursts near perihelion, bringing it up about 3 magnitudes (about 16 times) in brightness. This occurred most notably in 1946.
The light curve for Comet 21/P. The black dots represent actual observations and magnitude observations. Credit: Seiichi Yoshida’sWeekly Information on Bright Comets.
Comet 21/P Giacobini-Zinner is also the source of the Draconid (sometimes referred to as the Giacobinid) meteors, radiating from the constellation Draco the Dragon on and around October 7th and 8th. Feeble on most years, this shower can produce surprises, such as occurred in 1998, 2005 and most recently in 2011, when a Draconid outburst topped a zenithal hourly rate of 400 meteors per hour, flirting with ‘meteor storm’ status. And while we’re not expecting a meteor storm to accompany the 2018 perihelion passage of Comet 21/P Giacobini-Zinner, you just never know… it’s always worth keeping an eye out on early October mornings for the “Tears of the Dragon,” just in case. Note that the Moon reaches New phase on October 9th, just a few days after the meteor shower’s expected annual peak, a fine time to watch for any unheralded Draconid outbursts.
Prospects for Comet 21P
The comet is visible from the northern hemisphere through the remainder of August and all through September as it glides across Auriga, Taurus and Gemini and visits several well known celestial sights. In fact, it actually transits in front of several deep sky objects, including Messier 37 (Sept 10th), and Messier 35 (Sept 15th).
Comet 21P passes in front of open cluster Messier 35 on September 15th. Credit: Starry Night.
The comet will be moving at about two degrees per day when it’s nearest to the Earth, on and around September 11th.
We begin to lose the comet, as it heads southward in late October. Still, the comet is over 50 degrees above the eastern horizon at dawn come October 1st as seen from latitude 30 degrees north, having maintained a similar elevation throughout most of September. Not bad at all.
Here are some upcoming dates with destiny for Comet 21/P Giacobini-Zinner:
August 19: Crosses into the constellation Camelopardalis.
August 29: Crosses into the constellation Perseus.
August 30th: Crosses into the constellation Auriga.
The celestial path of Comet 21P from August 16th through September 15th. Credit: Starry Night.
September 2: Passes one degree from the bright star Capella.
Sept 7-8: Grouped 2 degrees from the open clusters M36 and M38.
Sept 10: Photo-Op: Skirts very near the open cluster M37. Also reaches perihelion on this date, at magnitude +7.
Sept 11: Passes closest to the Earth, at 0.392 AU distant.
Sept 13: Nicks the corner of the constellation Taurus.
Sept 14th : Enters the constellation Gemini.
Sept 15th: Photo-Op: crosses in front of the open cluster M35.
Sept 16: Crosses the ecliptic southward and near the +3.3 magnitude star Propus (Eta Geminorum).
Sept 17: Crosses into Orion.
Sept 21: Crosses into Gemini.
Sept 23: Crosses into Monoceros.
Sept 24: Passes near the Christmas Tree Cluster, NGC 2264.
The celestial path of Comet 21P from September 15th through October 15th. Image credit: Starry Night.
Oct 1: Crosses the galactic plane and the celestial equator southward.
Oct 7: Crosses in front of the open cluster M50.
Oct 10: Crosses into Canis Major.
Oct 31st: Passes near the bright star Aludra and may drop below +10th magnitude.
Binoculars are your best friend when you’re looking for comets brighter than +10th magnitude. With a generous field of view, binoculars allow you to sweep a suspect area until the faint fuzzball of a comet snaps into view. I like to ‘ambush’ a comet as it passes near a bright star, and a good time to spot comet 21/P Giacobini-Zinner is coming right up on September 2nd when it passes less than one degree from the bright +0.1 magnitude star Capella.
Don’t miss this year’s fine apparition of Comet 21/P Giacobini-Zinner, coming to a night sky near you.
Now, watch for a rare event this weekend, with the final eclipse for 2018 coming up on Saturday, August 11th, with a partial solar eclipse spanning northern Europe and the Arctic.
Circumstances for the August 11th, 2018 partial solar eclipse. Credit: NASA/GSFC/Fred Espenak
What’s so unique about this eclipse? Well, not only is it the last one for 2018, but it’s part of three eclipses in the second eclipse season of the year. Most seasons only feature two eclipses (one lunar and one solar) but every few years or so, it is possible to have a season with three: either lunar-solar-lunar (such as occurred in 2013) or solar-lunar-solar.
This is only possible when the middle eclipse occurs very near ascending or descending node along the ecliptic. The nodes are where the path of the Moon, inclined 5.1 degrees relative to the ecliptic plane intersect it—when these nodes are occupied by an alignment of the Earth, Sun and Moon (known as a syzygy, a fine word in Scrabble to land on a triple word score, though you’ll need a blank tile for the third ‘y’) a solar or lunar eclipse occurs. For an eclipse triple play, the middle eclipse needs to happen very near a node crossing, producing a fairly long eclipse. That’s exactly what happened on July 28th, when the Moon crossed through descending node just over an hour after crossing out of the Earth’s umbral shadow after the longest lunar eclipse for the 21st century.
This also leaves the Moon close enough to the opposite ascending node two weeks post and prior to July 28th on July 13th and August 11th to just nick the Sun for a partial solar eclipse, one over the Antarctic and one over the Arctic.
The animated path for the August 11th partial solar eclipse. Credit: A.T. Sinclair/NASA/GSFC
Eclipse Circumstances
Saturday’s partial eclipse touches down over the eastern coast of Canada at sunrise. From there, it sweeps eastward over Greenland, Iceland and the North Atlantic, with the Moon’s penumbra just grazing the northern United Kingdom before crossing over Scandinavia. Then, the shadow crosses over Asia, with a photogenic partial solar eclipse wrapping up at sunset over eastern China, the Koreas and the Russian far east.
Note that this eclipse is also a relative newcomer for its particular saros 155, as it is member 6 of a series of 71 eclipses. The saros just began less than a century ago on June 17th, 1928, and won’t produce its first total solar eclipse until September 12th, 2072 AD.
As of this writing, we’ve yet to see evidence of anyone carrying the eclipse live, though we’ll note it here if any webcast(s) surface.
When is the next one? Well, the next partial solar eclipse is on January 6th 2019, and the next total solar eclipse occurs on July 2nd, 2019.
Enter the Perseids
This weekend’s eclipse at New Moon also sets us up for a fine display of the Perseid meteors for 2018. This year, the Perseids are expected to peak on the morning of August 12th and August 13th. Watch for a zenithal hourly rate of 100 meteors per hour at the peak. A dependable annual favorite, the Perseids are debris remnants of period comet 109/P Swift-Tuttle.
The live webcast for the 2018 Perseid meteor shower. Credit: The Virtual Telescope Project.
Astronomer Gianluca Masi and the Virtual Telescope Project 2.0 will host a live webcast for the 2018 Perseids on August 12th starting at 20:30 UT.
Don’t miss the astronomical action worldwide this weekend, either live or online.
Comet C/2017 S3 PanSTARRS from July 22nd. Image credit and copyright: hodorgabor
Comet C/2017 S3 PanSTARRS from July 22nd. Image credit and copyright: hodorgabor
Comets are one of those great question marks in observational astronomy. Though we can plot their orbits thanks to Newton and Kepler, just how bright they’ll be and whether or not they will fizzle or fade is always a big unknown, especially if they’re a dynamic newcomer from the Oort Cloud just visiting the inner solar system for the first time.
We had just such a surprise from a cosmic visitor over the past few weeks, as comet C/2017 S3 PanSTARRSerupted twice, brightening into binocular visibility. Discovered on December 23rd 2017 during the PanSTARRS survey based on Haleakala, Hawai’i, S3 PanSTARRS is on a long-period, hyperbolic orbit and is most likely a first time visitor to the inner solar system.
The orbital path of comet S3 PanSTARRS through the inner solar system. Credit NASA/JPL.
S3 PanSTARRS was not only rocked by two new outbursts in quick succession, but seems to have undergone a tail disconnection event just last week, leveling off its brightness at around +8 magnitude and holding. This puts it in the range of binoculars under dark skies, looking like a fuzzy globular that refuses to snap into focus as it currently glides through the constellation of Camelopardalis the Giraffe the dawn sky.
The path of comet S3 PanSTARRS through August. Credit: Starry Night.
As July closes out, the time to catch sight of Comet S3 PanSTARRS is now, before it’s lost in the Sun’s glare. From latitude 40 degrees north, the comet sits 20 degrees above the northeastern horizon, about an hour before sunrise. By August 7th however, it drops below 10 degrees altitude. From there, the comet begins to circle the Sun as seen from the Earth beginning to favor southern hemisphere observers at dawn, who may be able to track it straight through perihelion on August 16th, if its brightness holds up. From there, northern hemisphere viewers may get a second view at dawn in September, again, if its brightness holds.
The dawn path of the comet looking northeast at dawn from latitude 35 degrees north, from July 31st through the first week of August. Credit: Starry Night.
You never know when it comes to comets. Here’s a brief rundown of the celestial happenings for comet C/2017 S3 PanSTARRS:
August
3- Crosses into the constellation Gemini.
4- Passes near the bright star Castor.
5- Passes near the bright star Pollux.
7- Crosses into the constellation Cancer.
7- Passes closest to the Earth, at 0.758 Astronomical Units (AU) distant.
8- Crosses southward over the ecliptic plane.
9- Passes just 4 degrees from the Beehive cluster, M44.
11- Passes 2 degrees from the open cluster M67.
12- Passes 10.5 degrees from Sun (1st apparent close pass as seen from the Earth)
13- Crosses into the constellation Hydra.
15- Reaches maximum brightness: the comet may top +2nd magnitude in mid-August.
16- Reaches perihelion at 0.21 AU from the Sun.
18- Crosses into the constellation Sextans.
30-Crosses into the constellation Leo.
31-Crosses the ecliptic plane northward.
The projected light curve for comet S3 PanSTARRS. The black dots are actual observations. Credit Seiichi Yoshida.
September
3- passes 4 degrees from the Sun.
25- Crosses into the constellation Coma Berenices.
From there, Comet C/2017 S3 PanSTARRS drops back below 6th magnitude in September, then below 10th magnitude in October as it heads back off into the icy realms of the outer solar system.
Be sure to nab this icy interloper why you can. The quote comet hunter David Levy, “Comets are like cats… they have tails, and they do exactly what they want.”
