A Big Hole in the Sun’s Atmosphere

Dec. 6, 2018:  A large hole in the sun’s atmosphere is facing Earth and spewing a stream of solar wind in our direction. NASA’s Solar Dynamics Observatory is monitoring the structure, shown here in a false-color UV image taken on Dec. 6th:

The hole (technical term: “coronal hole”) is so large it almost completely bisects the solar disk, stretching more than a million km across the sun’s equator.

We’ve seen this coronal hole before. It has been spinning around with the sun, lashing Earth with solar wind approximately once a month since September. Last month, the lashing commenced on Nov. 9th, lasted for almost 3 days, and caused sharp tremors in the geomagnetic field. Solar winds blowing faster than 600 km/s sparked an explosion of Phoenix-shaped auroras over Norway:

“The display over Senja, Norway, on Nov. 11th was nothing short of magical,” recalls photographer Adrien Mauduit. “Huge colorful pillars took the shape of a fiery bird.”

The same stream of solar wind will return on Dec. 8th or 9th and it may be even more potent this time because the underlying coronal hole has hrown larger in the intervening month. Arctic sky watchers, mark your calendars and warm your cameras. The Phoenix might rise again. Free: Aurora Alerts.

Realtime Aurora Photo Gallery

ZEN ASTRONAUT: Are the holidays stressing you out? Get your zen from the edge of space. On Dec. 2, 2018, the students of Earth to Sky Calculus launched a cosmic ray balloon to the stratosphere. This meditating spaceman pendant went along for the ride:

The students are selling the pendants to support their ballooning program. You can have one for $129.95. They make great gifts for space fans and are guaranteed to soothe holiday stress. Each premium stainless steel pendant comes with a greeting card showing the astronaut in flight and telling the story of its journey to the edge of space and back again.

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A Comet as Big as the Full Moon

Dec. 4, 2018: On Dec. 16th, Comet 46P/Wirtanen will approach Earth less than 11.5 million km away–making it one of the 10 closest-approaching comets of the Space Age. It’s a small comet, with a nucleus barely 1 km wide, but such proximity makes even a small things appear large. The comet’s gaseous atmosphere is now as wide as a full Moon. Mike Broussard of Perry, Louisiana, photographed the comet on Dec. 2nd and inserted the Moon for scale:

“The comet still has a couple of weeks before closest approach and it is already as big as a full Moon,” says Broussard, who could see the comet with his naked eye–“just barely using averted vision and only when it was in the darkest section of the sky,” he adds.

Despite its close approach, 46P/Wirtanen will never become a Great Comet like Comet Hayakutake in 1996 or Comet Hale-Bopp in 1997. Wirtanen’s relatively small core of dirty ice cannot produce enough gas and dust to create a really bright, flamboyant tail. The best case scenario is probably a big diffuse cloud of magnitude +3 or +4, barely visible to the unaided eye but an easy target for binoculars and small wide-field telescopes.

Last night in Cape Cod, Massachusetts, photographer Chris Cook didn’t even need a telescope to capture Wirtanen’s green glow. He took this picture using a Canon 6D digital camera with a 35 mm lens:

“This was my first sighting of Comet 46P/Wirtanen–just under naked eye visibility for my skies, but in 7×50 binoculars I could see a very large coma (ball of gas) almost 1° in diameter!” reports Cook. “It reminds me of Comet Hayakutake’s massive coma but not nearly as bright.”

Celebrated astrophotographer Juan Carlos Casado, who last night took his own HDR image of the comet above a church tower in Spain, offers some advice to novices: “Use Raw file format, a fast lens (at least f/2.8) and ISO settings between 1600 and 3200. The exposure will depend on the focal length. I normally use the 500 rule–that is, exposure = 500 / focal (mm) with a tripod. It also helps in areas with light pollution to use an antipollution filter. I am now using Optolong L-Pro clip filter which gives excellent color balance.”

On the nights of closest approach, 46P/Wirtanen can be found in the constellation Taurus rising in the east at sunset and high in the sky at midnight. Sky watchers in the northern hemisphere may orient themselves using these sky maps: Dec. 5, Dec. 6, Dec. 7, Dec. 8, Dec. 9, Dec. 10, Dec. 11, Dec. 12, Dec. 13, Dec. 14, Dec. 15, Dec. 16

More resources: orbital elements; ephemeris; 3D orbit; light curve.

Hyperactive Comet Approaches Earth

Nov. 26, 2018: Small but hyperactive Comet 46P/Wirtanen is approaching Earth and could soon become visible to the naked eye. On Dec. 16th, the kilometer-wide ball of dirty ice will be less than 11.5 million km away–making it one of the 10 closest-approaching comets of the Space Age. It already looks magnificent through amateur telescopes. On Nov. 26th, Gerald Rhemann took this picture using a 12-inch reflector in Farm Tivoli, Namibia:

“The comet is currently gliding through the southern constellation Fornax,” says Rhemann. “If you look carefully at the image, you can see galaxy NGC 922 near the comet’s head, and another galaxy ESO 479-2 on the left.”

Rhemann says that the comet’s emerald green atmosphere is 50 arcminutes wide. In other words–almost twice as wide as a full Moon. Its apparent diameter could double in the weeks ahead as the comet comes even closer. Because Wirtanen’s brightness is spread over such a wide area, it is diluted just below the limit of naked eye visibility, with a current magnitude near +6.0. We don’t yet know if the comet will ultimately become visible to the unaided eye–but it will certainly be an easy target for binoculars and backyard telescopes in December.

The nucleus of 46P/Wirtanen is small (~1 km) compared to greater comets such as Hale-Bopp (~30 km) and Halley (~15 km). It makes up for this deficit by hyperactivity. Recent measurements show that the core of 46P/Wirtanen is spinning once every 8.9 hours and spewing almost 1028 water molecules every second. This exceeds the expected production of such a small comet.

Comet Wirtanen passes through the inner solar system every 5.4 years. Right now it is just below the orbit of Earth, and the gap is narrowing. Click on the image above to explore the comet’s approach, courtesy of NASA/JPL.

More resources: sky map; ephemeris; 3D orbit; light curve.

Realtime Comet Photo Gallery