How Dark is Darksat? (Not Very)

Feb. 25, 2020: Last November, SpaceX tried to save astronomy. Among 60 bright and shiny Starlink satellites that blasted off from Cape Canaveral on Nov. 11, 2019, was one “Darksat”–a Starlink satellite with an experimental anti-reflective coating. Making Starlink satellites darker could head off a brewing confrontation between astronomers and internet entrepreneurs.

“But is ‘Darksat’ really darker?” asks astrophotographer Thierry Legault. “On Saturday morning at astronomical twilight, I filmed the passage of a group of Starlink satellites at their final altitude (550 km). Darksat is one of the brightest.”


“On the images covering more than 80° on the sky (from Lyra to Bootes), these satellites reached magnitude 2.5, which is even brighter than I expected! We are still waiting for effective albedo reduction measures, and in the meantime the launches continue…” he says.

Indeed, SpaceX has conducted three more launches since last November, bringing the total number of Starlink satellites in orbit to 300. Ultimately, the network of internet satellites could grow as large as 42,000. That’s a lot of artificial stars–and so far the Darksat experiment has not succeeded in reducing their visibility. Keep trying, SpaceX.

Betelgeuse is Brightening Again

Feb. 24, 2020: Call off the supernova watch. Betelgeuse is brightening again. Researchers from Villanova University, who have been leading the study of Betelgeuse’s unprecedented decline, have confirmed in a new Astronomical Telegram that the star has reversed itself. The turnaround was actually predicted, and suggests the recent dimming was an unusually deep excursion of the star’s natural 430-day periodicity.

Here are the latest data from the American Association of Variable Star Observers (AAVSO):


According to the light curve, Betelgeuse hit bottom during the week of Feb. 7th – 13th with a V magnitude slightly greater than +1.6. “Based on these and additional observations, Betelgeuse has definitely stopped dimming and has started to slowly brighten,” says Ed Guinan of Villanova University. “Thus, this ‘fainting’ episode is over.”

The monitoring should continue, however. Sensational images captured last month by the ESO’s Very Large Telescope in Chile revealed that one half of Betelgeuse was dimming more than the other. No one knows why. Additional imaging during Betelgeuse’s recovery might unravel the mystery.

The Changing Shape of Betelgeuse

Feb. 15, 2020: Betelgeuse isn’t just dimming, it’s also changing shape. Today, the European Southern Observatory released new images of Betelgeuse from the Very Large Telescope (VLT) in Chile’s Atacama desert. The unstable red supergiant is definitely lopsided:

A team led by Miguel Montargès of KU Leuven in Belgium took the picture in Dec. 2019, shortly after the star began its unprecedented dimming. They were able to compare it to a “normal” picture of Betelgeuse taken 11 months earlier. The change in shape is striking.

What’s going on? The researchers aren’t sure why Betelgeuse looks so different, but they suspect the involvement of dust. Red supergiants like Betelgeuse create and eject vast amounts of dusty material, losing mass even before they explode as supernovas. The lopsided shape and dimming of Betelgeuse might be explained if a cloud of dust is partially blocking its disk. Indeed, VLT infrared observations of Betelgeuse at the same time reveal lots of dust around the star:


Above: Clouds of dust around Betelgeuse. Credit: ESO/P. Kervella/M. Montargès et al. [more]

Mystery solved? Not necessarily. “Our knowledge of red supergiants remains incomplete, and this is still a work in progress, so a surprise can still happen,” notes Montargès. Other possibilities include magnetic activity on Betelgeuse’s surface (such as a giant starspot) and, of course, the early stages of a supernova explosion.

The Very Large Telescope with its adaptive optics instruments is one of the few facilities in the world capable of imaging the surface of Betelgeuse, located more than 600 light years away. More images from the Atacama desert may yet reveal what’s happening–if Betelgeuse doesn’t tell us first!

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The Problem with Satellite Mega-Constellations

Feb. 14, 2020: The night sky is in danger. This has been true for years as urban landscapes became increasingly light-polluted. But now there’s a new threat, one you can’t escape by driving into the countryside. It’s the “mega-constellation.” Some companies are planning to launch tens of thousands of internet satellites into low-Earth orbit. The recent launch by Space X of just 240 Starlink satellites has already ruined many astronomical observations.


Above: Astronomers at the Cerro Tololo Inter-American Observatory were trying to photograph nearby galaxies when 19 Starlink satellites intervened. [Full Story]

This week the International Astronomical Union (IAU) issued a new press release describing the impact of satellite mega-constellations on astronomy. IAU astronomers simulated 25,000 satellites similar in type to the satellites of SpaceX, Amazon, and OneWeb, and here are their results:

1. The number of satellites above the horizon at any given time would be between ~1500 and a few thousand. Most will appear very close to the horizon, with only a relative few passing directly overhead.

2. When the sun is 18 degrees below the horizon–that is, when the night becomes dark–the number of illuminated satellites above the horizon would be around 1000. These numbers will decrease during the hours around midnight when many satellites fall into Earth’s shadow.

3. At the moment it is difficult to predict how many of the illuminated satellites will be visible to the naked eye because of uncertainties in their reflectivity. Probably, the vast majority will be too faint to see. This depends to some degree on experiments such as those being carried out by SpaceX to reduce the reflectivity of their satellites with different coatings.

Trails made by Starlink satellites

Above: Starlink satellites photobomb the NGC 5353/4 galaxy group at Lowell Observatory [more]

4. Even if most satellites are invisible to the naked eye, mega-constellations pose a serious problem for professional astronomy. The trails of these satellites are bright enough to saturate modern detectors on large telescopes, and wide-field scientific astronomical observations will be severely affected.

5. The Vera C. Rubin Observatory currently under construction in Chile will be particularly hard-hit. The innovative observatory will scan large swaths of the sky, looking for near-Earth asteroids, studying dark energy, and much more. According to the IAU, up to 30% of the 30-second images during twilight hours will be affected. In theory, the effects of the new satellites could be mitigated by accurately predicting their orbits and interrupting observations, when necessary, during their passage, but this is a burdensome procedure.

There are no international rules governing the brightness of orbiting manmade objects. Until now, they didn’t seem to be necessary. Mega-constellations, however, threaten “the uncontaminated view of the night sky from dark places, which should be considered a non-renounceable world human heritage,” says the press release. Therefore the IAU will present its findings at meetings of the UN Committee for Peaceful Uses of Outer Space, bringing the attention of this problem to world leaders.

Surfing the Jet Stream Reduces Aviation Radiation

Feb. 12, 2020: Something strange is happening in the North Atlantic. For the past three years, airplanes have been flying across the ocean in record time. Credit the jet stream. It’s revved up, possibly by climate change, and planes that surf it are flying faster than ever before. The latest record was set just days ago. On Feb. 8, 2020, British Airways Flight 112 (BA 112) rocketed from New York to London in a mere 4 hours 56 minutes, at one point traveling faster than 825 mph.

This is good news, because airplanes surfing the jet stream absorb significantly less cosmic radiation.


(Top) The flight path of British Airways 112 and (Bottom) The transatlantic jet stream on Feb. 8, 2020.  Credit:

Researchers have long known that air travelers are exposed to cosmic rays. At typical cruising altitudes, passengers absorb 50 to 100 times more radiation than they would at sea level. This has led the International Commission on Radiological Protection (ICRP) to classify pilots and flight attendants as occupational radiation workers.

The jet stream can reduce this exposure. By propelling the passengers of BA 112 across the Atlantic at record speed, the jet stream lowered their radiation dose by about 30%. Two Virgin Atlantic flights following close behind the British Airways Boeing 747 experienced similar reductions.

These conclusions are based on E-RAD, a new model for aviation radiation. Since 2015, we ( and the students of Earth to Sky Calculus) have been collecting X-ray, gamma-ray, and neutron radiation data onboard airplanes. Our database contains more than 25,000 radiation measurements over 27 countries, 5 continents, and 2 oceans. E-RAD uses these measurements to predict dose rates on flights anywhere in the world.


British Airways flies from New York to London every day. We applied E-RAD to BA 112 on several dates, comparing dosages on Feb. 8th, when the plane surfed the jet stream, to nearby dates when it didn’t. Surfing the jet stream shaved almost 10 uSv (microSieverts) off the total radiation dose, a reduction equivalent to about 1 dental X-ray.

It’s not all good news, though. The jet stream can cause trouble. An active, fast-moving jet stream is often filled with turbulence, making flights miserable for buckled-in passengers. Planes dodging the rough air can actually increase their flight times, boosting cosmic ray exposure instead of reducing it.

Oh, and did you want to go home? Passengers returning to New York from London have to cross the Atlantic against the jet stream. Their flights will be slower, increasing exposure time. Indeed, we calculated the radiation exposure for British Airways flight 177 on Feb 8th, which flew in the opposite direction, from London to New York. Passengers onboard that aircraft received double the dosage: 34.4 uSv (London to New York) instead of the 17.7 uSv (New York to London) received by passengers on the BA 112 flight earlier in the day.

Climate change research suggests that all of these effects will intensify in the years ahead. A seminal study in 2016 found that changes in atmospheric dynamics would increase round-trip times between London and New York despite the quickening jet stream. Unless you fly to London and remain there, you’re going to absorb more and more “rads on a plane.”

End Notes:

(1) The radiation dosages mentioned in this story are not dangerous. Typically, they amount to 2 or 3 dental X-rays spread out over hours instead of the quick, intense pulse you receive in a dentist’s office. A single flight is not going to kill you. For frequent flyers and flight crews, however, repeated exposure adds up over time and may pose health risks that are still poorly understood.

(2) Transatlantic flights have been breaking records for the past three years. In Jan. 2018, a Norwegian passenger jet set a record when it went 779 mph during a trip from New York to London. In Feb. 2019, Virgin Atlantic went even faster: 801 mph from Los Angeles to London. Then, on Feb. 8, 2020, a British Airways Flight 112 shattered those records: 825 mph from New York to London.

(3) A key article on this topic is Transatlantic flight times and climate change. Environmental Research Letters, 2016; 11 (2): 024008 DOI: 10.1088/1748-9326/11/2/024008

According to the study, led by Dr Paul Williams, an atmospheric scientist at the University of Reading, “the average jet-stream winds along the flight route between London’s Heathrow airport and New York’s John F. Kennedy International airport are predicted to become 15% faster in winter, increasing from 77 to 89 km/hr (48 to 55 mph), with similar increases in the other seasons. As a result, London-bound flights will become twice as likely to take under 5h 20m, implying that record-breaking crossing times will occur with increasing frequency in future. On the other hand, New York-bound flights will become twice as likely to take over 7h 00m, suggesting that delayed arrivals will become increasingly common.”


Prediction: Betelgeuse Could Bounce Back

Feb. 9, 2020: For months, astronomers have been keeping a wary eye on Betelgeuse, the bright red star in Orion’s shoulder. What’s attracting their attention? All of a sudden, Betelgeuse isn’t bright anymore. Its visible luminosity has “fallen off a cliff”–a sign that the star could be on the verge of going supernova.

“The most recent measurements put the visual magnitude of Betelgeuse at about +1.66, the dimmest its been in our 25 years of photometry,” says Edward Guinan of Villanova University.

Above: The horizontal axis is Heliocentric Julian Date (HJD). For reference, Jan. 30, 2020, the date of the most recent measurement, has an HJD of 2458879.

Betelgeuse is a highly evolved red supergiant–the type of star that could collapse and explode at any moment. Indeed, the dimming of Betelgeuse could be explained if the star has suddenly contracted to about 92% of its previous radius. But that’s not the only possibility. Betelgeuse might be dimmed by a giant starspot–or maybe it is shrouded by an outburst of stardust from its own cool outer layers–or something else entirely. No one knows.

Answers might be forthcoming on Feb. 21st. Astronomers have long known that Betelgeuse is a variable star. It pulsates with many periods, as shown in this Fourier analysis of Betelgeuse’s light curve:

Above: A period analysis of 23 years (1995-2018) of Betelgeuse photometry. Credit: Peranso.

“This shows a dominant (probable pulsation) period of P = 430 days,” note Guinan and colleague Richard Wasatonic in a recent Astronomical Telegram. Given this result, “the minimum brightness is expected on 21 (+/-7d) February 2020.”

If Betelegeuse starts to bounce back on Feb. 21st, this whole episode might just be a deeper-than-average pulsation, and perhaps the supernova watch can be called off. However, notes Guinan, “even if the 430-day period is still working, this would indicate a minimum brightness near 0.9 mag–much brighter than the current value near 1.6 mag. So something very unusual is going on.”

Stay tuned for updates as Feb. 21st approaches.

High-altitude Balloon Photos of Polar Stratospheric Clouds

Feb. 4, 2020: On Jan. 22, 2020, something lucky happened. and the students of Earth to Sky Calculus were inside the Arctic Circle, preparing to launch a cosmic ray balloon. Moments before liftoff, an outbreak of Type 2 polar stratospheric clouds (PSCs) started developing over the launch site. PSCs are very rare. They form in the stratosphere only when the air temperature drops to a staggeringly-cold -85C. And they are extremely difficult to catch. Working quickly, we launched two balloons directly into the outbreak.


Operating 4 cameras, the two balloon payloads photographed the clouds from altitudes as high as 75,000 feet. We believe this is the first time polar stratospheric clouds have been photographed by a high-altitude balloon from their own habitat–the stratosphere. The footage reveals beautiful filamentary structures previously unseen from the ground.

How did we get so lucky? We had some help from the polar stratospheric vortex.

The polar stratospheric vortex is a jet stream in the Arctic stratosphere. This winter it has been very strong, bottling up cold air and preventing it from spilling to lower latitudes.  Just before we launched on Jan. 22nd, something unexpected happened to the polar vortex. It became elliptical and rotated around, sloshing a mass of super-cold stratospheric air over northern Scandinavia.

This is what the vortex looked like on Jan. 22-23, according to NASA’s Microwave Limb Sounder (MLS):


Note the purple blob over northern Sweden. That’s the cold air. PSCs can form inside the white contours. This animation of MLS data created by Lynn Harvey of the University of Colorado’s Laboratory for Atmospheric and Space Physics shows how the vortex evolved during our time in Sweden.

Type 2 polar stratospheric clouds are widely regarded as the most beautiful clouds on Earth. They are made of tiny ice crystals that diffract high-altitude sunlight, glowing with colors so vivid that some people mistake them for “daytime auroras.” Earth to Sky student Jordan Herbst’s photos of the Jan. 22-23 outbreak show how they look from the ground.  Now we know they’re beautiful from the stratosphere, too.

Stay tuned for more news from our trip to the Arctic Circle.