July 25, 2018: Friday, July 27th, is a big night for astronomy. Three reasons: First, Mars will be at opposition–directly opposite the sun and making a 15-year close approach to Earth. Second, Mars and the full Moon will be in conjunction–less than 10 degrees apart. Third, the Moon will pass through the shadow of Earth, producing the longest lunar eclipse in a century.

Graphic artist Larry Koehn of ShadowandSubstance.com created this montage of the eclipse:

An animation of the eclipse may be found here.

Almost everyone on Earth (except North Americans) can see the eclipse as the sunset-colored shadow of our planet swallows the Moon for almost 2 hours. During totality, the Moon will turn almost the same red color as Mars right beside it–an incredible sight.

Because Mars is opposite the sun, it will rise at sunset and stay up all night long.  The best time to look is around midnight when the Moon-Mars pair will be at their highest in the sky. The Red Planet will have no trouble being seen through the glare of the full Moon because Mars itself is so luminous–almost three times brighter than Sirius, the brightest star in the sky.

People in North America will not be able to see the eclipse. The shadow play happens mostly on the opposite side of the world. They can, however, witness the conjunction. Swinging a backyard telescope between the Moon and Mars in quick succession will reveal the dusty-red martian disk alongside lunar mountains and craters. It’s a special night. Enjoy the show!

July 17, 2018: As July 17th comes to a close, the sun has been without spots for 21 straight days. To find an equal stretch of spotless suns in the historical record, you have to go back to July-August 2009 when the sun was emerging from a century-class solar minimum. This is a sign that the sun is entering another solar minimum, possibly as deep as the last one.

Solar minimum is a normal part of the solar cycle. Every 11 years or so, sunspot production sputters. Dark cores that produce solar flares and CMEs vanish from the solar disk, leaving the sun blank for long stretches of time. These quiet spells have been coming with regularity since the sunspot cycle was discovered in 1859.

However, not all solar minima are alike. The last one in 2008-2009 surprised observers with its depth and side-effects. Sunspot counts dropped to a 100-year low; the sun dimmed by 0.1%; Earth’s upper atmosphere collapsed, allowing space junk to accumulate; and the pressure of the solar wind flagged while cosmic rays (normally repelled by solar wind) surged to Space Age highs. These events upended the orthodox picture of solar minimum as “uneventful.”

Space weather forecasters have been wondering, will the upcoming solar minimum (2018-2020) be as deep as the previous one (2008-2009)? A 21-day stretch of blank suns is not enough to answer that question. During the solar minimum of 2008-2009, the longest unbroken interval of spotlessness was ~52 days, adding to a total of 813 intermittent spotless days observed throughout the multi-year minimum. The corresponding totals now are 21 days and 244 days, respectively. If this solar minimum is like the last one, we still have a long way to go.

How does this affect us on Earth? Contrary to popular belief, auroras do not vanish during solar minimum. Instead, they retreat to polar regions and may change color. Arctic sky watchers can still count on good displays this autumn and winter as streams of solar wind buffet Earth’s magnetic field. The biggest change brought by solar minimum may be cosmic rays. High energy particles from deep space penetrate the inner solar system with greater ease during periods of low solar activity. NASA spacecraft and space weather balloons are already detecting an increase in radiation. Cosmic rays alter the flow of electricity through Earth’s atmosphere, trigger lightning, potentially alter cloud cover, and dose commercial air travelers with extra “rads on a plane.”

At the moment there are no nascent sunspots on the solar disk, so the spotless days counter is likely to keep ticking. Stay tuned for more blank suns and … welcome to solar minimum.

July 9, 2018: Last month, flight attendants got some bad news. According to a new study from researchers at Harvard University, the crews of commercial airlines face an elevated risk of cancer compared to members of the general population. The likely reason: cosmic rays. High energy particles from space hitting the top of Earth’s atmosphere create a spray of secondary radiation that penetrates the walls of airplanes above ~20,000 feet.

We have some new data pertinent to this topic. On June 19th, Spaceweather.com and students of Earth to Sky Calculus flew from California to New Zealand to launch a series of space weather balloons. Naturally, we took our radiation sensors onboard the aircraft. Here is what we measured:

Within minutes after takeoff from Los Angeles, radiation in the passenger compartment multiplied 25-fold and remained high until we landed again in Brisbane 13 hours later. Peak dose rates were almost 40 times greater than on the ground below. In total, we absorbed a whole body dose approximately equal to a panoramic dental X-ray.

Our sensors measure three types of radiation: neutrons, X-rays and gamma-rays. Using bubble chambers, we found that about 1/3rd of our exposure came from neutrons.

Each bubble pictured above is formed by an energetic neutron (200 keV – 15 MeV) passing through the chamber. Counting bubbles yields the total dose–about 8 uGy (micro-greys) of neutrons during the entire flight.

The remaining 2/3rd of our measured exposure came from X-rays and gamma-rays (10 keV to 20 MeV). We detected those forms of radiation using sensors based on Geiger tubes:

Adding it all together, we detected about 24.3 uGy of neutrons + X-rays + gamma rays during the Los Angeles to Brisbane leg of our flight. For comparison, a panoramic dental X-ray yields between 14 uGy and 24 uGy.

Now for more bad news. Cosmic rays at aviation altitudes are a cocktail of different things: e.g., neutrons, protons, pions, electrons, X-rays, and gamma rays spanning a wide range of energies. Our sensors sample only three ingredients of that cocktail (neutrons, X-rays, gamma-rays) at relatively low energies typical of medical X-rays and airport security devices. This means our data are only the tip of the iceberg. Flight crews and passengers absorb even more radiation than we can detect.

We have taken these sensors on airplanes before, many times. Last year, we took another long international flight, but instead of crossing the equator, we crossed the Arctic Circle. During a polar flight from Los Angeles to Stockholm in March 2017, our sensors detected 43.6 uGy of radiation, almost twice the 24.3 uGy we measured en route to Brisbane in June 2018. This difference is well understood: Earth’s polar magnetic field provides less shielding against cosmic rays, so we expect polar flights to be more “radioactive.”

What we didn’t expect was the difference in neutrons:  During our Arctic flight, neutron radiation made up fully half of our dose. During our equatorial flight, neutrons amounted to only 1/3rd. In other words, the cocktail changed. These are potentially important differences because neutrons are a biologically effective form of radiation of keen interest to cancer researchers.

Stay tuned for updates as we continue to process our haul of data from 5 plane flights and 3 balloon flights in New Zealand.