Rads on a Plane

by Dr. Tony Phillips (This article originally appeared on Spaceweather.com)

05 Nov. 2015: Spaceweather.com and the students of Earth to Sky Calculus regularly fly helium balloons to the stratosphere to measure cosmic rays. For the past six months, May through Oct. 2015, they have been taking their radiation sensors onboard commercial airplanes, too. The chart below summarizes their measurements on 18 different airplanes flying back and forth across the continental United States.

The points on the graph indicate the dose rate of cosmic rays inside the airplanes compared to sea level. For instance, the dose rate for flights that cruised at 40,000+ feet was more than 50 times higher than the dose rate on the ground below. No wonder the International Commission on Radiological Protection (ICRP) classifies pilots as occupational radiation workers.

Cosmic rays come from deep space. They are high energy particles accelerated toward Earth by distant explosions such as supernovas and colliding neutron stars. Astronauts aren’t the only ones who have to think about them; flyers do, too. Cosmic rays penetrate deep inside Earth’s atmosphere where airplanes travel every day.

Our radiation sensors detect X-rays and gamma-rays in the energy range 10 keV to 20 MeV. These energies span the range of medical X-ray machines and airport security scanners.

Cosmic Rays are modulated by solar activity. Solar storms and CMEs tend to sweep aside cosmic rays, making it more difficult for cosmic rays to reach Earth. Low solar activity, on the other hand, allows an extra dose of cosmic rays to reach our planet. This is important because forecasters expect solar activity to drop sharply in the years ahead as we approach a new Solar Minimum. Cosmic rays are poised to increase accordingly.

The plot, above, tells us what is “normal” in 2015. How will it change as the solar cycle wanes? Stay tuned for regular updates.

Space Weather Ballooning — results from Oct. 11, 2015

by Dr. Tony Phillips (this article originally appeared on Spaceweather.com)

Approximately once a week, Spaceweather.com and the students of Earth to Sky Calculus fly “space weather balloons” to the stratosphere over California. These balloons are equipped with radiation sensors that detect cosmic rays, a surprisingly “down to Earth” form of space weather. Cosmic rays can seed clouds, trigger lightning, and penetrate commercial airplanes. Our measurements show that someone flying back and forth across the continental USA, just once, can absorb as much ionizing radiation as 2 to 5 dental X-rays. Here is the data from our latest flight, Oct. 11th:

Radiation levels peak at the entrance to the stratosphere in a broad region called the “Pfotzer Maximum.” This peak is named after physicist George Pfotzer who discovered it using balloons and Geiger tubes in the 1930s. Radiation levels there are more than 80x sea level.

Note that the bottom of the Pfotzer Maximim is near 55,000 ft. This means that some high-flying aircraft are not far from the zone of maximum radiation. Indeed, according to the Oct 11th measurements, a plane flying at 45,000 feet is exposed to 2.77 uSv/hr. At that rate, a passenger would absorb about one dental X-ray’s worth of radiation in about 5 hours.

The radiation sensors onboard our helium balloons detect X-rays and gamma-rays in the energy range 10 keV to 20 MeV. These energies span the range of medical X-ray machines and airport security scanners.

Oct. 11, 2015, Balloon Flight Photo Gallery

Hey thanks! The cosmic ray research described above is 100% crowd-funded. Our Oct. 11th balloon flight was made possible by a generous donation of $500 from Spaceweather.com reader Vicki Brown. To say thanks, we flew Vicki’s parents, Betty and Earl, to the edge of space:

“I am so happy to help the young scientists, and it is cool to see my folks in the stratosphere!” says Vicki.

Readers, have you ever wanted to send a loved one to the stratosphere? You can make it happen by sponsoring a cosmic ray research flight. Contact Dr. Tony Phillips for details.

Space Weather Ballooning — Results from the Lunar Eclipse

by Dr. Tony Phillips

27 Sept. 2015: Once a week, and sometimes more often, Spaceweather.com and the students of Earth to Sky Calculus fly “space weather balloons” to the stratosphere. These balloons are equipped with radiation sensors that detect cosmic rays, a form of space weather important to people on Earth. Cosmic rays can alter the chemistry of the upper atmosphere, seed clouds, spark exotic forms of lightning, and penetrate commercial airplanes. This last point is of special interest to the traveling public. Our measurements show that someone flying back and forth across the continental USA, just once, can absorb as much ionizing radiation as 2 to 5 dental X-rays.

Here is an example of our data from a typical balloon flight:

This radiation profile was obtained on the evening of Sept. 27, 2015–incidentally, during a total eclipse of the Moon.  The altitude of the balloon is on the horizontal axis, radiation dose rates are on the vertical axis. Inset photos show scenes from the mission.

Radiation levels peak at the entrance to the stratosphere in a broad region called the “Pfotzer Maximum.” This peak is named after physicist George Pfotzer who discovered it using balloons and Geiger tubes in the 1930s. Radiation levels there are nearly 100x sea level.

Note that the bottom of the Pfotzer Maximim is near 55,000 ft. This means that some high-flying aircraft are not far from the zone of maximum radiation. Indeed, according to the Sept. 27th measurements, a plane flying at 45,000 feet is exposed to 288 uRads/hr. At that rate, a passenger would absorb about one dental X-ray’s worth of radiation in 5 hours.

The radiation sensors onboard our helium balloons detect X-rays and gamma-rays in the energy range 10 keV to 20 MeV. These energies span the range of medical X-ray machines and airport security scanners.

Rads on a Plane: Hot Seats in First Class

by Dr. Tony Phillips (Spaceweather.com)

July 30, 2015 — Many people think that only astronauts have to worry about cosmic radiation. Not so. Regular air travelers are exposed to cosmic rays, too. This week, Spaceweather.com’s Dr. Tony Phillips and the students of Earth to Sky Calculus flew across the United States to conduct a transcontinental launch of space weather balloons. They took radiation sensors on board the plane to find out how many cosmic rays they would absorb during the flight. Here are the data they collected flying east:

Radiation levels in the cabin of the Airbus 319 (Spirit Airlines flight 640) tripled within ten minutes after takeoff, and were nearly 30 times ground level by the time the plane reached cruising altitude at 39,300 feet. Summing over the entire flight, the sensors measured about 1 mrem of radiation–similar to a dental x-ray.

There was no solar storm in progress. The extra radiation was just a regular drizzle of cosmic rays reaching down to aviation altitudes. This radiation is ever-present and comes from supernovas, black holes, and other sources across the galaxy.

The Earth to Sky team consisted of five people who sat in three different locations: First Class, over the wings, and in the back row. Would they all absorb the same dose? No. On this particular flight, dose rates were highest in First Class and lowest near the toilets in the rear. The front-to-back ratio was as high as 13%. This gradient is not understood; presumably, it has to do with the way cosmic rays interact with the plane’s fuselage and fuel tanks.

Five days later, following a successful Transcontinental Balloon Launch, the team flew back to the west coast. Once again they flew on an Airbus 319 (Spirit Airlines flight 641), non-stop from Boston to Las Vegas. The results were similar:

As before, the First Class seats registered the highest dose of radiation–as much as 6% higher than the wings and rear of the plane. On this flight we added a second radiation sensor to First Class to confirm the effect. Both sensors agreed: ionizing radiation was slightly higher in the front of the plane.

Because cosmic rays come from space, radiation inside the airplane grows stronger as the airplane ascends. This plot shows how the dose rate changed as a function of altitude throughout the July 23rd flight:

Note how radiation levels remain low at altitudes below ~15,000 ft. Earth’s atmosphere does a good job shielding those altitudes from cosmic rays. Above 15,000 ft, however, dose rates climb rapidly as the plane ascends.

The radiation sensors are the same ones that Earth to Sky Calculus routinely flies onboard helium balloons to measure cosmic rays in the stratosphere. They detect X-rays and gamma-rays in the energy range 10 keV to 20 MeV. These energies span the range of medical X-ray machines and airport security scanners.

Growing Peril for Astronauts?

 NASA’s successful test flight of Orion on Dec. 5th heralds a renewed capability to send astronauts into deep space. A paper just published in the journal Space Weather, however, points out a growing peril to future deep space explorers: cosmic rays. The title of the article, penned by Nathan Schwadron of the University of New Hampshire and colleagues from seven other institutions, asks the provocative question, “Does the worsening galactic cosmic ray environment preclude manned deep space exploration?” Using data from a cosmic ray telescope onboard NASA’s Lunar Reconnaissance Orbiter, they conclude that while increasing fluxes of cosmic rays “are not a show stopper for long duration missions (e.g., to the Moon, an asteroid, or Mars), galactic cosmic radiation remains a significant and worsening factor that limits mission durations.” This figure from their paper shows the number of days a 30 year old astronaut can spend in interplanetary space before they reach their career limit in radiation exposure:

According to the plot, in the year 2014, a 30 year old male flying in a spaceship with 10 g/cm2 of aluminum shielding could spend approximately 700 days in deep space before they reach their radiation dose limit. The same astronaut in the early 1990s could have spent 1000 days in space.

What’s going on? Cosmic rays are intensifying. Galactic cosmic rays are a mixture of high-energy photons and subatomic particles accelerated to near-light speed by violent events such as supernova explosions. Astronauts are protected from cosmic rays in part by the sun: solar magnetic fields and the solar wind combine to create a porous ‘shield’ that fends off energetic particles from outside the solar system. The problem is, as the authors note, “The sun and its solar wind are currently exhibiting extremely low densities and magnetic field strengths, representing states that have never been observed during the Space Age. As a result of the remarkably weak solar activity, we have also observed the highest fluxes of cosmic rays in the Space Age.”

The shielding action of the sun is strongest during solar maximum and weakest during solar minimum–hence the 11-year rhythm of the mission duration plot. At the moment we are experiencing Solar Max, which should be a good time for astronauts to fly–but it’s not a good time. The solar maximum of 2011-2014 is the weakest in a century, allowing unusual numbers of cosmic rays to penetrate the solar system.

This situation could become even worse if, as some researchers suspect, the sun is entering a long-term phase of the solar cycle characterized by relatively weak maxima and deep, extended minima. In such a future, feeble solar magnetic fields would do an extra-poor job keeping cosmic rays at bay, further reducing the number of days astronauts can travel far from Earth.

To learn more about this interesting research, read the complete article in the online edition of Space Weather.