Van Allen belts

Moon-landing deniers say the Van Allen radiation belts would have killed the Apollo crews, and they say the man who found the belts, James A. Van Allen, is the one who proved it. He answered that claim in his own handwriting. The email he was confirming is next. The 1959 and 1961 magazine lines people quote are further down, with the clocks and the doses still attached.

The thorough account of the radiation claims, shielding, trajectory, dosimeters, and the arguments around them, is Jay Windley's Clavius page on the belts. This article stays with what Van Allen himself wrote.

What he wrote in his own hand

James A. Van Allen's handwritten note on Jay Windley's letter, 27 July 2004. The street address is blacked out on the public scan.

On 27 July 2004 Jay Windley, who ran the Clavius site, wrote to Van Allen. Windley quoted a sentence Van Allen had already sent to someone else, about a Fox television special: "The recent Fox TV show, which I saw, is an ingenious and entertaining assemblage of nonsense. The claim that radiation exposure during the Apollo missions would have been fatal to the astronauts is only one example of such nonsense." He asked Van Allen to confirm it.

Van Allen wrote on the letter: "This is a completely correct quotation from my letter. James A. Van Allen."

The handwriting covers that one sentence. The fifteen minutes in the belts, and the "less than 1 percent of a fatal dosage," are in the letter he was confirming.

The letter he confirmed

On 5 March 2003 Doug Lambert posted that letter, an email Van Allen had sent him, on page 1 of a CosmoQuest thread. Van Allen said he was forwarding a reply he had written to another inquiry about two months earlier. In his wording:

• The belts "do, indeed, pose important constraints on the safety of human space flight."
• The dangerous part is the very energetic protons in the inner belt, "tens to hundreds of MeV," and they are "the most difficult to shield against." Flights of many months have to stay below about 250 miles.
• A person in a shuttle, in a circular equatorial orbit at about 1,000 miles, in the most intense part of the inner belt, "would be subjected to a fatal dosage of radiation in about one week."
• Apollo's outbound and inbound paths "cut through the outer portions of the inner belt and because of their high speed spent only about 15 minutes in traversing the region and less than 2 hours in traversing the much less penetrating radiation in the outer radiation belt." The round trip, he wrote, "was less than 1% of a fatal dosage — a very minor risk among the far greater other risks of such flights." He had made those estimates "in the early 1960s and so informed NASA engineers who were planning the Apollo flights. These estimates are still reliable."
• "The recent Fox TV show, which I saw, is an ingenious and entertaining assemblage of nonsense. The claim that radiation exposure during the Apollo missions would have been fatal to the astronauts is only one example of such nonsense."

That last sentence is the one he later initialed for Windley. He dated the estimates to the early 1960s, the same years as the Space World article below, and he told NASA's Apollo planners the same thing. A week parked in the heart of the inner belt could be fatal. A fast pass along the outer edge of it was a minor risk among the other risks of the flight.

The magazine articles are what make this letter necessary. They are quoted as if Van Allen had declared every crossing fatal. Read with the sentences that follow the famous lines, they are the estimates he said were still reliable.

Where the "fatal dose" line comes from

Scientific American, March 1959.

In March 1959 Scientific American published Van Allen's "Radiation Belts around the Earth." Satellites and lunar probes, he wrote, had shown that Earth is encircled by two zones of high-energy particles. The opening paragraph, on page 39, says the finding is "troubling to astronauts" and that "somehow the human body will have to be shielded from this radiation, even on a rapid transit through the region."

That sentence is the one most often cut out and held up alone. The article goes on to say where the zones are and where they are not. They do not cover the planet like a skin. The closing paragraph, on page 47, says manned rockets "can best take off through the radiation-free zone over the poles" if no practical shield is available, and that the Moon "probably has no belt, because its magnetic field appears to be feeble."

Scientific American, March 1959, p. 47. Ten to 100 roentgens an hour, a two-day exposure, and a path over the poles.

The number quoted from that closing paragraph is the hourly rate. "Our measurements show that the maximum radiation level as of 1958 is equivalent to between 10 and 100 roentgens per hour, depending on the still-undetermined proportion of protons to electrons." The next sentence sets the clock. "Since a human being exposed for two days to even 10 roentgens would have only an even chance of survival, the radiation belts obviously present an obstacle to space flight."

Two days at 10 roentgens an hour is 480 roentgens. Van Allen's comparison is a stay measured in days, at the low end of the peak rate he had just given. He calls the belts an obstacle. The paragraph then gives the practical answer he had: shield the travelers, or leave through the radiation-free zone over the poles.

Where "steer clear" comes from

In December 1961 Space World ran "The Danger Zone: Earth is wrapped in deadly belts of radiation." On page 23 Van Allen wrote that the trapped particles were "of overwhelming biological importance for manned space flight." On page 49, behind about the shielding a lightly built spacecraft would have, he reported exposure "of some 20 r/hr" in the heart of the inner zone, and several times that in the heart of the outer zone. Page 53 puts the heart of the inner zone at "about 20 r/hr within a shield of 1 g/cm² of iron." Because those protons penetrate, he wrote, "effective shielding is quite beyond engineering feasibility in the near future. Hence, the inner zone must be classed as an uninhabitable region of space as far as man is concerned."

Space World, December 1961, p. 53. The inner zone called uninhabitable, and a prompt escape priced at about 10 roentgens.

"Uninhabitable" on that page means a place where a person should not remain. The next column is about leaving. Using Pioneer IV as the example, Van Allen wrote that the trapping region "can be completely traversed in about 6 hours with some 2 hours being spent in the high-intensity region. Hence the cumulative dosage on a trajectory of this nature would be of the order of 10 r in a lightly shielded craft (1 g/cm²)." He added that it "appears wise to evade the inner zone, and this is not difficult to do, since its latitudinal and radial extent is limited." The outer zone, he wrote, is harder to miss, but there are "cones of escape" over the north and south geomagnetic poles, opening outward with a half-angle of about 20 degrees.

The "steer clear" line is on page 54, after a different comparison. Vanguard I and Sputnik III had kept working for about two years. Van Allen treated that as evidence that electronic equipment can survive in space, and he wrote that the integrated exposure in those two cases was still below the point at which the hardware would seriously deteriorate. Then: "But, though mechanical and electronic equipment can operate within the high radiation areas, a living organism cannot survive this level of radiation damage. Hence, all manned space flight attempts must steer clear of these two belts of radiation until adequate means of safeguarding the astronauts has been developed."

"This level" is the exposure of machines that stayed up for years. The same article has already priced a prompt escape at about 10 roentgens in a lightly shielded craft, and has already said the inner zone can be dodged because it does not reach every latitude. Ten roentgens against the 1959 even-chance case, 480 roentgens over two days, is about 2 percent of the exposure Van Allen treated as a coin flip. Shielding, trajectory, and time are the safeguarding the last sentence says had to be developed. His 2003 letter is that same distinction applied to Apollo: a week in the heart of the inner belt on one line, fifteen minutes through the outer portion of it on the next.

Documents

• Handwritten confirmation on Jay Windley's letter of 27 July 2004, p. 1.
• Van Allen's emailed statement, posted by Doug Lambert on CosmoQuest, 5 March 2003, thread p. 1.
• "Radiation Belts around the Earth", Scientific American, March 1959. Opening quotes: p. 39. Dose, two-day clock, and the polar zone: p. 47. Scan: PDF.
• "The Danger Zone: Earth is wrapped in deadly belts of radiation", Space World, December 1961. Biological importance: p. 23. Heart-of-the-belt rates: p. 49. Uninhabitable inner zone, the 10-roentgen crossing, and the cones of escape: p. 53. "Steer clear": p. 54.

See also

• Moon landings