Van Allen belts

Scientific American, March 1959.

Moon-landing deniers treat the Van Allen radiation belts as a wall. The usual proof is two magazine articles by James A. Van Allen, the physicist whose instruments found the belts, and one sentence from the second article: manned flights must "steer clear" until astronauts can be safeguarded. The same pages give the dose, the time, and a way through. A letter Van Allen wrote four decades later, and then confirmed in his own handwriting, uses those numbers. It does not throw them out.

Two zones, not a solid shell

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 is the sentence most often cut out and held up on its own. The rest of the article is about what the zones are and where they are not. The inner zone and the outer zone do not cover the planet like a skin. Van Allen's 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."

The dose he was talking about

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

The number deniers quote is also in that closing paragraph, on page 47. "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 is the one that fixes 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. He does not say a crossing of minutes or hours delivers that two-day dose.

What "steer clear" is attached to

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" here means a place where a person should not remain. The next column on the same page 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 famous last 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 he had just assigned to machines that stayed up for years, not the 10-roentgen crossing he had just calculated for a prompt escape. The article says both things on purpose. Do not live in the heart of the inner belt. A fast exit, in a lightly shielded craft, is a dose on the order of 10 roentgens, and the inner zone can be dodged because it does not reach every latitude.

Ten roentgens is not the two-day figure from 1959. Set against that article's own even-chance case, 480 roentgens over two days, a 10-roentgen crossing is about 2 percent of the exposure Van Allen treated as a coin flip.

The letter, and the handwriting

On 5 March 2003 Doug Lambert posted, on page 1 of a CosmoQuest thread, an email Van Allen had sent him. Van Allen said he was forwarding a reply he had written to another inquiry about two months earlier. The points, 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 Jay Windley, who ran the Clavius site, asked Van Allen to authenticate. Windley's letter is dated 27 July 2004. It quotes the Fox line and asks for confirmation. Van Allen wrote on that one-page letter, in his own hand: "This is a completely correct quotation from my letter. James A. Van Allen."

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.

The handwriting covers one sentence, the Fox sentence. It does not restate the fifteen minutes or the 1 percent. Those figures are in the email Lambert published, which is the letter Van Allen told Windley he was confirming. He also wrote that the estimates were the ones he had given NASA in the early 1960s, the same years as the Space World article, and that he still stood by them. The later letter is not a reversal of 1959 or 1961. It is the Apollo case of the distinction those articles already drew: a week in the heart of the inner belt can be fatal; a fast pass through the outer edge of it is not.

What the claim needs the reader to skip

The denial version needs four omissions.

The 1959 "10 to 100 roentgens per hour" has to be quoted without the two-day clock Van Allen put in the next sentence, and without the radiation-free zone over the poles in the sentence after that.

The 1961 word "uninhabitable" has to be pulled off the heart of the inner zone, behind a thin iron shield, and applied to every path out of Earth orbit. The same column already prices a Pioneer IV-style escape at about 10 roentgens and says evading the inner zone is not difficult.

"Steer clear" has to be cut loose from the two-year satellite exposures it follows, and from the safeguarding Van Allen said had to be developed. Shielding, trajectory, and time are the safeguarding. He did not write that no safeguard would ever exist.

The 2003 email and the 2004 handwriting have to be waved off as a man talked into calling his own work nonsense. The nonsense he named was the Fox special's claim that Apollo's exposure would have been fatal. He did not withdraw the belts, the proton hazard, or the one-week fatal case for a ship parked in the inner zone.

Documents

• "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.
• Van Allen's emailed statement, posted by Doug Lambert on CosmoQuest, 5 March 2003, thread p. 1.
• Handwritten confirmation on Jay Windley's letter of 27 July 2004, p. 1.