This year, 2019, commemorates the 50th ceremony of i of humanity's nigh incredible achievements thus far: of people physically reaching the Moon. On July 20, 1969, Apollo 11 landed on the natural satellite and broadcast a live view of the lunar surface, Earth, space and of astronauts working on the surface.

Notwithstanding nonetheless, there are doubts these days that humans actually achieved this feat. For example, the following question appeared on Quora on Baronial 23, 2018: "When will the beingness of the Van Allen belt and our inability to penetrate its harsh radiations with today's technology force NASA to acknowledge it faked the moon landing?"

What surrounds Globe?

Some people believe we never went to the Moon because of the being of the Van Allen radiation belts. The thought is that whatsoever astronauts en route to outer space has to pass through these belts and, in and so doing, they would receive a lethal dose of radiations.

These belts are the outcome of Earth's magnetic field working like a magnetic trap – or an accelerator – confining the particles of the solar (more often than not) wind, and sometimes accelerating them on their orbits inside the belts to nearly the speed of light.

These belts are shaped like two nested doughnuts. Their sizes change depending on solar activity and, sometimes, on how nosotros model them. The inner belt starts at an altitude of 600-1,600 km, according to different sources, and extends till ix,600-13,000 km. The range of the outer belt varies similarly: from 13,500-19,000 km to about 40,000 km.

There is a gap between these belts chosen the slot region, which is mostly devoid of energetic particles. Sometimes, a third, transient appears in this region. Geosynchronous communications satellites orbit just inside the outer border of the outer radiations chugalug, and the low-Earth orbit (LEO), where the International Space Station and the Hubble infinite telescope are, is just below the inner edge of the inner belt.

The radiation belts are not the only structures surrounding World. Starting from most ane,000 km, at the very edge of the inner radiation chugalug, and partially pushing into the outer belt, there is a deject of charged particles called the plasmasphere. This is still another layer of protection Earth has against space radiations. Electrons nearly at lightspeed in the outer belt movement laterally along the magnetic field lines around Globe; a plasmasphere phenomenon called the plasmaspheric hiss – which is very low-frequency (VLF) electromagnetic waves – prevents them from moving down closer to Earth. This VLF hiss results in part from atmospheric lightning and in another function from ground-based transmitters communicating with submarines (Fig. 1).

Earth's plasmasphere in far-ultraviolet taken from the Moon by Apollo 16
Fig. 1: Globe'due south plasmasphere in far-ultraviolet taken from the Moon by Apollo 16

The electrons in the outer belt can motion into the slot region only when the Sun produces a strong solar storm or a coronal mass ejection. When they do that, the third chugalug appears.

The story of how these belts were discovered is every bit, if not more, interesting.

In the 1950s, as ideas for space exploration were picking up, nobody knew what the environment of the office of space surrounding Earth – i.east. near space – was similar. Many believed this region to exist more often than not a vacuum. However, scientists already knew about cosmic rays coming to Globe's temper from infinite, which they had studied with ground- and balloon-based instruments, and from rockets and rockoons (rockets launched from balloons at loftier altitudes).

In the 1950s' Usa, James A. Van Allen, so chairman of the Upper Atmosphere Rocket Inquiry Panel, launched hundreds of rockoons. He was investigating high-altitude areas well-nigh the magnetic poles. In 1953, he detected what they called 'soft' radiation – radiations that could be a mixture of charged particles and X-ray photons, chosen and then because information technology was hands captivated in the temper – in the auroral regions above sixty km.

"The 1953 expedition yielded a remarkable new finding, namely the first directly detection of the electrons which, nosotros surmised, were the primaries for producing auroral luminosity," Van Allen wrote. Then, the International Geophysical Year (IGY) of 1957-1958 was announced, and on October 4, 1957, the Soviet Marriage launched the first artificial satellite, Sputnik ane. Information technology showed for the outset time that orbital space is prophylactic – at least from meteoroids – for satellites.

Soviet scientists, led past Sergei Vernov at Moscow State University, weren't much behind the Americans. For many years, fifty-fifty from before World War II, they had been studying cosmic rays from the ground and from balloons, and even using captured German rockets designed by Wernher von Braun. In 1956, Vernov and his staff prepared their instruments for cosmic-ray measurements in space, set to fly on Sputnik 1. Yet, the launch was shrouded in secrecy, fifty-fifty from the eyes of Soviet scientists.

On November 3, 1957, the Soviet Union launched a more sophisticated spacecraft called Sputnik 2, which had equipment to actually measure the free energy of catholic rays. The two KS-v counters (Kosmicheskii Schetchik in Russian) consisted of SI-17 gas discharge tubes, commonly used in high-distance airship studies of cosmic rays. Sputnik 2 was launched into an elliptical orbit with a perigee of 225 km, apogee of i,671 km and a period of 103.75 minutes. It downlinked several sets of mission data: solar 10-ray data, cosmic-ray counts, biometric data on the dog Laika and temperature readings.

However, the X-ray team idea their experiment had failed because the photomultiplier tubes were mostly saturated past intense radiation independent of whichever way they were pointed in space. They realised only in hindsight that these were the effects of the radiation belts. The KS-5 information, obtained between November iii and ix, 1957, was of good quality. The increment of radiations was registered on November 7, when the satellite was at almost +60° latitude over Soviet territory (Fig. 2).

Fig. 2: Data from Sputnik 2 on November 7, 1957. The increased count rate was registered over the northern latitudes of the USSR. Photo: Logachev 2017
Fig. two: Information from Sputnik ii on November 7, 1957. The increased count rate was registered over the northern latitudes of the USSR. Photo: Logachev 2017

This was attributed to a weak solar flare that Vernov had reported in May 1958, in Russian and without explaining the source of the flux increment. The satellite had actually detected the infinite radiations, and the belts could have been called Vernov belts if not for the Cold War paranoia at that time.

The telemetry from Sputnik ii could only be received over territorial USSR, and the spacecraft had no recording device. The apogee of the orbit (at 1,671 km) occurred over the due south polar region, where information technology penetrated into the radiation belt, but that information was not received by Soviet stations. The Australian scientist Harry Messel recorded Sputnik 2 data when it passed over Australia, simply the Soviets refused to share the interpreting codes when he asked for them. And when the Soviets asked for the data from Messer, he refused.

As Fred Singer, a contemporary of Van Allen's, expressed in a letter to Alex Dressler (see appendix one), the director of the Infinite Science Laboratory at the Marshall Space Flight Centre, Alabama, "When they finally asked for the re-create of the recorded data, he told them to go to hell (equally only Harry Messel could)."

The US launched its kickoff satellite, Explorer 1, on January 31, 1958, with instruments to mensurate radiation. Just the cosmic ray detector did not annals any radiation in the parts where Van Allen thought information technology should be high, and then he figured the instruments were really saturated with particles trapped in Earth'south magnetic field. He reported this in May 1958 at the IGY lecture, attributing the particles to exist of auroral origin which had somehow leaked into the equatorial region.

Past March 1958, two months before, Explorer 3 had confirmed Explorer i'south data, but simply Sputnik 3 was able to determine the nature of these particles. This was because it also carried a scintillation counter – a xl × 40 mm sodium iodide crystal doped with thallium. It allowed scientists to decide that particles registered in polar regions were protons of ~100 Mev energy while those in the equatorial regions were electrons with energies ~100 Kev.

Explorer four confirmed the existence of two distinct belts; subsequently, it turned out that Sputniks two and 3 had discovered the outer belt – also chosen the well-nigh-polar belt – while the Explorers observed the inner belt – the equatorial belt, owing to the different launch latitudes.

Fig. 3: The different launch latitudes of Sputnik 2 and Explorer 3
Fig. 3: The unlike launch latitudes of Sputnik two and Explorer three

Apollo 11's trans-lunar injection

After establishing that radiation belts practice contain lots of highly energetic particles – protons and electrons – scientists realised they presented a problem for humans and satellites in infinite. The obvious solution was to limit orbits to beneath the inner belt, which begins at about 600 km higher up Earth's surface. For example, the maximum altitude of the first human flight by Yuri Gagarin had been 327 km.

The first recommendations for dealing with the Van Allen belts on a possible mission to the Moon came in the summertime of 1960, at a NASA Space Chore Group meeting. Scientists suggested that a moderate amount of protection could shield a crew from the outer Van Allen belt particles. In 1962, Van Allen – assertive that protons of the inner chugalug could seriously threaten man spaceflight missions – suggested clearing them away by setting a nuclear bomb off about the outer chugalug. The particles would and then have the extra free energy to escape World'south magnetic field.

The Americans conducted a series of nuclear tests in the 1960s chosen Operation Dominic. It included a group of atmospheric tests called the Fishbowl events, designed to  understand how nuclear weapon debris would interact with World'southward magnetic field in the issue of a nuclear war. The highest of the Fishbowl events was called Starfish Prime number – a 1.4-megaton nuclear bomb detonated at 400 km on July 9, 1962. However, instead of clearing the inner Van Allen belt, it actually added more than radiation to it. The Soviet tests in the aforementioned year increased the intensity of the inner belt past a million-times, and also destroyed several satellites that were already at that place.

1 of them was Telstar 1, which had been launched just the day after Starfish Prime. It relayed the first TV pictures, fax images and the first-ever transatlantic TV feeds. By Oct, the increased radiation from the Soviet test had burnt Telstar's transistors and it went out of service in 1963.

The loftier-energy electrons injected into the lower Van Allen chugalug had decayed to 1-twelfth of their mail-test superlative intensity just by 1969.

A remarkable letter in which James Van Allen confirms that the "claim that radiation exposure during the Apollo missions would have been fatal to the astronauts is only one example of such nonsense".
A remarkable letter in which James Van Allen confirms that the "claim that radiation exposure during the Apollo missions would have been fatal to the astronauts is only 1 example of such nonsense".

When NASA commenced its lunar spaceflight plan, its scientists already knew about the belts and their spatial and energy distribution. Apropos the energies: electrons beneath near ane MeV were unlikely to be dangerous, equally were protons below 10 MeV. For example, a proton with an energy of three MeV could penetrate well-nigh half dozen mm of aluminium (a typical spacecraft fabric) whereas 1 of 100 MeV could penetrate up to forty mm. And so engineers fashioned shielding that consisted of a spacecraft hull and all the instrumentation lining the walls.

Farther, knowing the belts' absence above the poles, the altitude of the lower edge of the inner belt being ~600 km (well above the LEO) and the location of the Due south Atlantic anomaly, where doses are at a high 40 mrads/24-hour interval at an distance of 210 km immune NASA to design the Apollo translunar injection (TLI) orbit in a way that the spacecraft would avoid the belts' near dangerous parts.

Apollo 11 bypassed the inner belt and only passed through the weaker part of the outer belt (Fig. four). According to NASA's 'The Apollo Spacecraft: A Chronology', the high-altitude nuclear tests would have had a significant impact on Apollo orbits but NASA scientists had accounted for this possibility in radiation-protection planning.

Fig. 4: This figure shows only the final leg of the path through the belts. Red marks indicate the time in 10-minute intervals of the Apollo 11 flight. The first red dot near Earth is the point of TLI. From AP-8 Trapped Proton Environment for solar maximum and minimum. Source: National Space Science Data Center, December 1976
Fig. 4: This figure shows but the final leg of the path through the belts. Scarlet marks signal the time in 10-minute intervals of the Apollo 11 flight. The first red dot near Earth is the point of TLI. Photo: Apollo xi's Translunar Trajectory

Several factors worked in favour of the minimum exposure trajectory. We all know that Earth'due south axis is tilted by 23.v° relative to the ecliptic plane. In 1969, the magnetic north pole was displaced from the geographical north pole by xi.four°. Therefore in 1969, the Van Allen radiations belts could accept had a maximum inclination of 34.9° (23.five°+11.4°) with respect to the ecliptic (Fig. 5).

Fig. 5: Schematics of radiation belts at maximum inclination to ecliptic (red line). The blue line depicts the geomagnetic plane. The last leg of Apollo 11's path was slightly above the ecliptic, avoiding the inner belt completely and only passing through the outer layers of the outer belt. The red ellipse encircles Apollo 11's path through the belts.
Fig. five: Schematics of radiation belts at maximum inclination to ecliptic (cerise line). The blue line depicts the geomagnetic plane. The last leg of Apollo 11'southward path was slightly above the ecliptic, avoiding the inner belt completely and only passing through the outer layers of the outer chugalug. The red ellipse encircles Apollo 11's path through the belts.

The ideal path for Apollo would've been through the north or south magnetic poles because that is where there are no belts. However, that would also accept meant called-for a lot of fuel to get into the lunar orbital aeroplane.

The geomagnetic airplane being inclined relative to the ecliptic effectively resulted in the location of the TLI point of Apollo 11 orbit being very shut to the descending node of the geomagnetic plane (see Fig. 5). The elliptical nature of the transfer orbit ways that its apogee will exist at the aforementioned latitude in the opposite hemisphere of its perigee. And then if the injection happened in the lunar orbit plane, it will stop up in the same aeroplane again when it reaches the Moon.

The Moon's orbit is inclined by 5° relative to the ecliptic, simply the Moon at the arrival of the Apollo 11 was nearly exactly in the ecliptic (at the entrance into the lunar orbit on July 19, 1969, 17:22 UTC, the Moon's position had the ecliptic coordinates 174° geocentric longitude, 0° geocentric latitude), then that the flight path after the TLI was also well-nigh in the ecliptic. In fact, on the webpage 'Apollo by the Numbers – Translunar Injection', the inclination is specified every bit 31.383ยบ.

Thus, later leaving the TLI, they travelled in the management where the geomagnetic aeroplane slopes away from the ecliptic. By the time Apollo eleven reached a distance of almost 3 Earth radii, the geomagnetic axis was tilted almost exactly in the direction of the spacecraft, thus the separation between Apollo 11 and the geomagnetic plane was at its maximum.

In Fig. iv, the path is shown with 10-minute increments (ruby dots). It is obvious that Apollo 11's flying path avoided areas with the highest flux. The Earth parking orbit is under the inner radiation chugalug; information technology traversed the inner zone of the outer chugalug in about 30 minutes and through the near energetic region in about ten minutes. On its manner back, its trajectory was optimised such that Apollo 11 would steer clear of the belts as much as possible. It approached Earth from the due south and was travelling at an even greater inclination and velocity than it had been on its way to the Moon. The astronauts were inside the fringes of the radiation belts for but about hour.

Based on data from the twin Van Allen Probes NASA launched in 2012, scientists constitute that the inner belt is made up typically of high-energy protons and low-energy electrons. They also found that the radiation here was much weaker than what they'd assumed it to be. The most dangerous highly relativistic electrons (with energies of 0.7-1.5 MeV) couldn't penetrate the slot region. Rather, they could just simply rarely, such as when they were helped along past two astringent solar storms in 2015.

NASA currently admits information technology overestimated the level of danger in the low and medium Globe orbits, as a issue spending also much coin on building heavily protected spacecraft.

However, once we're out of nearly space, outer space and the lunar surroundings could present their ain threats.

Radiation risk in outer infinite

Exterior World'south magnetic field, in that location are about 10-20 protons per cubic cm, travelling at about 400-650 km/south. For approximately five days in each of its orbits, the Moon is inside Earth'southward geomagnetic tail, where there are normally no particles from the solar wind. For some other approximately five days, the Moon is in the magnetosheath, where the particle flux is reduced and the protons are slower, moving at 250-450 km/s. Nosotros were able to obtain these continuous measurements using the Solar Wind Spectrometer that the Apollo 12 left behind on the Moon.

A relative view of Earth's magnetosheath. Credit: NASA
A relative view of Earth'southward magnetosheath. Credit: NASA

It remains possible for astronauts to be afflicted by solar storms. For example, if any of them had been travelling to the Moon during the solar event of August 1972, they would accept been exposed to life-threatening amounts of radiation.

The Apollo 11 crew had received 0.18 rem each on boilerplate ('rem' stands for 'roentgen equivalent man'). To compare, one CT scan delivers about i rem. To kill an adult human, you'd have to evangelize 300 rem or more in a very short bridge of fourth dimension, only if information technology is spread over weeks or fifty-fifty days, there will be few effects. Nigh fifty rem delivered at once will cause radiation sickness.

Even if in that location had been a solar tempest when Neil Armstrong and Edwin "Buzz" Aldrin, Jr. were on the Moon, they could accept been protected by the Apollo command module: it was built to attenuate 400 rem on the outside to less than 35 rem on the inside. Attenuation numbers are usually denoted in units of areal density, grams per sq. cm. A typical Apollo accommodate had a force of 0.25 chiliad/cm2 while the Apollo command module hull had a force of 7-8 g/cm2. The hull of the International Infinite Station, to compare, has a strength of xv g/cm2 in the specially shielded areas.

We were indeed lucky that the Baronial 1972 outcome happened between the last ii Apollo flights: Apollo sixteen had returned in April and Apollo 17 prepare off to the Moon in Dec. No major solar particle events have occurred during the Apollo 8 or 11 missions.

Margarita Safonova is a visiting scientist at the Indian Institute of Astrophysics, Bangalore. Her broad interests are the application of gravitational lensing in astrophysics and cosmology and UV astronomy from space.