by Lucie Green
There are some frequencies of radiation near the visible part of the spectrum that also make it down to the ground. Enough of the ultraviolet, infrared and radio radiation emitted by the Sun makes it through the atmosphere for it to be observed with telescopes. But some frequencies are missing. No X-rays or gamma rays make it to the ground. Which raises the questions: does the Sun simply not produce X-rays and gamma rays, or does it produce them but the atmosphere is filtering them out? The only way to check is to take a peek at the Sun from above the atmosphere. Simple.
10.1 Schematic illustration of the components of the electromagnetic spectrum.
Or, rather: not simple at all. To get past enough of the Earth’s atmosphere to make meaningful measurements of what is coming from the Sun requires getting to a height of at least 50 kilometres above the Earth’s surface. There are certainly no mountains tall enough for this. Most passenger jumbo jets fly at only 12 kilometres above the ground. Even Concorde, with its maximum altitude of just over 18 kilometres, didn’t make it. Fifty kilometres is a very long way up. Balloons could lift instruments to altitudes of up to around 40 kilometres, but beyond this was completely out of the reach of humans until the middle of the twentieth century, when rockets began to be used. The sudden introduction of rocket technology was motivated sadly not by curiosity about the Sun but rather by the Second World War.
The war was the catalyst for some revolutionary rocket research. The ambition for ballistic missiles that could deliver explosives across the borders of countries culminated in the development of a rocket-powered bomb: the V-2. The name stands for ‘Vergeltungswaffe Zwei’ – ‘vengeance weapon two’.
The V-1 was its noisy jet-engine-powered predecessor. It was the buzzing of these engines that caused the people of England to name it the ‘Doodlebug’. But while the V-1 may have been ‘jet-powered’ (using a pulse jet engine) it did not leave the Earth’s atmosphere. The V-2 was the first human-made object to reach the upper limit of our atmosphere. Just under 1500 V-2 rockets were targeted at southern England during the war. And they came in silently, travelling much faster than the speed of sound.After the war, though, these rockets were repurposed for scientific use.
When it was clear that the end of the war was imminent, the German engineers who had led their country’s rocket programme surrendered to the US. Almost 500 of them fled there. And this included the inventor of the V-2 rocket himself, Werner von Braun. Importantly, the US also got hold of around 100 rockets and immediately started launching them.
No civil space agency existed in America at that time so it was the job of the military to launch the rockets. The first launch took place on 16 April 1946, and as the rocket roared up from the White Sands Missile Range it reached an altitude of 6 kilometres. A good start to test the technology. It seemed that getting 50 kilometres up to look at the Sun would be possible. This was of interest to the Navy. Radio communications relied on favourable conditions in the atmosphere, conditions that were sometimes disrupted, with the Sun being the suspected culprit. Using a rocket could kill two birds with one stone: conditions in the atmosphere could be studied and astronomical observations made to see what role the Sun might play.
So scientists at the Naval Research Laboratory set out to use a rocket to answer a simple question: what does the solar spectrum look like at ultraviolet wavelengths when you get high up in the Earth’s atmosphere? From the ground we can detect wavelengths ranging from around 300 to 1100 nanometres, which includes ultraviolet, visible and infrared radiation. The plan was to look for wavelengths shorter than any solar radiation that had been measured from the ground. This would reveal whether the Sun’s radiation was affecting the upper atmosphere and might help shed some light on what was causing problems for radio communication. Scientists wanted to know because this would start to unravel the true range of radiation emitted by the Sun. Getting a detector to a high enough altitude, where the Earth’s atmosphere is sufficiently thin and not able to absorb the photons, was the only way to find out.
On 10 October 1946 a rocket was launched carrying a simple detector made by the Naval Research Laboratory. It quickly broke through the stratosphere and into the very thin gas in the mesosphere. Reaching a final altitude of 55 kilometres it confirmed that the Sun emits more ultraviolet radiation than we are able to detect at the surface of the Earth.
This simple observation 55 kilometres above the Earth’s surface was the birth of solar astronomy being done from space. The picture was not much to look at: a series of ultraviolet exposures from different altitudes. It was not even a picture of the Sun, just a measure of how much ultraviolet light it was producing. But it was the beginning of the scientific space age.
10.2 The ultraviolet spectrum detected during the 10 October 1946 rocket flight at different altitudes (© 1946, American Physical Society).
ROCKET MEN
The UK space programme didn’t begin as early as in the USA. There was no ready supply of rockets handed to the scientists but they had ideas on how to use them, and plans to get some were developing. Harrie Massey, a professor of physics at my own university, UCL, was keen to see the UK use rockets for research. He was also the Chairman of the Gassiot Committee at the Royal Society, which had been overseeing the Society’s stance on studies of the atmosphere. UK scientists working in this area were well placed to adopt studies using rockets. The Gassiot Committee organized a conference with engineers and scientists from across the UK, the American rocketeers and the UK’s Ministry of Supply, the Government department that was responsible for developing rockets so that the military could have ballistic missiles. But in the run-up to the conference things took an exciting turn.
On 13 May 1953, just as Massey was about to leave for the annual UCL staff–students cricket match, his phone rang. It was the Ministry of Supply calling to see if the committee he was chairman of was interested in using rockets for research. Massey said yes. The call reportedly made him late for the cricket match, but he would have had a good story to tell – the UK was going into space.
In 1955 the rocket that became known as ‘Skylark’ was approved, giving Britain its own national rocket programme to do science in space. With the funding in place, a sub-committee was set up at the Royal Society to advise on the programme. One of the founding members was Robert Boyd, who went on to establish the Mullard Space Science Laboratory at UCL, the department where I now work. The first Skylark launch took place from Woomera in Australia in 1957 carrying experiments to study the Earth’s upper atmosphere. And several rockets quickly followed.
The big goal was to take a photo of the Sun from space. The first detectors were only able to confirm that the Sun was giving off short-wavelength radiation. Not long after the first V-2 launch, another rocket launched in 1948 showed that the Sun emits X-rays: a radiation of even shorter wavelengths than ultraviolet (it also used a very simple detector: a photographic film placed behind a filter made of beryllium that only lets through light with a wavelength of less than 0.4 nanometres). We didn’t know what the Sun looked like in these wavelengths. But taking a photo is a challenging task because you need to have a stable camera to point at the Sun – very hard to do on a spinning rocket, but the US managed it. The first image of the Sun’s X-ray faint glow was captured in 1960.
This image was taken when a rocket was launched carrying a set of pinhole cameras up to a height of 220 kilometres. They stared at the Sun for 286 seconds. They were delightfully simple devices – the best image was captured through a pinhole that was just over 0.1 millimetres across and created an image on the photographic film that was 1.6 millimetres across. The im
age is slightly blurred because the rocket rolled a bit, but the Sun was clearly visible. And the images showed that the Sun’s X-rays are coming from the corona.
10.3 The 1960 X-ray image taken with a pinhole camera from a rocket. The rocket rolled and smeared out the X-ray features (© Richard Blake).
It wasn’t just the US and the UK that were busy developing rockets and space technology. By the end of 1957 the Soviets had launched two satellites: Sputnik 1, which didn’t do much more than beep (but analysis of its orbital path did reveal details of atmospheric conditions), and Sputnik 2, which barked – along with detectors for solar ultraviolet and X-ray emissions (and cosmic radiation) it also carried Laika the dog. Originally found as a stray dog on Moscow’s streets, Laika sadly only survived a few hours in space. The US soon joined the satellite club with the successful launch of Explorer 1 in January 1958. And in 1958 the almighty NASA formed. The ‘space race’ was officially on.
The UK joined the orbital club on 26 April 1962 with the Ariel 1 satellite, built and launched in collaboration with NASA. NASA were keen to work with other countries and the UK was perfectly positioned after its studies into the Earth’s upper atmosphere using rockets. Ariel 1 was designed to study how the Sun’s high-energy radiation was affecting the Earth’s upper atmosphere and carried instruments designed and built by scientists and engineers from the universities of Birmingham and Leicester, Imperial College London and, of course, UCL.
The team working on Ariel 1 included a young Ph.D. student who was later to become important in my career. His name is Len Culhane and he went on to become director of the Mullard Space Science Laboratory, where I now work. He was also my Ph.D. supervisor and my first mentor in space science. Massey had given responsibility for the rocket research at the Mullard Space Science Laboratory to Robert Boyd, and Boyd was Culhane’s Ph.D. supervisor. So Boyd’s Ph.D. student became my Ph.D. supervisor. I see myself as the scientific granddaughter of the original UK space scientists!
The research that began to be done with rockets was immediately fruitful. This was discovery science – like reaching the South Pole for the first time or delving down into the Mariana Trench. The first instruments were in a totally new environment and showed what no one had previously ever set eyes on. Before Sputnik, about one major discovery about the Sun was made every year; after Sputnik, this rose to about three per year and the importance of space vehicles for understanding our Sun was made very clear. The Earth’s atmosphere was no longer a barrier.
THE HIGHS AND THE LOWS
Perhaps one of the most ambitious space ventures is the story of Skylab – America’s first space station – which was launched on the afternoon of 14 May 1973. By then NASA had been to the Moon and back, but there was no way for astronauts to stay in space for long periods of time. So NASA repurposed the Saturn V, a three-stage rocket built under von Braun’s leadership to take humans to the Moon. But to place the astronauts in Earth orbit, the rocket had to reach an altitude of 435 kilometres, rather than traverse the 384,000 kilometres to our natural satellite, and this needed only two of the Saturn V’s rocket stages. This freed the third to become accommodation and a workshop for the crew. This was the basis of the USA’s first-ever space station: a cylinder 14.66 metres long and 6.7 metres in diameter.
Landing on the Moon had been NASA’s big success in the 1960s, but the Sun became the focus in the 1970s as NASA intended to use the space station as an orbiting solar observatory. On the outside of the workshop was the Apollo Telescope Mount, which comprised eight instruments to provide solar images and spectra in the visible, ultraviolet and X-ray wavelengths. Skylab would provide a giant leap forward in our knowledge of the Sun. But the launch didn’t go smoothly.
Heavy cloud cover obscured the view of the cameras monitoring the launch and the mission team didn’t see that as the first stage fired, delivering the 7 million newtons of thrust needed for lift-off, part of it fell off! The vibrations shook free a shield crucial to protect Skylab from space debris, small pieces of rock and dust (micrometeoroids), and to shield the workshop from the searing heat of the Sun, like a thermal blanket to keep the heat out. Skylab was successfully delivered into orbit, but without the shield in place the temperature rose to 38 degrees Celsius within just a few hours. There was no way that the astronauts could visit Skylab in this condition. The problem delayed the visit of the first three-man crew, but when they arrived they had brought with them a solution to the problem: a space parasol.
This parasol was unfurled and fitted successfully onto Skylab, and two ninety-three-minute orbits around the Earth later the temperature had dropped to a comfortable level. On 28 May 1973 the workshop was ready to use. It’s an interesting twist in the story to say that Owen Garriott was amongst the crew of three on that first mission to Skylab. Thirty-five years later his son, Richard Garriott, visited the International Space Station as a self-funded astronaut.
Across all the Apollo missions, NASA landed twelve humans on the Moon, who stayed on the lunar surface for a total of 12 days 10 hours 35 minutes and 47 seconds. The first Skylab crew smashed this total by spending 28 days and 49 minutes in orbit, conducting science experiments and observations from their unique perspective. They had even been trained in solar physics to make sure the very best observations were taken. In total three crews, each involving three astronauts, visited Skylab and spent a total of 171 days, 13 hours and 14 minutes in orbit. They gathered images that showed the X-ray glow of the solar atmosphere in a totally new way. (See plate 10.)
Rather than fuzzy patches of X-ray emission, the corona was shown to be full of plasma loops of different sizes that arched up from the surface and back down again. There were dark regions that seemed to emit no X-rays at all – these became known as ‘coronal holes’. There were sinuous, S-shaped features too. At these wavelengths the Sun doesn’t look like a smooth ball of plasma at all. Seen in X-rays the corona is highly structured and has no clear edge. And it was realized that the X-ray emission was brightest above the strongest magnetic fields in the photosphere – above the sunspots.
The third and final crew left Skylab on 8 February 1974. Although plans were discussed that would allow Skylab to operate for many years and have its orbit boosted by a visit from the Space Shuttle that was then being developed by NASA, they never materialized. After the final crew had left and returned to Earth the engineers at mission control made their preparations to monitor and control Skylab’s return home. With no orbital boost to keep the space station from being brought down by atmospheric drag, it began to lose altitude. Skylab slowly dropped as its orbit decayed and finally fell to Earth on 11 July 1979.
The re-entry was uncontrolled, although NASA engineers could make changes to the orientation of Skylab to try and influence the timing and location of the landing. Speculation and media interest in the fate of Skylab, which has a mass of almost 80,000 kilograms, soared. With the world watching, Skylab eventually came in over the Indian Ocean and scattered itself over the southern part of Western Australia.
The massive interest in Skylab led one American newspaper to offer a $10,000 reward for the first piece of debris to be delivered to them in San Francisco and NASA being issued with a littering fine by the town of Esperance, where some of the pieces fell. To this day there remain large pieces of Skylab in the local museum in Esperance. I made a pilgrimage there in 2011 to see what remains of the mission that transformed solar astronomy. (See plate 11.)
With the Apollo programme over and the phenomenal success of the Skylab mission under their belt, NASA launched their next solar observatory on 14 February 1980. This satellite was l
aunched at the peak of solar cycle 21, giving it its name: the Solar Maximum Mission. Weighing more than 2,000 kilograms, it was about 4 metres tall and over 2 metres wide. Not designed to house humans, this satellite carried eight instruments into space that together could detect photons across the gamma ray, X-ray, ultraviolet and visible parts of the spectrum and included a coronagraph so that the extended corona could be seen.
The Solar Maximum Mission was launched during the era of the Space Shuttle and it had been designed with this in mind. It was the first satellite that could be captured and serviced in space by astronauts working in the cargo bay of the shuttle if something went wrong. And unfortunately there were problems with the satellite right from the start.
The satellite was launched successfully from Cape Canaveral and placed into an orbit 400 kilometres above the Earth. But three of the instruments began to have problems and this was quickly followed by a major setback when the satellite’s orientation system failed: the satellite could no longer be controlled and pointed directly at the Sun. Just as with Skylab, some in-orbit repairs were needed. Despite having been designed to be visited by the shuttle it took three years of preparation and lobbying to NASA and Congress before a shuttle rescue mission was on its way. When the Challenger shuttle arrived in April 1984 history was made, as the Solar Maximum Mission became the first satellite to be repaired in space.
The repair was audacious and the satellite turned out to be a tricky object to catch. When Challenger reached the same orbit as the ailing satellite it began moving in unison with the satellite around the Earth. Now it was time for the astronaut George ‘Pinky’ Nelson (it seems ‘Pinky’ was a childhood nickname, nothing to do with his fingers) to leave the confines of the shuttle and use the Manned Maneuvering Unit to propel him towards the satellite and capture it. The Manned Maneuvering Unit was referred to as a flying armchair, an apt description, but it was an unsuccessful attempt and resulted in the satellite going into a spin that could have potentially put an end to any subsequent attempts at capture.
