A Hole Over Hobart
How the Space Shuttle’s engines helped scientists push the limits of radio astronomy
To Grote Reber, the idea must have seemed absolutely bananas.
Well, maybe not. After all, he had built his scientific career on unconventional thinking. Reber was one of the pioneers of radio astronomy. The 31-foot dish he constructed in his backyard in 1937 was the first telescope of its kind. He spent his spare time over the subsequent decade conducting the first searches of the sky at radio wavelengths, eventually ending up in Tasmania, where he built his own house while joining Graeme Ellis to pioneer low-frequency observations in the fields outside Hobart. In short, Grote Reber was no stranger to the weird, the wild and the wonderful.
In the mid-1970s, though, it seemed he might have met his match. Inspired in part by phenomena observed during a Saturn V launch, a group of radio astronomers including Ellis and Michael Papagiannis put together a daring plan to push the limits of low-frequency observing. It would require a new telescope, a Space Shuttle, and a healthy dose of good luck. After learning about the idea in 1979, Reber wrote that trying to use a Shuttle was “expensive and uncontrolled.”
Nonetheless, the plan eventually went ahead with support from even Reber himself, and on the night of August 4–5 in 1985, radio astronomers outside Hobart found themselves monitoring an array of antennas spread across an area the size of 19 football fields, waiting for a sign that the experiment had worked. Waiting for the Space Shuttle Challenger to poke a hole in the sky.
Let me back up a bit.
Earth’s atmosphere is helpful to astronomers in that it keeps us alive, but other than that, it’s a significant hassle. It absorbs light in many regions of the electromagnetic spectrum, notably at gamma-ray, x-ray, ultraviolet and infrared wavelengths. This is a major reason why we place infrared telescopes like Spacelab’s IRT in space — or, for that matter, high-energy observatories like Chandra and Swift. Even optical telescopes have problems, as turbulence in the atmosphere distorts the light they collect.
At radio frequencies, the atmosphere is relatively transparent, although some bands still give us trouble, thanks to absorption by oxygen and water vapor. This can be mitigated by building radio telescopes in dry places at high altitudes, like the Atacama Desert. Radio astronomers also face unique challenges in avoiding artificial emissions from everything from cell phones to Bluetooth to satellite transmissions, which has given rise to radio quiet zones around the globe, including the 10,000-square-mile National Radio Quiet Zone centered in West Virginia.
However, there is one seemingly fundamental radio astronomy limitation that no mountaintop observatory or transmission regulations can solve, and once again, the atmosphere is to blame. Beginning at an altitude of 50 miles and stretching ten times as high is the ionosphere, a region where radiation from the Sun has stripped electrons from atoms, forming a dynamic plasma that behaves in extraordinary ways. Waves within it are governed by a fundamental limit called the plasma frequency; below this frequency, waves are effectively blocked, placing a low-frequency cutoff beyond which radio astronomers cannot see. In the ionosphere, this cutoff typically lies somewhere between a few megahertz and 10 megahertz, depending on conditions. Hobart is one of two places in the world — the other being the North American Great Lakes — where the cutoff can get significantly lower, sometimes below 2 MHz.
There’s one way around this. The plasma frequency depends on the density of unbound electrons in the medium; decrease the number of free electrons and the plasma frequency decreases. A dramatic enough change might make a noticeable difference in the low end of the radio window. But what could cause changes in the upper atmosphere at the edge of space?
Well, it turns out firing the Space Shuttle’s engines might just do the trick.
On May 14, 1973, a modified Saturn V rocket had thundered into the sky over Florida. Its payload was Skylab, the first American space station. Australians probably remember Skylab best for scattering debris over Western Australia when it reentered Earth’s atmosphere six years later, but it has another connection to the country: Skylab’s launch indirectly led to the low-frequency measurements made at Llanherne in 1985.
After the Saturn V shot through the ionosphere, observers at Sagamore Hill, Massachusetts, monitoring Application Technology Satellite 3 noticed an ionospheric “hole”: a decrease in free electron density in the ionosphere, spanning an area 1000 kilometers in radius. Later analysis showed that oxygen ions in the atmosphere had reacted with water vapor and hydrogen in the exhaust from the rocket’s engines, capturing free electrons. This meant that, for a short while, the ionosphere’s plasma frequency had been decreased — enough to be noticed by telescopes on the ground.
This inspired the experiment central to this entire saga. The observations raised several tantalizing questions: could the effect be carefully, intentionally replicated, and could it be useful for radio astronomy? Launching another Saturn V for this purpose alone was clearly off the table, but the same effect should be achievable with other spacecraft. In 1977, astronomers Graeme Ellis and Michael Papagiannis proposed using the Space Shuttle, then four years away from its first flight. The Shuttle’s Orbital Maneuvering System (OMS) engines, typically used to perform orbital maneuvers, could be fired while in orbit, thereby avoiding the need for a special launch.
Grote Reber believed that the OMS would be unable to achieve the requisite precision and advocated using small, specialized rockets to reach comparable altitudes. However, his was a minority view, and although the OMS proposal won out, Reber was consulted on some of the observation particulars and offered to let the team use one of his arrays. By 3:00 AM Hobart time on August 5, 1985, new instruments had been built, teams were standing by on two continents, and the plan was going smoothly.
Almost.
There are many words you could use to describe STS-51-F, the mission Challenger was in the middle of that night, but “routine” is not among them. On July 29, 5 minutes and 43 seconds after launch, two temperature sensors failed, causing the orbiter’s center engine to shut down. Within seconds, Flight Dynamics Officer Brian Perry and others at Mission Control assessed the situation and instructed the crew to initiate an Abort to Orbit (ATO) maneuver, which would continue the mission but place Challenger in a lower orbit than planned.
Roughly two and a half minutes after the command was given, however, a third sensor failed, this time on the Number 3 engine. Booster Systems Operator Jenny Howard quickly called for the crew to “inhibit” the limits of the remaining sensor on that engine, potentially averting a catastrophic second shutdown. Almost exactly six months later, Challenger would suffer a tragedy which not even the quickest thinkers back at Kennedy Space Center could stave off. On this summer afternoon, however, disaster had been averted.
STS-51-F represented the only instance in the three-decade history of the Shuttle program in which astronauts had to perform an abort after launch. It also heralded the only spaceborne battle of the Cola Wars of the ’70s and ’80s. After the Coca-Cola Company was given permission to use a Shuttle flight to test out a soda dispenser designed to work in microgravity, PepsiCo executives complained to the White House; in the end, the crew of STS-51-F ended up sampling both Coke and Pepsi while in orbit. Coca-Cola won the well-publicized taste test, but after several experiments over the next decade showed that carbonation in space was a fundamentally terrible idea, both sides retreated back to Earth.
The highest-profile experiments conducted by the crew — fizzy drinks aside — involved Spacelab-2, the newest incarnation of a European module which originally flew on the second Shuttle mission back in 1981. While some variants of Spacelab included a pressurized laboratory that fit in the Shuttle’s cargo bay, Spacelab-2 mainly consisted of the Igloo, a cylindrical, 10-foot-long device weighing as much as a small car and containing a number of instruments.
Accompanying the Igloo on STS-51-F was the six-inch Spacelab Infrared Telescope (IRT)— which, ironically, needed anything but an igloo’s warmth. Infrared telescopes are particularly susceptible to contamination from thermal emission, even from ordinary objects — a good reason to place them in space. A cryostat supplied with liquid helium cooled most of the instrument below 10 K, and while the Shuttle itself was hot enough to contaminate much of the data at long wavelengths, the IRT still managed to explore about 60% of the sky covered by the Galactic plane.
Although Spacelab-2 and all its accoutrements were undoubtedly the scientific focus of STS-51-F, they overshadowed two scheduled firings of the Shuttle’s Orbital Maneuvering System (OMS), performed for the benefit of the radio astronomers waiting below. There would be a 16-second burn conducted by Challenger over Hobart, as well as two burns totaling 53 seconds over Millstone Hill in Westford, Massachusetts. The purpose of the Millstone burns was to study how underdensities in parts of the upper atmosphere form and evolve, using the Millstone Hill Incoherent Scatter Radar, with Boston University’s Mobile Ionospheric Observatory gathering optical data on Long Island.
The purpose of the Hobart burn was to actually apply these underdensities to low-frequency observing, by seeing whether astronomical signals would briefly get stronger as ionospheric effects diminished, and to see what insights this new ultra-low-frequency regime could reveal.
First, however, the astronomers needed to build a new telescope.
Despite the grandeur of famous facilities like the Arecibo Observatory and the Very Large Array, radio telescopes can be quite simple. The most basic type of antenna is a dipole, a pair of metal conducting rods. Even one dipole can detect radio waves, but much better results can be obtained from collections of hundreds, thousands, or even tens of thousands of dipoles, known as arrays. One such instrument was the Interplanetary Scintillation Array in Cambridge, England, used by Jocelyn Bell Burnell to detect the first pulsars. Although it might look like thousands of wooden posts strung together with wires, it played a pivotal role in the history of astronomy.
Many similar arrays were built at a site called Llanherne, located in a different town by the name of Cambridge — this one situated 10 miles east of central Hobart, near the city’s airport. The largest of these telescopes was the Llanherne Low Frequency Array (LLFA), spread over a third of a square kilometer. Opened in 1972, it took five years to build, a process made harder by marshy ground and overly inquisitive sheep. The LLFA would go on to produce maps of the Milky Way at several frequencies, as well as key observations of bursts coming from Jupiter.
The Llanherne arrays largely operated above 2 megahertz; for comparison, most radio telescopes operate at frequencies from hundreds of megahertz to tens of gigahertz. For the experiment Reber’s colleagues proposed, a new instrument was needed. The aptly-named 1.6-MHz array was constructed in 1985 roughly half a kilometer from the airport terminals; today, the site lies opposite Llanherne Golf Club on the other side of Grueber Ave. The 1.6-MHz array, like its companion telescopes, was a humble collection of dipoles, slightly more than one quarter the size of the LLFA and designed specifically to observe in the 1–2.75 MHz range.
The array up and working well before Challenger passed overhead, performing a survey of the southern sky at 1.6 MHz that would last into 1986. It looked like just another set of wooden poles, but it was about to do something very cool.
The abort-to-orbit maneuver during launch one week earlier had placed Challenger in a lower orbit than the roughly 380 km altitude planned. That alone would not have been problematic, but the maneuver also required dumping a significant amount of fuel. As a result, the crew was only able to spend 539 pounds of fuel during the OMS born over Hobart, just under one third of the 1797 pounds originally allotted. The experiment had been in the works for the better part of a decade, and yet the burn was carried out in just 16 seconds.
Fortunately, even the reduced quantities of water vapor, hydrogen and carbon dioxide in the exhaust proved to be sufficient. Within minutes, they began combining with free electrons in the ionosphere, producing a sharp dropoff in plasma frequency that slowly increased, lasting until daybreak 3–4 hours later. At its peak, the hole reached density levels 30% below its natural minimum — not a dramatic change, but enough to be detectable.
After three minutes, the low-frequency limit in the hole over Hobart had been lowered from 2 MHz to 1.8 MHz. An hour and a half later, it had been lowered even further. The astronomers monitored the sky at a range of frequencies, but the most dramatic effects were found at 1.7 MHz, where there was a significant increase in cosmic noise, as Ellis, Papagiannis, Reber and others had hoped. The two burns conducted six days earlier over New England, designed to prove ionospheric physics, were also successful, with significant airglow detected at the Long Island site. In short, the bizarre experiment had worked.
Radio astronomy has made leaps and bounds in instrumentation since in the four decades since the team pulled off this feat. While the Shuttle program ended in 2011, I like to think that the basic idea could be repeated in the future. After all, Reber himself advocated for using small, cheap rockets instead of the Shuttle. The astronomers were aided by the fact that 1985 lay near the end of solar cycle 21, almost at solar minimum; given the current solar cycle, the next opportunity for observations should be the early 2030s.
Does that give us time to push the limits of low-frequency observing even further? Possible. Ellis, pessimistic about human-made radio transmissions at these frequencies, wrote in 1986 that
over the past thirty years, the increase in the number of broadcast transmitters below 1.5 MHz has virtually eliminated the possibility of observations in this band … These observations are likely to be the last in this series and indeed the last to be made from the Earth. Future observations are likely to be made from the Moon or space in the twenty-first century.
I propose we take that as a challenge.
