Good afternoon, everyone! Last week, we covered the last three of the original six authorized Artemis missions. However, not every flight investigating the universe has to go quite that far from home, and this week we're returning to that topic: space-borne astronomy in the care of Workable Goblin. Hope you all enjoy it!
Eyes Turned Skyward, Part IV: Post #11
Although Hubble was still weeks away from reentering Earth’s atmosphere when the Henrietta Swan Leavitt Telescope left the launch pad in late 1994, the successful injection of the latter into its trans-Lagrange trajectory marked the passing of a torch from one era of large-scale space astronomy to another. For the past decade, Hubble had been the most capable telescope on or around the planet. Though not the largest, it was not far off, and in a much more advantageous position. Its capabilities in visual, ultraviolet, and infrared observations had been exploited to the fullest by a generation of astronomers. As it tumbled and burned in the atmosphere over the South Pacific, with it went space-based visual observation capabilities, since Leavitt was an X-ray telescope, optimized to focus on one of the bands of electromagnetic energy not visible from the Earth’s surface. The development of larger ground-based visual telescopes, such as the Kecks, and the technology needed to make them nearly as effective as Hubble had made space-based optical telescopes less interesting and important for astronomers, a fact made clear by the lost-in-the-woods status of the putative Hubble-replacing Large Optical Space Telescope over the next decade.
Nevertheless, astronomers lamented that Leavitt was not able to be launched and commissioned a year earlier--long enough that it could have made at least a few observations in conjunction with Hubble before the latter had to be retired. In the meantime, as it started observations in late 1995, astronomers were looking towards its first results, expected to be as large an improvement over the last x-ray telescope, the Einstein Observatory of the late 1970s, as Einstein had been over previous sounding rocket experiments. Early on, Leavitt focused on supernovas and supernova remnants, imaging the leftovers of these brilliant stellar explosions to explore the results of the different processes that can give rise to them, with observation campaigns focusing on the famous Crab remnant, SN 1987A, SN 1993J, and other, less prominent objects. At the same time, Leavitt was also being used to image a number of energetic and nearby galaxies. During the course of one of these campaigns, against the Circinus galaxy in the southern sky, astronomers detected an unusually bright object near the galaxy, one that resisted attempts to clear the “error” from Leavitt’s detectors. Suspicions that it might actually be a strange object of some kind were confirmed shortly after, when observations carried out by the Anglo-Australian Observatory confirmed that Leavitt had actually discovered the remnants of a supernova that had exploded earlier in the year, the first ever to be detected by x-ray observations instead of optical methods. More was yet to come, as Leavitt participated in observations of SN 1996X, a Type IA supernova detected early in the year and quickly marked as a priority target for further observations, to explore the x-ray afterglow left by the supernova and its structure and decay over time. Although not the only observation campaign that Leavitt took part in over the next year, it was one of the largest, coordinated with multiple ground-based observatories, and helped define Type IA x-ray light curves for future research, allowing improvements to the standard candle scale that Leavitt’s namesake had introduced with her research into Cephid variables.
While Leavitt was stretching her scientific wings, back on Earth work was proceeding on the Large Gamma-ray Observatory that had been approved in her wake. During the nearly two decades the concept had spent maturing, it had been refined from a catch-all gamma-ray mission building on the earlier HEAO missions to one focused primarily on imaging gamma-ray sources, pinpointing gamma-ray sources and beginning to understand their internal structure and how they correlated with astronomical objects visible in other bands by adapting techniques developed by particle physicists on Earth to observe gamma rays over the past fifty years. With decades of refinement behind the basic technologies involved, progress was relatively smooth; although the usual difficulties in adapting them to spaceflight arose, as well as certain unusual problems (dealing with background noise was a particular problem), the fundamental maturity of the underlying techniques meant that solutions could usually be quickly found to any new issues that arose. As the decade ground forwards, LGO’s construction seemed to move in tandem.
With work progressing on LGO in the foreground, in the background efforts on LOST and, in particular, the Large Infrared Space Telescope that was its chief competitor continued apace. After all, although neither was an approved program, LIST had been anointed the “next logical step” after LGO and to follow up Hubble by the National Academy of Sciences, while LOST was in all respects a direct successor to Hubble in capability, if bolstered by recent classified developments in large space-based optics systems. Both, however, had to confront serious problems, such as LOST’s lack of broad community support (or, indeed, any community support), a problem not easily solved or dealt with, and one that would continue to marginalize the concept. LIST’s issues, by comparison, were much simpler; essentially, although the United States had plenty of ground-based infrared telescopes, and had tested a number of aerial infrared telescopes, it had never actually launched a space infrared telescope, unlike the Europeans or Japanese. There was simply no experience in building or operating a cryogenically cooled, space-based telescope, creating technical risks that had played a part in preventing LIST from becoming the next major American astronomy mission. To remedy this issue, supporters of the LIST concept, and of space-based infrared astronomy more generally, had developed a cheaper, simpler concept, one that could lead up to the full-scale LIST.
Building on the results of the European IRAS satellite twenty years earlier, WISE--the Wide-field Infrared Survey Explorer--would bring newer technology and superior capabilities to bear on the survey mission. Higher resolution, additional observation bands, and more complete all-sky image sets would make for a new, superior map of the infrared sky. Cooler objects, such as asteroids and brown dwarfs, would also be visible to WISE, unlike IRAS, boosting research into entirely new classes of object as well. And, beyond the scientific results, WISE would provide valuable engineering data and operational experience similar to that needed for LIST. Built as part of the long-standing Explorer program for small Earth-orbiting satellites, WISE was launched in 1998 and operated for nearly fifteen months, several months longer than expected, before its coolant supply was depleted and it was retasked for asteroseismology research using its star tracker, a program that lasted a further year. In that time, it completed two full sky surveys and a substantial part of a third, discovering tens of thousands of asteroids, comets, and other small solar system bodies, along with dozens of brown dwarfs and other cool stellar objects. WISE’s data set also ruled out the existence of any unknown large bodies--planets or small stars--orbiting the Sun in the Oort Cloud, where previous surveys would not have been able to detect them. Besides these nearby targets, WISE was also able to facilitate research into interstellar dust, a strong source of infrared light, and into distant, infrared-bright galaxies, often merging galaxies or young galaxies experiencing their first bursts of star formation. Although much smaller and closer to home than LIST was planned to be, WISE was also completely successful in demonstrating critical engineering and design features of the bigger telescope, greatly increasing confidence that it could be built and flown successfully.
Aside from the type of gradual progress exemplified by WISE, where incremental developments in technology were used to build increasingly capable telescopes, the late 1990s saw the beginnings of a major revolution in astronomy, with the discovery of the first exoplanet around a main-sequence star, 70 Virginis b, in 1996. While astronomers had for centuries believed that planets most likely orbited other stars, repeated searches and occasional discovery claims, like the infamous claims by Peter van de Kamp of detecting gas giant planets orbiting the nearby red dwarf Barnard’s Star had always amounted to nothing. Verification, the ability to establish that there really were planets out there, triggered a rush of interest, and the initial trickle of discoveries soon widened to a stream, then a flood as finding the real Holy Grail of life-bearing planets abruptly appeared possible.
Nevertheless, there were problems in the new astronomy of exoplanets. Many of the first exoplanets discovered were giants with highly elliptical orbits--like 70 Virginis b--or which orbited extremely close to their stars, like 51 Pegasi b, one of the earliest exoplanets to be discovered. In either case, the formation of terrestrial planets, and hence life, would be unlikely. It was true that methods for discovering planets were, at that time, biased towards giants of this type; Jupiter would have been barely detectable, at best, had astronomers been studying the solar system. Nevertheless, it was possible that the Solar System was unusual, and that this type of system was being detected more often because it really was more common, not just because of a bias in the detection methods. The only way to tell would be to push the sensitivity limits--to build instruments capable of detecting smaller planets in less favorable orbits--and to scan more stars, many more stars, to ensure that a significant sample size would be captured even if only a few stars had planets.
Here, astronomers ran into a second, more serious problem in the limitations of their instruments. Planetary searches require precision measurement of slight variances in certain stellar properties, such as brightness or proper motion, which can be thrown off by the Earth’s atmosphere. Similarly, scanning large numbers of stars would be much easier in space, where the amount of the sky blocked off by various obstacles would be much less than at any terrestrial observatory. Finally, there were of course still many astronomers dedicated to studying stars and galaxies, not planets, who still wanted the always precious commodity of telescope time. Searching a large number of stars would therefore require a dedicated instrument, whether or not it was located in space. This point was driven home when data from WAPPE, the Wide-Area Precision Parallax Explorer, was reprocessed in order to see if any of its target stars had planets orbiting them, via direct astrometrical analysis, only to produce no viable results. Despite the high precision of the astrometrical data collected, WAPPE’s primary targets had been standard candles, objects with known luminosity which could be measured to better determine their distance as part of WAPPE’s mission of refining the cosmic yardstick used to measure distance to other astronomical objects. As standard candles are often unlikely at best to have planets around them, this meant that WAPPE’s dataset included few stars which were even candidates for study.
Early proposals for space-based planetary observatories, however, focused more on aspiration than these relatively prosaic near-term needs. The discovery of giant planets seemed to herald the coming detection of terrestrial planets, and multiple plans were floated for satellites which could not only do this, but go beyond to actually image exoplanets, or at least measure their spectra, and therefore, perhaps, discover life itself. Projects such as the so-called Terrestrial Planet Telescope of the European Space Agency, Japan’s ASTRO-T, or NASA’s Extrasolar Planet Imaging Camera all envisioned large systems capable of directly detecting and imaging even terrestrial-sized planets orbiting in the habitable zone of their stars. This initial enthusiasm was quickly tempered, as studies soon showed that any of the systems would be very costly and, moreover, very difficult to realize in practice. The interferometers favored for EPIC, for example, would require extreme precision in measuring telescope-to-telescope distance, perhaps too much to be practically realizable.
As such, these missions were downsized, reoriented towards the more modest goal of simply detecting terrestrial planets. In this, there were several techniques that were beginning to be available by the time their initial goals were being abandoned, such as the gravitational microlensing technique that had been developed by Polish astronomers through the decade. The most promising technique, and importantly one that had recently been demonstrated at the time, was the so-called transit method, where the brightness of a star would be continuously measured to detect small dips, possibly caused by the passage of a planet in front. Careful study could distinguish planet-caused dips in brightness, or transits, from other sources of variation such as starspots or flares, and it could be sensitive to much smaller planets than could possibly be detected through most other techniques. Although the random orientation of stellar spin axes would mean that most extrasolar systems would not be detectable through this method, that would impart no bias to the results overall, allowing scientists to make firm statistical conclusions about the frequency of different types of planets--of whether giants were really more common than smaller, more Earth-like planets or not.
While the Europeans continued to study the concept for their next mission selection round, NASA was able to move ahead more quickly with their Explorer program once EPIC was reoriented. The revised mission was selected in 2002 and launched in 2005 to begin a thorough search for new planets. And it delivered; although the data took several years to process, even the first few months of observations after it reached its operational orbit contained more planetary candidates than had been detected by every other detection campaign combined since 1996, with hundreds of confirmed and thousands of unconfirmed planets proving to exist within its field of view. In order to gather more statistics and confirm or rule out its remaining planetary candidates, observations continued for five years until an an electrical failure in 2010 spacecraft’s retirement, though not before several hundred more planets were verified. The enormous success of this mission despite its relatively limited field of view helped spur the approval during that time of the European Giordano Bruno spacecraft, which would be able to scan vastly more sky and detect enormously more planets than EPIC possibly could, providing a wider selection of targets for other follow-up observations.
Besides investing in new Earth-orbiting, free-space telescopes, since the announcement of Project Constellation in 1989 astronomers had been studying the potential of astronomical observations from the Moon. Compared to conventional space observatories, lunar basing offers several potential advantages, such as a stabler, unmoving platform for telescopes more sensitive to movement, additional baseline for Very-Long Baseline Interferometry radio observations, ultra-cold regions near the poles for emplacement of cryogenic telescopes without requiring coolant, and, perhaps most prominently, a far side shielded by thousands of kilometers of solid rock from the considerable radio noise generated by Earth by both human activities and natural phenomena. Nevertheless, despite these potential advantages the high cost and complexity of moving telescopes to the lunar surface had prevented any exploration of the possibilities, so that in 1989 they remained merely theoretical potential. From the outset, Artemis sought to change that, and at least demonstrate that the potential capabilities unlocked by lunar-based telescopes could be realized in the real world, soliciting payloads from astronomers beginning almost before the program itself for emplacement at one or more of the (at that time not yet firmly selected) landing sites.
Among the first payloads suggested by astronomers for Artemis flight were radio telescopes, the beginnings of what would become the FROST, for Farside Radio Observing Scanning Telescope, arrays. Although many concepts were proposed, ranging from dipole arrays focused on low-frequency observations impossible on Earth to elaborate, multipurpose dish arrays on the lunar Farside, the simplest idea, and the one that quickly gained the most support, was to emplace a few small radio telescopes at the landing sites for experimental observations. Out of necessity, the size and hence capability of the dishes carried on Artemis missions would be limited, but even small dishes would be able to at least demonstrate the basic concepts and produce useful results. Carried on the Artemis 7 and 8 missions to Mare Ingenii and the crater Antoniadi in 2002 and 2003, respectively, the FROST dishes integrated atomic clocks--the first ever emplaced on the lunar surface--to allow their integration into the Very Long Baseline Array, the largest and most capable very-long baseline interferometry system on the planet. By virtue of their position on the Moon, at widest separation the FROST dishes could be used to extend the VLBA baseline from a mere 8,600 kilometers to over 400,000, vastly improving angular resolution. Unfortunately, due to the small size of the FROST dishes, this capability could only be used on the brightest sources in the radio sky, and only for a small range of frequencies due to the limited mass of signal processing and detection equipment transportable on a lunar mission. Nevertheless, for those objects it could be used on, the enhanced Very Long Baseline Array, or eVLBA, was the most powerful imaging device in the Solar System. Additionally, FROST observations as part of eVLBA campaigns have been used as an independent check on laser ranging experiments measuring the distance between the Earth and Moon, much as similar observations are used on Earth to measure small crustal shifts and movements. Although the original FROST dishes have been shut down due to failures of their servomotor steering units and electronics after more than a decade of use, they wouldn't go unreplaced, with looser payload limits allowing the emplacement of a larger, more capable FROST-2 antenna and its associated data-processing elements, and the continuation of eVLBA operations.
Besides their main use as VLBA elements, the FROST and FROST-2 dishes have also been used in a number of independent observational programs, mostly when VLBA observations were scheduled on objects they were or are not capable of observing. As both FROST dishes were emplaced on the lunar farside, these have focused on radio frequencies not observable on Earth due to human-generated noise, producing the first maps of the radio sky in several bands. By far the most prominent of these observations, however, has been the SETI campaign over the summer of 2004, when both dishes were tasked to listen for emissions near the famous 1.42 GHz hydrogen hyperfine line. Besides searching for alien transmissions (none were detected, of course), this had the serious objective of studying the radio environment of the lunar farside around a frequency commonly used in radio astronomy, helping to characterize it for future telescopes, and was one of a number of observational campaigns in this vein carried out by the telescopes.
Although FROST had been a part of Artemis nearly since the beginning, the LIFT infrared telescope emplaced by Artemis 9 in 2004 was a much later breaking payload, only emerging after NASA confirmed that it would be sending at least one mission to explore one of the permanently shadowed craters discovered by the Lunar Reconnaissance Pioneer. The telescope’s origins stemmed from the realization that, quite aside from whether or not the permanently shadowed craters contained water ice or other volatile minerals, they were and are certain to be very, very cold. Cold enough, in fact, that an infrared telescope emplaced there could operate at near-optimum temperatures and enjoy the clarity of vacuum without needing elaborate sun-shielding or cryogenic coolant, as free-space telescopes needed. This, in turn, could allow much longer system lifetimes, and perhaps ongoing upgrades and improvements if located near a lunar base.
As ever, however, these theoretical advantages needed to be put to the operational test. To do so, astronomers at Goddard proposed that a small infrared telescope be emplaced at one of the permanently shadowed craters on the planned flight there. Although it would be no tremendously powerful observational instrument, it would at least be capable of proving the basic theory and, almost as importantly, developing techniques for operating systems in the intense cold of the permanently shadowed craters. To minimize the mechanical complexity of the system and avoid needing to design motors and drive systems capable of operating at temperatures at or below 100 Kelvin, some 170 degrees Celsius below zero, the Goddard scientists proposed that it should be a zenith or transit telescope, a type of telescope that sits fixed and allows the motion of the Earth--or Moon, in this case--to carry objects overhead. Due to the much smaller axial tilt of the Moon compared to the Earth, and its longer period of rotation, a transit telescope at one of the Moon’s poles will spend lengthy periods of time observing nearly the same portion of the sky, essentially acting as a permanent sentry for any changes in its small region of the sky. This limited scientific mission was, almost self-admittedly, a figleaf for the more critical engineering goals of the telescope, and despite lukewarm at best support from other astronomers, the Lunar Infrared Fixed Telescope was soon manifested by NASA for their polar crater mission, Artemis 9. Installed in the permanent chill of Shackleton with little fanfare or drama, LIFT soon began to produce, keeping a constant vigil on the sky around the celestial south pole for any fluctuations in the infrared sky. In doing so, it produced a constant stream of data showing not only that a passively cooled infrared telescope was a practical idea, but that the specific engineering concepts that had been developed to accommodate LIFT’s unique environment, like the specially designed and insulated ‘hot boxes’ needed to contain its electronics and RTG power source, were functional and effective. With a consensus growing around a polar site for any permanent base, plans were soon afloat to install a larger and more capable telescope--one with the cold-temperature mechanical systems that had been omitted from LIFT--if and when a mission returned to the poles and their permanently shadowed craters.
Even while astronauts were installing astronomical equipment on the Moon, slower-developing payloads like the Large Gamma Ray Observatory were finally finding their own way into space. After decades of study and development, its launch in 2005 atop a Saturn rocket, bound for a low Earth orbit just grazing the van Allen belts due to its massive size and weight compared with even Leavitt, was strangely anticlimactic, with astronomers already turning towards the next major observatory. Even as it had been prepared for launch, it had gained two things. First, it gained a name, as project leaders christened it the Compton Gamma Ray Observatory, after the prominent American physicist of the early 20th century who had studied gamma rays, then experimentally proved that light waves could also be regarded as particles, a then-controversial concept in physics circles, before finally playing a key role in the Manhattan Project and overseeing the integration of Washington University in St. Louis as its Chancellor. Second, and more importantly, it gained a successor, as LIST went from being the most favored new major space astronomy initiative in the 2001 decadal survey to an approved project. Even as Compton gathered its first photons, astronomers were already turning towards the task of applying lessons learned from it, Leavitt, WISE, and LIFT to the big telescope they had been hungering for for the last decade.
