Since the 1960s Astrophysics has been using a fleet of satellite based instruments to observe cosmic objects in the bands of the electromagnetic spectrum in which the atmosphere prevents or hinders observations from the ground. This effort has considerably enriched our knowledge of the cosmos.
Space astronomy is considered an expensive branch of science, and it is therefore legitimate to question its pursuit. This short note addresses this question from the viewpoint of an astrophysicist who used several space observatories in the last decades.
The cosmic objects
Space observations have allowed the discovery and the study of a large set of cosmic objects that had been neither known nor anticipated while only ground based observations had been possible.
In 1962 a rocket launched by R. Giacconi and his team registered a bright X-ray signal from a region close to the centre of our Galaxy. Nobody had predicted that such sources would exist, as the only celestial X-ray source then known, the Sun, would be much too faint to be observable at the distance of even the nearest stars with the instrument. The source discovered by Giacconi and his team was named Sco X-1[1], it was the first of many thousands. In subsequent years it was found that this and other X-ray sources are binary systems in which a compact object, a neutron star or a black hole, attracts matter from its companion. The matter is heated in the gravitational field of the compact object and radiates this energy in the form of X-rays.
X-ray binaries have been found to come in many guises. The diversity of their appearance is found to be very large. New types of them are still being discovered, notably by ESA’s INTEGRAL satellite.
X-ray binary systems vary in time by large factors over timescales that span seconds to weeks. They emit as much energy as 100,000 suns.
In the early 1960s the measurement of the position of radio sources became sufficiently precise to allow scientists to associate some of them with optical sources. Emission lines were found in these objects, that were named quasars or QSO, for quasi stars or Quasi Stellar Objects. M. Schmidt identified the lines in 1963 and thereby showed that the objects are at distances larger than the then known furthest galaxies. Since these objects appear relatively bright, they emit an enormous luminosity, up to 10,000 times that of a normal galaxy comprising several hundred billion stars. Their emission spans the electromagnetic spectrum from radio waves to gamma rays. They vary on all timescales from hour onward. In subsequent years other objects sharing some characteristics of quasars were discovered. All these classes of objects are grouped under the name of Active Galactic Nuclei. Many aspects of their very rich phenomenology are yet to be understood.
In 1967 Pulsars were discovered, and soon found to be rotating neutron stars. Neutron stars have the mass of the sun but a radius of only ten kilometres or so. Their density is far larger than anything known on Earth. Neutron stars emit radio waves in cones and appear somewhat like light-houses, each time that the cone sweeps over the Earth a radio pulse is registered. Pulsars can rotate as fast as several hundred times per second.
In the late 1960s gamma ray bursts were discovered while looking for gamma rays from nuclear explosions on Earth. In 1997 it was finally found that the objects emitting these bursts are as far away as the quasars. In these sources, a fraction of the mass of the Sun is transformed into radiation in a few seconds. This phenomenology takes place in jets moving at close to the speed of light.
X-ray observations in the 1970s indicated that there is more matter in the form of a very hot diffuse gas than in stars in the large ensembles of galaxies that are called clusters of galaxies.
In 1979 a new type of bursting sources was discovered, the soft gamma ray repeaters. Recent work shows that magnetic fields 1015 times that of the Earth are responsible for this phenomenology.
In 1995 planets orbiting solar like stars were discovered. The first exoplanets were found to be massive and close to their star, unlike the solar system, where massive planets are much further away. Observations performed since then from the ground, but also with a number of space missions, have shown that planetary systems have a large variety of dynamical properties.
In 2007 short radio pulses were discovered. No identification is yet available. We have no idea of how far these sources might be or what they might be.
In 2015 gravitational waves from coalescing black holes have finally been discovered, a century after their prediction.
Almost certainly, this story is not yet at an end. New instruments, new domains of the spectrum, better sensitivity aimed at in the coming years are likely to uncover as of now unknown classes of objects or new aspects of some of them.
ESA has launched not only missions exploring new spectral domains, it has also launched highly original missions to further the study of known objects. Hipparcos and Gaia observed and observe the positions and velocities of stars, measurements that allow scientists to develop entirely new models of the Galaxy and its populations. Planck measured the cosmic microwave background with unprecedented precision. The measurements that these missions performed and continue to perform have had a profound impact on our view of the Milky Way, our Galaxy, and the evolution and constituents of the Universe as a whole
The Physics
We have not only discovered completely unexpected types of cosmic objects or performed original measurements of known ones, we have also found that many more chapters of physics are needed to understand their behaviour than could be expected from the previous studies of stars and galaxies.
Jets, material ejected from a compact object in a narrow cone at velocities that can be as low as 10,000km/s or as high as to be within a few percent of the velocity of light, are often met in these environments. These jets seem to be very inhomogeneous, with some sections moving faster than others thus creating powerful shocks. Shocks in turn are environments where particles are accelerated to relativistic energies.
Electrons, often relativistic, interact with magnetic fields, causing the emission of radiation known as synchrotron radiation. Electrons interact also with photons, exchanging energy in the process.
Magnetic fields not only determine the geometry of emission regions, they can cause massive emission of light through a cascade of phenomena involving electron-positron pairs when their field lines are disturbed.
Electrons and positrons annihilate to create radiation. This is observed in the centre of our Galaxy. The origin of the positrons in this process is not known yet.
Gravity orchestrates most of the emission, as matter falls in the deep potential wells of the compact objects, where it sometimes creates disks. Matter is heated in these disks through friction and thus radiates its gravitational potential energy. Disks are probably at the origin of a much richer ensemble of phenomena, including the formation of planetary systems.
Time variations
Looking at the sky with X-ray sensitive instruments using some of our satellites reveals a sky that changes on all time scales. Nothing but the largest objects remains at the same intensity level. Sources appear, flicker, disappear, some in times as short as milliseconds, some in weeks or months. Those that remain observable for as long as observations have been possible, double or halve their luminosity on a range of timescales from hours to years.
Our Weltbild
It follows from these discoveries that the Universe is not the quiet, eternal, slowly expanding place that naked eye and ground based observations led us to believe. It is a place of constant violent activity, and it is reshaping its constituents on all timescales. Change is the rule, not the exception. Complex shapes are common, the sphere is not the rule. These are major changes of our perception of the cosmos.
Our view of the cosmic environment is one of the founding elements of our whole culture. In the same way as the Copernican revolution displaced the human being from the centre of the Universe, or the expansion of the Universe implied a beginning of time, findings of the last 50 years or so will alter our perception of time and evolution. Our appreciation of the place of humankind in the Universe will be affected by this. The Earth and the sub-lunar sphere are not the only cosmic environment “corrupted” by constant change, rather the whole world is constantly changing.
Modern Astronomy thus deeply influences our culture again, in a way similar to what it did in centuries past.
This is the legacy of modern Astrophysics, of which space observations play a major part.
Where next?
Some basic understanding of the conditions prevailing in the objects discovered in the last decades was gained in the years following their discovery. X-ray binaries were so named because the binary nature of the objects was found in the regular movement of the companion star. Quasars and Active Galactic Nuclei, soon found to vary on timescales of less than a year, had to be of small size. It became clear that accretion onto a black hole was the only possible way to generate their luminosity in such a small volume. But this conclusion was so far outside the general thinking that it was the object of controversies that lasted for two decades. Signatures of strong magnetic fields were observed in X-ray pulsars, which helped understand how the pulsars extract energy from their rotation to radiate it.
These arguments set the scene on which understanding of the phenomenology at work must be sought. This is where difficulties arise that are still far from being solved. The way is long between knowing that an X-ray source is a binary and understanding the physical conditions that prevail, what distinguishes the various classes of binaries or whether the compact object is a neutron star or a black hole. It is also difficult to establish what sequence of events leads to the formation of a given binary system, or how often this sequence takes place, in which environments in our Galaxy, with what companions. What these objects tell us about the evolution of stars and of our Galaxy in general, is also a question that must be answered to give us a clear picture of the Universe around us.
Taking it for granted that matter falling into a massive black hole generates the luminosity of quasars and Active Galactic Nuclei, one asks how this matter falls. It is well known that not all matter of our solar system falls onto the Sun, why is it then that in some galaxies large quantities of matter are funnelled to the centre? Some quasars are bright in the radio domain, while others not, so where does this difference come from? Present day models are often ridden by gross inconsistencies that are very difficult to eliminate. The evolution of these objects and the way through which the enormous quantities of energy that they inject in their surroundings influences their environment, are also still open questions.
This enumeration could be continued and lead us to wonder how much we want to know. Indeed it would be absurd to seek understanding of every glitch of a neutron star or the “weather” in a quasar. In the same way as studying all leaves in a tree individually makes no sense. Seeking understanding at a level that allows us to have a clear view of the evolution of galaxies and their populations is, however, what will allow us to tell the story the Universe and its constituents. This in turn is the information we need to set the scene for the solar system, life, and ultimately, for our story.
A large effort is still needed in order to achieve this goal. This effort requires a lively community of astrophysicists and a dynamic space science programme for many years ahead of us.
The Costs
Important space science missions nowadays may cost between several hundred million Euros and one or a few billion euros. The outcome of these missions is measured at first in terms of scientific papers in the main journals. In later times, the legacy of space missions lies in books, philosophy, and culture in general. These latter elements cannot be quantified. The first layer of productivity can, however, be at least roughly accounted for. ESA quotes for example that more than 4000 refereed scientific papers have been published based on observations performed by the XMM-Newton X-ray observatory and 1000 for the INTEGAL gamma ray observatory.
A rough estimate therefore shows that the mission cost invested in a paper is of the order of one to few hundred thousand Euros. Papers based on results obtained with observatory class missions, like XMM-Newton, are typically written by teams of few scientists (say 3 to 10). The time it takes to analyse the data, understand the implications of the results, and publish a paper can be estimated to be about a year and represents probably approximately one to few years of full time equivalent work. Assuming that a scientist’s year costs on average about 100,000 Euros, one sees that the mission cost is commensurate with that of the scientific work associated with its exploitation[2].
This analysis, rough as it may be, shows that while appearing large in absolute terms, the costs of scientific space missions is an important but not strongly dominant element of the research endeavour. When taking into account the fact that the space mission is the element that makes the discoveries at all possible, it appears like a very worthwhile investment.
Seen on the European scene, the costs of an ESA space mission amounts to some 2 Euros per person, spent over some 10 years.
What Use is this?
Too often when discussing the use of science, the discussion is restricted to technology progress. The impact of knowledge on our culture and the way in which culture then shapes our lives is left out. It is indeed not possible to quantify in any way the impact of the Copernican revolution on our societies. In a similar way we cannot assess the long term importance of the discoveries made from space and in space. However, we know that the images of the Earth as seen from the Moon have had a profound influence on our perception of our planet and on our efforts to strive towards a sustainable way of life. We therefore know that the cultural impact of space science is far from negligible. One could argue that it is the main reason for which our societies should pursue science in space.
The economic impacts of space activities and of fundamental research are difficult to assess. Analyses summarised in appendix 3 of EASAC’s report 25 “European Space Exploration: Strategic considerations of Human versus Robotic Exploration” suggest that the return on investment is a factor two to seven for space activities. Van Bochove studied the impact of fundamental research on the economy and suggests that the return on investment in the present conditions of our societies may be of 20 to 200 times the initial investment (CWTS–WP-2012–003,2012). Looking at applied research, the same study suggests a return on investment of a factor 5 to 25. The difference between these two figures illustrates that not only technological developments contribute to economic development, but also motivation, teaching, industrial practices, etc. All of these aspects are central in fundamental research.
Summary
The Cosmos that we can describe now to our children is vastly different from the story that our parents could tell us. This is mainly due to the pursuit of Astrophysics from space. The Universe is found to be diverse beyond anything imagined before and constantly changing. This will have profound implications for our culture.
The costs associated with a lively space science programme is in line with the expenditure of the associated science community. The cost of a major ESA space mission is at the level of about a Euro per person in Europe over five year.
While we may have the impression that, at some gross level, the understanding of the discoveries made during the last decades has been achieved, we are still discovering new objects and we are very far from being in a position to give a coherent image of the newly discovered phenomenology. Only a vigorous space programme can help us along this path.
The pursuit of a major space science programme in Europe will continue to bring major benefits, intellectual and down to earth, over many years.
The value to society of the Astrophysics performed in space includes not only direct and indirect economic benefits, but also a major contribution to culture.
[1]Named so because the source is in the constellation of Scorpius
[2]This analysis is made for an observatory type mission. These missions perform observations on behalf of a small team that is then responsible for the exploitations of the data. Survey type missions, like Gaia or Plank produce few large sets of data exploited by very large consortia including several hundred researchers working for several years. Here again, one might estimate that the mission cost is commensurate with the exploitation cost.