Two extra-Galactic surveys are considered. The first takes observations of nearly 200 GMCs across a small sample of local galaxies in the CANON CO survey. In spite of the local nature of the sample, results confirm essential facts about molecular gas distribution in galaxies other than our own, including a confirmed linear relationship between GMC virial mass and CO luminosity, which implies a constant CO-H2 conversion factor and supports a virialization assumption. The second survey, PHANGS-ALMA (Physics at High Angular Resolution in Nearby Galaxies with the Atacama Large Millimetre-submillimetre Array), maps CO emission from galaxies up to 17 Mpc away, with resolutions of 1″–1.5″ encompassing active star-forming galaxies down to total stellar masses ~5 × 109 M☉. Within 11 of those target galaxies considered here, the results offer tens of thousands of measurements at GMCs scales between 20 and 130 pc, comparable to Galactic-scale observations, and one outcome is confirmation of a positive correlation between GMC surface densities and velocity distributions.

The asteroid 101955 Bennu is just a pile of rubble, weakly held together by its own gravity, the remnants of a catastrophic event that occurred a billion years ago. But Bennu is also a bearer of both life and death, containing clues about the origins of life on Earth while, at the same time, having the potential to destroy humanity. For over time, the agencies of physics and chance have brought the 500-metre-wide asteroid onto an orbit very near to Earth.

Two HII region surveys are considered. The first is a multi-band survey of over one hundred hypercompact HII (HCHII) candidates using the Jansky-VLA. The second survey, the deep-resolution ALMA Three-millimetre Observations of Massive Star-forming regions (ATOMS-ALMA), studied just under 500 and identified 89 cores that cocoon HCHII or UCHII sources observed in H40α; 32 hot molecular cores (HMCs) showing more than 20 COMs; and 58 HMC candidates not associated with HII regions. The study shows how, in the vicinity of newly formed OB protostars and HII regions at an early stage of evolution, we can begin to understand the dynamics of infall, outflow, and rotational motions, as well as the feedback roles of outflows, stellar winds, and HII regions.

Until recently, those wanting to escape the effects of terrestrial light pollution could leave cities and travel to the countryside to observe the night sky. But increasingly there is nowhere, and therefore no way, to escape the pollution from the thousands of satellites being launched each year. ‘Mega-constellations’ composed of thousands or even tens of thousands of satellites are designed to provide low-cost, low-latency, high-bandwidth Internet around the world. This chapter outlines how the application of the ‘consumer electronic product model’ to satellites could lead to multiple tragedies of the commons, from the loss of access to certain orbits because of space debris, to changes to the chemistry of Earth’s upper atmosphere, to increased dangers on Earth’s surface from re-entered satellite components. Mega-constellations require a shift in perspectives and policies. Instead of looking at single satellites, we need to evaluate systems of thousands of satellites, launched by multiple states and companies, all operating within a shared ecosystem.

The Orion Bar as the canonical high-flux PDR is examined. In addition to a detailed description of the source, the estimation of physical parameters such as ionization fraction and observational indicators such as carbon recombination lines are considered. High-resolution observations point to the sensitivity of carbon chemistry to CR ionization and the apparent merging of C/C+/CO transition and H/H2 transition zones not readily predicted by theory. A wide range of molecular sulphur observations also presents the opportunity to rethink gas–grain reaction networks and model their consequences, with a following chapter looking at the low-flux PDR case of the Horsehead Nebula, through which the sulphur question will be further explored.

Space tourism began in 2001 when an American investment manager paid the Russian space agency US$20 million to travel to the International Space Station on a Soyuz rocket. In 2021, three US-based companies began launching tourists on their own rockets: Virgin Galactic, Blue Origin and SpaceX. The emergence of Space tourism raises difficult issues. One such issue is the environmental effects of launches on the atmosphere and the corresponding implications for climate change. Space tourism also raises difficult questions of international law, including, where does space begin? Who gets to call themselves an ‘astronaut’? Do states have a duty to rescue tourists stranded in space?

Through the emission observations of molecular species in the IRAS2 and IRAS4 locations in NGC 1333 in the Perseus Molecular Cloud (PMC), the distinctions between conditions favouring COMs or WCCC production in the immediate neighbourhoods of low-mass protostars are discussed. The current chemical modelling and that which will follow from accumulating higher-resolution observations using the latest generation of millimetre and submillimetre instrumentation are discussed.

Fourteen super star clusters (SSC) sites are identified in the central bar of the external galaxy NGC 253, and the factors influencing their star-formation efficiency are considered. Molecular emission clearly shows red-shifted emission and blue-shifted absorption line profiles (P-Cygni) characteristic of outflows. While separation of large-scale motions in CO along the line of sight is difficult, CS and HCN are identified as tracing localised and spatially resolved emission within the clusters rather than the foreground gas. The SSCs are shown to fragment into primary clusters surrounded by smaller satellite clusters. From column density and projected sizes, outflow mass and other physical parameters are estimated, and outflow drivers and feedback mechanisms are discussed. The application of chemical clocks, particularly involving sulphur species, is explored and wider molecular comparisons made.

The rapid development of mega-constellations raises difficult issues of international law, including liability for collisions involving satellites. Establishing ‘causation’ – that the actions of one satellite operator caused a specific collision with another space object and resulted in damage – could be a challenge, especially in the context of knock-on collisions where debris from an initial collision later collides with one or more spacecraft, including satellites. A further challenge is determining, in the absence of binding international rules on the design and operation of satellites, what is ‘reasonable’ behaviour and therefore what constitutes ‘negligence’. This chapter also addresses the interference to astronomy that is increasingly resulting from light and radio spectrum pollution from satellites. A full interpretation of the Outer Space Treaty leads to the conclusion that states are already required to take certain steps, including conducting an environmental impact assessment, before licensing mega-constellations, because of the obligation of ‘due regard to the corresponding interests of all other States Parties to the Treaty’.

Within the Large Magellanic Cloud, a hot core is observed associated with the embedded high-mass YSO (IRAS 05195˗6911), known as ST16. Comparative observations with molecular abundances typical of Galactic hot cores are discussed, as is the evidence for a rotating protostellar envelope and outflow cavity. A second LMC source, the prominent star-formation region N113, shows centrally focused star formation with associated point-like mid-infrared emission, masers, and compact HII regions superimposed on extended emission. Gas and dust appear compressed by a complex structure of ionized gas bubbles (prominent in Hα detections) engendered by massive stars in several young clusters. In both ST16 and N113 low-metallicity sources, warm dust appears to inhibit COMs formation and survival, while reaction routes appear broadly comparable with Galactic models.

The opening chapter introduces the most significant areas of contemporary research in the molecular astronomy of star formation: prestellar cores, hot cores, hot corinos, accretion, protoplanetary disks, photodissociation regions (PDRs), HII regions, stellar jets, disk winds, outflows, and masers. These sit within the wider considerations of dense molecular clouds on many scales, from the giant molecular clouds (GMCs) to fragments, filaments, and clumps. Our understanding of these molecular cloud environments depends on our understanding of molecular excitation, energy balance, gas and grain surface reaction kinetics, cosmic ray ionization, and photochemistries. Chemical modelling involving both gas-phase and grain-surface reactions is described, as are the observational and analytical essentials of antenna temperature, optical depth, velocity distribution, column density, beam dilution, relative abundance, rotation diagrams, and radiative transfer modelling.

Anti-satellite weapons that rely on violent impacts and create space debris are regarded as a major threat to the exploration and use of space, including the use of space assets for communications and Earth imaging. This chapter examines two ways in which the testing of such ‘kinetic’ weapons might already have become illegal. First, the accepted interpretation of Article I of the Outer Space Treaty may be evolving as a result of the changing practice of the parties to that treaty. In short, many states are behaving as if tests of anti-satellite weapons that create debris are contrary to the ‘freedom of exploration and use of space’. Second, the same practice and an accompanying opinio juris may be contributing to the development of a parallel rule of customary international law. This chapter also examines the legality of the use of kinetic anti-satellite weapons, as opposed to their testing. Two additional, separate bodies of international law are relevant here: the jus ad bellum governing the recourse to armed force, which includes the right of self-defence, and the jus in bello governing the conduct of armed conflict. A close analysis leads to the conclusion that any use of a kinetic anti-satellite weapon would be illegal today because of the growing crisis of space debris.

The low-mass star formation Lupus complex sits within the expanding HII shell of the Upper Scorpius OB cluster, with shock impacts triggering multiple star formation. IRAS 15398 in Lupus I-1 is considered as a WCCC source rich in COMs, molecular line emissions allowing distinctions between molecules particularly prevalent in either compact or extended regions. Molecular emissions from close to the protostar as well as from gas spreading in outflow material are involved. Within the latter are found distinguishable localized components (‘blobs’) that show likely shock enhanced chemistry. As is the case for IRAS 16293 and NGC 1333, disk emission is separable from envelope emission through characteristic species and levels of molecular excitation.

Two surveys of high-mass star formation (HMSF) are discussed. One is the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL), using the Atacama Pathfinder Experiment (APEX) 12 metre dish. The other uses the Institut de Radioastronomie Millimetrique (IRAM) Northern Extended Millimetre Array (NOEMA). The value of a representative survey of HMSFRs lies in learning what physical and chemical parameters are shared across a variety of sources. The results of statistically large samples of detected, or non-detected, sources such as that of ATLASGAL provide secure data from which to generalise about the typical star-formation process. The results of smaller but still multi-location studies such as the NOEMA sample give us greater specific details, albeit from a self-selecting sample, which may or may not be typical but that we can certainly say are common, at least until future wider surveys demonstrate error.

More than six decades after Sputnik, most rocket bodies used for space launches are still abandoned in orbit. In 2020, over 60 per cent of all launches to low Earth orbit resulted in at least one rocket body being abandoned in orbit. If that orbit has a sufficiently low perigee, drag from gas in the tenuous, uppermost regions of the atmosphere will gradually reduce the rocket body’s altitude and cause it to re-enter the denser, lower atmosphere in an uncontrolled way. This can occur at any point under its flight path, creating dangers for people on the surface and in aeroplanes. Moreover, many uncontrolled re-entries occur near the equator due to the trajectories of many of the abandoned rocket bodies. As a result, the cumulative risk from rocket body re-entries is higher in the states of the global South, as compared to the major spacefaring states. Yet launch providers have access to technologies and mission designs today that could eliminate the need for most uncontrolled re-entries, albeit at increased financial cost.