Even longer baselines: feasibility and observational implications

ALMA's high spatial resolution capabilities, provided by baselines of up to 16km length, have revolutionised observational millimeter/sub-millimeter astronomy. Although challenging in many respects, observations with the longest construction baselines have been highly successful. Offering even longer baselines is an axis of development supported for exploration by the ALMA 2030 Roadmap, but what are the possibilities for realistically achieving this, and what implications would there be on scientific observations? Here I will present the status of an ongoing ALMA development study investigating a simple baseline extension within the current operational model and with minimal technological changes. Results from a recent test campaign of interferometry between antennas at AOS and OSF sites (a 24km baseline with 2.2km altitude difference) will be presented. Beyond technical feasibility, the practicalities of locating new antenna stations, altitude differences and calibration will be discussed. The fundamental issue of surface brightness sensitivity for cool thermal emission (dust, thermal molecular lines) will be discussed, with a view to how this situation may develop in the scope of system upgrades supporting the 2030 Roadmap. Technically easier hot and non-thermal science cases such as the study of stellar photospheres and circumstellar masers will also be discussed.