What would it take to accelerate to Interstellar space by a NASA spacecraft deliberately, within a time that is short enough that the people who designed it and studied it will be available to analyze the findings?
The Voyager spacecraft is currently the only object that has been capable of performing its measurements in interstellar, and its aging instruments have made a planetary “grand tour” the first real sampling of the medium between stars in the history of humanity. NASA terms Voyager Interstellar Mission as a space exploration to investigate the interstellar space beyond the heliosphere an area which is not defined by its distance but by the physics of a boundary where the wind of the Sun gives way to the galaxy.
It is not a line but a layered interface, with well-defined regimes which are not only important to science but also to spacecraft design. The external structure of the heliosphere is made up of a termination shock which marks the sudden slowing of the solar wind, a turbulent heliosheath, and the heliopause or the entrance to interstellar space. Voyager 1 entered interstellar space at approximately 122 AU (approximately 11 billion miles), and the situation was complicated by the demise of one of its major instruments several decades before. The validation was to be based on other diagnostics, such as plasma wave measurements that could be used to deduce changing density around the spacecraft. The subsequent crossing of Voyager 2 was aided by the fact that the probe had a functional plasma instrument, which taught an important lesson to any future venture: interstellar boundary science is instrument-redundancy science, since it is the combination of multiple sensors that detects the existence of an invisible change in the environment.
That lesson is already being incorporated into missions closer in time that would consider the heliosphere a system to be mapped not crossed.
The Interstellar Mapping and Acceleration Probe (IMAP) of NASA is aimed at monitoring the solar wind and energetic particles and developing a more detailed image of the interaction of the heliosphere with the local interstellar environment. Located in L1, some 1 million miles toward the sun beyond Earth, IMAP is an integration of the boundary science with operational space-weather monitoring, giving roughly 30 minutes of notice of approaching dangerous conditions of the particles. Though not an interstellar probe, the approach to measurements of IMAP ten instruments based on concepts of particles and fields as well as signatures of interstellar material illustrates how modern heliophysics considers the heliosphere as a dynamic, protective “bubble” the shape of which and its permeability are of interest to technology and exploration near home.
In the case of a real interstellar precursor, propulsion is the constraint of the gate, since not only is the interstellar space far, but it is time-consuming. The main constraint is bluntly expressed by the NASA technical evaluation of the propulsion systems needed to bring near-interstellar precursors: near-interstellar travel cannot take longer than the career life of the research team. Within that pragmatic window, the ideas that best match flight-ready engineering are not some sort of exotic spacetime shortcuts but mixtures of already proven ideas: giant solar sails to take advantage of sunlight with no propellant spending; electric propulsion scaled to deep space cruise; and judicious applications of gravity or aerogravity assistance to bend up the trajectories with no use of fuel. All of these options transfer the challenge to other engineering fields- ultralight thin films and deployment to sails, long-lasting thrusters and a reliable power source to electric propulsion or thermal and navigation margins to close planetary approaches.
In addition to them, higher-energy ideas such as beamed-energy sails, fusion, and even antimatter which are still primarily theoretical due to their reliance on infrastructure or storage, or energy-handling facilities not yet developed to travel to space are highlighted by the same assessment. The near-term “could actually fly” route thus appears not to be so much as a one-breakthrough, but rather systems integration: durable multi-sensor heliophysics packages informed with the diagnostics hard-won during Voyagers flight, combined with propulsion systems architectures that trade novelty with reliability, and developed on a decade-scale time scale.
