**4.1 Solar system probes**

There have been numerous scientific probes launched primarily by the United States, the European Union, and Russia/Soviet Union since the late 1950s [19]. This list of participating countries has expanded to include many more nations as launch capabilities extend to additional nations. The probes have included a variety of purposes and target planets.

Missions include flybys of a planet, moon, or other space objects; orbiting these bodies; atmospheric entry; impact with the space object; and soft-landing on the surface. Target space bodies include all Solar System planets, the Moon, dwarf planets, asteroids, moons of planets, asteroids, and comets. Probes have also been launched into Solar orbits.

The probes have expanded our knowledge of planetary systems including their masses, atmospheric composition, surface characteristics, and temperature and pressure profiles. Additional studies focused on major moons as well as other orbiting space objects including comets and asteroids.

Ref. [19] provides a detailed historical listing of space probes during 1958–2016. Since 2016, the number of probes and their sophistication has improved. Although, Mars and the Moon are popular destinations, recent data, suggesting the appearance of phosphine in the atmosphere of Venus, has increased focus of that planet [21]. The number and scale of future probes will reveal additional information regarding these planets. For this reason, this chapter has been written in a general manner and does not address specific probe objectives or data. However, some of the chapters of this book illustrate selected probes and their capabilities.

### **4.2 Exoplanet probes**

The Exoplanet Exploration Program's roadmap of NASA's exoplanet missions provides a summary of existing and planned probes [20]. NASA has a number of ongoing exoplanet and planned probes. Current instrumentation includes the Hubble, Spitzer, and Kepler/K2 Space Telescopes. These instruments have significantly improved knowledge of exoplanetary systems though the discovery and characterization of transiting systems. Another device, the Transiting Exoplanet Survey Satellite (TESS) launched in 2018, adds an additional tool to expand the

knowledge of exoplanetary systems. The James Webb Space Telescope (JWST), having a projected late 2021 launch date, includes mid-infrared transit spectroscopy, and has the capability to directly image massive exoplanets. In addition, the Nancy Grace Roman Space Telescope tentatively planned for launch about 2025 includes infrared capability that could facilitate direct exoplanet imaging.

Additional missions are possible if oversight groups recommend their viability, and funding becomes available. These include the Large Ultraviolet Optical Infrared Surveyor (LUVOIR) and the Habitable Exoplanet Imaging Mission (HabEx) that have the potential to directly image and characterize Earth-like exoplanets. These devices also have the capability to detect the spectra of molecular forms including water and oxygen. The Origins Space Telescope (OST) is a conceptual infrared mission that is intended to measure the atmosphere of exoplanets. These additional space missions have the capable of observing the necessary characteristics for life to exist on exoplanets.
