Humanity’s fleet of robotic planetary explorers continues to evolve as missions complete their tasks and new ones are launched to replace them. Thematic emphasis also changes as priorities evolve and mission cadence progresses. Here we review the status of our planetary fleet with an emphasis on target regions.

Outer Solar System

As of 2022, only three missions are currently operating in the outer solar system, with one more on the way and three under construction. The oldest of these, Voyagers 1 and 2, continue to operate in deep space at >100 au, with the objective of mapping out the outer edges of the Sun’s influence. [One astronomical unit (au) is a unit of length effectively equal to the average, or mean, distance between Earth and the Sun, defined as 149,597,870.7 kilometers (92,955,807.3 miles)]. The aging spacecraft, now more than 45 years old, continues to show signs of degradation. Earlier this year, Voyager 1 required the most remote software upgrade ever attempted to correct garbled data transmissions.

New Horizons, exiting the solar system at ~54 au, continues to explore the Kuiper belt by remote observations of distant objects at higher phase angles than possible from Earth, as well as sampling of the solar environment at distances roughly halfway between Earth and the two Voyagers. On the nearside of the outer solar system, the Lucy mission, launched October 16, 2021, is in flight for encounters with Trojan asteroids orbiting in front of and behind Jupiter later this decade to examine the composition and evolution of these ancient bodies.

Juno, the only craft currently located among the four giant planets, is in its extended mission phase, which also includes new observations of Jupiter’s interior and new and higher latitude views of the giant planet as the spacecraft’s orbit evolves. That same changing orbit allowed Juno to conduct the first high-resolution observations of the large satellites Ganymede (in June 2021) and Europa (in September 2022). The Ganymede observations included some of the highest-resolution spectroscopic measurements, the first resolved microwave observations of Ganymede, and new views of areas poorly seen by previous spacecraft. The highly successful Europa encounter just last month should produce similar revelations. Two passes near Io are expected in 2023/2024.

Coming up, construction of both the European Space Agency’s JUpiter ICy moons Explorer (JUICE) and NASA’s Europa Clipper mapping missions is on schedule, with launches planned for 2023 and 2024. These missions will map Jupiter’s moons Ganymede and Europa in unprecedented detail using more than two dozen instruments in an effort to better understand the internal oceans on both bodies and the origins of the magnetic field on Ganymede. NASA’s Dragonfly mission, scheduled for launch in 2027, will explore the chemistry and habitability of Saturn’s largest moon, Titan, and currently continues in the development and instrument construction phases.

Looking to the near future, the National Academy of Sciences’ Decadal Survey of Planetary Sciences earlier this year identified an orbital mission to the ice giants Uranus (or Neptune) as the next national priority. A Uranus mission was the last decadal survey’s third-ranked flagship, and the new survey recommends that work begins on it as soon as budgets permit. It explains that Uranus is a compelling destination due to factors such as its extreme axial tilt, which suggests the planet suffered a catastrophic impact in its past, as well as the prospect that some of its moons may harbor a subsurface ocean similar to the one on Enceladus. While it is too early to say how such a mission will proceed, it should provide the revolutionary science that resulted from the Cassini mission to Saturn in the last decade.

Asteroid Belt

On September 26 of this year, NASA’s Double Asteroid Redirection Test (DART) mission successfully “encountered” Didymos’ small moon Dimorphos by directly impacting the small moon at ~6 kilometers per hour (~4 miles per hour). The objective was to attempt to change the small moon’s orbit in a test of asteroid deflection techniques in case a small asteroid should threaten Earth’s population. The impact was observed by terrestrial telescopes including the James Webb Space Telescope (JWST) and Hubble Space Telescope (HST), as well as a small cubesat that followed behind. While it will take months to fully understand the results of this test, all indications are that it occurred as planned, and the small moon’s orbital period was reduced by 32 minutes. For more information, read the article by Lori Glaze in this issue.

The asteroid sample mission Hayabusa2 (launched by the Japan Aerospace Exploration Agency, JAXA) completed its sample return mission from asteroid 162173 Ryugu in 2021 and is now en route to the rapidly rotating 1998 KY₂₆. OSIRIS-REx (NASA) is currently en route to Earth with its surface sample container from the small asteroid Bennu, with the container scheduled to return to Earth in September 2023. After delivering the sample container, the spacecraft will continue on its journey to visit and orbit Apophis, a small asteroid that will make a close pass of Earth in 2029.

NASA’s Psyche mission to the asteroid of the same name, due for launch this year, was delayed for a year due to software issues.

Mars

An international fleet continues to operate at Mars in orbit and on the surface. A fleet of orbiters from NASA [Mars Odyssey, Mars Atmosphere and Volatile EvolutioN (MAVEN), and Mars Reconnaissance Orbiter (MRO)], the European Space Agency (ESA) (Mars Express and Trace Gas Orbiter), the China National Space Administration (CNSA) (Tianwen-1), and the United Arab Emirates (UAE) Space Agency (Hope) continue to operate, completing the global mapping of Mars; monitoring atmospheric conditions, weather, climate, and atmospheric loss; and supporting the landers currently operating. India’s Mangalyaan mission ended in September when contact was lost.

The CNSA Zhurong rover, part of the Tianwen-1 lander, carries radar, magnetometer, and climatic instruments as well as cameras and spectrometers to search for evidence of ancient oceans at its Utopia Planitia landing site. The InSight geophysical lander continues to record marsquakes but nears the end of its operational lifetime. NASA’s Curiosity and Perseverance rovers continue to explore sedimentary deposits in Gale and Jezero craters, respectively, mapping out geochemical traces of past environments and searching for aquatic conditions suitable for life. Initial results from Perseverance indicate the floor of the crater includes extensive deposits of layered igneous rocks. It will eventually drive up onto what appears to be sedimentary delta-like deposits. The Ingenuity helicopter deployed by Perseverance continues testing the limits of remote-powered flight on other planetary bodies.

Future missions to Mars are planned by several nations, with one highlight being the attempt to return samples from the surface. The last decadal survey’s top recommendation for a flagship mission led NASA to develop its Perseverance rover, which landed on Mars a year ago. Now, NASA and ESA are developing the multi-vehicle Mars Sample Return (MSR) mission to retrieve those samples. MSR is currently NASA’s most expensive science mission project, and, although it was not directly recommended in the last survey, the new survey states that completing it should be the “highest scientific priority of NASA’s robotic exploration efforts this decade.”

Venus

Only one orbiter remains at Venus to observe and monitor Venus’ cloud physics and dynamics: JAXA’s Akatsuki, in orbit since 2015, which is using a suite of instruments to image the clouds in ultraviolet, infrared, microwave, and radio wavelengths and look for evidence of lightning and volcanism on the planet.

Three missions were selected in 2022 to fly to Venus in the early 2030s. These missions are in their early development stages and include NASA’s Venus Investigation of Noble gases, Chemistry, and Imaging (DaVINCI) and Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy (VERITAS) missions, which will examine water loss and other atmospheric processes with an orbiter and descent lander, and map the surface with radar to map out the planets tectonic and volcanic history in much greater detail. ESA’s EnVision mission will use radar to make high-resolution maps of the surface and look for volcanic surface changes.

Mercury

The only mission to Mercury currently in flight or in preparation is ESA-JAXA’s BepiColombo, which has completed several flybys of Venus and Mercury in the past two years. These encounters are decreasing the spacecraft’s momentum in preparation for orbit insertion in December 2025. The spacecraft will separate into two orbiters: JAXA’s Mercury Magnetospheric Orbiter, which will investigate the planet’s unusual magnetic field and its origins, and ESA’s Mercury Planetary Orbiter, which will help improve our understanding of Mercury’s geologic history and composition.

Moon

A total of seven spacecraft from three nations (U.S., China, and India) are currently in operation in lunar orbit or on the surface. China’s Chang’e-4 farside lander and rover are currently the only vehicles operating on the lunar surface. NASA’s Lunar Reconnaissance Orbiter and India’s Chandrayaan-2 orbiters continue to fill gaps in the global mapping of morphology, topography, and composition while mapping out potential landing sites for future manned and unmanned landers.

The Danuri orbiter (also known as the Korean Pathfinder Lunar Orbiter or KPLO) is currently en route to the Moon to map lunar resources. Launched on August 5, Danuri will study the Moon’s surface and help us plan future missions to the lunar poles. Danuri represents the first step in South Korea’s Moon exploration plans, which aims for a robotic landing mission and more.

Plans for future lunar exploration are in flux but include as many as 20 separate programs by at least 6 nations. Landing reattempts by India and Israel are planned within the next year, as well as commercial lunar missions in the near future. Some of these are technology demonstrators, while others such as NASA’s Volatiles Investigating Polar Exploration Rover (VIPER) mission will scout out lunar resources.

The Solar System at Large

Major advances have been made recently in our ability to study the planets with near-Earth observatories. The successful launch and deployment of JWST will open up a high-resolution study of the planets in the infrared, as the first images of Neptune and Jupiter (released this summer) demonstrate. HST continues to study the planets with its unique capabilities. Earth-bound observatories can now challenge these. The Very Large Telescope in Chile rivals JWST in resolving power and has started observing the solar system. Many other observatories also routinely monitor the solar system, but telescope time for these objects is severely limited, and amateur astronomers continue to make highly valuable contributions by monitoring the planets, supporting missions such as Juno or our Mars fleet by observing transient events such as dust storms or impacts into Jupiter’s cloud tops. These efforts will be the subject of an article in a future issue.