Images of moons and planets visited by RHU-heated and RTG-powered spacecraft

RHU-heated and RTG-powered spacecraft have visited many moons and most planets in our Solar System; two of them are in interstellar space. Credit: NASA

The first launch of a radioisotope power system (RPS) by the United States 60 years ago in June 1961 led to decades of historic RPS-powered missions by NASA. These spacecraft have explored every planet in our solar system from Venus outward—including the weather systems of Earth—as well as the polar regions of the Sun.

Several of these hardy robotic emissaries continue to probe the most distant fringes of our solar system, with Voyager 1 and 2 both having broken through to reach the eerie, empty vacuum of true interstellar space.

illustration shows the positions of NASA’s Voyager 1 and Voyager 2 probes outside the heliosphere, the region surrounding our star, beyond which interstellar space begins
This illustration shows the positions of NASA’s Voyager 1 and Voyager 2 probes outside the heliosphere, the region surrounding our star, beyond which interstellar space begins. Credit: NASA/JPL-Caltech

After traveling for more than four decades at thousands of miles per hour, the huge distance from the Voyagers back to Earth is truly mindboggling: It takes more than 42 hours to send a radio signal at the speed of light to Voyager 1 and then receive a response back. Voyager 1 is the most distant human-made object, more than 14 billion miles (22 billion kilometers) from Earth.

Blue dot
Known as "The Pale Blue Dot," this Voyager 1 image shows Earth as a bright speck within the sunbeam, and appears softly blue. Credit: NASA/JPL
Like all NASA missions (and many international ones), the Voyagers use the giant parabolic dishes of NASA’s Deep Space Network (DSN)—the most sensitive radio receivers in the world—to listen to their faint signals. Even the word “faint” is understatement: The power of the radio signal received from Voyager 1 by the 70-meter-wide (227-foot-wide) antenna of the DSN is so small that it would take a trillion years to accumulate enough energy to light up a refrigerator bulb for a single second!

While Voyager 1 gets the prize for most distant spacecraft, Voyager 2 (which is not so far behind) is exemplified by the enduring charms of its famous “Grand Tour.” This mission design took advantage of a fortuitous alignment of the outer planets that occurs every few hundred years to conduct legendary flybys of Jupiter, Saturn, Uranus, and Neptune, with amazing pictures and data provided live to the world on television as they were first received. Both Voyagers are expected to keep operating through at least 2025, as engineers on Earth continue to carefully use their slowly declining power levels so that their propellant lines and other systems remain just warm enough to avoid freezing in the cold vacuum of space.

Voyager's family portrait
This data visualization uses actual spacecraft trajectory data to show the family portrait image of the planets in our solar system from Voyager 1's perspective in February 1990. Credit: NASA/JPL-Caltech

The Voyagers are quickly being followed by NASA’s New Horizons mission, which is predicted to join them in interstellar space in the 2040s. (New Horizons just passed the milestone of reaching 50 times the distance from the Sun to the Earth.)

illustration of spacecraft in the solar system
An illustration of where RTG-powered spacecraft are. In April 2021, New Horizons reached 50 AU: 50 times the distance from the Sun to the Earth. Credits: NASA/JHUAPL/SwRI

After giving humanity its first clear images of mysterious Pluto and its intriguing moons, New Horizons has pioneered the detailed study of even more distant objects in a region known as the Kuiper Belt.

Pluto Up Close
This animation combines various observations of Pluto over the course of several decades.The video shows Pluto as it appeared upon its discovery by Clyde Tombaugh in 1930 and as seen by NASA's Hubble Space Telescope (1990s) and NASA's New Horizons spacecraft in 2015. Credit: NASA (Image courtesy Lowell Observatory Archives) Full image and caption ›

New Horizons has made unique observations of three dozen of these icy bodies, which are believed to be remnants of the formation of the solar system more than 4 billion years ago.

Arrokoth as seen by New Horizons
This composite image of Arrokoth was compiled from data obtained by New Horizons as it flew by the object in 2019. Credit: NASA/JHUAPL/SwRI/Tkachenko
These odd, dark objects seem to have a variety of compositions, shapes, and spin rates, with many of them likely to be binary pairs, such as the “snowman”-like object named Arrokoth that New Horizons flew by in 2019.

In addition, scientists working with imaging data from New Horizons found that the universe is a tiny bit brighter than expected; the source of this “extra light” in the background of stars and galaxies remains unclear, for now.

Both New Horizons and the Voyagers owe a deep debt to NASA’s Pioneer 10 and 11 missions of the early 1970s. The two Pioneers, powered by radioisotope thermoelectric generators, were the first to transform Jupiter and Saturn (and their many moons) from generally blurry blobs seen in Earth-based telescopes to true three-dimensional worlds, with features on their surfaces and in their atmospheres that can be compared and contrasted with similar ones on Earth and the Moon.

Jupiter and Pionner
Left: An illustration of Pioneer 10 during its encounter with Jupiter. Right: Composite of Pioneer 10 images of Jupiter and two of its largest moons, Europa (left inset) and Ganymede (right). Credit: NASA

Over the history of planetary science, the next step after doing a flyby of a planet is often the much harder task—from a propulsion standpoint—of going into orbit for long-term observations. NASA’s Galileo mission did this for Jupiter, and then NASA’s Cassini-Huygens mission did this for Saturn.

The most enduring discovery from Galileo is probably the confirmation of a likely subsurface ocean on Jupiter’s icy moon Europa, as suggested by the uplifted and shifted blocks of ice seen in the Conamara Chaos region. This profound result helped lead NASA to direct JPL to develop the Europa Clipper mission, which is under construction and testing now, and is due to orbit Jupiter and fly by Europa dozens of times in the 2030s to help scientists study the moon in incredible detail.

Europa up close
Galileo's images showed interesting features on Jupiter's largest moon, Europa. The presence of young fractures cutting through Conamara Chaos region indicate that the surface froze again into solid, brittle ice. Credit: NASA/JPL
One of the grandest missions in NASA's six decade-long history, the Cassini-Huygens spacecraft reached Saturn after a seven-year journey, passing by Venus, Earth, and Jupiter along the way.
Enceladus' plume seen by Cassini instruments
Cassini's surprising discovery of geologic activity on Enceladus prompted changes to the mission's flight plan to maximize the number and quality of encounters with the icy moon. Credit: NASA/JPL

Cassini observed Jupiter at the same time as the Galileo spacecraft, and this collaboration allowed scientists to compare results from two sources at once, providing a unique science perspective. Cassini spent 13 years orbiting Saturn 294 times, returning a bounty of information about Saturn's atmosphere, rings, and moons.

Shortly after its arrival at Saturn, Cassini released the Huygens probe, built by the European Space Agency; Huygens penetrated the hazy atmosphere of Saturn's intriguing moon, Titan, in January 2005, the first probe to land on an outer moon.

Even the ending of the mission was dramatic, featuring a carefully planned and executed series of close flybys where the spacecraft slid between the rings and cloud tops of the planet two dozen times before diving in to its final demise to protect any moons from an inadvertent future encounter with it.

Enceladus plumes - 1200
Narrow jets of gas and icy particles erupt from the south polar region of Enceladus, contributing to the moon's giant plume. Cassini flew by the moon many times, the closest approach within 16 miles (25 kilometers) of the moon's surface. Credit: NASA/JPL/Space Science Institute

Closer to Earth, RPS continue to power NASA’s two large rovers on the surface of Mars, Curiosity and Perseverance. Since landing in Gale Crater in 2012, Curiosity has confirmed that the crater had the right habitable conditions to support the possible existence of life on the Red Planet more than 3 billion years ago, at the same time life was beginning to flourish on Earth.

Perseverance on Mars
Like its twin rover Curiosity, Perseverance is powered by a Multi-Mission Radioisotope Thermoelectric Generator, or MMRTG, that converts heat from the natural radioactive decay of plutonium into electricity. Credit: NASA/JPL-Caltech/MSSS

This success gave NASA the confidence to design and build its closely related successor, Perseverance, which landed in February 2021.

After supporting the Ingenuity Mars helicopter as it made the first powered, controlled flights on another planet, Perseverance is beginning to explore the ancient river delta inside Jezero Crater. Eventually, it will gather carefully selected samples of Mars rocks and soil that could be returned to Earth by future missions, which are now in the detailed planning stages.

Apollo 14
An Apollo 14 astronaut deploys the Apollo Lunar Surface Experiments Package's power source (foreground) and Central Station (background), where the Lunar Dust Detector was mounted. Credit: NASA/JSC

Earth’s Moon represents the past achievements and future promise of RPS-powered exploration. RPS went to the Moon with every Apollo astronaut crew, providing power for a variety of surface experiments that studied the geophysical environment of the Moon; designed for only 1-2 years of operation, the experiments were shut down after up to eight years of making first-of-their-kind measurements.

Today, NASA is planning for a return of astronauts to the Moon, including the first woman. Part of this exploration could include a demonstration of the first dynamic RPS in space—this type of system uses a piston or other moveable element to generate electricity about three times as efficiently as the radioisotope thermoelectric generators that have been flown to date.

Venus seen by Galileo
This colorized picture of Venus was taken in 1990 about 6 days after Galileo's closest approach to the planet. It has been colorized to a bluish hue to emphasize subtle contrasts in the cloud markings and to indicate that it was taken through a violet filter. Credit: NASA/JPL

And while Venus has never been the focus of an RPS-powered mission, many past missions such as Galileo and Cassini flew by the cloud-covered planet on the way to their final destinations, helping provide data to sustain a community of researchers who are now beginning to anticipate a bounty of new information from the two recently selected NASA Discovery missions, VERITAS and DAVINCI+, and ESA’s newly announced EnVISION mission.

Looking further ahead, the next NASA mission that plans to use RPS power is Dragonfly, a dual-quadcopter that would explore a variety of locations on Saturn’s mysterious moon Titan. (Interestingly, NASA’s two Voyager spacecraft observed Titan in 1979 and 1980, but the thick organic haze in the atmosphere obscured the surface at visible wavelengths.) The dense, calm atmosphere and low gravity on Titan make flying an ideal way to travel to different areas. In under an hour, Dragonfly would cover tens of miles or kilometers, farther than any planetary rover has traveled.

Dragonfly 1200
Dragonfly mission concept of entry, descent, landing, surface operations, and flight at Titan. Credit: Johns Hopkins University APL

Dragonfly would sample surface materials and determine their composition in different geologic settings. This revolutionary mission includes the capability to explore diverse locations to characterize the habitability of Titan's environment, to investigate how far prebiotic chemistry has progressed, and even to search for chemical signatures that could indicate water-based and/or hydrocarbon-based life.

After six historic decades of use in space, it’s clear that radioisotope power systems will provide NASA missions with the power to explore for many, many years to come.

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