Discourse 159 – Journey through the Solar System and Beyond: The Odyssey of the Voyager 1 and 2 Probes | DOCUMENTARY

Journey through the Solar System and Beyond: The Odyssey of the Voyager 1 and 2 Probes | DOCUMENTARY

The Voyager probes' journey began on August 20, 1977, the launch date of Voyager 2, followed by Voyager 1 on September 5 of the same year. NASA, which had long been interested in studying the four most distant planets in the solar system (Jupiter, Saturn, Uranus, and Neptune), devised a mission that would take advantage of the favorable orbital alignment predicted for the late 1970s, calculated by engineer Gary Flandro. The use of gravitational assistance was necessary to pass from one planet to another and would have required launch within that period, as a similar opportunity would not arise again for another 176 years.

Gravitational assistance allows a space probe to change its speed and trajectory by exploiting the gravity of planets. To increase the chances of success, NASA sent two probes, designed with an expected lifespan of five years, which are still operational today. The main objective was to collect data on Jupiter and Saturn, including aspects such as cartography, geology, morphology, atmospheric composition, and the study of rings and satellites, particularly Titan. If the initial objectives were achieved, data collection would be extended to Uranus and Neptune, about which very little information was available at the time.

After launch, Voyager 1 had already reached 11.66 million kilometers from Earth on September 18, 1977, taking the first image that simultaneously captured Earth and the Moon. Thanks to a shorter trajectory, Voyager 1 passed Voyager 2 the following December. Both then crossed the asteroid belt, taking nine months to do so, thus achieving the first objective of the mission.

From 1979 to 1989, the Voyager probes explored the four gas giants. Jupiter, visible to the naked eye since ancient times and observed through a telescope by Galileo in 1610, is the largest planet in the solar system. In 1973, Pioneer 10 flew past Jupiter, followed by Voyager 1 and Voyager 2 in March and July 1979, respectively, approaching the planet to within 210,000 km (Voyager 1) and 570,000 km (Voyager 2).

The probes made it possible to observe Io, one of Jupiter's moons, previously considered to be geologically inactive. The images sent back showed a young, craterless surface, suggesting intense volcanic activity, which was confirmed by subsequent detections of nine active volcanoes. The plumes of material can reach speeds of over 1 km/s and heights of 300 km. Currently, over 400 volcanoes have been identified on Io, representing the first observation of this type outside Earth. However, the images captured at the time were of lower resolution than today's standards.

Over the past five years, researchers have collected images from the top of a Hawaiian volcano to create a detailed atlas of Jupiter's moon Io. They found that its volcanoes are distributed in an unusual way: the brightest eruptions are concentrated in the hemisphere facing Jupiter, contrary to theoretical models. It is unclear whether the subsurface is composed of an ocean of magma, several pockets, or a spongy layer. Understanding these puzzles may help study the formation of Earth.

The intense tides generated by Io's eccentric orbit and Jupiter's gravitational pull cause internal friction, producing large amounts of heat and giving rise to volcanic activity unique in the solar system. Io's average surface temperature is -130°C, but the lava reaches 1500°C because its thin atmosphere does not retain heat. Ten percent of the thermal energy comes from the Loki Patera volcano, observed with new and old data. Volcanic debris covers vast areas and feeds Jupiter's magnetosphere.

Io, the most geologically active body in the solar system, has also been studied by the Juno probe, which has closely observed its volcanoes and sulfur-rich lava lakes. The continuous collapse of the crust generates new eruptions.

Voyager 1 identified Jupiter's rings and discovered three new natural satellites: Thebes, Metis, and Adrastea. It also documented the main Galilean satellites, namely Io, Europa, Ganymede, and Callisto. Among these, Europa is of particular interest to the scientific community due to the presence of an icy surface that could conceal an ocean of water. The European Space Agency's JUICE and NASA's Europa Clipper missions are scheduled to explore this celestial body by 2030 with the aim of detecting liquid water beneath the icy crust. Probes have already flown over Ganymede in 1979, helping to deepen our knowledge of the satellite's size and morphological characteristics.

It was later determined that Ganymede exceeds Titan in size, making it the largest natural satellite. The Galileo probe provided further details about Ganymede during six flybys between 1996 and 2000. Voyager 1 also transmitted the first images of Jupiter's rings, revealing the presence of structures similar to those observed on Saturn, which had not been detected previously due to their thinness and low brightness; this required a significant approach to the planet. The rings are divided into three main sections: the innermost torus-shaped section, the main ring (about 30 km thick), probably composed of dust from the satellites Adrastea and Metis, and the gossamer ring, located furthest from the planet and consisting of debris from Amalthea and Thebe. The latter is remarkably thick, several thousand kilometers, and is gradually disappearing into interplanetary space. There is also a very thin outer ring orbiting Jupiter in a retrograde direction. The density of the Jovian rings is significantly lower than that of Saturn's, and the micrometric particles scatter light, making them difficult to see from Earth. The gaps in the rings coincide with the location of the small moons Metis and Adrastea, while two other diffuse rings surround the satellites Amalthea and Thebe. Observations suggest that these rings were formed from dust ejected from the lunar surfaces following impacts. Overall, the probes collected approximately 33,000 photographs of the Jovian system.

Thanks to Voyager 1's flyby at a distance of 350,000 km from Jupiter and the images obtained, scientists were able to confirm that the Great Red Spot, observed by Cassini in 1665, is a vast anticyclone composed of clouds over 160 km thick. It has also been found that the storm is gradually shrinking; its size has decreased from 40,000 km, as estimated by 17th-century scholars, to 23,000 km in 1979 (Voyager 2 detection) and to 16,500 km according to measurements by the Hubble Space Telescope in 2014. Although it is expected to disappear within seventy years, its spatial dynamics remain unpredictable. The variation in the shape of the spot, which is becoming increasingly round, indicates an evolution that is still poorly understood. Further studies have expanded our understanding of Jupiter's atmospheric and magnetospheric processes, including the polar auroras highlighted by sophisticated magnetic field measurements. Jupiter's magnetosphere is currently the largest and most powerful in the solar system, covering over seven million kilometers in the direction of the Sun, almost equaling Saturn's orbit in the other direction. This structure absorbs energy from the solar wind and releases it through phenomena such as magnetic substorms and auroras, generated by strong magnetic currents around the poles.

From an atmospheric point of view, Jupiter has the most significant composition of any planet in the solar system, consisting mainly of hydrogen and helium, with traces of methane, ammonia, hydrogen sulfide, and water. The atmosphere is 5,000 km thick and is divided into layers: the exosphere, thermosphere, stratosphere, and troposphere. The latter is home to a complex system of layered clouds and fog, mainly formed by ammonia and hydrosulfite, accompanied by light and dark bands whose chromatic origin is still being studied. These atmospheric elements are closely monitored, mainly thanks to data provided by the Juno mission, which has allowed us to appreciate the wide range of dynamic phenomena, cyclones, anticyclones, storms, lightning, the Great Red Spot, and the Ba Oval.

As of February 2024, Jupiter has 95 confirmed natural satellites. The moons are now divided into six main groups, including the Amalthea group with close orbits and minimal inclination. After Jupiter, the Voyager probes studied Saturn, analyzing its rings and providing fundamental images in the 1980s. Voyager 1 observed Titan at 6,490 km, Tethys (with a 2,000 km canyon), Mimas, Enceladus, and Hyperion. Voyager 2 revealed the large Odyssey crater on Tethys. Saturn, the second largest planet, is not very dense and has some of the fastest equatorial winds in the solar system. Its atmosphere, composed mainly of hydrogen and helium, shows parallel bands due to methane; the clouds vary with depth.

Saturn's magnetosphere, similar to Jupiter's, generates polar auroras and is rich in plasma thanks to the geysers of Enceladus. Voyager 1 discovered four new moons that act as shepherd satellites for the rings. Enceladus shows surface activity and geysers that are potentially interesting for the search for life, although no dedicated missions are currently planned.

Titan, outside Saturn's magnetosphere, is subject to solar wind. It has lakes and rivers of liquid methane and ethane and an extremely low surface temperature (-179°C). Its dense atmosphere makes the surface arid; even an asteroid impact and the temporary presence of liquid water would not be enough for life to develop. Some speculate that life forms exist in underground oceans, but the temperature seems too low to allow biological reactions.

The images from the probes motivated the Cassini mission, which landed the Huygens module on Titan in 2005. Saturn has at least 146 satellites. The rings were observed by Galileo in 1610 and identified as such by Huygens in 1655; they were later found to be composed of many icy particles. The Voyager and Cassini missions revealed their composition, structure, and the presence of shepherd satellites that delimit their extent.

Voyager 2 transmitted images of other icy moons such as Tethys and Dione. Despite some technical problems during the flyby, our knowledge of Saturn has increased considerably. Continuing on to Uranus, Voyager 2 sent back numerous photos and data on the planet, its moons, and its magnetic field, revealing details previously unknown, especially about the planet's southern hemisphere.

The southern hemisphere of Uranus is distinguished by the presence of a bright polar cap and dark equatorial bands; between these two regions emerges a bright band called the ‘southern collar’, believed to be, together with the cap, an area dense with methane clouds. Voyager 2 detected ten small bright clouds north of the collar, with no other significant activity observed on the planet. Uranus' rotation period is 17 hours and 14 minutes, while the average temperature of the upper atmosphere has been measured at -224°C, proving to be surprisingly uniform even at the poles, regardless of their exposure to the Sun.

Uranus is the coldest planet in the solar system, with an atmosphere composed mainly of hydrogen and helium; the atomic ratio between these elements was determined by spectroscopic analysis and occultation observations made by the Voyager 2 probe. The presence of methane gives the planet its characteristic blue color. Uranus' atmosphere is remarkably stable compared to other gas giants, as evidenced by the small number of cloud formations observed during the passage of Voyager 2.

Uranus' axis of rotation is strongly tilted, giving the impression that the planet is "rolling" along its orbit, positioning the north and south poles at the equator. During the mission, eleven new moons and additional rings were discovered; the moons discovered were named after literary characters, including Juliet, Portia, Cressida, Desdemona, Rosalind, Belinda, Perdita, Cordelia, Ophelia, and Bianca. Puck is the largest inner satellite, with a diameter of 162 km, and is the only one for which Voyager 2 images show significant details.

The first hypotheses about the existence of Uranus' rings date back to 1787, with a detailed description of the Epsilon ring; the system was officially confirmed in 1977. Voyager 2 subsequently identified two more rings during observations conducted between 1985 and 1986; in 2005, the Hubble telescope discovered more distant ones. These rings, relatively young compared to the age of the solar system, are thought to have originated from debris from the moons.

Before the probe's flyby in 1986, no data on Uranus' magnetosphere was available. Observations revealed an unusual magnetic field, with its origin offset by about 8,000 km from the geometric center and tilted by 59° from the axis of rotation, generating an asymmetrical magnetosphere. The magnetic intensity varies significantly between the poles, and in 2017, analysis of Voyager 2 data led scientists to identify a daily magnetic reconnection process between Uranus' magnetosphere and the solar wind. This particular configuration may be a common feature of ice giants, given the similarity with Neptune. Despite these peculiarities, Uranus' magnetosphere shows similarities to those of other planets, although the magnetic tail has a corkscrew shape due to lateral rotation; polar auroras appear as bright arcs around the magnetic poles.

In 2020, in-depth analysis of magnetosphere data conducted by NASA astronomers led to the discovery of a plasmoid: an autonomous structure consisting of plasma and a magnetic field, which gradually removes part of the planetary atmosphere. The presence of this magnetic bubble had already been recorded in 1986, but it was only effectively identified thirty years later.

Voyager 2 also provided images of Miranda in January 1986, Uranus's satellite, of which the best images are available thanks to the probe, although they are limited to the southern hemisphere due to darkness in the northern hemisphere during the flyby. Miranda is the smallest of Uranus' five main satellites, with a diameter of less than 500 km and a distance of 129,900 km from the planet. The surface appears to be composed of water ice mixed with silicates, carbonates, and ammonia, presenting a diverse geography with overlapping craters, cliffs, and canyons up to 20,000 m deep, and mountains up to 24,000 m high. This chaotic landscape suggests intense past geological activity, the causes of which remain under study. Some scientists speculate that Miranda may have been subjected to numerous meteor impacts or undergone incomplete differentiation of its interior.

The flyby of Uranus took place on January 24, 1986. Subsequently, observations conducted using the Hubble Space Telescope and major ground-based telescopes have provided more detailed images and detected seasonal changes, increased meteorological activity, and intense winds. Plans to send an orbiter to Uranus are currently being evaluated, although details have not yet been finalized.

Exploration then turned to Neptune, which was reached in 1989 with a flyby just 48,000 km from the planet. As Neptune was the last celestial body flown over by Voyager 2, there were no constraints on the trajectory for leaving the planetary system; the scientific team therefore chose a close pass over the planet's North Pole, taking advantage of gravitational assistance to approach the satellite Triton. Voyager 2 came within 4,950 km of the North Pole, a distance that made radio links increasingly difficult. In the periods leading up to the flyby, various measures were taken to strengthen the network of ground antennas.

This was the first time a probe had approached Neptune, arousing considerable interest both in the scientific community and among the general public thanks to the spectacular images transmitted. The planet's characteristic blue color is due to the absorption of the red component by methane, while other gases that have not yet been identified contribute to the blue-violet hue. In the absence of methane, Neptune would in fact have taken on a green color.

Voyager 2 sent back around 10,000 images, including one of a vast dark spot and clouds indicative of intense atmospheric activity, similar in dynamics to Jupiter's Great Red Spot. This spot represents a huge anticyclone over 1,000 km in diameter, extending into the upper atmosphere and generating white cirrus clouds composed of methane crystals, which sublimate within a few hours. These cloud phenomena persist even after 36 hours, equivalent to about two revolutions of the planet, and the large dark spot, although it has disappeared, has been replaced by other similar formations.

The probe also documented a small dark spot with a bright core, which grew during the approach. Neptune's atmosphere is distinguished by its high variability and is composed of 84% hydrogen, 12% helium, 2% methane, and traces of ammonia, ethane, and acetylene, making it similar to that of Uranus. The strongest winds in the solar system have been recorded on Neptune, with speeds exceeding 2,000 km/h. Atmospheric stratification causes very cold conditions in the outer sections and high temperatures at depth, generating bubbles that give rise to these violent winds. Recent measurements have established that the planet's mass is 0.5% less than initial estimates, corresponding approximately to the mass of Mars.

Weak polar auroras have been observed, as the magnetic field is inclined at 47° to the axis of rotation. Neptune's rotation period has been determined to be 16 hours and 6 minutes. Voyager 2 identified six new satellites: Despina, Galatea, Larissa, Proteus, Naiad, and Thalassa, bringing the total number of known moons to 14, later increased to 16. The innermost satellites orbit within Neptune's rings; only Proteus was discovered in time to plan detailed observations. The latter has an irregular shape and is the maximum possible size for an object of its density without assuming a spherical shape.

Triton, the main satellite, was discovered in 1846; recent data has allowed geysers with plumes up to 8 km high to be identified. Voyager 2 approached to within 39,790 km of the surface, providing basic information about this moon, including observation of about 40% of its surface, which was found to be relatively smooth. The southern polar cap, Hulanda Regio, was the first region examined by the probe, revealing an irregular, pinkish, highly reflective surface. Triton's diameter is currently estimated at 2,700 km.

Characterized by a retrograde orbit, Triton must have formed elsewhere and was subsequently captured by Neptune. The tidal interaction with the planet results in a gradual loss of energy, which in the long term could lead to the fragmentation or fall of the satellite. Voyager 2 detected a faint atmosphere around Triton, comparable to 1% of Earth's, and a minimum surface temperature of -235°C, the lowest ever recorded in the solar system. Traces of cryovolcanism have been found, due to the melting of ice, with emissions of liquid nitrogen, dust, and methane compounds from the subsurface. One image shows a jet rising up to 8 km, carried by winds for a further 140 km.

Triton's morphology indicates a long geological history, highlighted by the recent renewal of much of its surface and a peculiar axis of rotation, which influences the orientation of the polar and equatorial regions with respect to the Sun. Voyager 2 made its flyby while the south pole was facing our star. Triton's composition is believed to be about 25% water ice and the rest rocky material.

The mission also provided the first images of Neptune's rings, clarifying previously little-known aspects of the dark rings discovered in 1984. The images confirmed the existence of complete and numerous rings: the three main ones are Galle, Le Verrier, and Adams. The Adams ring includes arcs of material named Liberté, Egalité, Fraternité, and Courage, which correspond to the brightest sections. Neptune's rings are also considered relatively young, probably formed as a result of the collision of pre-existing moons with the planet's rocky boundary. Terrestrial observations indicate the unstable nature of the rings, with images from the VM Keck Observatory in 2002 showing degradation compared to Voyager 2 data: by 2003, the Courage arc was close to extinction and had completely disappeared by 2009.

Voyager 2 is currently the only spacecraft to have flown past Neptune, providing essential data that is the main source of current knowledge about this planet. The images collected were broadcast live during the "Neptune All Night" special.

The next phase of the exploration of Neptune and Triton is part of the flagship program; this mission, called Trident, could be launched by 2025 with the goal of reaching Triton in 2038. After observing the four gas planets, Voyager 2 followed a trajectory southward along the ecliptic, the plane of the orbits of the planets in the solar system.

In 1990, Voyager 1 received instructions from NASA to photograph the planets it had visited from the outside, including a portrait of Earth taken from a distance of 6.4 billion kilometers. In 1994, Voyager 2—also 6 billion kilometers from the Sun—was used to observe the impact of comet Shoemaker-Levy 9 on Jupiter, recording radio signals without directly detecting traces of the event.

On April 17, 2010, Voyager 1 was 112 astronomical units from Earth and was operating in interstellar space, having passed the heliosphere in 2012, followed by Voyager 2 in December 2018. The heliosphere, bounded by the so-called terminal shock, is a region where the solar wind slows down to become subsonic; Voyager 2 crossed this zone in 2007, entering the heliosheath, composed of hot gases emitted into the interstellar region by solar winds.

The data collected has made it possible to estimate the radius of the heliosphere at approximately 17 billion kilometers, including the Sun, the eight planets, and numerous smaller orbiting bodies. The boundary of the heliosphere, called the heliopause, is the point beyond which the solar wind is stopped by the interstellar medium. Both probes thus reached interstellar space, composed mainly of a mixture of gas, cosmic rays, and dust. Although there is general agreement on the density of the particles encountered, the data transmitted sometimes differ, suggesting new hypotheses about solar motions in the galaxy.

When Voyager 1 crossed the heliopause, the interstellar magnetic field was found to be between two and three times higher than expected, exerting greater pressure than previous estimates. The plasma measurement instrument on Voyager 1 had not been operational since 1980, but the one on Voyager 2 allowed detailed analysis in 2018, confirming that near the heliopause, plasma slows down, heats up, and increases in density. Both Voyager 1 and Voyager 2 have detected a porous nature of the heliopause, highlighting the possibility of interstellar particle filaments penetrating the heliosphere itself.

Observations have shown that the solar wind slows down significantly before the heliopause threshold, a finding confirmed by both probes. However, the overall shape of the heliosphere remains uncertain: it could be oval, round, crescent-shaped, or have a comet-like tail, depending on the influence of the interstellar medium.

The two Voyager probes have not left the solar system but are continuing their trajectory toward unexplored regions beyond the solar wind. This second part of the mission is designated as the Voyager Interstellar Mission. During the journey, various technical problems were encountered and solutions were found. For example, shortly after the launch of Voyager 2, one of the antennas did not close properly; the problem was solved by physically modifying the structure. In another case, it was necessary to reduce the number of engines used to two, thus optimizing fuel consumption.

During the maneuvers, further operational difficulties arose, such as Voyager 2 unexpectedly entering safety mode and communication problems due to the antenna being out of focus. Operators gradually adopted manual frequency control measures to restore functionality.

The hardware and software capabilities of the probes are limited by the technological constraints of the 1970s: the available memory amounts to 69 KB and the transmitter power is only 23 W. Today, a small team of specialists manages the remaining operations.

As the probes age, technical problems are increasing: some main thrusters have lost efficiency, but backup engines have been reactivated to adjust the orientation of the antennas. Since 2002, Voyager 1 has been using its secondary altitude control system, while isolated errors in the computer systems have necessitated several phases of remote maintenance, as demonstrated by episodes of temporary inactivity of Voyager 2 and problems with antenna orientation or internal data routing.

The Voyager probes have faced various technical difficulties, including thruster problems and antenna misalignment, which have been resolved by NASA engineers. Voyager 1 no longer sends reliable data but remains active; the search for solutions is hampered by the need to consult paper archives from the 1970s and long signal transmission times. Currently, Voyager 1 is 24 billion kilometers from Earth, Voyager 2 is 20 billion kilometers away. Both are still communicating, although many instruments have been turned off to save energy. Communication with Voyager 2 is expected to last for a maximum of five more years. The nuclear generators are gradually losing power, further limiting the probes' functions.

Both carry the Golden Record, a disc containing sounds and images of Earth, designed as a symbolic message to any extraterrestrial civilizations. The probes, designed to last five years, have been active for almost half a century, contributing enormously to our knowledge of the solar system thanks to tens of thousands of images and data collected.

According to estimates, Voyager 1 will pass close to the star a+79 3888 in forty thousand years, while Voyager 2 will pass close to Sirius in about 296,000 years. When the signal becomes too weak, the probes will continue their journey through space without communication with Earth.

The Milky Way is about 13 billion years old and, in four billion years, will probably collide with Andromeda. Recent studies using asteroseismology have allowed us to revise the age of its thick disk, confirming the ancient origin of our galaxy.

Overall, the two galaxies will merge to form a single structure.

Imagine the extraordinary stellar panorama that this new mega-galaxy could offer. Meanwhile, the Milky Way continues to arouse the interest of the scientific community. In 2010, astronomers identified emissions of enormous gas bubbles located on either side of its center. These structures, known as Fermi Bubbles, are characterized by a radiating shape similar to the number eight and oriented perpendicular to the galactic disk. Currently, the exact nature of these bubbles remains uncertain: some scholars speculate that their symmetry may be directly related to the galactic center. They could be manifestations related to the death of stars in the Sagittarius A* region, home to our galaxy's supermassive black hole.

The Milky Way evolves within the universe among a multitude of stars; it has been established that it belongs to a group known as the Local Group, which includes over sixty galaxies, mainly dwarfs, all bound by common gravitational forces. However, the Local Group is only a fraction of an even larger cluster, the Virgo Supercluster, whose outskirts are home to our galactic system. This supercluster comprises at least one hundred groups and clusters of galaxies and extends for about two hundred million light-years. It is itself part of an even larger structure, the Laniakea Supercluster, composed of over one hundred thousand galaxies and extending over five hundred and twenty million light years. Not all scientists agree on the gravitational cohesion of the galaxies in this latter structure; according to some theories, these subgroups could disperse over time.

Returning to the Local Group, despite its small size, it has a complete range of galactic types, with the exception of giant elliptical galaxies, which cannot develop in such limited contexts. The size of the Local Group is estimated at around ten million light-years, with a total mass of 2,300 billion solar masses. The galaxies furthest from the Solar System are about five million light-years apart. Although the group mainly contains dwarf galaxies, it also includes three large, massive galaxies: the Andromeda Galaxy, probably the largest, the Triangulum Galaxy, the smallest, and of course the Milky Way. The main members of the Local Group, the Andromeda Galaxy and the Milky Way, are surrounded by a variable number of satellite galaxies; however, some galaxies in the group appear to be independent and will be the subject of further study. All are connected to the gravitational center of the Local Group, located between the Milky Way and the Andromeda Galaxy.

The Milky Way is home to hundreds of billions of stars, with estimates ranging from 200 to 400 billion. It is believed that there are more than 100 billion planets within this vast cluster, whose diameter is between 100,000 and 200,000 light-years. Most of the stars are concentrated in the galactic center, while beyond 120,000 light-years their number decreases dramatically. Beyond the visible boundaries of the Milky Way lies a mixture of interstellar gas scattered throughout intergalactic space, but there may be other objects that have not yet been detected.

Interactions with other galaxies have significantly influenced the morphology of the Milky Way. Recent studies suggest that about 700 million years ago, a close passage caused major gravitational disturbances, traces of which are still visible in the galactic disk, which shows deformations and movements toward the constellation Pegasus at a speed close to 115,200 km/h (32 km/s). In addition, the Sagittarius dwarf galaxy, one of the closest satellites to the Milky Way, is identified as the main cause of the ripples observed in the outer edges of the galaxy and may have contributed to the formation of the Sun.

Collisions between galaxies do not manifest themselves through explosive events, but generate chain reactions and progressive dynamic changes that can substantially alter the structures involved. The Sagittarius dwarf galaxy has had numerous close encounters with the Milky Way over the last six billion years, inducing stellar ripples and activating star formation processes.

According to some astrophysical simulations, a violent collision six billion years ago favored the genesis of the Andromeda galaxy. The models faithfully reproduce the structural characteristics of this galaxy: the thin disk, the gas ring, the central bulge, the thick and extended disk, and the streams of ancient and bright peripheral stars. Studies indicate that Andromeda is the result of the merger of two galaxies, one slightly more massive than the Milky Way and the other three times smaller. The merger, which took place about 5.5 billion years ago, was the most significant event in the history of the local group, considering the large amount of baryonic matter involved.

The initial collision was of considerable intensity, causing the ejection into space of huge masses of gas and stars, equal to about one-third of the mass of the Milky Way, which formed tidal tails that subsequently dispersed. One hypothesis attributes the origin of the Magellanic Clouds to these tidal tails, considering both their gas richness and their irregular shape, consistent with the properties of formations resulting from galactic collisions.

These results confirm, on the one hand, that spiral galaxies developed through successive mergers and, on the other, that many dwarf galaxies are derived from tidal tails. However, the enigma of dwarf galaxies that do not appear to be associated with a host galaxy remains. It is therefore appropriate to analyze other members of the local group in depth, with particular reference to solitary galaxies.

The irregular dwarf galaxy Sagittarius, distinct from the famous spheroidal dwarf galaxy of the same name, is located in the constellation Sagittarius, about 4.2 million light-years from the Sun. Discovered in 1977 using photographic plates from the European Southern Observatory in Chile, the galaxy has a peripheral position relative to the local group, making it one of the most eccentric known galaxies, and is brighter than the dwarf galaxy in Aquarius. Its stellar population includes numerous intermediate-age stars and as many as 27 carbon stars, characterized by carbon-rich atmospheres and a reddish color due to the absorption of short wavelengths. Despite this, most of the stars present are relatively young, with ages ranging from 4 to 8 billion years.

Most of the stars in the irregular Sagittarius dwarf galaxy are metal-poor, with an iron metallicity index of less than 1.3. This galaxy, which has no defined shape, evolves independently at the edge of the local group and was captured by the Hubble Space Telescope in 2003, which photographed the trail of an asteroid next to it. The image shows 13 reddish arcs indicating the path of the asteroid, about 2.15 million light-years from the Milky Way in the constellation Ophiuchus.

Another irregular dwarf galaxy, IC10, discovered by Lewis Swift in 1889, was recognized as a galaxy in the local group in the 1960s thanks to its radial velocity of 350 km/s. Although close by, it is difficult to study because it is hidden by dust and stars, but it reveals a population of young stars and at least 27 planetary nebulae. IC10 is known for its high stellar activity; it hosts an average of 5.1 Wolf-Rayet stars per square kiloparsec and shows a long history of star formation, supported by a vast envelope of interstellar gas. The rotation of the hydrogen bubble surrounding IC10 is independent of the galactic rotation. In the core, X-rays come from a binary system consisting of a Wolf-Rayet star and a very massive stellar black hole.

The irregular dwarf galaxy Cetus (IC 16613 or Caldwell 51), located about 2.4 million light-years away in the constellation Cetus, is home to approximately 100 million stars, including some Cepheid variables that play a key role in measuring cosmic distances. Most of the stars in this galaxy originated about 7 billion years ago; star formation has now ceased. Discovered in the early 20th century, it appears as a faint diffuse patch unobscured by interstellar dust, a feature that allows for accurate study of its variable stars and precise determination of distances. Caldwell 51, which is dust-poor and easily observable, therefore allows for direct analysis of stellar properties.

RR Lyrae-type variable stars and red giants, which exhibit periodic changes in size and brightness, have made Caldwell 51 essential for distance estimates. Periodic variations in brightness are closely related to the intrinsic brightness of stars: by measuring these periodicities, astronomers can derive the true brightness of each star. Comparing this with the apparent brightness allows the distance to be calculated, thanks to the role these stars play as ‘standard candles’. This methodology has enabled the construction of a cosmic distance scale, used to estimate the position of numerous objects in the universe.

However, not all galaxies have conditions as favorable as Caldwell 51. A significant example is the dwarf galaxy Phoenix (PGC 6830), located 1.3 million light-years away in the constellation Phoenix and discovered in 1976 by Hans Emil Schuster and Richard Martin West. Initially classified as a globular cluster, Phoenix stood out from early observations due to its atypical characteristics compared to conventional patterns. The youngest stars occupy mainly the inner regions, distributed along the east-west axis, while the oldest stars are located in the peripheral areas along the north-south axis. Although not a spheroidal dwarf galaxy, Phoenix does not contain enough gas to sustain star formation, even though its core shows traces of regular activity during galactic evolution. The presence of a gaseous cloud of about 10 solar masses near the galaxy, probably ejected by repeated supernova explosions, highlights the influence of these events on recent star formation. This HI region of neutral atomic hydrogen remains gravitationally bound to the galaxy and should re-enter it over time.

As for the spheroidal dwarf galaxies of the Local Group, the Tucana galaxy, located 2.8 million light-years from the Milky Way, is an isolated member almost diametrically opposite to other galaxies relative to our Galaxy. Composed exclusively of very old stars, formed during a single evolutionary phase parallel to that of the Milky Way and its globular clusters, Tucana has a central dispersion velocity of 15.4 km/s and a peripheral rotation velocity of 16 km/s. Furthermore, its radial velocity relative to the Sun is 194 km/s, while it is moving away from the galactic center of the Milky Way at 98.9 km/s. Discovered in 1990, this galaxy, due to its isolation, has been frequently studied to better understand the dynamics and evolutionary history of the Local Group. It is also the second most distant galaxy in this group after the irregular dwarf Sagittarius, which is located about 3.6 million light-years from the barycenter. Only two other spheroidal dwarf galaxies are known to be so isolated: neither is close to the Milky Way or Andromeda. Tucana's isolated evolution allows scientists to investigate the remote history and influence of the surrounding environment on the evolutionary processes of dwarf galaxies.

Another interesting case among isolated galaxies is the Wolf–Lundmark–Melotte (WLM), an irregular dwarf galaxy located in the constellation Cetus, about 3 million light-years from the Sun. With a maximum extent of 8,000 light-years and a brightness slightly above the average for globular clusters, WLM maintains a chemical composition similar to that of the early universe. IC 16613 is located in its immediate vicinity, at a distance of one million light-years. First observed in 1909, it was included in the official list of globular clusters in 1926. Low-mass stars considered primitive have been detected, some of which formed over 12 billion years ago; about half of the stars date back more than 9 billion years. A subsequent slowdown in star formation was recorded, followed by a resurgence between 1 and 2.5 billion years ago. There are numerous old red stars in the outskirts, while young blue stars predominate in the central regions. The elongated morphology, isolation at the edge of the Local Group, and stellar peculiarities suggest that WLM has never interacted with other galaxies in the group or cosmic objects. However, anomalies in the concentration of oxygen have been detected in some supergiants, up to five times higher than in the interstellar medium, similar to those observed in other galaxies such as Barnard and the Small Magellanic Cloud. These results indicate similar stellar evolutionary paths between independent galaxies.

Not all galaxies in the Local Group, however, have followed an identical evolution. This is particularly true for the Leo A galaxy, also known as Leo III.

The irregular galaxy Leo A is located 2.54 million light-years from our solar system, in the direction of the constellation Leo. It extends for about 10,000 light-years and has a mass of about 80 million solar masses, of which about 80% is dark matter. According to scientists, this galaxy has unusual characteristics, as it has no well-defined structures and shows an approximately spherical stellar mass. Leo A is one of the most isolated galaxies in the local group and shows no obvious signs of recent merger or interaction with other nearby galaxies.

Almost 90% of its stars are less than 8 billion years old, while the presence of RR Lyrae variable stars also indicates a portion of the stellar population that is around 10 billion years old. Its prevalent content of young stars usually derives from recent galactic interactions, however Leo A does not seem to have followed a conventional evolutionary development. The stellar history of this galaxy raises questions about star formation processes compared to similar galaxies.

There are many other galaxies and peculiar phenomena in the local group. For example, the Large Magellanic Cloud, located in the constellations Dorado and Mensa, has been the subject of attention due to an event observed by the Hubble telescope: the supernova LMC N49, 160,000 light-years away. This supernova remnant, the result of a star explosion that occurred about 5,000 years ago, consists of luminous filaments extending for about 75 light-years. During the final phase of its life, the star ejected gas in an energetic manner, generating a shock wave that heated the surrounding material to very high temperatures.

A neutron star has been detected within this gaseous cluster, with a mass similar to that of the Sun but a diameter of only a few kilometers, classified as a pulsar due to its high density and rapid rotation. Thermonuclear explosions of this type are short-lived events but allow relevant data to be collected on the stellar life cycle and the dynamics of the interstellar medium.

The Magellanic Clouds are also associated with high-speed clouds, such as the Magellanic Stream. These clouds, composed of interstellar gas, move at speeds significantly different from the galactic rotation, sometimes reaching hundreds of kilometers per second. Some have low metallicity, suggesting an origin outside the host galaxy, while others have concentrations of heavy elements probably due to ejection caused by supernovae. The Magellanic Stream, discovered in 1965, orbits the Milky Way at a distance of 180,000 light-years and continues to be studied to understand its nature and origin.

The Magellanic Stream has aroused considerable interest among astronomers due to its peculiar characteristics. This formation, consisting of interstellar clouds extending for almost 230° across the celestial sphere, has a speed of close to 400 km/s. It is the second largest structure observed from Earth after the Milky Way. It took more than forty years of observations using the largest radio telescopes to understand the nature of this structure. A 2015 study described the physical mechanisms of its formation, thus clarifying its origins.

Investigations have revealed that the Magellanic Stream is composed of two significant filaments of gas, each originating from one of the two Magellanic Clouds. These filaments head towards the Milky Way, crossing its halo of gas heated to temperatures close to one million Kelvin, similar to the trajectory of the respective clouds accompanied by large vortices. The approach of the clouds to the Milky Way increases the pressure exerted by the galactic halo, which tends to repel them into space.

In addition, a considerable amount of smaller gas bubbles are currently falling into our Galaxy as a result of a shock that occurred about 250 million years ago between the two Magellanic Clouds; this same event gave rise to the bridge of matter that connects them. Numerical simulations suggest that the current formed in two distinct phases: initially, the larger cloud would have stolen gas from the smaller one when they were still distant, while as they approached the Milky Way, both galaxies would have been deprived of a significant fraction of their mass in favor of the current itself. Gravity then shaped the stream, giving rise to the current filamentous arc, explaining its morphology and total absence of stars.

In a more remote region of the cosmos, near the Triangulum galaxy, a high-velocity cloud (HVC 127-41-330) has been identified, located 2.28 million light-years from the Milky Way in the constellation Pisces. This structure, characterized by the presence of a halo of dark matter trapping a disk of gas and baryonic dust, shows possible gravitational interactions with the dwarf galaxy Pisces. Being composed mainly of neutral atomic hydrogen, it is classified as an H I region. Despite its impressive size—about 20,000 light-years—no stars have been detected within it. If confirmed as a dark galaxy, it would be the first of its kind in the Local Group.

Dark galaxies are thought to represent an early stage in the formation of today's bright galaxies, characterized by a very low ratio of baryonic matter to dark matter, varying between 0.01 and 0.15. This composition does not allow for the efficient formation of stars, making them extremely difficult to identify. According to some estimates, it would take over 100 billion years to convert all the interstellar matter present into stars, causing these structures to become low surface brightness galaxies.

In the early universe, similar dark galaxies are thought to have been widespread, and the study of such objects provides valuable information about the evolutionary processes of present-day galaxies. In order to detect them, in 2012 a group of researchers from the European Southern Observatory proposed a methodology based on illumination by nearby quasars, capable of bringing out the fluorescence of the gas belonging to dark galaxies, thus facilitating their identification.

Astronomers, therefore, do not limit their observations to the brightest and most massive objects, as every cosmic component contributes to our understanding of the universe.

NGC 2663 is an elliptical galaxy located in the constellation Compass, about 100 million light-years from the Milky Way.

Recent multi-wavelength studies have shown that it is a very large and massive oval object with a stellar mass of about 580 billion suns. Surprisingly, it emits two radio jets nearly 1,150 light-years long, generated by its supermassive black hole, 50 times larger than the galaxy itself.

Another example is NGC 1275, the main galaxy in the Perseus cluster, about 246 million light-years away. It is distinguished by its complex structure, intense core activity, and a network of hot gas filaments. This lenticular galaxy contains large amounts of molecular hydrogen and has significant star formation activity, fueled by its merger with a nearby HVS galaxy. Its central black hole contributes to the production of electromagnetic radiation and the formation of plasma bubbles that support the gas filaments.

In 2019, a cosmic merger was observed between a black hole with 23 solar masses and a compact object with over 2.6 solar masses; the nature of the latter remains uncertain (it is thought to be either a neutron star or a black hole). The event generated gravitational waves that were also detected on Earth. The peculiarity of this collision lies in the high mass ratio between the objects involved, which has never been recorded before and offers new insights into the processes of merger between compact bodies in the universe.

Recent studies have led to unprecedented gravitational tests, raising new questions about the formation of binary systems. The cosmos is populated by numerous astronomical objects with both common and extraordinary properties: recent discoveries include significant galaxies, the luminous remnant of the Large Magellanic Cloud, high-speed clouds, a possible dark galaxy, pulsars, and jets from supermassive black holes. These are some of the main extragalactic objects currently being investigated by the scientific community, although the universe still offers many phenomena to explore.

One of the most interesting objects is the quasar, a quasi-stellar source located billions of light-years from Earth. A quasar is the active core of a galaxy and is characterized by the presence of a supermassive black hole, sometimes billions of times more massive than the Sun. Not all galaxies with black holes host a quasar: it corresponds to the particularly compact and luminous surrounding region. When celestial material is attracted towards the black hole, an accretion disk is formed, composed of extremely high-temperature gas that generates intense radiation emissions.

Evidence suggests that quasars were more abundant between nine and ten billion years ago, showing considerable variability throughout their existence. Despite their small size, they are classified among the most active sources in galactic nuclei (AGN). To be defined as such, the black holes in question must consume at least one solar-type star per day. This process is favored by the high concentration of matter in the surrounding areas, which feeds the growth of the black hole itself and enhances the magnetic fields produced by the induced accelerations.

The expansion of the universe leads to lower stellar density, thus reducing the frequency of collisions and the availability of ‘raw material’, according to current models of galaxy evolution. It is believed that many galaxies have gone through a quasar phase in the past, especially the most massive ones, characterised by periods of activity estimated at between ten and one hundred million years.

A particularly noteworthy case is the quasar HE 0450-2958, identified in 2005 by a team from the University of Liège. Located more than three billion light-years from Earth, this quasar has a central brightness that obscures any host galaxy. Investigations using spectrometry and infrared imaging have not revealed any dust clouds or the presence of stars in the vicinity, thus justifying the name ‘naked quasar’.

The discovery of a very close companion galaxy (22,000 light-years away) characterized by intense star formation offers further food for thought. This galaxy, which is the target of one of the powerful jets emitted by the quasar, produces about 350 solar-mass stars per year. A bridge of matter connecting the quasar to the nearby galaxy has been identified, suggesting that the black hole's activity may have triggered star formation in the galaxy itself.

Ongoing investigations aim to clarify the origin of the exceptional energy released by these objects, considering the significant contribution of intergalactic gas filaments as the primary source for the quasar. Like all cosmic objects, the quasar HE 0450-2958 is in constant motion in space, offering new opportunities to study the dynamics and evolution of the universe.

Over the course of tens of thousands of kilometers, this object will eventually merge with the young galaxy, regardless of whether it is actually isolated or not. In the future, it will inevitably be enveloped by the billions of stars currently forming. These observations challenge the scientific community's previous understanding of such systems. Considering this phenomenon as a possible missing link, we could investigate why black holes are more massive in galaxies with a higher number of stars.

Future observations, aided by even more precise next-generation technological instruments, will focus on similar objects located at greater distances. It is conceivable that, in the future, we will be able to analyze the relationship between the formation of supermassive black holes and the birth of galaxies in the deep universe.

We have reached the end of this journey beyond the boundaries of the Milky Way. During our exploration of billions of light-years, we have seen cosmic wonders that defy the imagination, highlighting the complexity and beauty of the universe, as well as the extreme events that sometimes determine its characteristics to the limits of our solar system. We have also looked at the early stages of galactic formation, emphasizing how extraordinary and difficult to explain these phenomena can be.

The intensity and majesty of these processes prompt reflection on humanity's place in the universe, highlighting how extremely complex and enigmatic it remains. Although astrophysicists have managed to clarify some aspects, they continue to grapple with new questions. Technological innovations are enabling previously unattainable levels of observation and precision, suggesting that many discoveries still await us.

The tools of the future may enable us to fully understand the mechanisms of the cosmos, from its origins with the Big Bang to the future evolution of the universe. Although it is currently impossible to provide definitive answers to the main existential questions, it is certain that understanding the implications of space and planetary exploration is a crucial area for the scientific community and for many enthusiasts.

You Tube: Journey through the Solar System and Beyond / The Odyssey of the Voyager 1 and 2 Probes | DOCUMENTARY. (In Italian)