September 18

Looking Back at Home

1977 — Voyager 1 Photographs Earth and Moon Together

Image time not established

On September 18, 1977, NASA's Voyager 1 turned its camera back toward the world it had left behind and captured a historic portrait: Earth and the Moon together in a single photographic frame.

Voyager 1 had launched only thirteen days earlier, on September 5, beginning a journey that would take it past Jupiter and Saturn and eventually into interstellar space. When it made the photograph, the spacecraft was already approximately 11.66 million kilometers (7.25 million miles) from Earth.

From that distance, Earth and Moon appeared not as the enormous world beneath our feet and the brilliant object dominating our night sky, but as two small crescents suspended together in darkness.

The illuminated portion of Earth showed eastern Asia, the western Pacific Ocean, and part of the Arctic. Voyager's position placed it roughly above Mount Everest, although Everest itself was on Earth's night side.

Creating the familiar color image required more than a single exposure. Voyager recorded images through different color filters, which were subsequently combined and processed by the Jet Propulsion Laboratory's Image Processing Laboratory. Because Earth was much brighter than the Moon, the lunar image was artificially brightened by a factor of three so that both worlds could be clearly seen together.

Spacecraft had photographed Earth before. Mariner 10 had even photographed Earth and the Moon separately in 1973, and those images were combined into a composite. But Voyager 1's September 18 photograph was different: both Earth and Moon appeared together within the same camera frame.

The photograph anticipated an even more famous Voyager image. On February 14, 1990, from approximately 6.4 billion kilometers (4 billion miles) away, Voyager 1 would turn its camera toward home once again and record Earth as the tiny “Pale Blue Dot.”

But the September 18, 1977 image came first. It offered an early glimpse of something planetary exploration would repeatedly teach us: traveling outward changes the way we see home.

Why It Matters: Voyager 1's portrait was the first spacecraft photograph to show the complete Earth-Moon system together in a single frame. Scientifically, it documented our planetary system from a new perspective. Culturally, it helped establish a tradition of spacecraft looking homeward and showing Earth not as the center of our experience, but as one small world traveling through the universe.

 

Also on This Day

1959 — Vanguard III Begins a Mission to Study the Environment Around Earth

Launch time not established

On September 18, 1959, the United States successfully launched Vanguard III from Cape Canaveral, Florida, aboard the final Vanguard launch vehicle.

The small scientific satellite entered an elliptical Earth orbit carrying instruments designed to investigate Earth's magnetic field, solar X-rays, micrometeorites, radiation, and temperatures in the space environment.

One of its most successful instruments was a proton-precession magnetometer. During approximately 85 days of battery-powered operation, Vanguard III gathered measurements of Earth's magnetic field over a wide range of altitudes. The mission also helped define characteristics of the lower edge of the Van Allen radiation belts and collected information about micrometeorites and the environment surrounding Earth.

Not every experiment worked as intended. Instruments designed to observe solar X-rays and ultraviolet Lyman-alpha radiation were overwhelmed by energetic particles in Earth's radiation belts, preventing useful observations of those phenomena. Even that difficulty provided experience that influenced later space-based astronomy.

Vanguard III was also historically significant because its launch effectively marked the end of the Project Vanguard flight program. Vanguard had begun as the American effort to launch a scientific satellite during the International Geophysical Year. After Sputnik 1 dramatically reached orbit first, Vanguard became associated in the public mind with the early American struggle to catch up in the Space Race. Yet the program ultimately produced successful satellites and valuable scientific and engineering knowledge.

Why It Matters: Vanguard III belonged to the first generation of spacecraft that transformed the region above Earth into a place scientists could directly measure. Its instruments helped investigate Earth's magnetic environment and micrometeorite population while contributing to the developing techniques of scientific satellite research.

1968 — Zond 5 Carries Living Creatures Around the Moon

Closest-approach time not established

On September 18, 1968, the Soviet spacecraft Zond 5 swept around the Moon, carrying an extraordinary collection of passengers—including two Russian steppe tortoises.

Zond 5 had launched on September 14 UTC on a free-return trajectory around the Moon. Along with the tortoises, its biological payload included insects, plants, seeds, bacteria, and other living material. The mission was intended in part to investigate whether living organisms could survive a journey through deep space and back to Earth.

During its September 18 lunar encounter, Zond 5 passed approximately 1,950 kilometers (1,210 miles) above the Moon. The spacecraft also photographed Earth from tens of thousands of kilometers away.

No human being had yet traveled beyond low Earth orbit. Nevertheless, the living organisms aboard Zond 5 were now moving through space beyond the Moon.

Three days later, on September 21, the spacecraft returned to Earth and splashed down in the Indian Ocean. Despite an unusually severe ballistic reentry that subjected its biological passengers to high gravitational forces, the tortoises survived.

They had become the first terrestrial animals to travel around the Moon and return alive.

The mission occurred only three months before Apollo 8 carried Frank Borman, Jim Lovell, and William Anders on humanity's first crewed voyage around the Moon.

Why It Matters: Zond 5 demonstrated that living organisms could survive a circumlunar journey and return to Earth. It also showed how close the Soviet Union had come to achieving a crewed lunar flyby as the United States prepared Apollo 8.

 

The Bigger Picture

September 18 presents three different ways in which leaving Earth changed what humanity could know.

Vanguard III represents measurement. For most of astronomical history, scientists studied the heavens from beneath Earth's atmosphere. Early satellites allowed instruments to enter the environment they were investigating, directly measuring magnetic fields, radiation, particles, and micrometeorites.

Zond 5 represents travel. Nine years after Vanguard III entered Earth orbit, living creatures from Earth were already traveling around the Moon. The enormous distance between observing space and journeying through it was being crossed with astonishing speed.

Then Voyager 1 changed the direction of our gaze.

Its mission was outward-bound. Jupiter and Saturn lay ahead. Yet on September 18, 1977, Voyager looked backward.

The resulting photograph captured something that would become one of the great recurring themes of space exploration: the farther we travel from Earth, the more clearly we sometimes see Earth itself.

From the ground, the Moon appears to travel across our sky while Earth seems immense and stationary beneath us. From Voyager's perspective, both became small worlds sharing the same darkness.

Thirteen years later, Voyager would look home once more and reduce Earth to the Pale Blue Dot.

September 18 therefore reminds us that exploration has two directions. We look outward to understand the universe, and sometimes the universe gives us a new vantage point from which to understand home.

At a Glance

1959 — Launch time not established — Vanguard III launches from Cape Canaveral to investigate Earth's magnetic field, radiation environment, micrometeorites, and other characteristics of near-Earth space.

1968 — Closest-approach time not established — Zond 5 passes approximately 1,950 kilometers above the Moon carrying tortoises and other living organisms on the first successful biological journey around the Moon and back.

1977 — Image time not established — Voyager 1 photographs Earth and the Moon together in a single camera frame from approximately 11.66 million kilometers away, the first spacecraft image of its kind.

September 17

Enterprise Takes Its Name

1976 — NASA Unveils the Space Shuttle Enterprise

Time not established

On September 17, 1976, NASA formally unveiled the first completed Space Shuttle orbiter at the Rockwell International facility in Palmdale, California. The spacecraft bore a name that connected the emerging Shuttle era with one of popular culture's most enduring visions of space exploration: Enterprise.

The orbiter had originally been designated Constitution, with its public rollout deliberately planned for September 17—Constitution Day in the United States. But thousands of fans of the television series Star Trek organized a letter-writing campaign urging the White House to name the spacecraft after the fictional starship Enterprise. President Gerald Ford supported the change, and NASA adopted the name.

The connection became unmistakable at the rollout ceremony. Star Trek creator Gene Roddenberry and members of the original television cast attended as the new orbiter emerged from its assembly building. The Star Trek theme was played during the ceremony.

Enterprise looked like the orbiters that would later travel into space, but it was never equipped for orbital flight. It lacked main engines and many of the systems necessary for a space mission. Instead, NASA built it primarily to test whether the Shuttle's revolutionary reusable spacecraft design could actually fly and land like an airplane.

Beginning in 1977, Enterprise was carried aloft atop NASA's modified Boeing 747 Shuttle Carrier Aircraft. Some tests kept the orbiter attached to the aircraft; during others, Enterprise was released in flight so astronauts could glide it independently to a runway landing.

These Approach and Landing Tests provided crucial information about the Shuttle's aerodynamic performance, handling, and landing characteristics. Enterprise was later used for ground testing and fit checks at facilities prepared for operational Shuttle missions.

Although Enterprise never crossed the boundary into space, the orbiters that followed—Columbia, Challenger, Discovery, Atlantis, and Endeavour—built upon what NASA learned from it.

Why It Matters: Enterprise was the pathfinder for the Space Shuttle era. Its testing demonstrated that the unusual winged spacecraft could glide through the atmosphere and land safely after returning from space. Its naming also represents a rare and memorable interaction between science fiction and actual space exploration: a fictional spacecraft inspired the name of a real one that helped prepare humanity's next generation of spacecraft.

Also on This Day

1789 — William Herschel Discovers Saturn's Moon Mimas

Observation time not established

On September 17, 1789, astronomer William Herschel discovered Mimas, one of Saturn's moons, using his enormous 40-foot reflecting telescope at Slough, England.

Herschel had completed the telescope only weeks earlier. Its primary mirror measured approximately 1.2 meters—48 inches—in diameter, making it an extraordinary instrument for its era. The telescope's light-gathering power allowed Herschel to detect objects beyond the reach of smaller instruments.

Mimas is relatively small, only about 396 kilometers (246 miles) across. Its most visually striking feature, unknown to Herschel, is the enormous Herschel crater, named in his honor. At roughly 130 kilometers across, the crater dominates the moon's appearance and gives Mimas its famous resemblance to the fictional Death Star from Star Wars.

Modern observations have revealed that Mimas is much more interesting than its battered, icy exterior initially suggested. Analysis of measurements from the Cassini mission has provided evidence that a global liquid-water ocean may exist beneath its icy crust.

Why It Matters: Herschel's discovery expanded the known Saturnian system and demonstrated the power of increasingly large telescopes to reveal previously unseen worlds. More than two centuries later, spacecraft observations continue to transform those tiny telescopic points of light into complex planetary bodies—and perhaps, in the case of Mimas, ocean worlds.

1857 — Konstantin Tsiolkovsky Is Born

Time unknown

On September 17, 1857, Konstantin Eduardovich Tsiolkovsky was born in Izhevskoye in the Russian Empire.

Long before rockets could reach space, Tsiolkovsky worked out much of the theoretical foundation necessary to make spaceflight possible. A largely self-taught teacher and theorist, he explored concepts involving rockets, orbital travel, space stations, and human journeys beyond Earth.

His landmark 1903 paper, generally translated as “Exploration of Outer Space by Means of Rocket Devices,” described the use of liquid-propellant rockets for spaceflight and presented the mathematical relationship now known as the Tsiolkovsky rocket equation.

That equation relates the change in velocity a rocket can achieve to the velocity of its exhaust and the ratio between its initial and final mass. It expresses one of the fundamental realities of rocketry: a spacecraft must carry propellant to accelerate, but carrying that propellant itself adds mass that must also be accelerated.

Tsiolkovsky also envisioned multistage rockets as a practical way of overcoming this problem. Although he never built the large rockets he imagined, later engineers recognized the importance of his theoretical work.

Why It Matters: Tsiolkovsky helped establish the mathematical and conceptual foundations of astronautics decades before practical spaceflight existed. His career demonstrates that exploration often begins not with a machine but with an idea—and with someone demonstrating that what appears impossible is consistent with the laws of physics.

1962 — NASA Announces Its Second Group of Astronauts

Time not established

On September 17, 1962, NASA announced the selection of nine new astronauts, the second astronaut group in American spaceflight history.

They were Neil Armstrong, Frank Borman, Charles “Pete” Conrad, James Lovell, James McDivitt, Elliot See, Thomas Stafford, Edward White, and John Young.

Unlike the original Mercury Seven, all of whom were military test pilots, the new selection criteria allowed qualified civilian test pilots to apply. Neil Armstrong, a NASA research pilot, became the group's first civilian astronaut.

The nine would play central roles in Gemini and Apollo. Armstrong and Conrad would walk on the Moon. Borman, Lovell, and Anders would make humanity's first voyage around the Moon aboard Apollo 8. Stafford commanded the American spacecraft during the Apollo-Soyuz Test Project. Young eventually walked on the Moon and later commanded the first Space Shuttle mission.

Edward White became the first American to walk in space but later died with Virgil “Gus” Grissom and Roger Chaffee in the Apollo 1 fire. Elliot See died in an aircraft accident in 1966 before making a spaceflight.

Why It Matters: NASA's second astronaut group supplied many of the people who would transform President Kennedy's lunar goal into reality. Their careers linked Mercury-era experimentation with Gemini, Apollo, international cooperation, and eventually the Space Shuttle.

1996 — Shannon Lucid's Record-Setting Stay Aboard Mir Nears Its End

Crew exchange began after docking at 03:13 UTC, September 19

On September 17, 1996, Space Shuttle Atlantis, launched the previous day on STS-79, was approaching Russia's Mir space station carrying astronaut John Blaha, who would replace Shannon Lucid as NASA's long-duration resident aboard Mir.

Lucid had been living aboard the Russian station since March. Launch delays extended her stay well beyond its original schedule. By the time she returned to Earth, she had spent 188 days in space, then the longest spaceflight by an American and the longest by a woman.

September 17 itself was primarily a rendezvous day rather than the crew-exchange milestone—the Shuttle docked with Mir on September 19 UTC. For that reason, this event is better treated as contextual rather than as a principal September 17 anniversary.

 

The Bigger Picture

September 17 offers an especially clear progression from seeing other worlds, to imagining how to reach them, to preparing the people and machines that actually would.

In 1789, William Herschel used one of the largest telescopes ever constructed to discover Mimas. Humanity's relationship with Saturn was entirely observational; even the existence of the little moon had previously been unknown.

In 1857, Konstantin Tsiolkovsky was born. He would spend much of his life thinking about a question that still seemed fantastic: could human beings build machines capable of escaping Earth and traveling through space?

His mathematics said yes.

In 1962, NASA introduced nine astronauts who would help put those principles into practice. Among them was Neil Armstrong, who less than seven years later would become the first human being to step onto another world.

And in 1976, Enterprise emerged from its assembly building. By then, reaching space was no longer an extraordinary experiment undertaken by a handful of pioneering astronauts. NASA was preparing a reusable spacecraft intended to make journeys between Earth and orbit a regular part of human activity.

There is also a delightful cultural thread running through the day. Tsiolkovsky imagined voyages beyond Earth when such ideas belonged largely to speculation. Generations later, Star Trek imagined humans routinely traveling among the stars. Fans of that fictional universe then persuaded their government to give the name Enterprise to a real spacecraft.

September 17 therefore captures one of the recurring patterns in the history of astronomy and space exploration:

We observe what is out there. We imagine reaching it. We determine whether the journey is possible. And eventually, we build the machines and choose the people who will try.

At a Glance

1789 — Observation time not established — William Herschel discovers Mimas, a moon of Saturn, using his newly completed 40-foot telescope.

1857 — Time unknown — Konstantin Tsiolkovsky, future pioneer of astronautics and rocket theory, is born in the Russian Empire.

1962 — Time not established — NASA introduces its second group of nine astronauts, including Neil Armstrong, Frank Borman, Pete Conrad, Jim Lovell, Tom Stafford, Ed White, and John Young.

1976 — Time not established — NASA publicly unveils its first Space Shuttle orbiter, Enterprise, at Palmdale, California.

September 16

A Lifeline in Space

1994 — Astronauts Test a New Way to Rescue Themselves in Space

EVA time: approximately 08:42–15:33 UTC

On September 16, 1994, astronauts Mark C. Lee and Carl J. Meade stepped outside Space Shuttle Discovery during mission STS-64 and deliberately did something spacewalkers normally work very hard to avoid: they floated freely away from their spacecraft.

They were testing a new device called the Simplified Aid for EVA Rescue, better known as SAFER.

SAFER was conceived as an emergency rescue system. Astronauts performing extravehicular activity normally remain connected to their spacecraft by safety tethers. But engineers had to consider a frightening possibility: What would happen if a spacewalker somehow became completely untethered and began drifting away?

Without some means of propulsion, the astronaut might have no way to return.

SAFER provided an answer. The compact unit was worn like a backpack over the astronaut's spacesuit and used small jets of compressed nitrogen controlled by the astronaut. Unlike the much larger Manned Maneuvering Unit used during several Shuttle flights in 1984, SAFER was not intended primarily as a vehicle for routine untethered maneuvering. It was an emergency system—a kind of life jacket for space.

During their six-hour, 51-minute spacewalk, Lee and Meade took turns flying the device untethered near Discovery. They maneuvered above and around the Shuttle's payload bay while evaluating SAFER's controls and handling characteristics. The tests demonstrated that an astronaut who accidentally became separated from a spacecraft could potentially maneuver back to safety.

The technology subsequently became part of normal spacewalking safety procedures. Astronauts conducting spacewalks outside the International Space Station wear SAFER units as an emergency backup should their tethers fail.

Why It Matters: SAFER addressed one of the most dangerous conceivable emergencies during a spacewalk: an astronaut drifting helplessly away from the spacecraft. The September 16 test transformed an emergency-rescue concept into demonstrated technology and helped make future extravehicular activity safer.

 

Also on This Day

1978 — A Total Lunar Eclipse Crosses Earth's Shadow

Greatest eclipse: 19:04:12 UTC

On September 16, 1978, the Moon passed completely through the darkest portion of Earth's shadow, producing a total lunar eclipse.

The eclipse began when the Moon entered Earth's penumbral shadow at approximately 16:22 UTC. The partial phase began around 17:21 UTC, and totality began at approximately 18:25 UTC.

At 19:04:12 UTC, the eclipse reached its greatest phase. Totality lasted approximately 78 minutes and 39 seconds, ending around 19:44 UTC. The Moon finally left Earth's penumbral shadow at approximately 21:46 UTC.

A lunar eclipse occurs because Earth moves directly between the Sun and the Moon. During totality, the Moon does not normally disappear completely. Sunlight passing through Earth's atmosphere is refracted into Earth's shadow, while shorter blue wavelengths are scattered more efficiently. The remaining reddish light can illuminate the lunar surface, producing the familiar copper or red appearance of a totally eclipsed Moon.

The September 1978 eclipse had an umbral magnitude of approximately 1.327, meaning the Moon passed deeply enough into Earth's umbra that its entire disk was immersed in the central shadow.

Why It Matters: Lunar eclipses are among the most accessible demonstrations of celestial mechanics. Long before spacecraft existed, observations of Earth's curved shadow crossing the Moon helped reinforce the understanding that Earth is spherical. Modern eclipse predictions also demonstrate the remarkable precision with which the motions of Earth, Moon, and Sun can be calculated.

1996 — Atlantis Launches on the First American Crew Exchange at Mir

08:54:49 UTC

On September 16, 1996, at 08:54:49 UTC, Space Shuttle Atlantis lifted off from Kennedy Space Center on mission STS-79, beginning the fourth Shuttle docking mission with the Russian space station Mir.

The mission represented an important development in American-Russian cooperation in space. Astronaut John Blaha traveled aboard Atlantis to begin a long-duration stay on Mir, while astronaut Shannon Lucid, already aboard the Russian station, would return to Earth aboard the Shuttle.

Lucid had arrived at Mir in March. Delays in launching STS-79 extended her mission to 188 days, establishing at the time both a new American space-duration record and a world record for a woman.

STS-79 also carried the first double SPACEHAB module, providing additional room for equipment and supplies. During the docked portion of the mission, the Shuttle and Mir crews transferred thousands of pounds of water, supplies, experiments, and equipment between the two spacecraft.

The launch itself had encountered unusual delays. Atlantis had been rolled back from its launch pad twice because of threats from Hurricanes Bertha and Fran—the first time a Shuttle had undergone two hurricane-related rollbacks during a single processing cycle.

Why It Matters: STS-79 conducted the first exchange of American long-duration crew members aboard Mir. The Shuttle-Mir program required American and Russian flight controllers, engineers, astronauts, and cosmonauts to learn how to operate together for extended periods—a body of experience that helped prepare both nations for their partnership aboard the International Space Station.

2015 — A Sounding Rocket Fires a Record 44 Rocket Engines

Time not established

On September 16, 2015, a NASA Black Brant XI sounding rocket launched from Andøya, Norway, carrying the Charged Aerosol Release Experiment II, or CARE II.

The flight investigated dusty plasmas—mixtures containing electrically charged particles and small grains of matter that occur naturally in portions of Earth's upper atmosphere and elsewhere in space.

But the mission also achieved an unusual engineering record.

During the flight, 44 rocket engines were fired, earning recognition from Guinness World Records for the greatest number of rocket engines fired during a single flight. Only three provided the principal propulsion for launching the vehicle. Four others served as spin motors, while many of the remaining motors were used to create the artificial exhaust cloud required for the scientific experiment.

The mission is a useful reminder that not all significant space research requires an orbital spacecraft. Sounding rockets can carry experiments above most of Earth's atmosphere for relatively brief flights, allowing scientists to investigate phenomena that are difficult or impossible to reproduce at ground level.

Why It Matters: CARE II combined an unusual engineering achievement with scientific investigation of the upper atmosphere. Sounding rockets remain an important—and comparatively economical—bridge between laboratory research on Earth and longer-duration experiments conducted in orbit.

 

The Bigger Picture

September 16 reminds us that exploring space requires more than simply finding ways to travel farther from Earth.

The 1978 lunar eclipse represents astronomy in one of its oldest forms: watching the predictable motions of celestial bodies from the ground and learning about the geometry of our solar system.

The 1994 SAFER experiment addressed a very different problem. Once humans began leaving their spacecraft to work in the vacuum of space, engineers had to devise ways of protecting them when something went wrong.

Two years later, STS-79 demonstrated another essential development: cooperation. The United States and Russia, whose earlier space programs had developed largely as rivals, were exchanging crew members aboard Mir and learning how to conduct long-duration missions together.

And the CARE II mission reminds us that even in an era of sophisticated planetary probes and enormous orbital laboratories, relatively small sounding rockets continue to make valuable contributions to space science.

Together, these events illustrate four complementary aspects of humanity's relationship with space: observing it, learning to survive in it, learning to work together within it, and continuing to devise new ways to study it.

The history of space exploration is therefore not simply a succession of destinations. It is also the story of gradually acquiring the knowledge, technologies, safeguards, and partnerships necessary for human beings to operate beyond their home world.

At a Glance

1978 — 19:04:12 UTC — A total lunar eclipse reaches greatest eclipse; totality lasts approximately 78 minutes and 39 seconds.

1994 — Approximately 08:42–15:33 UTC — Astronauts Mark Lee and Carl Meade conduct an STS-64 spacewalk and perform the first in-space tests of the SAFER emergency maneuvering system.

1996 — 08:54:49 UTC — Space Shuttle Atlantis launches on STS-79, beginning the first American long-duration crew exchange at Russia's Mir space station.

2015 — Time not established — NASA's CARE II sounding-rocket mission launches from Norway and fires 44 rocket engines during the flight, establishing a world record.

September 15

Farewell to Cassini

2017 — Cassini Plunges into Saturn

10:31–10:32 UTC — spacecraft event time

On September 15, 2017, NASA's Cassini spacecraft entered the atmosphere of Saturn, deliberately bringing one of the most scientifically productive planetary missions in history to an end.

Cassini had left Earth nearly twenty years earlier, on October 15, 1997. After a journey of almost seven years, it entered orbit around Saturn on July 1, 2004, and began an extraordinary thirteen-year exploration of the planet, its rings, its magnetic environment, and its diverse family of moons.

By 2017, Cassini's supply of maneuvering propellant was nearing exhaustion. Mission planners faced an unusual problem created in part by Cassini's own discoveries. The spacecraft had revealed that Enceladus possesses a subsurface ocean and ejects water-rich material through enormous plumes, while observations of Titan had revealed lakes and seas of liquid hydrocarbons and an extraordinarily complex organic environment.

Scientists did not want an uncontrolled Cassini eventually crashing into one of these worlds and potentially contaminating an environment of astrobiological interest with terrestrial microorganisms. NASA therefore chose to dispose of the spacecraft safely by sending it into Saturn itself.

Cassini's final phase became known as the Grand Finale. Beginning in April 2017, the spacecraft made 22 daring passages through the previously unexplored region between Saturn and its innermost ring. These final orbits allowed Cassini to make measurements that could not have been attempted earlier in the mission.

Then came September 15.

At approximately 10:31 UTC, Cassini began encountering Saturn's atmosphere. Its attitude-control thrusters fired increasingly hard as the spacecraft struggled to keep its high-gain antenna pointed toward Earth. About a minute later, the thrusters reached full capacity. Atmospheric forces overwhelmed Cassini's ability to maintain its orientation, its antenna turned away from Earth, and the spacecraft's signal disappeared. Moments later, Cassini broke apart and burned in Saturn's atmosphere.

But observers on Earth did not know immediately that Cassini was gone. Saturn was so distant that Cassini's radio signals required approximately 83 minutes to cross the solar system. The final signal reached NASA's Deep Space Network station in Canberra, Australia, at about 11:55 UTC.

Cassini continued gathering and transmitting scientific measurements during its descent. Instead of storing its final observations for later transmission—which would have been impossible—the spacecraft sent data directly to Earth as it sampled Saturn's upper atmosphere.

It was doing science until almost the final moment of its existence.

Why It Matters: Cassini transformed our understanding of the Saturn system. Among its many discoveries, it revealed active jets and evidence for a global subsurface ocean at Enceladus, explored Titan's methane-and-ethane lakes and seas, studied Saturn's extraordinarily complex rings and atmosphere, and observed interactions among the planet, rings, moons, and magnetosphere. Its intentional destruction also established an important principle of planetary protection: exploration carries a responsibility to protect potentially habitable environments for future scientific investigation.

 

Also on This Day

1857 — Astronomer Anna Winlock Is Born

Time unknown

On September 15, 1857, Anna Winlock was born in Cambridge, Massachusetts. She would become an astronomer and one of the earliest women employed as a scientific computer at the Harvard College Observatory.

Winlock was the daughter of astronomer Joseph Winlock, director of the Harvard College Observatory. After her father's death in 1875 left the family in financial difficulty, she approached the observatory seeking work. She became one of the women whose painstaking mathematical calculations and analysis of astronomical observations contributed substantially to Harvard's research.

Her work included reducing observations made with the observatory's meridian circle and helping produce astronomical catalogues. She became particularly associated with the Harvard Zones, observations used in compiling an extensive catalogue of stars near the celestial poles. She also performed calculations concerning asteroids.

Winlock's career preceded the better-known generation of women at Harvard—including Williamina Fleming, Antonia Maury, Annie Jump Cannon, and Henrietta Swan Leavitt—whose work became fundamental to modern stellar astronomy.

Why It Matters: Anna Winlock represents an often-overlooked part of astronomical history. Long before women were routinely admitted to professional scientific careers, women working as astronomical computers performed the calculations, measurements, and cataloguing upon which major observatories depended. Their work helped transform enormous quantities of observations into usable astronomical knowledge.

1938 — James Christy Is Born

Time unknown

On September 15, 1938, American astronomer James W. Christy was born in Milwaukee, Wisconsin.

Four decades later, while working at the U.S. Naval Observatory, Christy would make a discovery that fundamentally changed astronomers' understanding of Pluto.

In 1978, Christy was examining photographic plates of Pluto when he noticed that the distant object appeared slightly elongated. Rather than dismissing the irregularity as a flaw in the photographic image, he examined additional plates and discovered that the elongation appeared in different positions according to a repeating pattern.

Christy correctly concluded that Pluto had a previously unknown companion.

The object was subsequently named Charon. The discovery allowed astronomers to determine the orbital characteristics of the Pluto-Charon system much more accurately and ultimately led to substantially improved estimates of Pluto's mass and size.

Decades later, when NASA's New Horizons spacecraft flew through the Pluto system in 2015, Charon was revealed as a remarkable world in its own right, with enormous canyons, varied terrain, and evidence of a complicated geological history.

Why It Matters: Christy's discovery transformed Pluto from an apparently solitary distant object into a complex planetary system. It also demonstrates a recurring lesson in astronomy: important discoveries sometimes begin when a scientist notices an apparent imperfection and asks whether it might actually be telling us something about the universe.

1966 — Gemini XI Returns from a Record-Setting Mission

13:59:35 UTC

On September 15, 1966, at 13:59:35 UTC, astronauts Charles “Pete” Conrad Jr. and Richard F. Gordon Jr. splashed down in the western Atlantic Ocean, completing the ambitious Gemini XI mission.

During their three days in space, Conrad and Gordon had demonstrated several capabilities that would soon be essential for Apollo.

Gemini XI achieved rendezvous and docking with an Agena target vehicle during its first orbit, demonstrating how rapidly two spacecraft could meet in space. Gordon performed two extravehicular activities, and the docked Gemini-Agena combination used the Agena's engine to climb to an altitude of approximately 1,374 kilometers (854 miles)—a human altitude record at the time.

The astronauts also experimented with connecting Gemini and Agena by a tether and creating a small amount of artificial gravity by rotating the two spacecraft around their common center of mass.

After completing 44 Earth orbits, Gemini XI performed an automatically controlled reentry and landed only a few kilometers from its planned recovery point. Conrad and Gordon were recovered by the USS Guam.

Why It Matters: Gemini XI demonstrated techniques—rendezvous, docking, maneuvering two joined spacecraft, extravehicular activity, and precision reentry—that helped prepare NASA for Apollo. Gemini's achievements sometimes stand in the shadow of the Moon landings that followed, but Apollo depended upon skills developed and tested during these demanding missions.

2006 — Cassini Photographs Saturn from Its Shadow

Observation sequence began September 15 UTC

Eleven years before Cassini's mission ended on September 15, the same spacecraft used an extraordinary alignment to produce one of the most revealing views ever made of Saturn's rings.

On September 15, 2006, Cassini passed into Saturn's shadow, placing the enormous planet between the spacecraft and the Sun. With Saturn blocking the Sun's intense glare, Cassini could look back toward the planet and its rings under lighting conditions impossible to reproduce from Earth.

Over nearly three hours, Cassini's wide-angle camera obtained 165 images that were later assembled into a spectacular panoramic mosaic. The spacecraft remained in Saturn's shadow for roughly twelve hours, giving its instruments an exceptional opportunity to study extremely faint material in and around the ring system.

The backlighting revealed delicate structures normally overwhelmed by sunlight, contributing to the identification and study of previously unknown faint rings. The resulting panorama also contained something almost inconspicuously small: Earth, visible as a tiny point of light beyond Saturn's rings.

Why It Matters: The observation demonstrated how profoundly viewing geometry can affect astronomical discovery. By placing Saturn between itself and the Sun, Cassini saw structures that otherwise remained nearly invisible. It also produced one of the great visual reminders of planetary exploration: our enormous home world reduced to a tiny point of light seen from the vicinity of Saturn.

 

The Bigger Picture

September 15 offers an unusual opportunity to see the history of astronomy and space exploration as a continuous story.

Anna Winlock represents the painstaking work of measuring and cataloguing the sky. Astronomy advanced because generations of observers and computers transformed individual observations into organized knowledge.

James Christy represents discovery through careful observation. By recognizing that a small irregularity in images of Pluto was real rather than accidental, he revealed another world.

Gemini XI represents learning how to operate in space. Rendezvous, docking, spacewalking, maneuvering, and controlled reentry were skills humanity had to master before astronauts could attempt voyages to the Moon.

And then there is Cassini, appearing twice on this date.

On September 15, 2006, Cassini hid in Saturn's shadow and looked outward, using an extraordinary perspective to reveal the planet's rings as they had never been seen before.

Exactly eleven years later, on September 15, 2017, the same spacecraft turned toward Saturn for the last time and disappeared into the world it had spent thirteen years exploring.

There is something fitting in that coincidence. Astronomy begins with looking carefully. Space exploration allows us to carry that act of looking to places our ancestors could never reach. And eventually every mission ends, leaving behind its observations for others to study.

September 15 therefore reminds us that exploration is not simply about reaching distant places. It is about seeing what was previously unseen, preserving what we learn, and passing that knowledge to those who come next.

At a Glance

1857 — Time unknown — Astronomer Anna Winlock is born in Cambridge, Massachusetts; she later becomes one of the earliest women employed as a scientific computer at Harvard College Observatory.

1938 — Time unknown — Astronomer James W. Christy is born; four decades later he will discover Charon, Pluto's largest moon.

1966 — 13:59:35 UTC — Gemini XI splashes down in the Atlantic after a mission that demonstrated rendezvous, docking, EVA, high-altitude flight, and other techniques important to Apollo.

2006 — Observation sequence began September 15 UTC — Cassini enters Saturn's shadow and conducts an extraordinary series of backlit observations of the planet and its rings.

2017 — Approximately 10:31–10:32 UTC — Cassini enters Saturn's atmosphere and loses attitude control, ending nearly twenty years in space and thirteen years of exploration at Saturn.

2017 — Approximately 11:55 UTC, Earth-received time — Cassini's final radio signal reaches the Deep Space Network in Canberra, Australia, approximately 83 minutes after the spacecraft's destruction at Saturn.

September 14

Around the Moon and Back

1968 — Zond 5 Begins a Historic Journey

21:42:10 UTC

On September 14, 1968, at approximately 21:42:10 UTC, the Soviet Union launched Zond 5 from Site 81 at the Baikonur Cosmodrome aboard a Proton-K rocket. The spacecraft was placed first into Earth orbit and then sent toward the Moon on a free-return trajectory.

Zond 5 was a version of the Soyuz 7K-L1 spacecraft being developed as part of the Soviet effort to send cosmonauts around the Moon. No people were aboard this flight, but the spacecraft carried an unusual collection of living passengers—including two Russian steppe tortoises, along with insects, plants, microorganisms, and other biological specimens. Scientists hoped to learn more about how living organisms would respond to a journey beyond Earth's immediate environment.

Four days later, on September 18, Zond 5 swept around the Moon, passing approximately 1,950 kilometers (1,210 miles) above its surface. The spacecraft then headed back toward Earth. On September 21, it reentered Earth's atmosphere and splashed down in the Indian Ocean, where Soviet recovery vessels retrieved it. The tortoises and other biological specimens survived the journey.

The mission achieved an extraordinary milestone: living creatures from Earth had traveled around the Moon and returned safely. It also demonstrated important technologies required for human lunar missions, including navigation across hundreds of thousands of kilometers and high-speed atmospheric reentry after returning from the Moon.

The timing was particularly significant. Only three months later, in December 1968, Apollo 8 astronauts Frank Borman, Jim Lovell, and William Anders would become the first humans to travel around the Moon.

Why It Matters: Zond 5 demonstrated that a spacecraft could travel from Earth, pass around the Moon, and return living passengers safely to Earth. It represented an important technological bridge between robotic lunar exploration and human voyages to the Moon.

 

Also on This Day

1712 — Giovanni Domenico Cassini Dies

Time unknown

On September 14, 1712, the influential Italian-born French astronomer Giovanni Domenico Cassini died in Paris.

Cassini was one of the outstanding observational astronomers of the seventeenth century. Among his achievements, he discovered four moons of Saturn—Iapetus, Rhea, Tethys, and Dione—and identified the prominent division between Saturn's A and B rings that still bears his name: the Cassini Division.

He also made important observations of Jupiter and Mars and contributed to efforts to determine the scale of the solar system.

Centuries later, his name would travel to Saturn aboard the Cassini spacecraft, the NASA-European Space Agency-Italian Space Agency mission that entered orbit around Saturn in 2004 and transformed our knowledge of the planet, its rings, and its remarkable family of moons.

Why It Matters: Cassini helped establish planetary astronomy as the careful study of other worlds as physical places with measurable characteristics. His discoveries remain part of the language of astronomy more than three centuries after his death.

1926 — J. L. E. Dreyer Dies

Time unknown

On September 14, 1926, Danish-born astronomer Johan Ludvig Emil Dreyer, usually known as J. L. E. Dreyer, died in Oxford, England, at age 74.

Dreyer's most enduring contribution was the New General Catalogue of Nebulae and Clusters of Stars, published in 1888 and universally known as the NGC. The catalogue contained 7,840 objects, with thousands more subsequently included in Dreyer's supplementary Index Catalogues.

His system remains part of the working vocabulary of astronomy. Familiar objects continue to be identified by their NGC numbers: the Andromeda Galaxy is NGC 224, the Orion Nebula is NGC 1976, and the Whirlpool Galaxy is NGC 5194.

Dreyer was also an important historian of astronomy. He wrote about the development of astronomical thought and devoted considerable scholarly attention to the life and observations of Tycho Brahe.

Why It Matters: Dreyer helped organize the deep sky. His catalogues gave generations of astronomers a common system for identifying thousands of objects beyond the solar system—a system that remains in widespread use today.

1958 — Mohr Sounding Rockets Reach the Upper Atmosphere

Time unknown

On September 14, 1958, three experimental sounding rockets designed by German engineer Ernst Mohr were successfully launched near Cuxhaven, West Germany. Their dart-like payloads reached an altitude of approximately 50 kilometers (31 miles).

The achievement followed unsuccessful launch attempts in June. Mohr's single-stage, solid-fueled rocket accelerated a narrow payload dart that separated after burnout and continued upward because of its more efficient aerodynamic shape. The successful September flights reached their intended altitude.

Fifty kilometers is below the 100-kilometer Kármán line commonly used to define the boundary of space, but sounding rockets occupy an important place in the history of space science. They allow instruments to investigate portions of Earth's atmosphere that are too high for balloons while avoiding the complexity and expense of placing a spacecraft into orbit.

Why It Matters: Mohr's experiments formed part of the postwar development of sounding-rocket technology—an important scientific tool for studying Earth's upper atmosphere and the environment approaching space.

 

The Bigger Picture

September 14 brings together three stages in humanity's effort to understand and eventually travel beyond Earth.

Cassini represents observation. With increasingly capable telescopes, astronomers discovered that the planets were not simply wandering points of light but complex worlds surrounded by rings and moons.

Dreyer represents organization. As telescopes revealed thousands of objects beyond the solar system, astronomy required systematic catalogues so that scientists could identify, compare, and communicate what they observed.

The sounding rockets of 1958 represent experimentation beyond the ground. Astronomical and atmospheric instruments increasingly could be carried above the dense lower atmosphere.

And Zond 5 represents exploration. Only ten years after the Mohr rocket experiments reached 50 kilometers above Earth, another spacecraft departed Earth altogether and began a journey of hundreds of thousands of kilometers around the Moon and back.

The progression captures much of the story at the heart of On This Day in Astronomy and Space: humanity observed the heavens, learned to map and understand them, developed machines capable of rising above Earth, and eventually began sending those machines to other worlds.

At a Glance

1712 — Time unknown — Giovanni Domenico Cassini, pioneering observer of Saturn and its moons, dies in Paris.

1926 — Time unknown — J. L. E. Dreyer, compiler of the New General Catalogue (NGC), dies in Oxford.

1958 — Time unknown — Ernst Mohr's experimental sounding rockets successfully reach approximately 50 kilometers above West Germany.

1968 — 21:42:10 UTC — Zond 5 launches from Baikonur on the historic mission that will carry living organisms around the Moon and safely back to Earth.

September 13

 Humanity Reaches Another World

1959 — Luna 2 Reaches the Moon

22:02:04 UTC

On September 13, 1959, at approximately 22:02:04 UTC, the Soviet spacecraft Luna 2 struck the Moon, becoming the first human-made object to reach the surface of another celestial body.

Launched from the Soviet Union the previous day, Luna 2 was placed on a direct trajectory toward the Moon. After traveling through space for roughly a day and a half, the spacecraft impacted the lunar surface in the region between Mare Imbrium and Mare Serenitatis, near the crater Autolycus. NASA places the impact roughly 160 miles from the site where Apollo 15 astronauts would land twelve years later.

Luna 2 was not merely intended to demonstrate that the Moon could be reached. The spacecraft carried scientific instruments for investigating cosmic rays, radiation, magnetic fields, and the space environment. Its measurements found no significant lunar magnetic field or radiation belts comparable to Earth's. The spacecraft also carried metallic spheres bearing Soviet emblems, which were intended to scatter upon impact.

The achievement came less than two years after Sputnik 1 inaugurated the Space Age. Sputnik demonstrated that humans could place an artificial object into orbit around Earth. Luna 2 crossed another threshold: something built by human hands had now traveled from Earth to another world.

Why It Matters: Luna 2 transformed reaching another celestial body from an aspiration into an accomplished fact. Every successful planetary landing and impact mission that followed—from the Moon and Mars to asteroids and comets—belongs to a history whose first physical contact with another world occurred on September 13, 1959.

 

Also on This Day

1850 — John Russell Hind Discovers 12 Victoria

Time unknown

On the evening of September 13, 1850, English astronomer John Russell Hind discovered the asteroid 12 Victoria while observing at George Bishop's Observatory in London.

Hind was comparing his observations in the constellation Pegasus with a recently prepared star chart when he noticed an object that did not belong among the mapped stars. He correctly recognized it as a previously unknown member of the solar system.

Victoria was only the twelfth asteroid known to astronomy. When the first asteroid, Ceres, had been discovered in 1801, astronomers had no idea that the region between Mars and Jupiter contained a vast population of small worlds. Discoveries such as Victoria gradually revealed the true nature of what we now call the asteroid belt.

The asteroid's name produced an interesting nineteenth-century controversy. Officially, Hind named it for Victoria, the Roman goddess of victory, but the name also honored Britain's reigning Queen Victoria. Some American astronomers objected because the developing convention discouraged naming asteroids after living people, and the alternative name Clio appeared for a time in American publications.

Hind eventually discovered ten asteroids, becoming one of the most successful asteroid hunters of his generation.

Why It Matters: The discovery of Victoria was part of a profound change in humanity's picture of the solar system. What had once seemed to consist principally of the Sun, planets, and their moons was increasingly understood to contain a much larger and more diverse population of smaller worlds.

1985 — An F-15 Destroys a Satellite in Orbit

Time not established

On September 13, 1985, U.S. Air Force Major Wilbert D. “Doug” Pearson climbed rapidly in a specially modified F-15A Eagle and launched an ASM-135A anti-satellite missile toward an object hundreds of kilometers above him.

The target was Solwind P78-1, an aging scientific satellite that had been used to study the Sun. The missile released a miniature kinetic-kill vehicle that homed in on the satellite and destroyed it through a high-speed collision. The test marked the first—and ultimately only—time the ASM-135 system destroyed an actual satellite in orbit.

The achievement demonstrated something technologically extraordinary and strategically troubling: a weapon carried by an aircraft in Earth's atmosphere could successfully strike an object traveling at orbital velocity high above the planet.

The test also illustrated another danger. Destroying a satellite at orbital velocity created a cloud of debris. Such fragments can remain in orbit and pose collision hazards to other spacecraft, making destructive anti-satellite testing an enduring concern in the increasingly crowded environment around Earth.

Why It Matters: The 1985 test demonstrated that space had become more than a realm of scientific discovery and peaceful exploration. It had also become strategically important enough to be treated as a potential arena of military conflict—and showed that actions in space could leave consequences long after an individual mission ended.

 

The Bigger Picture

September 13 presents three remarkably different moments in humanity's changing relationship with the universe.

In 1850, John Russell Hind stood at a telescope and discovered another small world orbiting the Sun. Astronomy was still fundamentally an activity conducted from Earth: humans looked outward and gradually learned what populated the heavens.

In 1959, that relationship changed. Luna 2 did something no telescope could do. Humanity sent a machine across the gulf between Earth and Moon and physically touched another celestial body for the first time.

By 1985, human activity had expanded into near-Earth space so extensively that satellites had become important not only for science and communication but also for national security—and nations were developing technologies capable of destroying them.

These events reveal both the promise and the responsibility that accompany technological progress. We discovered other worlds, learned how to reach beyond our own, and eventually confronted the consequences of bringing human competition into space with us.

September 13 therefore tells more than a story of technological advancement. It reminds us that our increasing ability to reach the universe also raises an enduring question: What will humanity choose to do with the power to operate there?

At a Glance

1850 — Time unknown — John Russell Hind discovers asteroid 12 Victoria at George Bishop's Observatory in London.

1959 — 22:02:04 UTC — Luna 2 impacts the Moon, becoming the first human-made object to reach another celestial body.

1985 — Time not established — An ASM-135A missile launched from an F-15 destroys the Solwind P78-1 satellite, the first satellite destroyed by a weapon launched from an aircraft.

Europa Clipper Mission Teleconference

 On Monday, September 9, NASA will hold a media teleconference to provide an update on the Europa Clipper mission, that will study whether Jupiter’s moon Europa could be hospitable to life. The teleconference will occur after a key decision point meeting earlier that day regarding next steps for the mission. The teleconference can be watched here: https://www.nasa.gov/live/

If you would like information on the Europa mission and the spacecraft, I have a book for you! The book is called Journey to Europa: The NASA Europa Clipper Mission, and it is available in Kindle and paperback form. Click on the image below to check it out!

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