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.