September 30

Rosetta Comes to Rest on a Comet

2016 — Rosetta Ends Its Mission on Comet 67P

Approximately 10:39 UTC — spacecraft event time

On September 30, 2016, the European Space Agency's Rosetta spacecraft completed one of the most ambitious journeys in the history of robotic exploration. It did not return to Earth. Instead, after more than twelve years in space and more than two years accompanying a comet around the Sun, Rosetta descended slowly toward the surface of Comet 67P/Churyumov–Gerasimenko. The spacecraft continued observing almost until the moment it touched the comet. Rosetta had launched from Kourou, French Guiana, on March 2, 2004. Its destination could not be reached directly with the available launch vehicle, so the spacecraft followed an extraordinary path through the inner solar system. It flew past Earth three times and Mars once, using the planets' gravity to reshape its orbit and gradually acquire the trajectory needed to meet its comet. Along the way, Rosetta also flew past asteroids 2867 Šteins and 21 Lutetia. Then, in June 2011, far from the Sun, the spacecraft entered deep-space hibernation. For 31 months, most of its systems remained shut down while Rosetta traveled through the cold outer portion of its orbit. On January 20, 2014, an onboard alarm clock awakened it. Seven months later, Rosetta reached Comet 67P. The comet proved astonishing. Its nucleus consisted of two distinct lobes joined by a narrow neck, giving it an appearance sometimes compared with a rubber duck. Its dark surface contained cliffs, pits, boulders, fractures, smooth plains, and deposits shaped by the comet's repeated passages near the Sun. Rosetta became the first spacecraft to rendezvous with a comet and accompany it as it traveled around the Sun. Rather than obtaining only the brief snapshot possible during a high-speed flyby, Rosetta watched 67P change. As the comet approached the Sun, sunlight warmed its icy nucleus. Gases escaped, carrying dust into space and creating the coma and tails characteristic of an active comet. Rosetta was there to watch the transformation. In November 2014, the mission achieved another historic first when the small Philae lander separated from Rosetta and descended toward the nucleus. Philae reached the surface but its anchoring systems failed to secure it. The lander bounced several times before coming to rest in a poorly illuminated location. Despite the difficult landing, Philae returned valuable scientific measurements before exhausting its primary battery. Rosetta continued its own observations. Its instruments investigated the comet's dust, gases, plasma environment, surface composition, structure, and interaction with the solar wind. Among the mission's most important questions was whether comets like 67P could have delivered a significant portion of Earth's water. Rosetta measured the ratio of deuterium to ordinary hydrogen in the comet's water and found it substantially different from Earth's ocean water. The result suggested that comets of 67P's type were probably not the dominant source of Earth's oceans. By 2016, however, the comet was moving outward from the Sun again. Solar energy reaching Rosetta's enormous solar panels was declining. Maintaining the spacecraft and communicating with it would become increasingly difficult. Mission planners chose a scientifically productive ending. Rosetta would descend to the comet. On September 29 at 20:50 UTC, the spacecraft performed its final maneuver from an altitude of approximately 19 kilometers and began a slow, controlled fall toward the nucleus. There would be no braking burn. Rosetta spent its final hours doing science. Its instruments measured gas, dust, plasma, temperature, and other properties increasingly close to the surface. Its cameras returned images at resolutions impossible from the spacecraft's normal operating distances. The target was in the Ma'at region on the comet's smaller lobe, near pits associated with cometary activity. ESA's reconstruction places Rosetta's arrival on the surface at approximately 10:39 UTC. The spacecraft came to rest only about 33 meters from its intended target point. The moment Rosetta contacted the surface, its systems automatically shut down. But Earth did not know immediately. Comet 67P was then approximately 720 million kilometers from Earth. Radio waves traveling at the speed of light required about 40 minutes to cross that distance. At ESA's European Space Operations Centre in Darmstadt, Germany, controllers watched Rosetta's signal. At 11:19:37 UTC, it disappeared. Rosetta's journey was over.

Why It Matters: Rosetta transformed the study of comets from brief encounters into sustained exploration. It became the first spacecraft to rendezvous with and accompany a comet around the Sun, delivered the first lander to a cometary nucleus, and observed 67P as increasing solar warmth transformed an inactive icy body into an active comet. Its final descent extended that science almost to the moment the spacecraft touched the world it had spent more than two years studying.

 

Also on This Day

1880 — Henry Draper Takes the First Successful Photograph of a Nebula

Evening observation; precise time not established

On the evening of September 30, 1880, American physician and astronomer Henry Draper pointed his telescope toward one of the most magnificent objects in the winter sky: the Orion Nebula. He attached photographic equipment to the telescope and exposed a plate. What appeared on it represented an important turning point in astronomy. Draper had obtained the first successful photograph of a nebula. Astronomers had been experimenting with photography for decades. The Moon, Sun, and bright stars were comparatively straightforward targets because they provided enough light to register on the insensitive photographic materials then available. Nebulae were much more difficult. They were faint and diffuse. The Orion Nebula—catalogued today as M42—was therefore an obvious but challenging target. Draper's September 30 photograph required an exposure of roughly 50 minutes. The resulting image was modest by modern standards, showing principally the bright central region around the Trapezium. But the important fact was that the nebula had left a permanent record on a photographic plate. That changed what an astronomical observation could be. For most of history, an astronomer looked through a telescope and drew or described what was visible. Photography allowed the sky to record itself. An image could be preserved. It could be measured later. Different photographs could be compared to detect change. And photographic plates could accumulate light for longer than the human eye could, eventually revealing objects too faint to see directly through the same telescope. Draper continued improving his photographs of Orion. Other astronomers soon pushed astronomical photography much farther. Long exposures revealed extraordinary structures within nebulae, star clusters, and galaxies, while photographic surveys recorded millions of stars. Eventually glass plates gave way to electronic detectors, especially CCDs, but the principle remained the same. Modern observatories—including the Hubble and James Webb space telescopes—are heirs to this transformation.

Why It Matters: Henry Draper's photograph of the Orion Nebula helped establish photography as a scientific tool for deep-sky astronomy. The telescope had extended the human eye; photography allowed astronomers to preserve, measure, compare, and eventually see beyond the limits of the eye itself.

1966 — Surveyor 2's Failure Is Formally Assessed as the Lunar Program Continues

No distinct September 30 event established

Surveyor 2 is sometimes associated with late-September space-history chronologies, but its historically meaningful events occurred earlier in the month: it launched on September 20 and crashed on the Moon on September 23 after a failed midcourse correction caused the spacecraft to tumble. Under our project's dating convention, it therefore does not belong in the September 30 chronology. Its exclusion is worth noting because anniversary lists frequently repeat ongoing missions or administrative aftermath across several dates. Our approach is to assign the event to the date on which the historically significant action actually occurred.

2009 — Soyuz TMA-16 Launches a New International Space Station Crew

07:14 UTC

On September 30, 2009, a Soyuz TMA-16 spacecraft lifted off from the Baikonur Cosmodrome in Kazakhstan. Aboard were Russian cosmonaut Maxim Suraev, NASA astronaut Jeffrey Williams, and Canadian spaceflight participant Guy Laliberté, founder of Cirque du Soleil. The launch began another crew transition aboard the International Space Station. Williams and Suraev would become members of Expeditions 21 and 22, eventually spending 167 days aboard the station. Their arrival came during a period when the International Space Station was changing rapidly. Earlier in 2009, the station's permanent crew complement had increased from three people to six, dramatically expanding the amount of scientific work that could be performed aboard the laboratory. The station had also become genuinely multinational in its daily operation. American, Russian, European, Japanese, and Canadian spacecraft, laboratories, robotic systems, astronauts, cosmonauts, and ground-control teams were increasingly functioning as parts of one enormous orbital complex. Laliberté's presence represented another aspect of the changing human presence in orbit. He traveled as a privately funded spaceflight participant under an agreement with the Russian space agency. During his 11-day journey, he promoted awareness of global water issues in what he described as a poetic social mission. Soyuz TMA-16 docked with the station on October 2. Williams and Suraev remained aboard until March 2010, while Laliberté returned to Earth in October aboard another Soyuz.

Why It Matters: Soyuz TMA-16 illustrates the increasingly international and varied character of human spaceflight in the early twenty-first century. The International Space Station was evolving from a construction project into a mature laboratory supporting long-duration crews while also opening limited opportunities for privately funded participants.

 

The Bigger Picture

September 30 provides a fitting conclusion to this month's journey because its events are fundamentally about changing the way we observe. Henry Draper changed astronomy by placing a photographic plate behind a telescope. For the first time, the faint light of a nebula could create a permanent image. The observer no longer had to rely entirely upon what the eye could see in a particular moment. The universe could leave a record of itself. Rosetta represents the same impulse carried almost unimaginably farther. Astronomers once studied comets as transient visitors crossing Earth's sky. Photography allowed them to record those appearances. Spectroscopy revealed the chemicals in their comae and tails. Spacecraft eventually flew past them. Then Rosetta did something fundamentally different. It stayed. For more than two years, the spacecraft accompanied Comet 67P as both traveled around the Sun. It watched the comet awaken. It measured gases escaping from its surface. It photographed cliffs and pits. It released a lander. And in its final hours, Rosetta descended into the environment it had been studying, continuing to observe until observation itself was no longer possible. The progression from Draper's photograph to Rosetta is therefore not as great a conceptual leap as it might first appear. Both represent humanity finding a way to overcome a limitation in observation. The eye can look only for a moment. So we invented photography. A spacecraft flying past a comet can observe it only briefly. So we built one capable of traveling alongside it. There is another important connection to September's earlier entries. On September 15, 2017, Cassini ended its exploration of Saturn by deliberately entering the planet's atmosphere. On September 21, 2003, Galileo deliberately entered Jupiter. And on September 30, 2016, Rosetta descended onto its comet. The destinations and reasons differed, but all three missions demonstrate something important about mature planetary exploration: how a mission ends can itself be part of the mission. Rosetta did not simply run out of power and disappear. Engineers transformed its ending into one final scientific experiment. Its last journey—only 19 kilometers long—became one of the most scientifically intimate portions of a voyage that had carried it billions of kilometers through the solar system. September therefore ends where so much of astronomy begins: with an observer moving closer to something mysterious and asking for one last, better look.

At a Glance

1880 — Evening; precise time not established — Henry Draper photographs the Orion Nebula, obtaining the first successful photograph of a nebula.

2009 — 07:14 UTC — Soyuz TMA-16 launches from Baikonur carrying Maxim Suraev, Jeffrey Williams, and Guy Laliberté toward the International Space Station.

2016 — Approximately 10:39 UTC, spacecraft event time — Rosetta reaches the surface of Comet 67P/Churyumov–Gerasimenko after gathering scientific observations throughout its final descent.

2016 — 11:19:37 UTC, Earth-received time — Rosetta's final signal disappears at ESA mission control, confirming the end of the mission approximately 40 minutes after the event occurred at the comet.