September 27

 A Journey to Two Worlds

2007 — Dawn Begins Its Journey to Vesta and Ceres

11:34 UTC

On September 27, 2007, NASA's Dawn spacecraft lifted off from Cape Canaveral aboard a Delta II Heavy rocket and began an extraordinary journey into the asteroid belt. Its destination was not one world. It was two. Dawn had been designed to investigate Vesta and Ceres, two of the largest bodies in the main asteroid belt between Mars and Jupiter. They had formed in the same general region of the early solar system, yet they evolved into remarkably different worlds. Vesta is dry, rocky, and heavily cratered—a protoplanet, or surviving planetary building block, whose interior differentiated into layers much as the terrestrial planets did. Ceres is larger, richer in water-bearing minerals and volatile materials, and massive enough for gravity to have pulled it into a nearly spherical shape. Today it is classified as a dwarf planet. By studying both with the same spacecraft and instruments, scientists hoped to understand why two bodies formed relatively close to one another could follow such different evolutionary paths. Dawn's journey depended upon a technology that had already proved itself on another remarkable spacecraft: ion propulsion. Instead of burning large quantities of chemical propellant to produce enormous thrust for a short time, Dawn electrically accelerated ions of xenon gas to extremely high speeds. The resulting thrust was extraordinarily gentle. At maximum thrust, Dawn would require about four days to accelerate from zero to 60 miles per hour. But an ion engine can continue operating for weeks or months. The result is cumulative. A tiny push maintained for a very long time can eventually change a spacecraft's velocity by an amount difficult to achieve with conventional chemical propulsion. Dawn carried three ion thrusters and approximately 425 kilograms of xenon propellant. Its enormous solar arrays—each about 8.3 meters long—generated the electrical power necessary to operate them. That propulsion system made Dawn's unprecedented itinerary possible. After launch, Dawn spent years gradually reshaping its orbit around the Sun. It flew past Mars in February 2009, using the planet's gravity to assist its journey toward Vesta. In July 2011, Dawn entered orbit around Vesta. For more than a year, it mapped and studied the protoplanet. Its observations revealed an enormous impact basin near Vesta's south pole, complex geological structures, and evidence confirming connections between Vesta and a class of meteorites found on Earth. Then Dawn did something no spacecraft had ever done. It left orbit around one extraterrestrial body and traveled to another. In September 2012, Dawn departed Vesta. Its ion engines gradually carried it farther through the asteroid belt. On March 6, 2015, Dawn entered orbit around Ceres, becoming the first spacecraft to orbit a dwarf planet. There it discovered a world far more geologically interesting than many scientists had anticipated. The famous bright deposits in Occator Crater proved to contain salts, evidence of briny material that had reached the surface. Dawn also found widespread evidence for water-related minerals and helped reveal Ceres as a chemically and geologically complex world. The spacecraft remained in orbit around Ceres until 2018, when it exhausted the hydrazine needed to control its orientation. NASA deliberately left Dawn in a stable orbit rather than allowing it to crash onto Ceres, partly because of planetary-protection considerations. The spacecraft may remain there for decades.

Why It Matters: Dawn was the first spacecraft to orbit two extraterrestrial destinations and the first to orbit a dwarf planet. Its long-duration ion propulsion system turned what would otherwise have required separate missions into a single voyage of comparative planetary exploration, revealing Vesta and Ceres as two very different survivors from the formation of the solar system.

 

Also on This Day

2003 — SMART-1 Begins Europe's First Mission to the Moon

23:14:46 UTC

On September 27, 2003, the European Space Agency's SMART-1 spacecraft lifted off aboard an Ariane 5 rocket from Europe's spaceport in Kourou, French Guiana. Its name stood for Small Missions for Advanced Research in Technology. Like NASA's Deep Space 1 before it—and Dawn after it—SMART-1 was both an explorer and a technology experiment. Its destination was the Moon, but getting there was part of the experiment. SMART-1 became ESA's first lunar mission and its first mission to use solar-electric propulsion as its primary means of traveling through space. The spacecraft carried only about 82.5 kilograms of xenon propellant for its electric propulsion system. Instead of following the relatively rapid trajectories used by Apollo spacecraft, SMART-1 began in an elongated Earth orbit and used its low-thrust electric engine to gradually enlarge that orbit. The process took months. SMART-1 fired its engine repeatedly, slowly spiraling farther from Earth while also exploiting the gravitational influence of the Moon. It was an extraordinarily fuel-efficient way to travel. The journey took more than a year, but SMART-1 was eventually captured by lunar gravity and entered orbit around the Moon. Once there, the spacecraft conducted scientific observations with a suite of miniaturized instruments. It mapped the lunar surface, investigated its mineral composition, studied the chemical elements present in lunar rocks, and searched for evidence related to water near the lunar poles. SMART-1 also tested technologies in communications, spacecraft autonomy, and compact scientific instrumentation that could be useful on future European deep-space missions. After completing its work, controllers deliberately lowered SMART-1's orbit. On September 3, 2006, the spacecraft struck the Moon in a controlled impact, allowing astronomers on Earth to observe the resulting flash and ejecta.

Why It Matters: SMART-1 was Europe's first lunar mission and demonstrated that solar-electric propulsion could be used for travel beyond Earth orbit. Its combination of technology demonstration and lunar science helped prepare ESA for more ambitious planetary exploration.

2008 — Zhai Zhigang Conducts China's First Spacewalk

Approximately 08:40–08:58 UTC

On September 27, 2008, Zhai Zhigang opened the hatch of China's Shenzhou 7 spacecraft and emerged into space. He became the first Chinese citizen to perform a spacewalk. Shenzhou 7 had launched two days earlier carrying Zhai, Liu Boming, and Jing Haipeng. China had already demonstrated that it could independently launch people into orbit. Yang Liwei had made China's first crewed orbital flight aboard Shenzhou 5 in 2003, and Shenzhou 6 had carried two crew members in 2005. Now China was attempting another essential capability: learning to work outside the spacecraft. Zhai wore the Chinese-developed Feitian spacesuit. The name, meaning roughly “flying in the heavens,” had deep roots in Chinese art and culture. Technically, however, the suit was a highly sophisticated personal spacecraft. It had to maintain pressure around Zhai's body, supply oxygen, remove carbon dioxide and heat, provide communications, and protect him from the vacuum and thermal extremes of space. Liu Boming assisted from the hatch wearing a Russian-derived Orlan spacesuit. At approximately 08:40 UTC, Zhai emerged from the orbital module. Television viewers in China and around the world watched live as he moved outside the spacecraft and waved a Chinese flag. During the excursion, he also retrieved a sample of solid lubricant that had been mounted outside Shenzhou 7 before launch. Scientists could examine the material after its exposure to the space environment. The spacewalk lasted only about 20 minutes. But its importance was much greater than its duration. Extravehicular activity is essential for many complex human-spaceflight operations. The United States and Soviet Union had used spacewalks to conduct experiments, repair spacecraft, service satellites, and ultimately construct large orbital complexes. China was developing the same capability. Years later, Chinese astronauts would perform increasingly complex spacewalks while assembling and maintaining the Tiangong space station.

Why It Matters: Zhai Zhigang's excursion made China the third nation, after the Soviet Union and United States, to independently conduct a spacewalk. It demonstrated spacesuit, airlock, life-support, and operational capabilities essential to China's later development of a permanently crewed space station.

 

The Bigger Picture

September 27 is a remarkable date because all three of its major stories concern something space exploration must eventually learn to do: move beyond the simplest way of getting somewhere.

The earliest planetary spacecraft generally used chemical rockets to receive most of the velocity they needed near the beginning of their journeys. Once released toward their destinations, they largely coasted along carefully calculated trajectories, occasionally making relatively small corrections. Ion propulsion introduced another possibility. A spacecraft could continue accelerating—not dramatically, but patiently. SMART-1 demonstrated how that approach could slowly carry a European spacecraft from Earth orbit to the Moon.

Four years later, Dawn began an even more ambitious application of the idea. Its engines allowed the spacecraft not merely to fly past Vesta and Ceres, but to enter orbit around Vesta, leave that orbit, cross the asteroid belt, and enter orbit around Ceres. That is a profound change in what a single planetary spacecraft can do. Shenzhou 7 tells the human version of a similar story. Reaching orbit is one capability. Living and working beyond the protective shell of a spacecraft is another. Zhai Zhigang's brief excursion outside Shenzhou was therefore not simply a spectacular demonstration. It was one step in China's progression: reach orbit → send multiple crew members → conduct a spacewalk → rendezvous and dock → build a space station.

September 27 consequently reminds us that exploration develops through capabilities. A new propulsion system changes the destinations a spacecraft can reach. A new spacesuit changes where an astronaut can work. A new navigation system changes what a spacecraft can attempt without constant guidance from Earth. And once a capability has been demonstrated, the next mission can build upon it. There is also a particularly satisfying historical connection between SMART-1 and Dawn. On September 27, 2003, Europe launched a spacecraft whose gentle electric engine would slowly carry it toward the Moon. Exactly four years later, on September 27, 2007, Dawn launched with ion propulsion that would eventually carry it into orbit around two separate worlds. The technology had progressed from experiment to extraordinary scientific tool.

At a Glance

2003 — 23:14:46 UTC — ESA's SMART-1 launches from French Guiana, beginning Europe's first lunar mission and a major demonstration of solar-electric propulsion.

2007 — 11:34 UTC — NASA's Dawn spacecraft launches from Cape Canaveral to explore Vesta and Ceres.

2008 — Approximately 08:40 UTC — Zhai Zhigang emerges from Shenzhou 7 and begins China's first spacewalk.

2008 — Approximately 08:58 UTC — Zhai completes the historic excursion and returns inside Shenzhou 7.