Dawn Mission

FIELDS OF STUDY: Sub-planetary Astronomy; Space Technology

ABSTRACT: The Dawn Mission was a space mission organized by NASA to study the celestial bodies Vesta and Ceres. The mission began in 2007 and ended in 2018. The Dawn Mission included several historic firsts. The mission’s spacecraft, Dawn, became the first spacecraft in history to orbit an object in the main asteroid belt between Mars and Jupiter. Dawn became the first probe ever to orbit a dwarf planet. It also became the first spacecraft to use solar-electric propulsion of over 25,000 miles per hour (mph) and had an active powered flight of around 5.9 years.

Mission History and Goals

The Dawn Mission was initially proposed by the National Aeronautics and Space Administration’s Discovery Program in 2001. NASA is a US agency focused on space exploration, space technology, and aeronautics. The agency was established by President Dwight D. Eisenhower in 1958.

The Dawn Mission was led by Discovery Program principal investigators (PIs). The PIs also lead the scientific team, consisting of twenty-one members from the United States, Germany, and Italy. The Dawn Science Center, which is located at the University of California, Los Angeles, organizes science and instrument operations. The center is directed by the PIs. The Jet Propulsion Laboratory (JPL) became responsible for project management. Scientific instruments for the mission were donated by Germany and Italy. Both countries were also involved in design, fabrication, and testing.

The mission was meant to study celestial bodies in the main asteroid belt, a field of asteroids, or large rocky objects, between the planets Mars and Jupiter. Specifically, its purpose was to study the asteroid Vesta and the dwarf planet Ceres. The mission would first take the mission’s spacecraft, named Dawn, to Vesta. After remaining in orbit around Vesta for fourteen months, performing various tasks to study the asteroid, Dawn would then head for Ceres. The spacecraft would orbit Ceres and study the dwarf planet until June 2016.

By studying Vesta and Ceres, NASA expected to gain an understanding of the evolution of the two bodies. NASA also expected to be able to compare their evolutionary paths. Furthermore, because Vesta and Ceres are believed to be remnants of the formation of the solar system, NASA hoped a study of the celestial bodies would provide breakthroughs in understanding the solar system’s origins.

The Dawn Mission was cancelled on March 2, 2006, after a congressional budget hearing revealed cost and technical issues. By March 16, NASA made the decision to review the mission’s termination after the JPL presented new evidence to support the mission. Initially, NASA refused to discuss the evidence or provide any commentary to the press. The Dawn Mission was later reinstated after the JPL resolved all technical issues and volunteered to restaff the project. The JPL also became responsible for all costs and resources, as well as the completion of the project.

The Dawn Spacecraft

The Dawn spacecraft was categorized as an orbiter. It used many modern technologies and consisted of both new components and spare components from previous space missions. One of the unique characteristics of Dawn was its ion propulsion system. The system accelerated xenon ions to produce very small amounts of thrust. Although this means that Dawn accelerated very slowly, the spacecraft could ultimately reach incredible speeds because its ion drive allowed the engines to keep firing for extended amounts of time—even as long as several years. To complete the entire mission, the spacecraft required about 2,100 days of thrust time. Three ion propulsion engines were therefore needed to provide enough thrust time.

Dawn was also equipped with backups for most of its systems in the event of a problem with a system. Certain software onboard could sense the problem and then try to replace the faulty system with a backup. Dawn measured 2.36 meters (7 feet, 9 inches) in length when its solar array was in the retracted position. When the solar array was extended, the spacecraft measured 19.7 meters (65 feet) in length.

To complete the Dawn Mission, the Dawn spacecraft was equipped with instruments that would be able to characterize Vesta and Ceres and also provide gravity field data and data regarding bulk properties and the internal structure of the bodies. The instruments included a camera, a visible mapping spectrometer, an infrared mapping spectrometer, a gamma-ray spectrometer, and a neutron spectrometer.

Vesta and Ceres

The targets of the Dawn Mission, Vesta and Ceres, are the two largest objects in the main asteroid belt between Mars and Jupiter. Vesta was discovered by German astronomer Heinrich Wilhelm Olbers (1758–1840) in 1807. It has a diameter of about 530 kilometers (330 miles). The asteroid rotates every five hours and twenty minutes. Vesta was the fourth asteroid to have its orbit calculated, so its official name is "4 Vesta."

Ceres was discovered by Italian astronomer Giuseppe Piazzi (1746–1826) in 1801. The dwarf planet has a diameter of about 950 kilometers (590 miles). Ceres rotates every nine hours, four minutes, and thirty seconds. It was the first asteroid or dwarf planet ever discovered and its orbit computed; therefore, its official name is 1 Ceres. Ceres has a long history of changing categorization. It was first considered a planet. As more objects in the asteroid belt were discovered, Ceres became regarded as an asteroid before finally being categorized as a dwarf planet.

Mission Details

The $473 million Dawn Mission began when NASA launched Dawn from the Cape Canaveral Air Force Station in Florida on September 27, 2007. On February 17, 2009, Dawn passed Mars and continued on to its first target, Vesta. The spacecraft entered orbit around Vesta on July 15, 2011. Dawn continued to orbit it for the next fourteen months. During this time, Dawn took pictures of Vesta, imaged its surface, established its interior structure, performed various measurements of its composition and temperature, and searched for possible moons. The spacecraft was also able to drop down to an altitude of 210 kilometers (130 miles), which was made possible by its ion propulsion system. On September 4, 2012, Dawn left Vesta and set out for Ceres. On March 13, 2015, Dawn entered orbit around Ceres and began investigations. It acquired adequate data about Ceres by mid-2016. The spacecraft ran out of hydrazine fuel in October 2018. At that time, it was floating at a distance of 35 kilometers (22 miles) above the surface of Ceres. NASA officially ended the mission on November 1, 2018, but researchers said that the powerless Dawn would continue to orbit Ceres for decades.

Significance of the Mission

Although the Dawn Mission was the ninth project in NASA’s Discovery Program, it was the first mission to orbit a main belt asteroid and the first to orbit a dwarf planet. The Dawn Mission was also the first mission to orbit two different extraterrestrial bodies that were neither the earth nor the moon.

The Dawn Mission has made several important findings regarding Vesta. The mission found that Vesta has a dense core that is enveloped by a mantle and crust. Vesta also resembles a terrestrial planet more than it resembles an asteroid. The mission further revealed that Vesta has produced more meteorites on Earth than the moon or Mars has. Another finding was that Vesta has a large crater with a mountain that is twice as tall as Earth’s Mount Everest. Lastly, the Dawn Mission found that Vesta has a network of about ninety chasms, some with dimensions similar to Earth’s Grand Canyon.

After completing the mission, around three hundred bright spots called faculae were revealed on Ceres. Other findings on Ceres include deposits of carbonate minerals in its Occator crater, possible geological activities, high internal density, and the presence of the mountain Ahuna Mons, which may have been formed by cryovolcanic activity that released salty water, ammonia, or methane instead of molten rock.

PRINCIPAL TERMS

• asteroid: a rocky, irregularly shaped space body that is thought to be a remnant of the formation of the solar system.
• Ceres: a dwarf planet in the main asteroid belt, an oval-shaped ring of objects between the orbits of Mars and Jupiter. Ceres is being studied on the Dawn Mission.
• ion propulsion: a system found on certain spacecraft that gives xenon atoms a negative electrical charge, accelerates them, and then emits them to propel the vehicle through space.
• NASA: abbreviation for the National Aeronautics and Space Administration; the US government agency responsible for developing aviation and space technologies and conducting both manned and unmanned space missions to research Earth, its solar system and galaxy, and the universe.
• Vesta: the largest asteroid in the main asteroid belt, an oval-shaped ring of objects between the orbits of Mars and Jupiter. Vesta was studied on the Dawn Mission.

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