Dawn (spacecraft)
Dawn is a retired space probe that was launched by NASA in September 2007 with the mission of studying two of the three known protoplanets of the asteroid belt: Vesta and Ceres. In the fulfillment of that mission—the ninth in NASA's Discovery Program—Dawn entered orbit around Vesta on July 16, 2011, and completed a 14-month survey mission before leaving for Ceres in late 2012. It entered orbit around Ceres on March 6, 2015. In 2017, NASA announced that the planned nine-year mission would be extended until the probe's hydrazine fuel supply was depleted. On November 1, 2018, NASA announced that Dawn had depleted its hydrazine, and the mission was ended. The derelict probe remains in a stable orbit around Ceres.
Dawn is the first spacecraft to have orbited two extraterrestrial bodies, the first spacecraft to have visited either Vesta or Ceres, and the first to have orbited a dwarf planet.
The Dawn mission was managed by NASA's Jet Propulsion Laboratory, with spacecraft components contributed by European partners from Italy, Germany, France, and the Netherlands. It was the first NASA exploratory mission to use ion propulsion, which enabled it to enter and leave the orbit of two celestial bodies. Previous multi-target missions using rockets powered by chemical engines, such as the Voyager program, were restricted to flybys.
Project history
Technological background
The first working ion thruster in the US was built by Harold R. Kaufman in 1959 at NASA's Glenn Research Center in Ohio. The thruster was similar to the general design of a gridded electrostatic ion thruster with mercury as its propellant. Suborbital tests of the engine followed during the 1960s, and in 1964 the engine was tested on a suborbital flight aboard the Space Electric Rocket Test 1. It successfully operated for the planned 31 minutes before falling back to Earth. This test was followed by an orbital test, SERT-2, in 1970.Deep Space 1, which NASA launched in 1998, demonstrated the long-duration use of a xenon-propelled ion thruster on a science mission, and validated a number of technologies, including the NSTAR electrostatic ion thruster, as well as performing a flyby of an asteroid and a comet. In addition to the ion thruster, among the other technologies validated by the DS1 was the Small Deep Space Transponder, which is used on Dawn for long-range communication.
Discovery Program selection
Twenty-six proposals were submitted to the Discovery Program solicitation, with budget initially targeted at US$300 million. Three semi-finalists were downselected in January 2001 for a phase-A design study: Dawn, Kepler, and INSIDE Jupiter. In December 2001 NASA selected the Kepler and the Dawn mission for the Discovery program. Both missions were initially selected for a launch in 2006.Cancellation and reinstatement
The status of the Dawn mission changed several times. The project was cancelled in December 2003, and then reinstated in February 2004. In October 2005, work on Dawn was placed in "stand down" mode, and in January 2006, the mission was discussed in the press as "indefinitely postponed", even though NASA had made no new announcements regarding its status. On March 2, 2006, Dawn was again cancelled by NASA.The spacecraft's manufacturer, Orbital Sciences Corporation, appealed NASA's decision, offering to build the spacecraft at cost, forgoing any profit in order to gain experience in a new market field. NASA then put the cancellation under review, and on March 27, 2006, it was announced that the mission would not be cancelled after all. In the last week of September 2006, the Dawn mission's instrument payload integration reached full functionality. Although originally projected to cost US$373 million, cost overruns inflated the final cost of the mission to US$446 million in 2007. Christopher T. Russell was chosen to lead the Dawn mission team.
Scientific background
The Dawn mission was designed to study two large bodies in the asteroid belt in order to answer questions about the formation of the Solar System, as well as to test the performance of its ion thrusters in deep space. Ceres and Vesta were chosen as two contrasting protoplanets, the first one apparently "wet" and the other "dry", whose accretion was terminated by the formation of Jupiter. The two bodies provide a bridge in scientific understanding between the formation of rocky planets and the icy bodies of the Solar System, and under what conditions a rocky planet can hold water.The International Astronomical Union adopted a new definition of planet on August 24, 2006, which introduced the term "dwarf planet" for ellipsoidal worlds that were too small to qualify for planetary status by "clearing their orbital neighborhood" of other orbiting matter. Dawn is the first mission to study a dwarf planet, arriving at Ceres a few months before the arrival of the New Horizons probe at Pluto in July 2015.
Ceres comprises a third of the total mass of the asteroid belt. Its spectral characteristics suggest a composition similar to that of a water-rich carbonaceous chondrite. Vesta, a smaller, water-poor achondritic asteroid comprising a tenth of the mass of the asteroid belt, has experienced significant heating and differentiation. It shows signs of a metallic core, a Mars-like density and lunar-like basaltic flows.
Available evidence indicates that both bodies formed very early in the history of the Solar System, thereby retaining a record of events and processes from the time of the formation of the terrestrial planets. Radionuclide dating of pieces of meteorites thought to come from Vesta suggests that Vesta differentiated quickly, in three million years or less. Thermal evolution studies suggest that Ceres must have formed some time later, more than three million years after the formation of CAIs.
Moreover, Vesta appears to be the source of many smaller objects in the Solar System. Most V-type near-Earth asteroids, and some outer main-belt asteroids, have spectra similar to Vesta, and are thus known as vestoids. Five percent of the meteoritic samples found on Earth, the howardite–eucrite–diogenite meteorites, are thought to be the result of a collision or collisions with Vesta.
It is thought that Ceres may have a differentiated interior; its oblateness appears too small for an undifferentiated body, which indicates that it consists of a rocky core overlain with an icy mantle. There is a large collection of potential samples from Vesta accessible to scientists, in the form of over 1,400 HED meteorites, giving insight into Vesta geologic history and structure. Vesta is thought to consist of a metallic iron–nickel core, an overlying rocky olivine mantle and crust.
Objectives
The Dawn mission's goal was to characterize the conditions and processes of the Solar System's earliest eon by investigating in detail two of the largest protoplanets remaining intact since their formation.Although the mission has finished, the data analyses and interpretations will continue for many years. The primary question that the mission addresses is the role of size and water in determining the evolution of the planets. Ceres and Vesta are highly suitable bodies with which to address this question, as they are two of the most massive of the protoplanets. Ceres is geologically very primitive and icy, while Vesta is evolved and rocky. Their contrasting characteristics are thought to have resulted from them forming in two different regions of the early Solar System.
There are three principal scientific drivers for the mission. First, the Dawn mission can capture the earliest moments in the origin of the Solar System, granting an insight into the conditions under which these objects formed. Second, Dawn determines the nature of the building blocks from which the terrestrial planets formed, improving scientific understanding of this formation. Finally, it contrasts the formation and evolution of two small planets that followed very different evolutionary paths, allowing scientists to determine what factors control that evolution.
Instruments
NASA's Jet Propulsion Laboratory provided overall planning and management of the mission, the flight system and scientific payload development, and provided the ion propulsion system. Orbital Sciences Corporation provided the spacecraft, which constituted the company's first interplanetary mission. The Max Planck Institute for Solar System Research and the German Aerospace Center provided the framing cameras, the Italian Space Agency provided the mapping spectrometer, and the Los Alamos National Laboratory provided the gamma ray and neutron spectrometer.- Framing camera – Two redundant framing cameras were flown. Each used a f/7.9 refractive optical system with a focal length of 150 mm. A frame-transfer charge-coupled device, a Thomson TH7888A, at the focal plane has 1024 × 1024 sensitive 93-μrad pixels, imaging a 5.5° x 5.5° field of view. An 8-position filter wheel permits panchromatic and spectrally selective imaging. The broadest filter allows imaging at wavelengths from 400 to 1050 nm. The FC computer is a custom radiation-hardened Xilinx system with a LEON2 core and 8 GiB of memory. The camera offered resolutions of 17 m/pixel for Vesta and 66 m/pixel for Ceres. Because the framing camera was vital for both science and navigation, the payload had two identical and physically separate cameras for redundancy, each with its own optics, electronics, and structure.
- Visible and infrared spectrometer – This instrument is a modification of the visible and infrared thermal-imaging spectrometer used on the Rosetta and Venus Express spacecraft. It draws its heritage from the Saturn orbiter Cassinis visible and infrared mapping spectrometer. The spectrometer's VIR spectral frames are 256 × 432, and the slit length is 64 mrad. The mapping spectrometer incorporates two channels, both fed by a single grating. A CCD yields frames from 0.25 to 1.0 μm, while an array of HgCdTe photodiodes cooled to about 70 K spans the spectrum from 0.95 to 5.0 μm.
- Gamma Ray and Neutron Detector – This instrument is based on similar instruments flown on the Lunar Prospector and Mars Odyssey space missions. It had 21 sensors with a very wide field of view. It was used to measure the abundances of the major rock-forming elements and potassium, thorium, uranium, and water in the top 1 m of the surface of Vesta and Ceres.