Dawn

Description

Dawn launched successfully at 11:34 UT (7:34 a.m. EDT) on Thursday, 27 September.

Dawn is a mission designed to rendezvous and orbit the asteroids 4 Vesta and 1 Ceres. The scientific objectives of the mission are to characterize the asteroids' internal structure, density, shape, size, composition and mass and to return data on surface morphology, cratering, and magnetism. These measurements will help determine the thermal history, size of the core, role of water in asteroid evolution and what meteorites found on Earth come from these bodies, with the ultimate goal of understanding the conditions and processes present at the solar system's earliest epoch and the role of water content and size in planetary evolution. The data returned will include, for both asteroids, full surface imagery, full surface spectrometric mapping, elemental abundances, topographic profiles, gravity fields, and mapping of remnant magnetism, if any.

Spacecraft and Subsystems

The Dawn spacecraft is generally box-shaped (1.64 x 1.27 x 1.77 m) and made of aluminum and graphite composite with a dry mass of 747.1 kg and a fueled launch mass of 1217.7 kg. The spacecraft core is a graphite composite cylinder, with the titanium hydrazine and xenon tanks mounted inside. Mounting, access, and other panels are aluminum core with aluminum facesheets. Two solar panel wings extend 19.7 m tip-to-tip and are mounted on opposite sides of the spacecraft. A parabolic fixed 1.52 m high gain dish antenna is mounted on one side of the spacecraft in the same plane as the solar arrays. Three low gain antennas are also mounted on the spacecraft. A 5 m long magnetometer boom extends from the top panel of the spacecraft. Also mounted on the top panel is the instrument bench, holding the cameras, mapping spectrometer, laser altimeter, and star trackers. A gamma ray / neutron spectrometer is mounted on the top panel as well.

The two 2.3 x 8.3 meter solar arrays, composed of InGaP/InGaAs/Ge triple-junction cells, provide 10.3 kW at 1 AU (1.3 kW at end-of-life at 3 AU) to drive the spacecraft (22-35 V) and the solar electric ion propulsion system (80-140 V). Power is stored in a 35 Ah NiH2 battery. The ion propulsion consists of three ion thrusters and is based on the Deep Space 1 spacecraft ion drive, using xenon which is ionized and accelerated by electrodes. The xenon ion engines have a maximum thrust at 2.6 kW input power of 92 mN and a specific impulse of 3200 to 1900 s. The 30-cm diameter thrusters are two-axis gimbal mounted at the base of the spacecraft. The xenon tank holds 425 kg of propellant at launch.

Attitude control is maintained by reaction wheels and twelve 0.9 N hydrazine engines placed around the spacecraft. The hydrazine tank holds 45.6 kg propellant at launch. The hydrazine thrusters can also be used to help orbit insertion maneuvers. Attitude knowledge is provided by star trackers and gyros The thermal control system consists of ammonia-based heat pipes and louvers, and requires roughly 200 W at 3 AU. Communications are in X-band for both uplink and downlink, through the body-fixed high and medium gain antennas and a low gain omnidirectional antenna, utilizing a 100 W traveling wave tube amplifier. The command and data handling system utilizes a RAD6000 processor, 8 Gb mass memory, and a Mil-Std-1553B data bus. Uplink data rates range from 7.8 b/s to 2.0 kb/s and downlink rates from 10 b/s to 124 kb/s.

Mission Profile

Launch from Cape Canaveral on a Delta 2 (7925-H) took place on 27 September at 11:34 UT (7:34 a.m.. EDT). Transfer into a trajectory towards the asteroid belt took place approximately 1 hour later. After a four year heliocentric cruise including a Mars flyby and gravity assist on 4 February 2009, Dawn will reach Vesta on 14 August 2011 and go into orbit for 7 months. One high orbit period at 2700 km altitude is planned, followed by a lower orbit of 950 km and then a lower (460 km, 4 hour) orbit. Dawn will depart Vesta in 22 May 2012 and reach Ceres on 1 February 2015 where it will go into orbit for 5 months, a high orbit at 6400 km, a medium orbit of 1800 km, and a low orbit of 1180 km (altitude above surface about 690 km, period of 9 hours). The end of the primary mission takes place in July of 2015. After the end of the mission Dawn will remain in orbit around Ceres. It is expected that 288 kg of xenon will be required to reach Vesta and 89 kg to reach Ceres. The hydrazine thrusters will be used for orbit capture.

An announcement was made that the mission was cancelled on 2 March 2006, but the mission was reinstated on 27 March 2006. A review concluded the mission should be performed if the technical difficulties can be overcome. The original budget for the mission was $373 million, cost overruns bring the total cost to $446 million.