Dawn (Discovery 9)

Launch data: Designation  32249 / 07043A Launch date  27 Sep 2007 - 11:34 UT Launch site  CC, SLC-17B Launch vehicle  Delta-7925H Mission  Deep Space probe Heliocentric orbit Perihelion / Aphelion   Incl. to ecliptic   ° Period   min

Spacecraft data: Prime contractor  Orbital/Dulles Operator  JPL Platform  Star-2-derived bus Mass at launch  1217.7 kg Dry Mass  747 kg Basic shape  Box Dimension  1.64 x 1.27 x 1.77 m Solar array  two 2.3 x 8.3 meter solar arrays, 19.7 m-span Stabilization   DC power   Design lifetime  Still in Vesta orbit

Description:

• 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 details:

• The Dawn space probe was launched towards the asteroid belt on Sep 27, 2007.
• Dawn is the 9th NASA Discovery mission and will orbit the dwarf planet Ceres and the large asteroid Vesta; it follows on from the successful Deep Space 1 technology mission in using ion drive for NASA planetary exploration. It has 3 NSTAR xenon ion thrusters, with 425 kg of xenon propellant for interplanetary travel and astreroid orbit insertion; and 12 0.9-Newton MR-103G hydrazine thrusters with 46 kg of propellant for safe mode entry. The spacecraft was built by Orbital/Dulles using a Star-2-derived bus with a dry mass of 747 kg, and is 1.8 x 1.3m in size with two large 19.7-meter-span solar arrays to provide power for the ion drive. Dawn has three main instruments: the Framing Cameras, the VIR visible/IR mapping spectrometer, and the GRaND gamma ray and neutron spectrometer. The mission is managed by JPL.
• United Launch Alliance carried out the launch on a Boeing Delta 7925H at 1134 UTC (7:34am. EDT) on Sep 27. At 1142 UTC the Delta second stage reached a 185 x 186 km Earth parking orbit; the second burn was at 1225 UTC. After a third burn, the second stage was in a 170 x 6776 km x 26.7 deg orbit. The PAM-D solid third stage fired at 1229 UTC to accelerate the probe to an escape velocity of 11.50 km/s; at 1236 UTC the stage separated after releasing two small despin weights on cables. Dawn's hyperbolic escape orbit of 209 x -83420 km x 28.9 deg had an asymptotic velocity of 3.36 km/s (specific energy C3 = 11.319 km**2/s**2).
• Dawn passed lunar orbit at about 1430 UTC on Sep 28, 2007 and left the Earth's sphere of influence on Sep 30. Dawn is now in a 1.00 x 1.62 AU solar orbit inclined 0.5 deg to the ecliptic; after a Mars flyby in Feb 2009 it will reach Vesta in 2011. The first test burn of the ion engine on Oct 6 was successful.
• En route to the asteroid belt, the Dawn space probe flown past Mars. Closest approach of 543 km at a Mars-relative velocity of 5.31 km/s was at 0028 UTC (SCET) on Feb 18, 2009. Dawn's hyperbolic orbit relative to the Mars equator was about 543 x -20665 km x 119.5 deg; its solar ecliptic orbit was changed from 1.22 x 1.68 AU x 1.8 deg to 1.37 x 1.84 AU x 6.1 deg. Dawn entered Mars' nominal gravitational sphere of influence on Feb 15, 2009 (1200 UTC) and left it at Feb 20 (1300 UTC).
  Dawn's next appointment is its arrival in Vesta orbit during Sep 2011 and Ceres in 2015.
• The Dawn asteroid probe is in a 1.50 x 1.94 AU x 6.5 deg solar orbit and was 1.75 AU from the Sun on Jan 1, 2010. It continues to use its ion engines to enlarge its orbit and is expected to reach Vesta in 2011.
• On June 2010, the Dawn space probe is approaching 2.0 AU from the Sun as it continues its ion-drive-propelled uphill climb towards asteroid (4) Vesta, which it will reach in Jul 2011. Dawn is currently in a 1.75 x 2.07 AU x 6.8 deg orbit around the Sun.
• The NASA/JPL Dawn spacecraft is slowly approaching minor planet (4) Vesta as it continues to operate its ion propulsion system. As of May 24, 2011 it was 640000 km away from Vesta, and is due to enter orbit around the body on July 16. (4) Vesta is on the boundary between dwarf planets whose mass is enough to make them quasi-spherical and small asteroids whose gravity is insufficient to overcome the structural strength of the rock they are composed of; it is an irregular ellipsoid 458 x 560 x 578 km in size. Vesta orbits the Sun in a 2.15 x 2.57 AU x 7.1 deg path in the main asteroid belt.
• NASA's Dawn probe is approaching Vesta. On Jun 27, 2011 it entered the planetoid's gravitational sphere of influence (Hill sphere) whose radius is 127000 km; by Jul 5, 2011 it was only 60000 km from Vesta.
• The Dawn space probe entered a bound orbit around Vesta on Jul 16, 2011. It continues to use its ion engine to lower the orbital height, and by Jul 22 it was at an altitude of 5400 km.
• The Dawn probe continues to operate in polar circumVestal orbit.
  According to data on the Dawn web site, orbit capture was at 0348 UTC on Jul 16, 2011 at about 16000 km from Vesta's surface; ion engine thrusting was suspended from Jul 23 to Jul 29 in a 5200 km orbit, followed by descent to the survey orbit of 2700 km altitude which was reached on Aug 2. From Aug 31 to Sep 18 the orbit was lowered again to 680 km for the high altitude mapping orbit (HAMO) phase. On Nov 2 thrusting resumed to leave HAMO for the low altitude mapping orbit which will be at 180 km; by Nov 16 altitude was 339 km. Of course Vesta is far from spherical; altitudes are expressed relative to a mean Vesta radius of 265 km.
• The actual target orbit for Dawn is 210 km above the nominal Vesta radius of 265 km. Dawn reached this Low Altitude Mapping Orbit (LAMO) on Dec 8, 2011 and began science observations on Dec 12.
• On May 1, 2012 the Dawn spacecraft began to raise its 210 km low altitude orbit around Vesta to begin its second high altitude mapping phase at around 680 km above the surface.
• Dawn will depart Vesta on May 22, 2012 and reach Ceres on February 01, 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.
• Departure of the Dawn probe from Vesta is underway, after a brief delay thanks to a problematic reaction wheel. Dawn is expected to escape Vesta and enter solar orbit on Sep 5, 2012 en route to Ceres.
• Dawn escaped Vesta's gravity at 0626 UTC on Sep 5, 2012 and is now in solar orbit on its way to Ceres, which it should reach in 2015.
• On March 6, 2015, NASA′s Dawn spacecraft will arrive at Ceres, marking the first time that a spacecraft has ever orbited two solar system targets. Dawn previously explored the protoplanet Vesta for 14 months, from 2011 to 2012, capturing detailed images and data about that body. Dawn has entered its approach phase toward Ceres, and the next couple of months promise continually improving views prior to arrival. By the end of January 2015, the spacecraft′s images and other data will be the best ever taken of the dwarf planet.
  
  

  Dawn trajectory map

  
  
• White Spot on Ceres.
• One of the most anticipated aspects the Dawn spacecraft being in orbit around Ceres HAS to be finding out what this spot is. It could be ice, it could be a cryovolcano or geysers, or it could be something else. But we do know fairly certain that it is a real feature and not an image artifact, since it shows up in most of the recent Hubble images and now the Dawn images.
• Planetary scientists have long suspected that water ice may be buried under Cere’s crust. A few things point to subsurface ice: the density of Ceres is less than that of the Earth′s crust, and because the surface bears spectral evidence of water-bearing minerals. Scientists estimate that if Ceres were composed of 25 percent water, it may have more water than all the fresh water on Earth. Ceres′ water, unlike Earth′s, would be in the form of water ice and located in the mantle, which wraps around the asteroid′s solid core.
• And then last year (2014), the Herschel space telescope discovered water vapor around Ceres, and the vapor could be emanating from water plumes, much like those that are on Saturn′s moon Enceladus, or it could be from cryovolcanism from geysers or icy volcanoes. Without huge a planet or satellite nearby tugging on it, the mechanism for how Ceres is active is also intriguing.
• The Dawn probe, en route from Vesta to Ceres, safemoded due to a radiation event on Sep 11, 2014. Ion engine thrusting resumed by Sep 16; the probe is currently in a 2.45 x 3.03 AU x 9.6 deg solar orbit, reducing aphelion and increasing inclination on its way to Ceres arrival in 2015.
• Dawn entered the Ceres Hill sphere (where Ceres gravity wins out over the Sun's, mumble mumble caveat equivocate...) on Jan 26, 2015 and flew past Ceres at 19:00 UTC Feb 23, 2015 at a distance of 38600 km. Slowing down as it arced out, it became bound to Ceres at 12:39 UT on Mar 6 at a distance of 60000 km. Initial orbit was 38000 km pericereal height at an inclination of 34 deg to the Ceres equator. Dawn reached an apocereal height of 75850 km on Mar 19 and slowly reapproached Ceres as the ion drive twisted the orbital inclination to a pure polar orbit of 15900 x 62900 km x 90.0 deg by Apr 13. The next predicted pericereal pass is at 15970 km on Apr 18. The bright spots seen in early Dawn images of Ceres remain mysterious at this writing.
• NASA′s Dawn space probe, orbiting the dwarf planet (1)Ceres, fell silent on Oct 30, 2018 as it ran out of propellant. Dawn was launched in 2007 and was the first mission to use ion propulsion to visit multiple worlds.
  The probe orbited Vesta from Jul 2011 to Sep 2012, and Ceres from Mar 2015 till now.

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