OrbitalHub

ESA dixit:

“This short animation explains the relative sizes of the Rosetta spacecraft and comet 67P/Churyumov–Gerasimenko. Rosetta is 32 m from tip to tip of the solar wings. Assuming the comet measures about 4 km across, that’s 125 times the width of Rosetta. Unlike typical artist’s impressions, this image is scaled to convey the vast difference in size between Rosetta and the comet, even when the spacecraft is in a close 10 km orbit, as depicted here. Rosetta arrives at the comet at an altitude of 100 km in the first week of August, and will move progressively closer over the following two months, with the intention to orbit at an altitude of just 10 km, depending on the comet’s activity. For Philae’s deployment in November, Rosetta will come to within a few kilometres of the surface. The comet depicted in this animation is an artist’s impression.”

Credit: ESA

Credits: ESA

On July 10, 2010, the European comet chaser Rosetta will perform the second asteroid flyby of its mission. The first flyby was performed on September 6, 2008, when Rosetta had a close encounter with the asteroid 2867 Steins. Rosetta will skim by the asteroid 21 Lutetia at approximately 3,000 km. The speed of the spacecraft relative to the asteroid will be around 54,000 km/h.

The asteroid Lutetia was discovered on November 15, 1852, by the German astronomer Hermann Goldschmidt. Besides the characteristics of its trajectory, few things are known about the asteroid. From the preliminary observations made by Rosetta, scientists were able to estimate the diameter of the asteroid to 134 km, but the actual shape and composition still remain to be determined.

During the flyby, the spacecraft will operate in a special Asteroid Flyby Mode. This will allow the spacecraft to control its attitude and keep the asteroid in the field of view of the imaging instruments carried onboard.

Rosetta has to follow a complicated trajectory that includes three Earth gravity assists and one at Mars, in order to accelerate to the speed needed for reaching its final destination. The last gravity assist maneuver occurred on November 13, 2009, when Rosetta swung by Earth.

After 6 years into the mission, the systems on the spacecraft are doing very well, and the best is yet to come: the rendezvous with the comet 67/P Churyumov-Gerasimenko in 2014. Rosetta will deploy a small lander on the surface of the comet, and it will continue to fly alongside the nucleus of the comet for more than one year.

OrbitalHub will re-cast the live webstream from ESOC, ESA’s European Space Operations Center, in Darmstadt, Germany. The program starts July 10, 2010, at 20:00 GMT. The closest approach will occur at 20:10:07 GMT. Come back and watch the events unfold!

Credits: ESA OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA

On November 13, 2009, at 8:45 AM CET, ESA’s comet chaser Rosetta swung by Earth. Rosetta passed just South of the Indonesian island of Java at an altitude of 2481 km. Its speed relative to Earth was 13.34 km/s. The maneuver provided a boost of 3.6 km/s.

Rosetta’s OSIRIS (Optical Spectroscopic and Infrared Remote Imaging System) narrow-angle camera was used to image the Earth once every hour for 24 hours.

Rosetta will meet asteroid 21 Lutetia in 2010. The final destination of Rosetta is the comet 67P/Churyumov-Gerasimenko, which the spacecraft will reach in 2014. Rosetta will deploy a lander to the surface of the comet, and will also orbit the nucleus of the comet and fly alongside as it heads towards the inner Solar System. Most of the time, the probe will hibernate with the majority of its systems shut down in order to optimize the power consumption.

You can find more information about Rosetta’s swing by on ESA’s Rosetta Blog website.

Credits: ESA/AOES Medialab

On November 13, 2009, the Rosetta spacecraft will swing by Earth for the last time. This maneuver will provide the boost needed by the spacecraft to reach the outer Solar System. The critical swingby events are described on ESA’s web site.

Rosetta’s mission began on March 2nd, 2004, when the spacecraft lifted off from Kourou, French Guiana. In order to optimize the use of fuel, the probe has a very complicated trajectory to reach its final target, the comet 67/P Churyumov-Gerasimenko. The long trajectory includes three Earth-gravity assists (2004, 2007, and 2009) and one at Mars (2007). The probe uses the gravity wells of Earth and Mars to accelerate to the speed needed for the rendezvous with the comet.

Read more about Rosetta…

Credits: NASA/JPL

The Dawn spacecraft is currently performing the Mars flyby phase of its mission. The purpose of the Mars flyby is to alter the trajectory of the spacecraft in order to rendezvous with its first scientific target in the main asteroid belt.

The spacecraft will come within 549 km of the surface of Mars on February 17, 2009, at 4:28 PST.

The flyby is a gravity assist maneuver used in orbital mechanics to alter the trajectory of a spacecraft. The gravity assist is also known as a gravitational slingshot. The first ever gravity assist maneuver was performed by Mariner 10 in February 1974, and most of the interplanetary missions have made use of it since then.

The scientific objective of the Dawn mission is to answer important questions about the origin and the evolution of our solar system. The currently accepted theory about the formation of our solar system states that Jupiter’s gravity interfered with the accretion process, thereby preventing a planet from forming in the region between Jupiter and Mars. This led to the formation of the asteroid belt.

The asteroids chosen as scientific targets for the Dawn mission are Vesta and Ceres. Due to their size, they have survived the collisional phase, and it is believed that they have preserved the physical and chemical conditions of the early solar system. The asteroids have followed different evolutionary paths and have dissimilar characteristics, which makes them perfect research subjects.

Credits: NASA/JPL

The design of the Dawn spacecraft is based on Orbital’s STAR-2 series, and uses flight-proven components from other Orbital and JPL spacecraft: the propulsion system is based on the design used on Deep Space 1, the attitude control system used on Orbview, a hydrazine-based reaction control system used on the Indostar spacecraft, and command and data handling, as well as flight software, from the Orbview program.

The core structure of the spacecraft is a graphite composite cylinder, while the panels are aluminum core with aluminum/composite face sheets.

The central cylinder hosts the hydrazine and xenon tanks. The hydrazine tank can store 45 kg of fuel, while the xenon tank has a capacity of 450 kg.

The attitude control system (ACS) uses star trackers to estimate attitudes in cruise mode. A coarse Sun sensor (CSS) allows ACS to keep the solar panels normal to the Sun-spacecraft line. ACS also uses the hydrazine-based reaction control system for the control of attitude and for desaturation of the reaction wheels.

Credits: NASA/George Shelton

The solar panels are capable of producing more than 10 kW at 1 AU and 1 kW at 3 AU (on Ceres’ orbit).

The command and data handling system (CDHS) is based on a RAD6000 board running VxWorks. The software is written in C. There are 8GB available on the board as storage for engineering and scientific data.

The scientific payload consists of the Framing Camera (FC), the Gamma Ray and Neutron Detector (GRaND), and the visible and infrared (VIR) mapping spectrometer.

The FC will be used for determining the bulk density, the gravity field, for obtaining images of the surface, and for compiling topographic maps of Vesta and Ceres. In addition, the FC will capture images for optical navigation in the proximity of the asteroids. For reliability purposes, the payload includes two identical cameras that can run independently.

GRaND will serve for the determination of the elemental composition of the asteroids. GRaND is the result of the expertise accumulated during the Lunar Prospector and Mars Odyssey programs.

Credits: NASA/Jack Pfaller

VIR will help map the surface mineralogy of the asteroids. The instrument is a modified version of the visible and infrared spectrometer flying on the Rosetta mission.

The Dawn spacecraft uses ion propulsion to make its journey to Vesta and Ceres. Ion propulsion will also be used by Dawn during the low altitude flights over the asteroids.

While the fact that Dawn’s engines have a thrust of only 90 mN can hardly impress a reader, the important detail to mention when discussing propulsion systems is the specific impulse. Dawn’s engines have a specific impulse of 3100 s. For a chemical rocket, the specific impulse ranges from 250 s for solid rockets to 450 s for bipropellant liquid rockets. The only drawback (if this can be regarded as a drawback) is that the ion engines must be fired for much longer in order to achieve an equivalent trajectory.

With such high specific impulse engines, Dawn makes use of the fuel onboard in a very efficient way. The fuel used is xenon, a heavy noble gas placed in group 8A of the periodic table. The power produced by the large solar panels is used to ionize the fuel and then accelerate it with an electric field between two grids. In order to maintain a neutral plasma, electrons are injected into the beam after acceleration.

Credits: NASA/Amanda Diller

Dawn was launched from Cape Canaveral Air Force Station and injected on an interplanetary trajectory by a Delta II launch vehicle.

The main contributors to the Dawn mission are the University of California in Los Angeles (science lead, science operations, data products, archiving, and analysis), the Jet Propulsion Laboratory (project management, systems engineering, mission assurance, payload, navigation, mission operations, level zero data), and the Orbital Sciences Corporation (spacecraft design and fabrication, quality assurance, and payload integration).

The scientific payload was provided by the Los Alamos National Laboratory, the German Aerospace Center, the Max Planck Institute, and the Italian Aerospace Center. The Deep Space Network is responsible for data return from the spacecraft.

For more information about Dawn, you can visit the Dawn Mission Home Page on the JPL web site.