OrbitalHub

Wikipedia dixit:

“Ceres is the largest object in the asteroid belt, which lies between the orbits of Mars and Jupiter. Its diameter is approximately 945 kilometers (587 miles), making it the largest of the minor planets within the orbit of Neptune. The thirty-third largest known body in the Solar System, it is the only one within the orbit of Neptune that is designated a dwarf planet by the International Astronomical Union (IAU). Composed of rock and ice, Ceres is estimated to comprise approximately one third of the mass of the entire asteroid belt. Ceres is the only object in the asteroid belt known to be unambiguously rounded by its own gravity. From Earth, the apparent magnitude of Ceres ranges from 6.7 to 9.3, and hence even at its brightest, it is too dim to be seen with the naked eye, except under extremely dark skies.

Ceres was the first asteroid discovered, by Giuseppe Piazzi at Palermo on 1 January 1801. It was originally considered a planet, but was reclassified as an asteroid in the 1850s when many other objects in similar orbits were discovered.

Ceres appears to be differentiated into a rocky core and icy mantle, and may harbor a remnant internal ocean of liquid water under the layer of ice. The surface is probably a mixture of water ice and various hydrated minerals such as carbonates and clay. In January 2014, emissions of water vapor were detected from several regions of Ceres. This was unexpected, because large bodies in the asteroid belt do not typically emit vapor, a hallmark of comets.

The robotic NASA spacecraft Dawn entered orbit around Ceres on 6 March 2015. Pictures with a resolution previously unattained were taken during imaging sessions starting in January 2015 as Dawn approached Ceres, showing a cratered surface. Two distinct bright spots (or high-albedo features) inside a crater, incorrectly reported as observed in earlier Hubble images, were seen in a 19 February 2015 image, leading to speculation about a possible cryovolcanic origin or outgassing. On 3 March 2015, a NASA spokesperson said the spots are consistent with highly reflective materials containing ice or salts, but that cryovolcanism is unlikely. On 11 May 2015, NASA released a higher resolution image showing that, instead of one or two spots, there are actually several.”

Video credit: NASA JPL

Credits: NASA/JPL

Juno is a NASA spacecraft scheduled to start its journey to Jupiter in a few days. Juno will help scientists understand the origin and evolution of Jupiter. While the dense cover of clouds helps Jupiter keep its secrets away from Earth observers, Juno will get close enough to Jupiter so that fundamental processes and conditions characteristic to the early solar system will be revealed.

First, Juno will try to determine if Jupiter has a solid planetary core. While this is an important piece of the puzzle, it might also help determine how Jupiter’s magnetic field is generated (by the way, scientists are still unclear how Earth’s magnetic field is generated, and there are several theories trying to explain it). Juno will also map Jupiter’s magnetic field, study the auroras, and determine the amount of water and ammonia in the atmosphere.

The launch vehicle to lift off with Juno is the most powerful Atlas rocket ever built, the United Launch Alliance Atlas V 551. In this configuration, an Atlas V launch vehicle can lift 18,810 kg to Low Earth Orbit (LEO) and 8,900 kg to Geosynchronous Transfer Orbit (GTO). However, the Atlas V 551 is not powerful enough to put Juno on a direct trajectory to Jupiter. In order to get as far as Jupiter’s orbit, Juno has to perform a gravity assist maneuver.

Juno will orbit Jupiter in a polar orbit and get as close as 5,000 km above the planet’s top clouds. This will allow the spacecraft to do science below the radiation belt of the planet and allow for a complete coverage of the planet. The low altitude will allow for a detailed analysis of the planet’s atmosphere. The orbit will also allow Juno to take a very close look at the auroras that are forming at the north and south Jovian poles.

The scientific payload carried by Juno includes a gravity/radio science system, a microwave radiometer, a vector magnetometer, particle detectors, ultraviolet and infrared spectrometers, and a color camera to capture images of the Jovian poles.

One interesting feature of the spacecraft is the electronics vault. Even if Juno’s highly elliptical orbit avoids the deadly radiation belts by approaching the planet at the north pole, skimming the clouds below the radiation belts, and exiting over the south pole, as an additional protection measure the onboard electronics are protected by a radiation shielded vault. This will ensure that the computers will not malfunction due to single events, and that the electronics will meet the requirements for the mission lifespan.

While the previous missions to the Jovian system have been powered by Radio Thermal Generators (RTGs), Juno will benefit from advances in solar power cell design. The cells used for Juno’s solar panels are far more efficient and radiation tolerant than the cells available to space systems engineers decades ago. Three solar panels that extend more than 10 meters from the hexagonal body of the spacecraft will provide the power required by the scientific instruments.

The mission is scheduled for launch on August 5, 2011. After coasting for more than two years, in October 2013, Juno will swing by Earth. The gravity assist maneuver will provide the delta V necessary for the spacecraft to reach Jupiter’s orbit. Juno will arrive at Jupiter in July 2016. After performing the Jupiter Orbital Insertion (JOI) maneuver, the spacecraft will start to collect and send back home scientific data.

Juno will send back science and telemetry data through the Deep Space Network (DSN), a network of powerful antennas located in Madrid, Spain; Barstow, California; and Canberra, Australia.

At the end of the mission, planned for October 2017, and after 33 complete revolutions around Jupiter, Juno will fire up its thrusters and decrease its velocity, enter the upper atmosphere of Jupiter, and get incinerated. Why such a tragic end to the Juno mission? Remember the Prime Directive? While the Prime Directive is known only to Star Trek fans… and it might get serious consideration only from Star Fleet officers, the possibility of having Juno crashing on one of the Jovian satellites (especially Europa) has to be eliminated. NASA scientists take contamination of other worlds very seriously.

You can find out more about the Juno mission on NASA’s dedicated web site. The Juno mission is managed by NASA’s Jet Propulsion Laboratory in Pasadena, California. The Principal Investigator for the Juno mission is Dr. Scott Bolton of Southwest Research Institute in San Antonio, Texas. The spacecraft was designed and built by Lockheed Martin of Denver, Colorado.

Credits: ESA – D. Hardy

On June 30, 2009, the Ulysses mission came to an end, one year after the predicted mission end date. Ulysses is one of the longest space missions to date, and holds the record for the longest running ESA operated spacecraft, with a total mission duration of 6,842 days (18 years, 8 months, and 24 days).

A joint venture of NASA and ESA, Ulysses was launched on October 6, 1990, from the STS-41 Space Shuttle Discovery mission. Being an Out-Of-The-Ecliptic (OOE) mission, the Ulysses mission studied the Sun at all latitudes. The initial gravity assist at Jupiter on February 8, 1992, injected the spacecraft in an orbit around the Sun with an inclination to the ecliptic of 80.2 degrees.

Besides studying the north and south poles of the Sun, Ulysses also made observations on Jupiter and the comets Hyakutake and McNaught-Hartley.

The spacecraft is box-shaped, 3.2×3.3×2.1 m in size. Three external features of the spacecraft are the High Gain Antenna (HGA), which is a 1.65 m diameter parabolic dish, the Radio-isotope Thermoelectric Generator (RTG), and the two 35 m antennae for the Unified Radio and Plasma (URAP) instrument. The HGA was used for communicating with ground-based stations in both X-band and S-band radio frequency bands.

Credits: ESA

If you are passionate about spacecraft design, an overview of the Ulysses spacecraft, with subsystem schematics and descriptions of all units, is available.

The link contains presentations of the Attitude and Orbit Control Subsystem, the Telemetry, Tracking, and Command Subsystem, the Data Handling Subsystem, and the Power and Thermal Subsystem. It is 1980s technology, but very tasty food for an engineer’s brains.

During its long-duration mission, Ulysses made observations above and below the poles of the Sun. Fundamental scientific discoveries and contributions to our understanding of the Sun and the heliosphere were made. Due to the characteristics of its orbit, Ulysses was able to perform direct measurements of interstellar dust and gas.

You can find out more about Ulysses on ESA’s and NASA’s websites.

Planetary Radio released an interview with Nigel Angold, the ESA Ulysses Mission Operations Manager. Find out how engineers kept the Ulysses spacecraft alive for so long. I invite everyone to listen to it.

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.