The Dawn of Ion Engines

Ion thrusters are becoming a bigger and bigger part of modern satellite design. Over 100 geosynchronous Earth Orbit communication satellites are being kept in the desired locations in orbit using this revolutionary technology. This post is about its most amazing achievement to date, the Dawn Spacecraft. Just reported that it is at the end of its second extension of the mission it has a few records under its belt. It is the first spacecraft to orbit two different celestial bodies, and the first to orbit any object in the main asteroid belt between Mars and Jupiter. It is also a record breaker for electric speed. Travelling over 25,700 mph it is 2.7x faster than the previous fastest electric thrusted spacecraft. That is a comparable speed to the Delta 2 launch vehicle that got it to space in the first place.

Delta 2 launch
The Dawn spacecraft launching on a Delta 2 rocket from Cape Canaveral Air Force Station SLC 17 on Sept 27th, 2007. Credit: NASA/Tony Gray & Robert Murra

The Dawn mission was designed to study two large bodies in the main asteroid belt. This is to get a deeper insight into the formation of the solar system . It also has the added benefit of testing the ion drive in deep space for much longer than previous spacecraft. Ceres and Vesta are the two most massive bodies in the belt, and are also very useful protoplanets from a scientific standpoint. Ceres is an icy and cold dwarf planet whereas Vesta is a rocky and dry asteroid. Understanding these bodies can bridge the understanding of how the rocky planets and icy bodies of the solar system form. It could also show how some of the rocky planets can hold water/ice. In 2006 the International Astronomical Union (IAU) changed the definition of what a planet is, and introduced the term “dwarf planet”. This is the change that downgraded Pluto from its planet status, although that has been argued to be wrong by Dr. Phil Metzger in a recent paper. Ceres is classified as a dwarf planet. As Dawn arrived at Ceres a few months before New Horizons reached Pluto, Dawn was the first to study a dwarf planet.

Dawn prior to encapsulation at its launch pad on July 1, 2007. Credit: NASA/Amanda Diller

The ion engine is so efficient that without them a trip to just Vesta would need 10 times more propellant, a much larger spacecraft, and therefore a much larger launch vehicle (making it much more expensive). The ion propulsion system that it uses was first proven by Deep Space Mission 1, along with 11 other technologies. Dawn has three 30 cm diameter (12 inch) ion thrust units. They can move in two axis to allow for migration of the center of mass as the mission progresses. The attitude control system can also use the movable ion thrusters to control the attitude. The mission only needs two of the thrusters to complete the mission, the third being a spare. All three have been used at some point during the mission, one at a time. As of September 7th 2018 the spacecraft has spent 5.9 years with the ion thrusters on, which is about 54% of its total time in space. The thrust to its first orbit took 979 days, with the entire mission being over 2000 days. Deep Space 1’s mission in contrast lasted 678 days before the fuel ran out.

An artist’s impression of Dawn with its ion thrusters on. Credit: NASA

The thrusters work by using electrical charge to accelerate ions from xenon fuel to speeds 7-10 times that of chemical engines. The power level and the fuel feed can be adjusted to act like a throttle. The thruster is very thrifty with its fuel, using a minor 3.25 milligrams of xenon per second, roughly 280g per day, at maximum thrust. The spacecraft carried 425 kg (937 pounds) of xenon propellant at launch. Xenon is a great fuel source because it is chemically inert, easily stored in compact form. Plus the atoms are very heavy so they provide large thrust compared to other comparable candidate propellants. At launch on Earth the xenon was 1.5 times the density of water. At full thrust the ion engines produce a thrust of 91 mN, which is roughly the force needed to hold a small sheet of paper. Over time these minute forces add up and over the course of years can produce very large speeds. The electrical power is produced by two 8.3 m (27 ft) x 2.3 m (7.7 ft) solar arrays. Each 18 meter squared (25 yard squared) array is covered in 5,740 individual photo voltaic cells. They can convert 28% of the sun’s energy into useful electricity. If these panels were on Earth they would produce 10 kW of energy. Each of the panels are on gimbals that mean they can turn any time to face the sun. The spacecraft uses a nickel-hydrogen battery to charge up and power during dark points in the mission.

The dawn mission patch.  This logo represents the mission of the Dawn spacecraft. During its nearly decade-long mission, Dawn will study the asteroid Vesta and dwarf planet Ceres Credit: NASA.

Vesta was discovered on March 29th 1807 by astronomer Heinrich Wilhelm Olbers, and is named after the Roman virgin goddess of home and hearth. The Dawn mission uncovered many unique surface features of the protoplanet ,twice the area of California, that have intrigued scientists. Two colossal impact craters were found in the southern hemisphere, the 500 km (310 miles) wide Rheasilvia basin, and the older 400 km (250 miles) wide Veneneia crater. The combined view of these craters was apparent even to the Hubble telescope. Dawn showed that the Rheasilvia crater’s width is 95% of the width of Vesta (it’s not perfectly spherical) and is roughly 19 km (12 miles) deep. The central peak of the crater rises to 19-25 km (12-16 miles) high, and being more that 160 km (100 miles) wide, it competes with Mars’ Olympus Mons as the largest mountain in the solar system. The debris that was propelled away from Vesta during the impacts made up 1% of its mass, and is now beginning its journey through the solar system. These are known as Vestoids, ranging from sand and gravel all the way up to boulders and smaller asteroids. About 6% of all meteorites that land on Earth are a result of this impact.


The brave new world of 4 Vesta, courtesy of NASA’s Dawn spacecraft. Credit: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA

Dawn mapped Vesta’s geology, composition, cratering record and more during its orbit. It also managed to determine the inner structure by measuring its gravitational field. The measurements were consistent with the presence of an iron core of around 225 km (140 miles), in agreement with the size predicted by
howardite-eucrite-diogenite (HED)-based differentiation models. The Dawn mission confirmed that Vesta is the parent body of the HED meteorites, by matching them with lab based measurements. These experiments measured the elemental composition of Vesta’s surface and its specific mineralogy. These results confirm that Vesta experienced pervasive, maybe even global melting, implying that differentiation may be a common history for large planetesimals that condensed before short-lived heat-producing radioactive elements decayed away. The pitted terrains and gullies were found in several young craters. This could be interpreted as evidence of volatile releases and transient water flow. Vesta’s composition is volatile-depleted, so these hydrated materials are likely exogenic (formed on the surface).

A colour coded topographic map from the Dawn mission of the giant asteroid Vesta. Credit: NASA/JPL

The first object ever discovered in the main asteroid belt was Ceres. Named after the Roman goddess of corn and harvest, it was discovered by Italian astronomer Father Giuseppe Piazzi in 1801. Initially classified as a planet, it was later classified as an asteroid as more objects were found in the same region. In recognition of its planet like properties (being very spherical) it was designated a dwarf planet in 2006 along with Pluto and Eris. Observed by the Hubble telescope between 2003 and 2004, it was shown to be nearly spherical, and approximately 940 km (585 miles) wide. Ceres makes up 35% of the mass of the main asteroid belt. Before Dawn there were plenty of signs of water on Ceres. First, its low density indicates that it is 25% ice by mass, which makes it the most water rich body in the inner solar system after Earth (in absolute amount of water). Also, using Hershel in 2012 and 2013, evidence of water vapor, probably produced by ice near the surface transforming from solid to gas (known as sublimating).

Dwarf planet Ceres is shown in these false-color renderings, which highlight differences in surface materials. Credit: NASA/JPL-CalTech/UCLA/MPS/DLR/IDA

Acquiring all the data it needed by the middle of 2016, Dawn measured its global shape, mean density, surface morphology, mineralogy, elemental composition, regional gravity and topography at exceeded resolutions. The imaging from the mission showed a heavily cratered surface with bright features. Often referred to as “bright spots” they are deposits of carbonates and other salts. Multiple measurements showed an abundance of ice at higher latitudes. However the retention of craters up to 275 km (170 miles) in diameter argue for a strong crust, with lots of hydrated salts, rocks and clathrates (molecules trapped in a cage of water molecules). Gravity and topography data also indicated that that Ceres’ internal density increases with depth. This is evidence for internal differentiation resulting from the separation of the dense rock from the low density water-rich phases in Ceres history. The rock settled to form an inner mantle overlain with a water-rich crust. This internal differentiation is typical of small planets like Ceres and Vesta that Sets them apart from asteroids.

Thank you for reading, take a look at my other posts if you are interested in space, electronics, or military history. If you are interested, follow me on Twitter to get updates on projects I am currently working on.

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Delta II Launch Site Demolished

Delta II launch
The launch of the GRAIL mission from Launch Complex 17 by a Delta II. The final launch from SLC-17. Credit: NASA/Tom Farrar and Tony Gray

At 11:00 UTC on the 12th of July 2018 the two launch towers of Space Launch Complex 17 were demolished by controlled explosions. The crowd of onlookers cheered as the towers fell, and took some great images and videos of the demolition. The launch site had not been used since 2011 when Delta II 7920H-10C fired NASA’s GRAIL spacecraft towards the Moon. The launch complex had two pads named 17A and 17B. The site is now to be reused as a test bed for potential lunar landers made by Moon Express. Boasting some very prestigious missions well beyond Earth SLC-17 will be remembered as an important part of the history of American space.

Delta Echo 1
A delta Rocket carrying NASA’s Echo 1 satellite launching August 12th 1960. The Echo satellite inflated in orbit to reflect signals back to Earth. Credit: NASA.

It was built in 1956 for use as a launch site for the PGM-17 Thor missile. This was the first operational ballistic missile that the United States had in their arsenal. The first launch of a Thor missile from 17A was 3rd of August 1957, with the first launch from 17B being 25th of January 1957. In the early 1960s the site was upgraded to support a variety of Expendable Launch Vehicles, all of which were derived in some way from the Thor booster. We now know this family of rockets as the Delta rockets used by the United Launch Alliance. Thirty five early Delta rocket missions were launched from LC-17 between 1960 and 1965. At that point operated by the US Air Force. In 1965 the operation of the site was transferred to NASA.

View of LC-17
View of LC-17 viewing East. A fairly old photo taken by the U.S. Army Corps of Engineers. Credit: Martin Stupich

In 1988 the site was returned to the Air Force to support the Delta II program. The site had to be modified to facilitate the new more powerful rocket, with new platforms being installed and the D=Ground Service Tower was raised by 10 ft. The program entered service in 1989 after worries about the shuttle due to the Challenger disaster. Pad 17B was modified in 1997 to support a newer more powerful launch vehicle the Delta III which made its maiden flight on 26th of August 1998. Ending in failure, the next three attempts were failures in some sense and the programme was abandoned in late 2000. The Delta II continued to launch, with it’s fairly cheap price tag, and amazing track record it has been a favourite for NASA on a number of big projects. This post by NASA explains how the layout of the site and the small teams allowed LC-17 to be efficient and consistent over it’s 50 year lifespan. Some Delta II launches could be within days of each other because the launch crews were so effective.

Space Launch Complex 17
A view of Space Launch Complex 17, pads A and B taken in 2007. Delta II rocket with THEMIS aboard sits on Pad B. Credit: NASA/George Shelton

There have been some very famous spacecraft launched from SLC-17 in the years, mostly by Delta I and II rockets. Among them the Explorer and Pioneer space probes studying the physics of our solar system, and exploring some of it. All of the Orbiting Solar Observatories between 1962 and 1975 were launched from this site, as well as the Solar Maximum mission in 1980. Some of the first weather satellites like TIROS and later GOES were launched from SLC-17 allowing much better understanding of weather and improving (mainly military) weather reports. My personal favourite launches are those of the Mars Exploration Rovers in 2003. Both spirit and Opportunity (still going) were launched from this important launch site.

Spirit lifting off
A Delta II launching from pad SLC-17A with the MER-A or Spirit Rover towards Mars on June 10th 2003. Credit: NASA/KSC

Space Launch Complex 17 is also famous for being the last site where you had to press a button to launch the rocket. Most pads had a computerized auto-sequencer, much like the space shuttle, and in the modern world of rocketry it makes much more sense to do that. Even after 1995 when they got rid of the button (sadly) a human needed to press go on a computer to say launch. Bill Hodge, an electrical engineer at the launch complex said “If you didn’t push that button, it didn’t launch.” Tom Mahaney, project manager for the closeout of the complex described the site as “hectic, but not dysfunctional.” This is the best description I can find of this massively important historical site. In its time it has supported a total of 325 Thor and Delta rocket launches!

Thank you for reading, take a look at my other posts if you are interested in space or electronics, or follow me on Twitter to get updates on projects I am currently working on.