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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Taking a Selfie on Mars

Curiosity in a dust storm
An image shared by Seán Doran on Sunday of the Mars Curiosity in the middle of a dust storm reported to cover an area the size of the US and Russia Combined. CredIt: NASA/JPL/Seán Doran.

Curiosity is a famous, car sized rover currently exploring Gale Crater on Mars. Famous because it has an impressive track record. Landing on Mars in August 2012, the rover was designed to last 687 days/668 sols (martian days) but was extended to indefinitely in December 2012. Although at the time of writing it is trying to wait out a dust storm that has forced Opportunity into a deep sleep, it is still going strong to this day, and has managed to even take a selfie while waiting for it all to blow over. That is over 2100 earth days, still functioning and completing chemical analysis on soil from 560 million km (350 million mi) away!

Mars Curiosity Rover MAHLI
The Mars Hand Hand Lens Imager (MAHLI) on NASA’s Curiosity Rover, taken by Curiosities Mast Camera on the 32nd martian day. Credit: NASA/JPL.
Curiosity first space selfie
The first selfie that Curiosity took of itself with its MAHLI camera with it’s dust cover closed. Taken September 7th, 2012. Credit: JPL/NASA.

Even though this impressive piece of engineering has been collecting samples and completing scientific experiments for over 5 years, the rover still has time to take the occasional selfie. It has a 2.1m robotic arm, and a sophisticated camera (MHLI) mounted on the end of it. The obvious thing you will notice about the images is that you can’t see the arm taking the image. To many of the NASA sceptics and flat earthers this is conclusive proof that the rover is in a film studio somewhere in California rather than on our nearest neighbour planet. At first glance you can understand the problem, where is the arm? The first clue is that the arm isn’t in the picture at all, and when you see the images taken of it here on Earth you can see it is a very prominent feature.

Mars Rover selfie October 2012
The Curiosity Rover taking a selfie at “Rocksnest” a sand patch on the surface of Aeolis Palus, between Peace Vallis and Aeolis Mons (“Mount Sharp”) Taken in October 2012, not long after landing. Credit: NASA/JPL.

The simple answer was explained by NASA/JPL when these questions came up after the first self shot. As the Curiosity camera has a limited view, it cannot get the entire rover into one shot, and even when it does, it looks slightly odd depending on the angle. This is also a problem that they have when taking images of the martian landscape. To get round it, the camera takes many images at differing angles. The images can then be stitched together in photoshop by engineers. They did something similar when putting together images of the moon taken by satellites. As the following image posted by NASA shows, the arm has to move during the changes in camera location, often moving out of frame. Even when the arm is slightly in an image they tend to cover it with another image, so it doesn’t confuse the people looking at it. The selfie would look odd if it had more than one arm showing.

Even though they take care to put together the images in a way that dont look like many stitched together there are still sometimes some inconsistencies. Notice that in the next image the shadow of the arm is still in the image, and there is a slight ghost of the arm below the rover. As you can see below this shot too 72 images stitched together to be made. 20 of those images, over 2 tiers just make up the horizon. Selfies are generally taken at each new drill site, as part of an overall effort to document the trip and of that site. The entire picture taking sequence has now been automated, and tested rigorously on the second identical rover that is here on Earth. If the rover were to take the multiple pictures from individual commands the process would be too long and drawn out.

Mars Rover Selfie August 2015
The Mars rover from a different lower angle. Taken at “Buckskin” on Aeolis Mons on
Mars. Taken on Aug. 5, 2015, during the 1,065th Martian day. Credit: NASA/JPL.
Mars rover selfie component images
The 72 images taken by the rover over the period of an hour. Credit: NASA/JPL/MSSS/Emily Lakdawalla.

There are at least 7 of these selfies taken over the years, all from a very similar angle. The big thing to notice is the difference in the rover itself. Over time it slowly gets covered in more and more dust, starting to blend in with the martian soil behind it. The saddest part to see is the slow deterioration of the wheels. There are small holes developing and getting bigger in the metalwork on the wheels, and in some images they can seem prominent. Either way, these selfies show a slight human side to the robot. There are many people throughout Twitter that anthropomorphize Curiosity and its predecessors, wishing them well on their journey.

Mars Rover selfie September 2016
A slightly newer selfie taken at “MurrayB” a named rock on
Aeolis Mons in Gale Crater. An awesome image taken in September 2016. Credit: NASA/JPL.

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.