Feature | August 21, 2018

How Dawn Works

By Zoë Webb-Mack

Dawn has given us valuable information about planet-like worlds Vesta and Ceres, providing insight into the history of our solar system. Dawn found, for example, that both Vesta and Ceres are differentiated into layers—Vesta with a formerly magmatic rock interior and Ceres with an icy interior. But Dawn didn’t just slice them open to take a look, so how do we know this? Let’s explore how the Dawn spacecraft and instruments work to return such insightful data about Vesta and Ceres. (You also can find more detailed information about Dawn’s spacecraft and instruments in the Dawn Journal or elsewhere.)

Dawn flies using an ion propulsion system. Its solar arrays, at 65 feet (19.7 meters) from wingtip to wingtip, are large enough to capture the solar energy needed to operate the spacecraft even when Dawn is at its farthest, 3 astronomical units from the Sun (three times the distance from Earth to the Sun). This energy excites the electrons of the solar panels’ semiconductor material, resulting in a current. From there, some of this electrical energy goes on to power the spacecraft’s computers, radio, heaters, instruments, and other devices. Most of the power is used for the ion propulsion system, which shoots electrons at xenon atoms, ionizing (charging) the atoms so they can be accelerated across a voltage provided by two parallel charged electric grids. The action of the relatively heavy xenon ions shooting out from the grids causes the spacecraft to move in the opposite direction as a reaction. This is due to conservation of momentum, also known as Newton’s third law.

Dawn also has three reaction wheels (plus a fourth for redundancy). Reaction wheels make use of the conservation of angular momentum to adjust the attitude (orientation) of the spacecraft by spinning in the opposite direction of its intended motion. Since they are lighter than propellant-based thrusters, they are often used to reduce mass on spacecraft. However, when these wheels failed, Dawn switched over to its hydrazine thrusters, small chemical thrusters that were originally on board to desaturate the reaction wheels.

Now that we have a spacecraft in motion, let’s go over some of its instruments. First up is Dawn’s framing camera, which functions like a camera one might use here on Earth and has captured tens of thousands of stunning images of Vesta and Ceres over the course of the mission. The camera also has seven filters that can isolate certain visible and infrared wavelengths. Dawn uses this camera to navigate, referencing the position of Vesta or Ceres against background stars to help pin down its location. Dawn also takes pictures in stereo (i.e., from different angles) so that we can create three-dimensional maps of the bodies’ surfaces and measure the depths of their features by comparing these images. From Dawn’s images, for example, we know that Ceres’ Ahuna Mons is around 2.5 miles (4 kilometer) high. Images also reveal that its shape is similar to volcanoes on Earth, suggesting that it formed in a similar way.

Next, we have the visible and infrared mapping spectrometer (VIR). This instrument records electromagnetic spectra, which depend on the composition of the chemicals reflecting them. The physical structures of chemicals interact with light differently, absorbing some wavelengths while reflecting others. By comparing spectra detected by VIR to those found in laboratory tests, these spectra can tell us about the mineralogical composition of Vesta and Ceres. Dawn’s VIR data, for example, showed that both carbonates and ammonia are present on Ceres’ surface, raising questions about Ceres’ history.

However, VIR can only detect spectra reflected by the surface of a body. Dawn’s gamma ray and neutron detector (GRaND), on the other hand, can detect the spectral signatures of elements up to about 3 feet (1 meter) below the surface since the high-energy cosmic rays that cause materials to emit gamma rays and neutrons can penetrate the surface. Though cosmic ray strikes are needed to be able to detect most elements, naturally gamma ray-emitting radioactive isotopes of potassium, uranium and thorium can be detected as well. When cosmic rays, highly energized atomic particles traveling near the speed of light, collide with atoms, they energize them, causing them to emit gamma rays (a high energy form of electromagnetic radiation) or release high speed secondary particles like neutrons. When a neutron strikes a massive atom, it bounces off at a high speed. Hydrogen atoms, however, have nearly the same mass as neutrons. So, when a neutron strikes a hydrogen atom, the neutron transfers more of its kinetic energy to the hydrogen atom and bounces off at a lower speed (due to conservation of momentum). The spectrometer detects this as a low-energy (low-temperature) neutron. The greater the concentration of hydrogen, the more low-energy neutrons the spectrometer detects. Because neutron spectrometers can easily detect hydrogen, and hydrogen is most common in water molecules, neutron spectrometers are often used to look for near-surface water on distant worlds. Based on GRaND’s detection of hydrogen, Ceres appears to have a relatively high concentration of near-surface water, likely in the form of hydrated materials and ice. Some liquid water may even remain from an ancient ocean.

Finally, Dawn takes gravity data in order to map the gravitational fields of Vesta and Ceres, which correspond to how mass is distributed. Since the mass of a large body affects the speed of orbiting bodies, gravitational field strength can be calculated by tracking changes in Dawn’s speed as it passes over the body in question. So, if Dawn approaches a region with greater mass, like a mountain, for example, it speeds up due to the stronger gravitational pull in this region. But how do we track Dawn’s speed? Light emitted by an object appears differently depending on the relative motion of the object with respect to the observer. This effect, the Doppler effect, results in the emissions of objects moving away from Earth having an apparent “redshift” (i.e., light appears to have longer wavelengths, shifting it toward the red side of the spectrum) and the emissions of approaching objects having a “blueshift” (shorter wavelengths). This shift can then tell us the speed at which the object in question, in this case, Dawn, is moving, which allows us to map the gravitational field of the body. Since the gravitational field of a uniform sphere is different than that of an equally massive but layered solid, these data can help determine how mass is distributed within the body. While inferences about a body’s interior can sometimes be made based on its mineralogy and surface features, gravity data provide a better look. Gravity data from Dawn’s flight over Vesta confirm its differentiated interior. These data suggest that Vesta is rather dense, with a rocky outer layer and iron core. Ceres, on the other hand, has a relatively low density, likely due to a high amount of water ice.

Vesta and Ceres, two previously unexplored worlds, look like little more than white specks when viewed through a standard telescope. No human has ever visited them. Yet a surrogate explorer from Earth has brought us closer to the two worlds. The spacecraft is an elegant application of years of scientific and technological knowledge. Armed with fundamental scientific principles, Dawn has given us new perspectives on Vesta, Ceres and our solar system, an extraordinary feat demonstrating the beauty of scientific discovery and the power of curiosity.