Draper designs precision spacecraft navigation technology to explore challenging, unknown places and return safely home.
To enable an uncrewed spacecraft to travel to an unexplored asteroid, skim its surface to collect a regolith sample and then return to Earth, Draper engineers would need to use several different navigation technologies to navigate successfully all stages of such a trip. These range from the familiar star sighting to new image-based navigation techniques to map unfamiliar terrain in real time in close orbit around the asteroid—even to navigate to its surface. Spacecraft must rely on autonomous navigation for critical activities since communication lag makes Earth-based ground control of the spacecraft impractical. Precision navigation systems will be key to journeying to distant unknown places, landing on them and returning with useful information.
Challenges of navigating relative to asteroids
The distance of asteroids from Earth and the lack of information about them leaves many uncertainties. Spacecraft sensors will need to collect data during the mission to answer the following questions—and feed relevant information into the navigation system to refine trajectory.
- What is the exact orbit of asteroid? What is solar influence on that?
- What is the actual shape of asteroid and landscape of its surface?
- What is the composition of asteroid: how much metal and rock versus dust and where?
- What is the overall strength of gravity on asteroid and pattern of variation in gravity field across the asteroid? Will rocks move after mapping due to low gravity?
- How do we operate a spacecraft in low-gravity environments?
- What is the spin rate of the asteroid?
Types of navigation technology for missions to unknown places
Successfully navigating to an unknown place, such as an asteroid, requires a spacecraft to know its position and use that information to follow a planned trajectory toward a destination—while making course adjustments via thruster firings during the mission when new data for navigation become available. While a spacecraft is closer to Earth it navigates relative to Earth for the best position accuracy. When it is nearer to the asteroid, it navigates relative to the asteroid. Draper has used these technologies in numerous space missions, from the Apollo Program in the 1960s to today’s Orion.
Inertial navigation: Using acceleration sensors (accelerometers) and rotation sensors (gyroscopes), an inertial navigation system (INS) calculates and tracks position, orientation and velocity (direction and speed of movement) of a craft without taking external measurements. INS is relative to a known starting location. Errors accumulate over time (i.e., drift) unless there is an absolute location update from GPS, star sightings or other aiding technique.
NASA’s Deep Space Network: Radio telescope arrays based on Earth send radio metric tracking data and state updates to spacecraft. The farther the spacecraft is from Earth, the longer the signal travel time and communication delay—and the more difficult the navigation.
Star Sighting: While on the surface of a celestial body, taking angular measurements between its horizon and another celestial body in the sky can determine one’s position.
Star Tracker: imaging device used to measure position of star(s). When the precise positions of stars are listed in star catalogues, a spacecraft can use a measurement to a star or star field to determine its own orientation (or attitude) relative to those stars.
Star-based Center Finding: The center of a celestial body (e.g., asteroid) is determined from an image of the body in relation to a background of known stars, providing updates on the path to the destination.
Centroiding: finding geometric center of a plane figure (e.g., image of Earth or asteroid); Draper’s Autonomous Deep Space Navigation for this mission would be used for asteroid centroiding on approach to the asteroid to calculate range and bearing to the asteroid.
Landmark-based: Using maps generated by taking pictures of an unknown celestial body’s surface; combining images taken with known spacecraft locations and any other available surface data generates a topographic map. The map enables definition of unique landmarks that can be detected reliably in images. When the spacecraft’s processing software detects these landmarks in real time in images it takes during flight, the associated known locations of the landmarks provide absolute navigation updates for the spacecraft relative to the asteroid’s surface.
Vision-aided navigation: Combining analysis of surface images and inertial navigation is a form of relative navigation. With a known location as a starting point, INS drift can be minimized by detecting and tracking unique features in the image data. Since the 3D location of these features in unknown at the time they are detected (unlike landmark features), Draper’s algorithms solve for the locations of these features and track them in the images to further constrain the INS solution. Draper has demonstrated ability to limit the resulting drift to less than 1 percent of distance traveled using available cameras and inertial measurement units (IMUs)—a large improvement over INS-only drift, especially for less-expensive/less-precise INS sensors. Incorporating landmark features with known 3D locations provides absolute position updates and further minimizes these errors. Rovers also could use this approach to navigate the asteroid surface.
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