Finding an Asteroid to Visit
Processing water from near-Earth asteroids (NEAs) promises to be a key approach for delivering propellant to Earth orbit. Two proposed systems include TransAstra's Queen Bee spacecraft and Honeybee Robotics' WINE system. Before either of these missions can be launched, they will need to know where they are going and what to expect. Unfortunately you can't simply search for which NEA you should send a mission to. How will TransAstra and Honeybee Robotics decide where to go? Through lots of remote observations, a bit of data science, and talking with experts.
The NASA JPL Small Body Database has data on nearly 800,000 asteroids and comets, including almost 20,000 NEAs. This database includes the orbital and physical properties for these objects. There are millions more objects not yet discovered, with most less than a kilometer in diameter. The tricky part is understanding the properties of a tiny, often dark object millions of kilometers away from Earth. Surveying asteroids can be broken into three steps: identify the object, calculate the orbit, and characterize the physical properties.
Asteroid identification involves finding (or rediscovering) objects orbiting the Sun. This hundred year old process requires monitoring the night sky, watching for points of light that change location over a short time span. Modern identification methods often use automated systems that perform sky surveys every night.
Two such surveys include the Catalina Sky Survey (CSS) and Pan-STARRS (recently released the largest single astronomical data release ever at 1.5 PB or 1.5 million GB) which identify hundreds of objects a year. The Large Synoptic Survey Telescope (LSST) will become the largest single sky survey telescope when it starts operation in 2022. Identifying asteroids is critical for knowing what our options are, and for knowing if an asteroid might collide with Earth!
Orbital determination is the procedure of tracking an object and calculating an orbit based on the observed points. The more points used for orbit determination, the more accurate the orbit. Unfortunately, these orbits are not static. The influence on other bodies in the Solar System can perturb the orbit of an asteroid, causing the previously calculated orbit to change. Therefore, asteroid orbits must be periodically updated to account for any change that may have occurred due to perturbations, collisions, or out-gassing.
Characterizing the physical properties of asteroids is the most challenging part of asteroid surveying. The prior two steps only need to track a point of light across the sky. Cataloging physical properties of an asteroid requires very detailed observations of the object. Some of the most important properties include the object's dimensions (including shape model), rotation characteristics, surface features (including spectra and texture), and sub-surface measurements. Remote sensing (usually ground based) provides the majority of asteroid observations. In-situ sensing (done at the object itself) provides a tiny subset of data, but the resolution and accuracy is unparalleled.
Remote observations often use telescopes on Earth. The cost of operating a ground based telescope is much cheaper than building and operating a spacecraft. But the large distance to the objects and the small size of the targets limit the details ground based telescopes can observe. Despite these limitations, ground telescopes have collected information on asteroid spectra, albedo, rotation periods, and shape models (via radar returns).
Asteroid spectra allows scientists to classify asteroids into groups, such as the Tholen and SMASS classification schemes. These classifications give rise to the main asteroid groups, including C-group (carbonaceous), S-group (stony), X-group (an umbrella group containing low-albedo P-type, high-albedo E-type, and metallic M-type), and others. Spectra is also useful for identifying volatiles on asteroids, which is critical for water prospecting (resource exploration) missions.
In-situ observations occur at the object itself using a deep-space spacecraft. Despite the severe weight restrictions placed on spacecraft, the sheer closeness to the target allows them to gain optimal measurements with high detail. These in-situ spacecraft are able to capture high resolution images of the surface, make interior observations, and potentially analyze/return samples. A direct result of higher fidelity observations is the increase in mass accuracy, which is important for understanding the structure of the asteroid. The JAXA Hayabusa2 and NASA OSIRIS-REx missions are prime examples of in-situ observation spacecraft.
Direct observations from asteroid missions is the holy grail for asteroid science. However, these missions require significant investments and time. With millions of asteroids it is also infeasible to visit each one and directly observe their features. Possible solutions to these hurdles include performing more remote sensing, developing more accurate models to simulate asteroid features, or accept the risk and send water prospecting spacecraft to asteroids without complete knowledge.
Water prospecting missions need to verify that their spacecraft is compatible with the asteroid surface, water exists in a form they can process, and the water rich material exists in sufficient quantities. These requirements may be satisfied via remote sensing and clever analysis, so that strategy hold great potential.
The key question for asteroid prospecting and mining firms is: How much data do they require before sending a mission to a NEA? This is no simple question, as the answer ultimately depends on how much risk they want to take and how much money they have.
Zellner, B., D. J. Tholen, and E. F. Tedesco. "The eight-color asteroid survey: Results for 589 minor planets." Icarus 61.3 (1985): 355-416.