The increased number of asteroid discoveries over the past few decades as well as the expectation that the number of known asteroids will increase by five times when the Large Synoptic Survey Telescope (LSST; https://www.lsst.org/) is expected to come online in 2022 bring an increased number of potentially hazardous asteroid (PHA) discoveries. PHAs are near-Earth objects – either asteroids or comets – for which the closest points between their orbits and the Earth’s orbit are less than 0.05 astronomical units (19.5 times the distance between the Earth and Moon) apart and with diameters of approximately 460 ft (140 m) or greater. An object this large is big enough to cause devastation to a populated region in the case of a land impact or a major tsunami for an impact into the ocean.
Not all PHAs are likely to impact the Earth in the foreseeable future, but their orbits put them in close enough proximity to the Earth that they need to be monitored in case their impact probability changes. One way the chances of an impact can change is if a PHA comes sufficiently close to a planet for it to gravitationally tug on the asteroid, changing its orbit enough that its future trajectory puts it on an impact course with the Earth. Fortunately, however, the threat due to asteroid impacts is a natural disaster we have the ability to avoid.
One of the projects supported by the DiRAC Institute is the Asteroid Decision Analysis and Mapping (ADAM) platform being developed by the Asteroid Institute, a program of B612. B612 is a non-profit organization dedicated to protecting the Earth from asteroid impacts as well as advising and advancing decision-making on planetary defense issues on a world-wide scale (https://b612foundation.org).
The ADAM platform is being developed to answer questions such as “How long after discovery does it takes for typical Earth-impacting asteroids to be labeled as impact threats?” and “How far in advance do we need to deflect such asteroids to avoid a collision?”. To answer these questions and others, ADAM is being built in Google Cloud, which allows the required computations to be run on a large-scale platform that provides ample data for analysis. One of the goals of ADAM is to make these computations accessible to the greater scientific community, not only in scale and accuracy, but in ease of use. ADAM is being developed as open-source software that upon completion of initial development and testing will be available to the scientific community to both use and contribute to its computational abilities.
One capability of ADAM is to compute large-scale asteroid orbit propagations that predict the orbital characteristics and locations of a large set of asteroids at times in the future given their current orbital characteristics. The animation shown here is of an orbit propagation of a synthetic Earth-impacting asteroid that shows the orbital motion of the four terrestrial planets (Mercury, Venus, Earth, and Mars) as well as the asteroid (labeled 129_2011_04_DeltaV) over a period of roughly 8 months. The animation ends when the asteroid impacts the Earth. This animation was produced using the tools upon which ADAM’s orbit propagation is based; visualization of orbit propagations will eventually be a benefit of computing propagations with ADAM.
Along with computing large-scale orbit propagations, ADAM can calculate the deflection impulse, or nudge, needed to avoid an asteroid impact. Such a nudge could be imparted to an asteroid using a spacecraft called a kinetic impactor, which would rendezvous with an Earth-impacting asteroid before it were to collide with the Earth to gently push the asteroid an amount large enough to avoid the collision. The goal of such a maneuver would be to avoid the asteroid and the Earth being at the point where their orbits intersect at the same time, thus avoiding a collision. This is similar to either stepping on the brakes or the gas in your car to avoid a traffic collision.
In a study1 recently submitted to the journal Icarus and expected to be published in early 2020 by members of the Asteroid Institute and the DiRAC Institute lead by Dr. Sarah Greenstreet using ADAM to determine the distribution of nudges needed by a large sample of synthetic Earth-impacting asteroids to avoid collision with Earth, researchers found that required nudges range from a few hundredths of an inch per second to a few inches per second, depending on the time before impact available to impart the nudge. In terms of the amount of energy this would impart to the asteroid, for a 450-ft-diameter asteroid made of typical rocky material, a nudge of roughly half an inch per second is the equivalent of the energy required to power a 60 W light bulb for one hour.
The researchers found that the required deflection impulse, or nudge, typically changes roughly as the inverse of the time before impact that the deflection impulse can be applied. This means that a nudge applied to an asteroid 20 years before impact needs to be approximately half the size of a nudge applied 10 years before impact to miss the Earth by the same distance.
Another finding of the study described above is that a small fraction of the synthetic Earth-impacting asteroid population studied require either 10 times more or less velocity change (nudge) than the median value. This means some asteroids are either much harder or easier to deflect than the typical Earth-impacting asteroid. These types of impact scenarios are important to study in addition to the typical cases to best understand the full breadth of the threat due to asteroid impacts.
An additional capability of ADAM currently being developed is determining the evolution of the probability of impacts for a large sample of synthetic Earth-impacting asteroids. For a given asteroid, as further observations are made of the asteroid after discovery, the orbit of the asteroid evolves. Each new observation adds additional data that can be used to compute an orbit for the asteroid, with each new calculation producing a different orbit until the evolution stabilizes and further observations do very little to change the orbit. As the determined orbit evolves, so does the probability of a future Earth-impact for the computed orbit. Like the evolution of the asteroid’s orbit, the impact probability changes with additional observations. Studying the impact probability evolution of a large sample of synthetic Earth-impacting asteroids can provide a better understanding of how long it can take to say with confidence that an impact is expected to occur for a wide range of Earth-impact scenarios.
Altogether, the capabilities of ADAM are helping us to better understand the threat due to asteroid impacts and what we can do to avoid them. This information can further future discussions and decisions regarding impact hazard mitigation on a global scale, as is the mission of the Asteroid Institute.
1Greenstreet, S., Lu, E., Loucks, M., Carrico, J., Kichkaylo, T., & Jurić, M., “Required deflection impulses as a function of time before impact for Earth-impacting asteroids”, 2019, Icarus, reviewed.
Dr. Sarah Greenstreet is a joint postdoctoral fellow with the Asteroid Institute, a program of B612, and the DiRAC Institute at the University of Washington. Her research interests include the study of orbital dynamics and impacts of small bodies in the Solar System.