On 20 June 2018, the White House released the latest update for the National Near-Earth Object Preparedness Strategy and Action Plan. This document describes the implementation plan for the strategy laid out in 2016, including design concepts for deflection technologies and coordination with FEMA if an asteroid impacted the US. In particular, one chart describes how many asteroids have been discovered compared to how many asteroids are expected to be out there. As a data manager myself, I think there are couple of things that could be tweaked to make it less confusing.
However, I do feel that it serves as a good high-level visualization. I'd like to break this down a bit to make it a little easier to understand what's going on.
Asteroid Size (km) and Impact Devastation
The X-axis on the bottom the most important axis because it lists asteroid sizes1. As you can probably guess, the impact of a large asteroid can cause much more damage than a small one. This directly corresponds to the color-coding at the top, which are separated by rough scales of destruction. A 10 meter asteroid will burn up in the atmosphere with negligible surface effects. A 200 meter asteroid will cause regional destruction, and a 4 kilometer asteroid would be a serious global-scale catastrophe. The 3 examples listed are some well-known asteroid impacts to give you a sense of scale. Here are some links to learn more about the Chelyabinsk Meteor, Tunguska Event, and K-T Impact.
Size Frequency Distribution
The Y-axis on the left is the logarithmic scale showing number of asteroids. The red line that angles downward from left->right is the plot of the size frequency distribution. By following this red line, you can see how many asteroids are expected for any given size. Statistical models account for what has already been discovered plus the overall dynamics of the solar system to estimate the total amount of asteroids left to discover. This shows that there are lots of really small asteroids and only a few really large ones.
Percentage of Completeness
The Y-axis on the right is the scale showing percentage of completeness. The blue line shows the percentage that have been found based on the number of asteroids expected of that particular size. This shows that most of the asteroids larger than 1km have been found.
In 1998, NASA was tasked with finding 90% of asteroids 1km or larger, and in 2005 expanded to finding 90% of asteroids larger than 140 meters.
A 2017 paper by the Planetary Science Institute2 estimates that of the NEO population, 920 ± 10 are larger than 1km and 27,100 ± 2,200 are larger than 140m. When these estimates are checked against the Center for Near Earth Object Studies (CNEOS) totals for mid-June 2018, completion percentage for these goals are 97% and 30% respectively. Literally as I wrote this, a notification popped up regarding a paper3 that refines those estimates even more.
Note: This report only accounts for Near Earth Objects, which are defined as having an orbital perihelion < 1.3 AU and a semi-major axis < 4.2 AU. This does not account for the myriad of other bodies that could potentially pose a risk, as these populations are not well-characterized or constrained. This includes long-period comets, Centaurs, Trans-Neptunian objects, hyperbolic comets, or interstellar objects which do occasionally cross Earth's orbit. It will be a very long time before these types of hazards are understood well enough to track with a moderate level of consistency.
Near Earth Object Discovery Totals from the Center for Near Earth Object Studies (NASA/JPL)
These results give us an opportunity to continually improve asteroid detection techniques. Asteroid impacts are one type natural disaster that we can realistically prevent, and it's important to continuing pushing for funding and prioritizing these efforts through NASA and across the world. That's what Asteroid Day is all about; raising awareness and highlighting the fact when the universe tries to kill us, we can (and will!) fight back.
When asteroids are first observed, they only appear as a point of light with a measured magnitude of brightness. Objects with the same magnitude of brightness can be larger or smaller depending on the amount of light they reflect (albedo). Albedos are typically not known because the measurement is time-consuming and takes specialized instrumentation. Diameters for specific asteroids are typically listed as a range (ex. 150m-500m). This is discussed with a bit more detail in the Planetary Defense - Characterization post. ↩