Planetary Defense Conference Exercise - 2021

The 2021 PDC Hypothetical Asteroid Impact Scenario

Although this scenario is realistic in many ways, it is completely fictional and does NOT describe an actual potential asteroid impact. The scenario begins as follows:

• An asteroid is discovered on April 19, 2021, at apparent magnitude 21.5, and confirmed the following day. It is assigned the designation “2021 PDC” by the Minor Planet Center. (To reinforce the fact that this is not a real asteroid, we are using three letters in the designation, something that would never be done for an actual asteroid.)

• The day after 2021 PDC is discovered, JPL’s Sentry impact monitoring system, as well as ESA’s similar CLOMON system, both identify several future dates when this asteroid could potentially impact the Earth. Both systems agree that the most likely potential impact occurs on October 20, 2021 - just 6 months away - but the probability of that impact is low, about 1 chance in 2500. With only two days of tracking on this object, no better estimate of impact probability can be made.

• Very little is known about the physical properties of 2021 PDC. Its size, in particular, is highly uncertain. Based on its mean apparent visual magnitude, the asteroid’s absolute (intrinsic) magnitude is estimated to be H = 22.4 +/- 0.3. If 2021 PDC’s albedo (reflectivity) is 13%, a typical mean value, this H value implies a mean asteroid size of about 120 meters. But the true albedo is not known and the asteroid’s size could therefore range anywhere from as small as 35 meters to as large as 700 meters.

• The asteroid’s orbit is eccentric, extending from a distance of 0.92 au from the Sun at its closest point to 1.60 au at its farthest point, just outside the orbit of Mars. (The abbreviation “au” stands for “astronomical unit”, which is the mean distance of the Earth from the Sun, 149,597,870.7 km, or 92,955,807 miles.) The asteroid’s orbital period is 516 days (1.41 years), and its orbital plane is inclined 16 degrees to the Earth’s orbital plane.

• When first detected, the asteroid is about 0.38 au (57 million kilometers or 35 million miles) from Earth, approaching our planet at about 5 km/s (3 mi/s or 11,000 mph), and slowly getting brighter. 2021 PDC is observed extensively during the week after discovery, and as the observational dataset grows from one day to the next, the impact probability increases. The asteroid brightens only slightly in the days after discovery, and reaches a peak brightness of only magnitude 21.35 on April 23.

• 2021 PDC approaches the Earth for three weeks after discovery, reaching its closest point of about 0.35 au on May 9. The asteroid is too distant to be detected by radar, and will not come within radar range until its potentially impacting approach in October.

• Astronomers continue to track the asteroid every night after discovery, and the impact probability steadily increases. As of April 26, 2021, the first day of the 2021 Planetary Defense Conference, the probability of impact has climbed to about 5%. The rest of the scenario will be played out at the conference.

Here are additional details on what we know about 2021 PDC on the first day of the conference:

• The following diagram shows the orbits of 2021 PDC and Earth along with their positions at the time the asteroid was discovered on April 20, 2021. The point at which the orbits intersect, and where an impact is possible, is highlighted in red. In three dimensions, the asteroid’s orbit is inclined by about 16 degrees: this diagram is a projection of the asteroid orbit into the plane of the Earth’s orbit. The two orbit planes intersect along a line defined by the Sun and the Earth position at the potential impact point. The asteroid makes roughly half an orbit about the Sun between discovery and the potential impact, and for that entire time it is above the plane of this diagram.

Orbit of asteroid 2021 PDC

• The following diagram zooms in on the intersection point of the orbits of 2021 PDC and the Earth, and shows the current uncertainty in the predicted position of the asteroid when the Earth crosses through the intersection point on October 20, 2021. The uncertainty region is traced out by random-sample cases whose position are shown as red dots. The region is clearly much larger than the Earth, its full length being three-quarters the diameter of the Moon’s orbit. As the asteroid is observed over subsequent months of 2021, its orbit will become better determined and the uncertainty region will shrink in size.

Current uncertainty in predicted position of asteroid 2021 PDC on October 20, 2021

• The asteroid’s position uncertainty region at the time of the potential impact is much larger in both length and breadth than the size of the Earth. Impact could occur anywhere on the forward hemisphere during the hour or so when the Earth crosses the asteroid orbit and sweeps through the uncertainty region. The red dots on the following Google Earth image show some of the possible impact points. This diagram was produced by filling the uncertainty region with thousands of random-sample cases and computing where those cases would impact when the Earth sweeps through the region. They cover not only an entire hemisphere of the Earth but also extend beyond the limb because gravity will cause trajectories that might otherwise miss to curve towards an impact. Note that while there are gaps between the dots in this image, the region at risk is actually a continuum that covers this entire hemisphere and goes well beyond the limb.

Potential impact hemisphere for 2021 PDC

A Google Earth kml file for the full set of impact points is available here.

• The following image is a projection of Earth surface features onto so-called “b-plane” of 2021 PDC. This is essentially a flattened image of the Earth as viewed from the oncoming asteroid, showing all the regions that could be impacted by potential trajectories surrounding that of 2021 PDC. (While the central part of this image is at risk because it’s on the hemisphere facing the oncoming asteroid, the outer ring in this image shows the regions at risk because gravity would bend those trajectories towards impact.) When the uncertainty region is much larger than the Earth, as is the current case, all regions in this projection are at risk of a potential impact. Conversely, regions of the Earth not within this image (e.g. Australia and Indonesia) are not at risk.

Earth surface projected into the b-plane of 2021 PDC

• A table of the impact circumstances of impact points shown in the above Google Earth image can be found here. The columns of this table are as follows:

  • xi & zeta are the Opik b-plane coordinates of the trajectory, in kilometers

  • Lat & ELon are the latitude and East longitude of the impact point, in degrees

  • Vel is the velocity at impact, in km/s

  • Az & El are the azimuth (measured eastwards from North) and elevation of the incoming velocity vector, in degrees

  • Time is the UTC time of the impact on the impact date, 2021-10-20.

• Impact probability is estimated from the available astrometric (sky-position) observations, and it depends most critically on the length of time over which an asteroid has been observed. 2021 PDC has been tracked for only one week, yielding only a few dozen observations. Observations must now cease for a few days because of sky glare from the full moon, but when they resume the impact probability will likely change dramatically. If the asteroid is on an impact trajectory, the probability will continue to rise, reaching as high as 30% by the end of the week, 70% by next week, and 90% during the following week. (These probability upper bounds are based on the expected accuracies of state-of-the-art optical observations.) If the asteroid is not on an impact trajectory, the impact probability may still rise for a time but will eventually drop to zero, once the asteroid has been observed for a long enough period.

• Asteroid 2021 PDC will remain continuously observable over the entire time from now until the potential impact in October, although it will be fainter than 23rd magnitude from June through September, requiring large-aperture telescopes such as the 4-meter Canada-France-Hawaii Telescope (CFHT). The asteroid will not get brighter than 22nd magnitude until just a few weeks before the potential impact in October.

• As the observation dataset grows in the coming weeks and months, the orbit of 2021 PDC will become increasingly well determined, and if the asteroid is on an impact trajectory, the regions of the globe at risk will narrow considerably, first to an increasingly narrow corridor wrapping around much of the Earth, and then to a specific “footprint” location on the Earth.

• Sky-image archives are now being searched for possible pre-discovery serendipidous observations of the region of sky the asteroid may have traversed 7 years ago, when it made a distant pass by our planet. If the asteroid can be detected in those images, the observed timespan would immediately extend to 7 years, and the impact probability would immediately jump to either 0% or 100%, and in the latter case, the impact location would immediately become well defined.

• As mentioned previously, the size of 2021 PDC is highly uncertain, ranging from as small as 35 meters to as large as 700 meters. This estimate is based on the asteroid’s brightness, its estimated distance, and the wide range of possible albedos (reflectivities). Little is known about other properties of the object, such as composition and density. As a result, the potential impact damage and population risk is also highly uncertain. Based on these estimates, the possible energy released on impact could range from 1.2 Mt to 13 Gt (TNT equivalent). The predominant hazard is an airburst causing blast overpressures possibly reaching unsurvivable levels. The size of the potential blast damage area could range from local (a few kilometers) at the small end at the possible range of asteroid sizes, to regional (hundreds of kilometers) at the large end. Unfortunately, there is little prospect for gaining more information on the asteroid’s physical properties until just before the October encounter, since the asteroid will remain too far away for detailed observation for that entire time.

• A special version of the JPL orbit viewer has been created for this object and can be accessed here.

• The orbit for a “worst case” trajectory for 2021 PDC has been loaded into JPL’s HORIZONS system, and can be accessed via the name “2021 PDC” or “PDC21”. This trajectory is not the current best estimate for 2021 PDC, which will change from day to day as observations are added, but rather the trajectory within the uncertainty region that passes closest to the geocenter on Oct. 20, 2021 (in other words, the trajectory that impacts at the center of the b-plane). HORIZONS can be accessed with this object preloaded via this web-interface here.

• For those familiar with the SPICE Toolkit software, an SPK file for this same orbit has been created and is available here:

  https://ssd.jpl.nasa.gov/ftp/xfr/2021-PDC/2021_PDC-s11-merged-DE431.bsp

  The SPK file is consistent with and contains additional DE431 planetary ephemeris information over the time-span 1998-Jan-01 through impact on 2021-Oct-20, permitting retrieval of object state vectors at any arbitrary instant within that timespan.

• The orbit for the “worst case” trajectory for 2021 PDC, described above, has been loaded into the JPL/Aerospace Corp. NEO Deflection App. This on-line tool allows users to study the velocity change (delta-v) required to deflect the 2021 PDC trajectory away from the Earth, as a function of deflection time. Specific amounts of impulsive velocity change can be applied at specific times before impact and the resulting deflection in the impact b-plane is shown. The App can also be configured to calculate kinetic impactor spacecraft trajectories, as well as the spacecraft masses that can be launched onto those trajectories by various launch vehicles. The App calculates the delta-v applied to the asteroid when the kinetic impactor hits it, and determines the Opik b-plane coordinates, xi and zeta, of the deflected trajectory. Using these coordinates, you can roughly determine the impact point of the deflected trajectory by interpolating in the table of impact circumstances given above. A complete description of the app is available here.

  The 2019 PDC trajectory is also loaded into the App along with trajectories of many other simulated Earth impactors.