A hypothetical comet impact scenario has been prepared for use at the 2019 IAA Planetary Defense Conference (PDC), to be held in College Park, Maryland, USA, April 29 - May 3, 2019. This scenario is for a long-period comet and is NOT part of the main 2019 PDC impact scenario, which will be presented as a hypothetical asteroid impact exercise at the 2019 PDC. As with other recent PDC scenarios, this comet scenario is technically realistic in many ways, but is completely fictional and does NOT describe an actual potential comet impact. The scenario begins as follows:
A comet is discovered on April 4, 2019, at total magnitude 21.1, and confirmed the following day. During the confirmation process, several pre-discovery (“precovery”) observations are found in images taken over the previous few weeks, which help establish a rough orbit for the new object.
In its announcement of the new comet, the Minor Planet Center assigns it the designation “C/2019 PDC”. (To reinforce the fact that this is not a real comet, we are using three letters in the designation, something that would never be done for an actual comet. We will also avoid assigning a name to the comet, even though that is customarily done in the discovery announcement.)
Even the early orbit calculations clearly show C/2019 PDC is in a highly eccentric, nearly parabolic orbit extending hundreds of astronomical units (au) from the Sun at its farthest point. (An “astronomical unit” is a standard unit of distance within the solar system equal to the mean distance of the Earth from the Sun, 149,597,870.7 km, or 92,955,807 miles.)
The orbital period of the comet is calculated to be several thousand years, which puts it in the category of a long-period comet (LPC) .
C/2019 PDC’s perhelion (closest point to the Sun) is calculated to be 0.92 au, well within the Earth’s orbit. The famous long-period comet C/Hale-Bopp had a similar perihelion distance. As is typical with long-period comets, the orbit is highly inclined to the plane of the solar system. In fact, the orbit of C/2019 PDC has an inclination of 129 degrees, making it retrograde. Comet Siding Spring (C/2013 A1), which approached very close to Mars in October 2014, was a long-period comet with a very similar orbit inclination.
The calculated orbit of C/2019 PDC very nearly intersects the orbit of the Earth, and initial orbit predictions indicate that the comet will make a very close approach to Earth in late February 2021, only 22 months away.
When first discovered, C/2019 PDC is far away, roughly 8 au from Earth, almost as far away as the planet Saturn. The comet is moving towards the inner solar system at a modest speed of about 15 kilometers per second, but that speed is predicted to increase dramatically as the comet approaches the Sun, reaching a speed relative to the Earth at close approach of roughly 65 km/s.
An initial orbit analysis of the motion of C/2019 PDC puts the probability of impact at roughly 1 chance in 100,000. As the comet is tracked almost every night through the month of April, 2019, the impact probability rises somewhat to about 1 chance in 3,000 (or about 0.03%).
Even though the comet will be observable almost continuously from discovery through the potential impact 22 months later, except for brief periods in late 2019, and late 2020, the observations are much less precise than they would be for an asteroid. First of all, the nucleus of the comet is difficult to detect within the diffuse coma, a problem which will grow as the cometary activity increases. But even if the nucleus could be unambiguously identified, measurements of its position provide considerably less orbit information than they would for an asteroid, simply because the object is so far away (over 7 au through much of 2019). The observations will become more powerful in 2020 when the comet gets closer to Earth.
Further complicating orbit predictions are the so-called non-gravitational accelerations caused by cometary outgassing. The strength of these “non-grav” accelerations can only be estimated after their effect on the motion of the comet has been observed. The strength of these accelerations varies roughly as the inverse square of the heliocentric distance, but the scaling factors for the non-gravs vary significantly from one comet to another. Orbit predictions must account for uncertainty in the non-gravs, which will remain large until the comet gets closer to the Sun and Earth. Even if the comet were on an impact trajectory, the impact could remain uncertain until the final few months, largely due to the unpredictability of the non-gravs.
The physical properties of the comet’s nucleus can only be guessed, especially before it has been observed in detail. Some inferences can be drawn from measurements of the amount of outgassing activity observed at the current considerable distance from the Sun. Drawing from what is known about the sizes of other long-period comet nuclei, and their activity levels at similar distances, astronomers speculate that the nucleus of C/2019 PDC is on the small end of the typical size range for LPCs, possibly only about 1 kilometer in size. But that estimate is highly uncertain and could easily be off by a factor of several.
Here are additional details on what we know about C/2019 PDC on the first day of the conference:
|
| Orbit of Comet C/2019 PDC |
|
| Potential impact hemisphere for C/2019 PDC |
A Google Earth kml file for these impact points is available here.
A table of the impact circumstances for the impact points shown in the above diagram 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-Feb-28.
A special version of the JPL orbit viewer has been created for this object and can be accessed here.
The orbit for the “worst case” for C/2019 PDC has been loaded into JPL’s HORIZONS system, and can be accessed via the name “c2019 PDC” or “cPDC19”. This trajectory is not the most likely one for C/2019 PDC, it is simply selected as one of those within the uncertainty region that passes closest to the geocenter on Feb. 28, 2021. 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:
ftp://ssd.jpl.nasa.gov/pub/xfr/2019-PDC/c2019_PDC-merged-DE431.bsp
The SPK file is consistent with and contains additional DE431 planetary ephemeris information over the time-span 1998-Jan-01 though impact on 2021-Feb-28, permitting retrieval of object state vectors at any arbitrary instant within that timespan.
The orbit for the “worst case” trajectory for C/2019 PDC 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 C/2019 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 object 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 trajectory for the 2019 PDC asteroid impact scenario is also loaded into the App along with trajectories of many other simulated Earth impactors.