The Second Part in a Series
|An artist’s rendition of 2016 WF9 as it passes Jupiter’s orbit inbound toward|
the Sun. Image: Courtesy NASA/JPL-Caltech
Now I plan to give some background on the subject of virtual asteroids and virtual impactors. While asteroid researchers know this topic well this is for the non-researchers, I am going to explain the subject by way of a silly story,
The Observatory's cat and the Lost Car Key
There was an observatory that hosted a monthly Astronomy and Ice Cream Night. This event would consist a free talk on Astronomy and ice cream(at a nominal cost). At the end of one of the events the professor who gave that night's talk discovers he had lost his car key. Everyone knew that the key had to be at the observatory. First, they sent security out to keep an eye on the parking lot, then they started checking under the tables, by the display cases, the podium, and the trash cans. Many other keys were found when asked "is this your key?" the professor said "no." One thing that set this observatory apart was it was adopted by a cat named OC(A.K.A Observatory Cat). OC loved ice cream, and OC was found with the key licking ice cream off it when asked "is this your key?" the professor said "yes" and drove home.
If we think of the story this way, there is a large number of "virtual" keys. One for each "possibility" where the "real" key "may be," as places were searched the "virtual" keys were eliminated. They could not rule out that someone had the key and would steal the car this risk would be a great risk if the vehicle is easy to find in a small lot. On the other hand, if the lot were enormous the and the car was hard to locate the risk would be lower. There may be many keys so one can not assume that they found the key because only one key will start the car. The cat would be the Yarkovsky Effect and gravitational perturbations, the cat moved the key but kept it at the observatory,
Now Back to Asteroids
One thing to remember is unlike, car thieves, cats, and lost car keys asteroids follow the laws of planetary motion. Asteroids move through an extensive solar system in elliptical orbits. An asteroid's orbit and position within its orbital path determines where in the sky the asteroid can be seen from a location at a given time. When astronomers(professional and amateur) observe an asteroid, they record its coordinates(sky position) along with the day and time, apparent magnitude, and a code for their location. An asteroid's orbit is determined by finding an orbit that best places it in the sky as it was observed from the given location at the given time.
When it comes to orbit determination there is no such thing as "the" orbit for any asteroid, tiny observational errors come into play. The solution is to generate an enormous number of slightly different orbits that fit the observations acceptably well. Each orbit has it own virtual asteroid. There is an uncertainty region containing the virtual asteroids. The "real" asteroid is somewhere within the uncertainty region. After each set of new observations, the orbits are re-generated, and like reality game shows contestants, many virtual asteroids are eliminated. As time moves forward, the virtual asteroids will move apart from each other. If the asteroid goes unobserved for an extended time, the uncertainty region can become enormous and sometimes can wrap around the solar system more than once.
When reading the risk lists, one should keep in mind that the risk lists are NOT a prediction of an impact or even a close-approach. The "real" asteroid could be on the other side of the solar system when the virtual asteroid "impacts." Also keep in mind as new observations are reported the asteroid will most likely be removed from the risk lists; however, the risk may increase before it drops off the list if the "real" asteroid is making an exceptionally close approach to Earth on the date in question, this is normal.
2012 TC4 beyond the 2017-Oct-12 Close-Approach
On 2017-Oct-12 the Earth will be outside of the uncertainty region of 2012 TC4 this rules out an impact from this asteroid on this date. However, based on all available observations made to date the Earth will pass through the uncertainty region of 2012 TC4 on 2020-Oct-11.72, and there will be a 1 in 1,613,000 chance of impact. The risk does not stop there from 2020 to 2114 there will be 79 Potential Impacts of 2012 TC4 with a cumulative risk of 1 in 12,000 chance of impact. 2012 TC4 could be observed during this year's apparition. If "new" observations are taken the orbits will be re-generated, and like reality game shows contestants, many virtual asteroids will be eliminated. Most likely any risk for the next 100 year will be ruled out.
[!!!Note you are reading this after Fall of 2017 Check for Updates!!!]
Background and Sources
(as of 2017-01-21)
- Object: 2012 TC4
- Orbit Type: Apollo [NEO]
- Approximate Diameter: 15 m - 33 m ( 49.2126 feet to 108.268 feet)(Absolute Magnitude: H= 26.7)
- On the Sentry Risk Table: Yes
- Torino Scale 0
- On the NEODyS CLOMON2 risk page: Yes
- Discovery observation was made: 2012 10 04.467661
- Discovery observation was made by Pan-STARRS 1 (MPC Code F51) The Discovery M.P.E.C.: MPEC 2012-T18 : 2012 TC4
- Data-Arc Span (publish): 7 days
- Number of Optical Observations(published):301
- Observatories Reporting (Published) Observations(MPC Code):
- Perihelion Distance: 0.9337184081730526(AU)
- Aphelion Distance: 1.877515914032821
- Goldstone Asteroid Schedule: Yes 2017 Oct ( Needs Astrometry: Yes Physical Ob
- Near-Earth Object Human Space Flight Accessible Targets Study (NHATS): Yes