You Don’t Need to Duck

FAS Astronomers Blog, Volume 32, Number 3.

Just recently, the folks at Live Science published a few articles about the risk of a dangerous asteroid hitting the Earth (see below). Is it time for us to start worrying?

Have you heard of NEOs, PHAs, and PHOs? Well, these are Near-Earth Objects (NEOs), Potentially Hazardous Asteroids (PHAs), and Potentially Hazardous Objects (PHOs). It seems that one of these fairly large sized space rocks zips by the Earth every few weeks.

Once in a great while, a large meteor or asteroid does strike the Earth. Remember the one that broke up over Chelyabinsk, Russia a few years ago? How about another that flattened trees for miles around Tunguska, Russia in 1908? Have you been to the Barringer Meteor crater in Arizona? That was formed by the Canyon Diablo meteorite around 50,000 years ago. Ask any dinosaur and they will tell you about the asteroid that landed near the present day town of Chicxulub, Mexico some 66 million years ago.

You might mark April 13, 2029 on your calendar. Yes, it is a Friday. This is when the 1,100-foot-wide asteroid Apophis will fly within around 20,000 miles of the Earth. Folks have been concerned about Apophis since its discovery in 2004. But not to worry! 

• In March 2021, NASA concluded there was no chance of Apophis striking the Earth within the next 100 years. NASA had previously ruled out a collision in 2029 and 2036. Now we look good for the 2068 close encounter as well. 
• Just to make sure, Paul Wiegert and Ben Hyatt released a study in March 2024 showing that there is no chance of Apophis striking another asteroid and then deflecting toward the Earth for its April 2029 encounter.

The good news is that the Earth isn’t that large compared to, well, space. It is very difficult to hit it. However, in any event, the folks at NASA are working on the problem. They are on the lookout for anything that might come a little too close. Their Center For Near Earth Objects (CNEOS) tracks many of the asteroids that venture close to the Earth. NASA’s Planetary Defense Coordination Office (PDCO) uses this information to monitor and evaluate potential threats from these Near-Earth Objects. They publish a monthly summary of the status of the Near-Earth Asteroids showing the total number (over 34,000 as of February 2024), and the number that have passed closer to the Earth than The Moon in the last 30 and 365 days.

Just in case they need to move an asteroid, NASA recently crashed the spacecraft DART into the moon Dimorphos of the small asteroid Didymos and actually altered its orbit. There is a nice video of the impact on YouTube.

To keep track of everything, they have ranking systems that measure the potential hazard of an object hitting the Earth. JPL’s Sentry System continually tracks and assesses the potential impact of many objects. JPL’s rankings include the potential hazard using both the Palermo and Torino measurement systems. So far, nothing has been ranked even close to being much of a hazard.

Palermo Technical Impact Hazard Scale. This is a more technical scale ranging from positive numbers (high risk) to between 0 and -2 where there is some risk and less than -2 where there is no risk.

The Palermo Technical Impact Hazard Scale was developed to enable NEO specialists to categorize and prioritize potential impact risks spanning a wide range of impact dates, energies and probabilities. Actual scale values less than -2 reflect events for which there are no likely consequences, while Palermo Scale values between -2 and 0 indicate situations that merit careful monitoring. Potential impacts with positive Palermo Scale values will generally indicate situations that merit some level of concern. The scale compares the likelihood of the detected potential impact with the average risk posed by objects of the same size or larger over the years until the date of the potential impact. This average risk from random impacts is known as the background risk. For convenience the scale is logarithmic, so, for examples, a Palermo Scale value of -2 indicates that the detected potential impact event is only 1% as likely as a random background event occurring in the intervening years, a value of zero indicates that the single event is just as threatening as the background hazard, and a value of +2 indicates an event that is 100 times more likely than a background impact by an object at least as large before the date of the potential impact in question. The primary reference for the Palermo Technical Scale is a scientific paper entitled “Quantifying the risk posed by potential Earth impacts” by Chesley et al. (Icarus 159, 423-432 (2002)).

“Palermo Technical Impact Hazard Scale.” NASA/JPL-Caltech Center for Near Earth Studies.

Torino Impact Hazard Scale. This is a simpler scale ranging from 0 to 10. It also has an associated color code chart indicating the hazard level for each scale.

The Torino Scale, adopted by the IAU in 1999, is a tool for categorizing potential Earth impact events. An integer scale ranging from 0 to 10 with associated color coding, it is intended primarily to facilitate public communication by the asteroid impact hazard monitoring community. The scale captures the likelihood and consequences of a potential impact event, but does not consider the time remaining until the potential impact. More extraordinary events are indicated by a higher Torino Scale value. For more information see, Morrison, D., Chapman, C. R., Steel, D., and Binzel R. P. “Impacts and the Public: Communicating the Nature of the Impact Hazard” In Mitigation of Hazardous Comets and Asteroids, (M.J.S. Belton, T.H. Morgan, N.H. Samarasinha and D.K. Yeomans, Eds), Cambridge University Press, 2004.

“Torino Impact Hazard Scale.” NASA/JPL-Caltech Center for Near Earth Studies.

If they do discover something headed our way, well … there’s not much we can do … so, you might get ready to duck … but not right now.