Another two harmless asteroids being shared in the sensationalist press. I am getting PM's from some really scared people about these. These two asteroids not only are harmless, they never were even considered for the Sentry table. And no they are not planet killers. They are not big enough to do more than have minor global effects like a few months of cooler weather because of dust in the atmosphere. Large enough to have some effects on the yields of our crops for one year. But that is an academic question becaue they can't hit.

To debunk:


You can check that here with my non techy interface to the NASA Sentry table here:

So you should cross both Business Insider and Daily Express off your list of reliable sources for asteroid news.

These stories are due to a confusion, that they do not know what is meant by a “Potentially Hazardous Asteroid” or PHA.

They then wrap up their ignorance in pseudoscientific gobbledegook about asteroids swerving in space due to the Yarkovsky effect and gravitational keyholes.

These are borrowed words, they are real effects but these non scientific journalists do not know how to use them; they do not apply here.

This is another article I'm writing to support people we help in the Facebook Doomsday Debunked group, that find us because they get scared, sometimes to the point of feeling suicidal about it, by such stories.

Do share this with your friends if you find it useful, as they may be panicking too.

What is a potentially hazardous asteroid (PHA) or comet

Short summary: it means it has an orbit that comes within a certain distance of Earth. It does not mean it is a real hazard. For instance 2010 TK7 (150 to 500 meters in diameter) orbits permanently a third of an orbit ahead of Earth. It is no danger to us because it is always a third of an orbit ahead of us. However, because it is constantly tracing the same orbit as us, it gets this classification PHA.

There are many others that sometimes cross our orbit but always at great distances from Earth and they also are PHA’s but no threat to us.

Most usually they have a few percent chance of hitting us some time in the next several tens of million of years but can’t hit us right now.

If it is not in the Sentry table it can’t hit us this century and most likely can’t hit us for many centuries.

Techy term: It just means an asteroid larger than 150 meters in diameter that comes within a certain distance of Earth's orbit (0.05 au which is 7.48 million kilometers, or 4.65 million miles). For a comet then it also has to have a period of at most 200 years.

Whenever they do close flybys of Earth this deflects them into slightly different orbits. It is really hard to hit Earth because it is so tiny, so they usually will end up being deflected away into other orbits that are far from Earth. Earth clears its orbit over a period of a few tens of millions of years, most of them being sent elsewhere.

Most of them will eventually hit the sun after many flybys of several planets, or hit Jupiter or be ejected from the solar system.

A few percent eventually hit other planets and that includes some that will eventually hit Earth, most likely thousands or millions of years into our future.

Most come from the asteroid belt originally, and a few, less than one percent, come from beyond Jupiter after first getting deflected by Jupiter into short period Jupiter family comets.

The largest known PHA is Swift-Tuttle at 26 km across, which has a one in a million chance of impact in 4479. That's plenty of time to deflect it with whatever technology we have thousands of years from now.

The sensationalist press often run stories about PHA’s. If it is not in the sentry table then it is not a risk.

You can search for the object to see if it is in one of the other tables. E.g.

Search for 1990MU

Search for 1998 OR2

Then click through to the ESA page and if they have it, it will say “not in risk list”

But if they don’t have it (as in this case) you can click through to the JPL page and they will usually have it, this is mainly just to check you entered it in correctly - if you got the name right and it isn’t in the Sentry table or removed objects list then it was never considered to be any risk to Earth.

Wikipedia’s page is also often useful for asteroids:

What is a NEO?

Meanwhile a NEO just means an object whose perihelion (closest point to the sun) is less than 1.3 au, i.e. 1.3 times Earth's orbital radius. Most are not even PHA's.

According to one estimate, the largest undiscovered NEO is likely to be about 3.5 km.

From Table 2-1. NEO population, impact frequency, and projected completion from Update to determine the feasibility of enhancing the search and characterization of NEO's There the numbers less than 1 are interpreted as a probability so for instance the 0.02 for objects from 7.94 to 10.0 km in diameter means there is only a 2% chance of finding even one more object of that size. Most of these will not be PHA's.

The table is complete from 10 km upwards (the size of asteroid that ended the dinosaur era). There are only four asteroids and four comets at this size.

The only other NEO of 10 km across or larger that seems to have a chance of hitting Earth even thousands of years into the future is the asteroid 433 Eros (16.84 km).

It could hit us some time after a million years from now, with a 5% chance, but is not likely to hit us before 100,000 years from now.

Summary details of all eight objects: NEO asteroids and comets of 10 km or more.

How does the Sentry table work?

The Sentry table is automated. New objects go their automatically as soon as they are identified as a NEO. The name consists of the year it was first observed, e.g. 2007 then a code of two letters and a number. The first letter gives the half month in which it was found, e.g. A is Jan 1-15, B is Jan 16 - 31 and so on. So F is Mar 16 - 31. Then the last bit, say, T3 in 2007 FT3, is assigned in a cycling system, first asteroid in that half month gets letter A, through to 25th gets letter Z (omitting the letter I), then it goes through A1 to Z1, and so on.

When they get the first observations, often they don't know if it is an asteroid or not (might just be a data glitch). If so, it goes into the Scout table where it gets a temporary name. As soon as it is confirmed to be a NEO it is then transferred to the Sentry table if there is any risk, or just to the JPL and ESA tables. When an object is first discovered then the orbit is not well known, and there are many possible virtual orbits. Only one of them is the real orbit but which it is isn't known at this stage. The impacts in the risk list are for virtual orbits. When an object is removed from the table it's usually because the orbit is better known and the virtual orbits that would impact can be ruled out.

The smaller asteroids can change orbit over years to decades due to minute effects such as absorbing sunlight and re-radiating it as heat in a different direction as it spins. Comets can change orbit slightly due to outgassing and jets. But normally the reason an object gets removed is just because its orbit is better known.

This video explains how that works:

(click to watch on YouTube)

Asteroids can’t abruptly swerve in their orbits

The Yarkovsky effect is minute and it is mainly relevant for smaller objects up to a few hundred meters across. It can shift objects by minute amounts over decades but it adds up and if something is coming really close to Earth it could make all the difference, so it is relevant for Bennu for instance and Apophis. It is due to the sunlight warming a rapidly rotating asteroid which then radiates the heat away on the dark side as it rotates out of the sunlight. That minute pressure of heat radiation is enouth to chainge its position slightly over years and decades.

It is just silly to talk about it turning a miss into a hit for an object that passes by at a distance of millions of kilometers and for big objects like these two.

The gravitational keyholes only apply to very close flybys of Earth to the point where the next encounter with Earth can depend on precisely where they fly past in Earth’s gravitational well.

Again it is just silly for an object that never comes close to Earth.

How to check

You can use my page to do a unified search of the Sentry table, Removed objects, and then links to the pages at JPL, ESA and NEODys2 (I can add other pages if anyone knows of other databases that I can link to using the object name).

If it isn't in the sentry table or removed object table you know it is not a risk and never has been.

NASA Near Earth Asteroids and Comets Sentry Table - As words instead of techy numbers

Checking the distance at JPL for the encounter is a natural thing to do but is not reliable because it doesn't show potential impacts.

e..g. for 2007 FT3 then because we have only 1 day of observation it could be almost anywhere which gives a minute chance of an impact like throwing 9 or 10 dice and getting a six every time.

2007 FT3

If you click through to the JPL page, it doesn't even show a close approach in 2019 because it's likely to be the opposite side of the sun from us.

Yet it shows a flyby in 2068 with a fair bit of uncertainty but not that much given that there is a tiny chance it hit Earth before then.

The reason is that the uncertainty distance there is for the most likely flyby and doesn't include minute vastly improbable events. It would be a certain number of standard deviations away - I am not sure what, like e..g. could be 5% or 1% chance it is outside of that range.

If only the JPL page included a section about potential impacts like the ESA page.

The ESA page doesn't say much but it does say if it is in the risk list or the priority list or not.

Did you know?

If the Egyptians had build an asteroid detection telescope back at the time of the Great Pyramid of Giza, over 4,500 years ago - it would still be waiting for its first "city killer" asteroid.

Combines photo of Kheops-Pyramid with one of the two Keck ten meter telescopes Linked Hawaiian Telescopes Catch a Nova Surprise (I know the top of a pyramid in Egypt is hardly the best place for a telescope, this is just for visual effect to show the idea). They were only a few key insights away from the industrial revolution in some ways. It's a not impossible alternative history :).

Many telescopes scan the sky every day to help keep you safe

Some of the telescopes looking out for asteroids every single day

All the asteroid tracking is done in the open and it can’t be hidden. That would be like trying to hide news of a hurricane. Anyway that’s the job of the CNEOS, minor planet center, ATLAS, ADAM, main reason for Pan STARRS to give us early warnings. To not tell us after Congress and others worldwide have given them hundreds of millions of dollars of funding to give us early warning of asteroids would be like weather forecasters saying “Everything is fine, forecast a clear sunny day tomorrow” as a hurricane bears down on Miami. Just doesn’t happen outside of movies. They’d be out of a job in minutes if they tried to do that and nobody would even want to.

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