• Patrick Shober

How small rocks miss the Earth based on data of rocks hitting the Earth

Updated: Aug 25

Our model of centimeter-sized rocks encountering the Earth show a net-positive flux of asteroidal debris onto comet-like orbits.

In 2017, the Desert Fireball Network observed a grazing fireball that sent an asteroidal rock onto a comet-like orbit. The only reason we saw this event happen is because the object got so close that it burned its way through the atmosphere. We wanted to figure out how rare a close encounter like this actually is...

Small debris have close encounters with the Earth all the time. However, we can usually only see the larger stuff when this occurs (often tens of meters or more). Using atmospheric impact data collected by the Desert Fireball Network, we created a model of close encounters of centimeter-sized bits.

Simply put, we assumed that the flux near the Earth should be pretty similar to the flux on the Earth. In principal, this is quite logical, however, we needed to account for several biases like:

  • gravitational focusing

  • the flux is slightly higher on Earth because of gravity, and the flux has a higher proportion of slow debris because they are deflected more

  • only night-time observations

  • decreases in camera sensitivity due to the Moon being above or below the horizon

  • seasonal variations

  • etc.

If you want to see how we accounted for these problems, check of the 'Addressing Observational Biases' section in the paper.

So, where were we? Oh yes, the model! So what we did was take each DFN fireball produced by meteoroids > 10 grams and make thousands of clones in nearby orbits. Once we had all these clones, we threw them at the Earth and recorded what happened. Below is an animation showing how this looks for clones generated from one DFN fireball.

In total we produced ~2.3 million particles based on impact data.

Small objects orbiting in the solar system can be classified two ways: by their physical properties or their orbits. In this study, we are focusing on the orbital classification. For example, a comet-like orbit is usually more eccentric and larger than an orbit for a typical asteroid which comes from an area closer to the Sun.

So what did we learn?

Well most of the particles in the model were negligibly affected by the close encounter with the Earth. Less than 1% of the total flux have encounters that send them on a different classification of orbit. However, in our model we predict there should be a net-positive flux of material from asteroid orbits to comet ones. This transferred material is on comet-looking orbits now, but most of it does not behave the same as actual comet-stuff.

Comets usually move around the Sun in a hectic way. Asteroids on the other hand are comparatively predictable. Although, for the particles transferred from an asteroid-like to a comet-like orbit, they mostly evolve like asteroids still. They do this by avoiding the close encounters with Jupiter. These close encounter with the gas giant cause normal comet-material to act all crazy over short periods of time.

"Okay cool, why do I care? "

This stuff at first glance looks kinda cometary, but since it is relatively stable, the number of objects on these orbits at any given time can be quite large (~10^13 objects). So much so that the DFN probably observes a handful of these objects every year!

This is consistent with our observations, where we see durable rocks coming from comet-like orbits. So we must be careful when we see debris on comet-like orbits, as to not interpret it as being representative of material from the cold outer solar system.

Other side notes from the model:

  • predicts ~10^7 objects transferred onto Aten-type orbits annually

  • these are objects that orbits primarily within Earth's orbit

  • also predicts a modest 10^4 objects ejected from the solar system due to encountering the Earth, but due to small-number stats this is a tenuous value

If you are interested you can check out the paper here!