How Do We Know If An Asteroid is Going to Hit the Earth?

In recent weeks, there has been news about the asteroid 2024 YR4 potentially hitting the Earth. One important figure that you might have noticed is the impact probability, and it fluctuated from around 1% to as much as 3% before finally converging to near 0% in the latest analysis. How are they analyzing the data to assess how likely it is for an asteroid to hit Earth? Let’s find out in this article.

To look out for asteroids, sky surveys (examples?) take images of the night sky every day. The images are then sent to a computer system, in which bright spots are identified and their relative positions on the celestial sphere are mapped. Then, the bright spots are compared against images to see if any spots have moved from image to image relative to other background stars, with close to linear and constant-rate motion. These trajectories are then checked against a database, and if it’s a known object, it adds to the collection of observations collected on that known object. Otherwise, it’s flagged as a new object, and the orbit determination process begins.

There are many algorithms (such as the open-source find_orb) that can carry out orbit determination and predict the future (and past) positions of an object. If an object gets close to the Earth (and the range of possible object positions can coincide with the Earth’s position), the object is flagged for more observation, and an impact probability is assigned based on the probability distribution of the range of possible orbits that the asteroid can take, based on observational uncertainties.

However, these algorithms are met with uncertainties in the asteroid’s orbit predictions from different sources and phenomena.

However, there are a number of inherent sources of uncertainty. First of all, there’s the dimensionality reduction problem. Remember that there are 6 parameters governing an asteroid’s orbit at any given time (note that these parameters are not constant because of external perturbations), one of those sets being the asteroid’s coordinates (x, y, z) and velocity (vx, vy, vz). But by projecting the 3D mapping into a 2D celestial sphere, you actually lose information about how far the asteroid is from you (i.e., the information on the component of the direction parallel to the line between the observer and the object).

This essentially turns the point into a line, an infinitely extending line stretching through all possible positions of the object that will lie on the same location of the celestial sphere. And the velocity information along that direction is lost as well. If the object is moving directly away from you, you will not be able to see it with a change in location of the object in your view. Thus, you won’t be able to get the full information from the 3D coordinates and velocities to interpolate its orbit, and you need more observations to constrain the orbital parameters of the object.

Even if you were able to perfectly determine the orbital parameters of the asteroid and accounted for all the planetary perturbations, you would still not have a full picture of where it is going to go. That’s because the shape of the asteroid also plays a part in the long-term evolution of the asteroid’s orbit.

(include yarkovsky effect image)

Specifically, there’s the Yarkovsky effect, which is the effect that reradiation of the absorbed heat of an asteroid can impose an acceleration on the asteroid. This acceleration changes with the asteroid’s shape and rotational period, and without accounting for that, even with the perfect information we described above, the range of possible orbits that an asteroid can take on will still get wider and wider as time goes on. These effects are small, so the propagation of error is not as serious as those coming from actual observational uncertainties (and hence the effect is only actually observed in the last 20 years or so). But still, when it comes to predicting chances of asteroid impact, this effect plays a major role. Here’s why.

Asteroids that can potentially hit Earth can also experience previous close approaches to our planet. That’s because the asteroid’s orbit nearly intersects the Earth’s orbit. These close approaches act as gravity assists — using the gravitational force of the planet, the close approach changes the velocity vector of the asteroid and changes it into a different orbit.

Close approaches are events that greatly magnify uncertainties in orbit determination. If an asteroid gets close to a planet, the side that it passes the planet can influence the direction of the change in orbit. If it passes in front of the planet, it slows down and is caught into a lower orbit. Otherwise, if it passes behind the planet, it speeds up and is flung out into a higher orbit. Obviously, the difference between passing in front and behind are very fine, so fine that the addition of small uncertainty values can completely change the subsequent orbit of an asteroid and thus its impact chances with the Earth.

The orbital uncertainties depend a lot on the observation arc of the asteroid, the amount of time elapsed between the first and last known observation of the object. Think about this: assume that, through some observations in a given time frame, you have a range of orbits, and a range of positions to look out for the asteroid in future (and past) observations. Remember that the uncertainties magnify over time (i.e., the further into the time dimension you look, the further your observations are from the expected value). Therefore, if you only wait a small amount of time and observe, you are not going to get a lot of information as the asteroid is pretty much where you expect it to be. However, if you look for observations from a few years ago and find the asteroid in an archival image, it can drastically narrow down the orbital uncertainties with a longer observation arc.

This process of lengthening observation arcs through archival images is known as a precovery process. Precoveries are often used to get better orbits for asteroids, and thus hopefully lower the chances for an impact.

But often, all we can do is wait for more observations and a longer observation arc for the impact probability to be gradually adjusted. That’s because most of the objects in interest are very small (like for the case of 2024 YR4, it is only about 40 to 90 meters wide). Therefore, even in times of previous relatively close approaches, it might not actually reflect enough sunlight to be captured in the detectors of the sky survey telescope, or that light might have blended into the random noise that the telescopes generate (read noise and dark current are examples of such noises).

And therefore, it might be the case that the current favorable observation window would simply be the first time in history that the object has ever been confidently captured in telescope images, and we would have to wait for a longer time to pinpoint the range of its orbital parameters. That’s why the asteroid impact probabilities can take a long time to decrease — a lot of the times, we simply have to collect more observations.

In this article, we’ve conceptually talked about the process of discovering a new near-Earth asteroid, how we evaluate its chances of impacting the Earth, and the uncertainties involved in the process.