how science detects asteroids and comets

If you’ve ever seen the movie “Don’t Look Up”, which premiered on Netflix in early December, you must have looked up, pardon the pun, and thought of the risk of having an unwanted space visitor, in the form of a giant rock, making a last minute crash landing on our beloved planet.

Nothing better, therefore, than talking to those who really know the subject to find out about how comets and asteroids are detected and if, in fact, there is a risk of discovering that one of these celestial bodies is about to collide with Earth.

The problem has a name: NEO

No, we’re not mixing the movies here and talking about the character Neo from the “Matrix” series. NEO are actually the initials for “Near Earth Objects”, or near-Earth objects. Despite what the name suggests, this does not mean that these bodies are necessarily close to our planet, but rather that their trajectory passes close to the orbit that the Earth develops around the Sun.

NEOs are objects whose point in their orbit closest to the Sun is 1.3 astronomical units. Each astronomical unit corresponds to the average distance of the Earth’s orbit from the Sun, that is, about 150 million kilometers. If these objects are more than 140 meters in diameter, they are considered PHOs (initials for “Potentially Hazardous Object”, or potentially dangerous object).

“The orbit is the decisive characteristic when it comes to risk and it is these bodies that are carefully monitored”, says Roberto D. Dias da Costa, professor at the Astronomy Department at the Institute of Astronomy, Geophysics and Atmospheric Sciences at the University of São Paulo (IAG-USP).

Detecting an asteroid or comet with these characteristics is a relatively simple, albeit laborious, process. Basically, what you do is the opposite of the Netflix movie title. In other words, look up.

scene of "Don't Look Up" - Disclosure/Netflix - Disclosure/Netflix

Scene from “Don’t Look Up”

Image: Disclosure/Netflix

There are monitoring programs dedicated exclusively to the task of detecting moving bodies in the Solar System. They are telescopes that continuously photograph the sky, creating a kind of mosaic of images.

“Each photo is compared with the previous one from the same position. When a moving body appears, it changes position from one photo to the next, while the stars obviously do not move. Once simple false positives such as aircraft or meteorological balloons, the suspicious bodies are continuously monitored, that is, they are followed, in order to determine their distances and orbits”, explains Costa.

These telescopes can be either on the Earth’s surface or in our planet’s orbit. NEOWISE fits into this second case. It is a space telescope launched in 2009 — still under the name WISE (initials of Wide-Field Infrared Survey Explorer, or open-field infrared analysis explorer, in free translation) — whose initial purpose was to analyze the sky. for the detection of asteroids, stars and galaxies. In 2013, it was put back into operation and started to monitor the so-called NEOs.

“It observes the entire sky at wavelengths from 3 to 25 micrometers. This telescope was responsible for observing the comet NEOWISE, which took the same name as the telescope and was observed in Brazil in 2020. This telescope takes pictures of the sky every 11 There are more than 7500 images per day that can be compared with each other in different types of wavelengths in the wavelength range that the telescope observes”, says Roberta Duarte, PhD student in Astrophysics at the Astronomy Department of the Instituto de Astronomy, Geophysics and Atmospheric Sciences at the University of São Paulo (IAG-USP).

Once the object is detected, its orbit can be determined using several techniques. Duarte explains that, among them, is the use of other wavelengths, both visible and radio, in addition to measurements via photometry (analysis of the flow or intensity of electromagnetic radiation from an astronomical object) and spectroscopy (which observes the spectrum of radiation object’s electromagnetic

“This makes it possible to analyze the light curve and understand how the body’s brightness is changing. From there, the orbit is calculated using astrometry tools, the area of ​​astronomy that studies measurements of bodies in the sky as well as the positions and movements of these bodies . In the case of an object very close to Earth, radars can also be used that can detect at short distances or after they enter the Earth’s atmosphere”, adds Duarte.

On the website of NASA’s Jet Propulsion Laboratory you can find a tool that shows the orbit of known bodies of interest. Just click here and add the object you want. For a list of NEO names, just visit here.

The bigger the better

It may seem counterintuitive, but the larger the asteroid or comet detected, the better. This is because, normally, larger bodies are usually detected when they are still very far from Earth.

“That cliché from disaster movies that an object is found and will collide with Earth in a few days or hours is false. Due to the great distances involved, these objects are typically found years before they pass close to us. small, with sizes on the order of ten meters or less, is that they are found very close or sometimes not seen beforehand and are only detected when they fall”, points out Costa.

An example of a small object (at least in astronomical terms) is the Chelyabinsk meteor, which fell in 2013 in Russia. It is estimated that it was between 15 and 17 meters in diameter, which prevented its previous detection. Still, it did damage by exploding in the sky, which shows how dangerous these objects can be. Similar passages took place in Russia, in 1908, with the Tunguska Event, and in Brazil, in 1930, with the Rio Curuçá Event.

Larger objects are therefore detected earlier. But what to do if a large object is found and ends up on Earth?

At the moment, the answer to that is a little scary: nothing. This is because there is still no standard procedure to act in these cases.

Record of the launch of the NASA probe developed for the Dart Mission, which intends to divert the asteroid's path - Reproduction/Nasa - Reproduction/Nasa

Record of the launch of the NASA probe developed for the Dart Mission, which aims to divert an asteroid’s path

Image: Reproduction / NASA

This situation, however, tends to change. On November 24, 2021, NASA launched a space mission called DART (Double Asteroid Redirection Test), which aims to test a system capable of changing the standard motion of bodies such as asteroids and comets and, thus, avoid possible impacts in the future.

The target is the moon of 65803 Didymos, a binary asteroid whose main body is about 800 meters in diameter and whose orbit is most of the time between Earth and Mars – at its closest point to Earth, it is about 20 times the distance between our planet and the Moon. The idea is to impact a probe on its moon, which is about 160 meters in diameter, and see how much that changes its trajectory. If the result is satisfactory – even if the deviation is minimal, this tends to cause considerable changes in trajectory in the medium and long term -, this technique has everything to be the main way to defend our planet.

Asteroid or comet: which is more dangerous?

Considering an impact of a celestial body with the Earth, would it be more dangerous if that body were an asteroid or a comet?

In practice, the answer is straightforward: whatever. What matters most in this case is size. It goes without saying that the larger the object colliding with Earth, the more damage it will do.

“To cause damage on a global scale, a body with hundreds of meters or kilometers would be needed. In general, most objects observed to date are considerably small and do not present risks. Another relevant point is that most of them already have their orbits. calculated for the next 100 years and there is no risk of a dangerous impact with the Earth”, says Duarte.

In general, asteroids and comets are very similar in general terms, with the biggest difference being their composition. While comets include ice and dust in their composition — which helps explain the formation of a “tail” — asteroids are often composed of metals.

Image of Comet Hale-Bopp - Getty Images - Getty Images

Comet Hale-Bopp image

Image: Getty Images

Now if we are going to talk about risk having as a criterion the chance of a collision with Earth, the balance ends up weighing in favor of asteroids. That’s because, in the Solar System, most of them are concentrated in a belt between the orbits of Mars and Jupiter. In astronomical terms, that means they are close to us.

Comets, in turn, are rarer, but they have a characteristic that ends up bothering them: their unpredictability.

“It must be remembered here that the planets and most asteroids move around the Sun in the same plane as if they were on a table. This plane coincides with the plane of the Sun’s equator and is due to the common origin of all bodies When the protosolar nebula condensed, the Sun was formed at the center and an equatorial disk was formed around it. From the disk formed the planets, satellites and asteroids, so everything moves in approximately the same plane, with one or the other exception due to collisions at the time of formation”, details Costa.

Comets, in turn, are “stored” in the Oort Cloud, which is approximately spherical in shape – as if it were a “shell” of the Solar System – and is very far from the Sun, although gravitationally bound to it. “Too far” in this case is about a thousand times farther than the planets are from the Sun.

“So, when a comet enters the inner solar system, where we live, it can appear from any direction, without taking into account the typical orbit of objects present within the system. For this reason, astronomers are still surprised by comets that have already are relatively close to us”, concludes Costa.

About Raju Singh

Raju has an exquisite taste. For him, video games are more than entertainment and he likes to discuss forms and art.

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