As every year, this June 30 is a day – or rather a night – to spend with your eyes to the sky: it’s International Asteroid Day, which since 2015 has been celebrated by the entire community of astronomers and enthusiasts to spread knowledge and endless curiosities about asteroids and minor bodies, as well as to raise awareness about the potential risks and related protections we have to avoid impacts and space threats.

A complex topic and perfect for entertaining at least once in a lifetime over some summer dinner, but do we really know what an asteroid is? How realistic are the risks of impact? What is the Torino Scale, and what tools and expertise can we rely on to monitor our safety in space?

We interviewed Daniele Gardiol, National Coordinator of the PRISMA network, First Italian Network for the Systematic Surveillance of Meteors and Atmosphere coordinated by INAF – National Institute of Astrophysics with the support of Fondazione CRT.

Let’s start at the beginning: what exactly is an asteroid? Where does it come from and how does it form?

An asteroid is a small rocky body that orbits around the Sun. Most asteroids are found in the asteroid belt, a region between the orbits of Mars and Jupiter. Unlike planets, asteroids are much smaller and non-spherical in shape, and they can vary in size from a few meters to hundreds of kilometers. They are considered remnants of the primordial solar system that never aggregated to form a planet.

When an asteroid, or an asteroid fragment, enters Earth’s atmosphere, it releases its energy in the form of light. This phenomenon is called a meteor; when meteors are very bright, they are also called bolides.

What are the potential risks associated with asteroids for Earth?

We know that Earth has always been hit by asteroids, sometimes large, more often small. The same happens to other planets, such as Mercury and Mars, or the Moon, as evidenced by the numerous impact craters we observe on these celestial bodies. These objects have cosmic speed of tens of kilometers per second and carry enormous energy. Naturally, the larger the asteroid, the more severe and significant the consequences: from very localized damage to the possible destruction of a city, up to a catastrophe on a continental or even planetary scale.

What technologies and strategies can we rely on to monitor and mitigate these risks?

Astronomers use special telescopes to observe the sky, every night when possible, trying to discover asteroids and monitoring known ones. To identify an asteroid, two images of the same area of the sky are made at successive times: comparing the two images will show that stars, being far away, will appear stationary, while an asteroid, due to its own motion, will be visible as a moving dot.

The observations allow us to calculate their orbits, predict their position in the near future, and assess any risk of impact with Earth. To increase accuracy, even discovered need to be continuously monitored for up-to-date data on their orbits. The databases of NASA – National Aeronautics and Space Administration and ESA – European Space Agency report all available data, from size to orbital parameters.

Several scales can be used to determine the hazard of an asteroid. One of these, internationally recognized, is the Torino Scale, which assigns a value from 0 to 10. 0 means no danger, 10 certain and catastrophic impact. Named during an international conference held in Turin in 1999, the Torino Scale ranges from risk 0 (white) for objects so small that they burn up in the atmosphere, to risk 10 (red) for objects with a certain collision, capable of causing a global catastrophe.

Are there recent examples of asteroids that have posed a high risk? How were they managed?

On June 30, 1908 in Tunguska, Siberia, an asteroid about 50 meters in diameter exploded with a power of about 1,000 Hiroshima atomic bombs. It leveled an area of 2,000 square kilometers to the ground and felled more than 80 million trees. Fortunately, the event occurred in an almost uninhabited area.

More recently, on February 15, 2013, in Chelyabinsk, also in Russia, an asteroid of about 15 meters in diameter caused 2 million square meters of windows to shatter as a result of an explosion, injuring more than 1,500 people. Unfortunately, these kinds of events, caused by small asteroids, are usually unpredictable.

In 2004, a newly discovered asteroid, Apophis, reached a value of 4 on the Torino Scale for a few days, with an impact probability of nearly 3% in 2029. Subsequent observations ruled out the risk of impact, but Apophis will still pass so close to Earth, visible to the naked eye!

The most terrible impact we have evidence of is the asteroid about 10 kilometers in size that struck Earth 66 million years ago in the Yucatan area in the Gulf of Mexico. It caused a crater about 200 kilometers wide, raised a cloud of dust that reached the stratosphere, enveloping the Earth and obscuring the Sun for more than 10 years. This inhibited photosynthesis and oxygen production, breaking the food chains and causing the extinction of three-quarters of the species presenta at the times, including dinosaurs.

What are the main challenges PRISMA faces in monitoring asteroids?

Astronomically speaking, most asteroids are “small” objects. With the telescopes currently available, it is difficult to be able to observe those smaller than one kilometer in size. For this reason, most of the dangerous asteroids, which may hit Earth in the near future, have not yet been discovered. Networks like PRISMA, promoted and coordinated by INAF and supported by Fondazione CRT, observe meteors, which are usually generated by larger asteroid fragments.

Observations can help identify small and invisible but still potentially dangerous asteroids and estimate their possible direction of origin. Additionally, by calculating the point of fall of any debris it is possible to recover meteorites, something PRISMA has already done twice, in 2020 in Cavezzo, Modena, and in 2023 in Matera. In such cases, the characteristics of the celestial body can be studied in depth, allowing us to learn more about our “enemy.”

What does the future of asteroid research foresee and what are the next big questions to answer?

Besides mitigating the risk to Earth, the recovery of fresh meteorites thanks to PRISMA is of great importance for studying the remote period when our Solar System formed over 4.5 billion years ago.

In particular, we have two fascinating hypotheses yet to be confirmed: the first concerns the presence of water on our planet. It is thought that water was brought by meteorites that bombarded Earth about 3 billion years ago. The second hypothesis suggests that even life itself came from space, carried by meteorites. To be able to confirm either or both of these hypotheses would be an epic achievement.

What paths would you recommend for those interested in learning more about asteroids and meteors?

PRISMA runs an educational program in schools and public outreach. It is a participatory project involving associations, schools, researchers and private citizens who contribute to meteorite search and the functioning of the cameras. There are also some excellent websites, mostly in English, but I would recommend the website ESA – NEOCC (Near Earth Objects Coordination Centre) for up-to-date news and data on potentially hazardous asteroids.