The oldest known to science

Scientists have accurately determined that an interstellar object recently discovered in the Solar System is a comet. The object clearly has an icy core and a faint tail of gas and dust, which removes it from the category of asteroids. As an asteroid, it would have been named C/2025N1, but thanks to the ATLAS robotic observatory in Chile, it was possible to determine that the comet appeared from interstellar space.

So the comet was named after the ATLAS system, and the letter I in its name indicates the interstellar origin of the object (interstellar). The number 3 in the name is the ordinal number of the object opened in this rare category. Only two objects that came from distant stars have been seen in the Solar System in the entire history of observations: the asteroid Oumuamua and Comet Borisov.

There are probably more of them, but it's extremely difficult to notice them: they are dim and very fast. As a matter of fact, their speed gives them an interstellar origin. The guest of 2025 3I/ATLAS flies about 60 kilometers per second, which significantly exceeds the typical speeds of objects in the Solar System.

Mikhail Nevsky, head of the educational observatory at the SFedU Faculty of Physics, said that the object probably formed in another star system and was ejected from it. This suggests that the object has been traveling through the galaxy for many billions of years, making the comet perhaps the oldest known to science.

"We usually expect that gravitational interactions or collisions will push a new comet out of the clouds of icy bodies that exist at the edge of the Solar System. We have two large asteroid belts and hundreds of small clusters. But we usually do not expect interstellar visitors, and each such object is of great interest to scientists, because it brings new information from the depths of space," explained Mikhail Nevsky.

Scientists are already preparing an ambitious observation program: the Hubble and James Webb space telescopes will help determine the size, chemical composition and rotation of the comet.

Now 3I/ATLAS is located at a distance of four astronomical units. One astronomical unit is the distance from the Earth to the Sun, that is, 150 million km. In October, the comet will pass at the closest distance from the Sun — 1.4 AU.

"And in December, it will get as close to the Ground as possible, to about 3 AU and, possibly, will be visible to the naked eye. He will fly by, say hello and go to other stars!" shared Mikhail Nevsky.

Before the face of the Apollons

Three astronomical units sounds safe, but how often do celestial objects approach the Earth at a distance less than to the Sun? Feel free to take it higher — asteroids regularly fly so close to the Earth that the distance to them is less than to the Moon.

So, on May 21, 2025, an asteroid from the Apollo group, 2025 KF, measuring 20 meters, flew at a distance of about 115 thousand kilometers from Earth.

"The Apollo group includes those asteroids whose orbits cross the Earth's orbit and have a semi-axis of more than 1 AU. Potentially dangerous to the Earth are those that are larger than 140 meters in diameter and fly closer than 750 thousand kilometers from our planet," explained Mikhail Nevsky.

On September 15, 2025, the asteroid "2024 ON" with a diameter of 220 m will arrive to us. It potentially belongs to the dangerous class, but in reality there is nothing to worry about — it will fly at a distance of three lunar distances. Even seeing it through a telescope will be difficult, but for large telescopes it is still possible.

"Close approaches of asteroids to Earth at a distance less than the Moon occur regularly for small objects, several times a month, and for large objects much less frequently, about once every few years or decades," Mikhail Nevsky added.

Humanity does not have a single plan of action in case it turns out that a large asteroid is heading directly towards our planet, but research in this direction is underway. Many scientists are thinking about how to prevent a collision and what protocol to propose if the danger turns from theoretical to real. So far, monitoring of celestial bodies is a priority.

"Modern monitoring systems make it possible to track most space objects and warn of potential threats. Even an interstellar comet, which is so far four times farther than the Sun, has been spotted by just such a monitoring system. Currently, ground-based observatories do the main job of detecting asteroids, but they can miss objects flying from the Sun. That's why in 2027, NASA will launch the NEO Surveyor telescope into space to detect dark asteroids invisible from Earth," said Mikhail Nevsky.

The essence of the NEO Surveyor project is to create a systematic and comprehensive program for the search and monitoring of near-Earth objects, especially those that are in orbits close to Earth and may pose a risk of collision.

Hide the nuclear bomb

Mankind realized the reality of such risks 117 years ago, when the Tunguska meteorite collided with the Earth on June 30, 1908. It produced an explosion with a capacity of 10-50 megatons — comparable to a powerful hydrogen bomb — and toppled about 80 million trees over an area of more than 2,000 km2.

Light anomalies were observed all over the world that day: due to the scattering of particles in the atmosphere, the nights were so bright that it was possible to read newspapers without artificial lighting. At the same time, the meteorite did not leave a crater, which means that the asteroid's body exploded in the air at an altitude of 5-10 km.

"And then, a hundred years ago, potential methods of countering meteorites arose at the theoretical level: destroying an asteroid and taking it out of orbit. The destruction method has never been tested, but to test the effectiveness of the kinetic impact method, the NASA space probe carried out the DART mission in 2021," said Mikhail Nevsky.  

The DART (Double Asteroid Redirection Test) mission successfully ended with a collision with the Dimorphos asteroid on September 26, 2022. Her goal was to change the orbital period of Dimorphos' rotation around its larger companion, the asteroid Didymus. The period before impact was 11 hours and 55 minutes, and after the collision with the probe it was reduced by 32 minutes, which significantly exceeded the minimum effect expected by scientists in 73 seconds.

The choice of a kinetic impact probe method instead of an explosion, for example, a nuclear device for the DART mission is due to several key factors. Firstly, kinetic impact is a more predictable and safer method. An explosion, especially a nuclear one, could create uncontrolled debris that in itself would become a threat to the Earth or spacecraft.

Secondly, the use of nuclear devices in outer space is limited by the Outer Space Treaty of 1967, which prohibits the deployment of weapons of mass destruction in orbit. Although theoretically a nuclear explosion could be effective for large asteroids, its application requires complex legal and diplomatic approvals.

Thirdly, most asteroids are not exactly rocks, but rather "piles of rubble" — a loose accumulation of stones loosely bound by gravity. A relatively weak kinetic impact can cause the release of gas and dust and create the effect of a jet stream that will change the trajectory of the asteroid, and an incredible atomic explosion can disperse debris without changing the orbit.

The DART experiment proved that kinetic impact is a viable method of deflecting asteroids, especially for objects 100-500 meters in size, which are the most dangerous due to their prevalence. A small change in the speed of an asteroid can indeed significantly change its trajectory, but only with early exposure.

It is better to detect space objects that are potentially dangerous to the Earth several years before a potential collision, which explains all the resources invested in monitoring systems.