Astronomers have discovered more than a million asteroids in our solar system, and knowledge about them is constantly growing. There are many reasons for studying asteroids. The first is to determine their orbits to find out whether any of them pose a collision risk to Earth.
– Investigating their composition constitutes an important aspect of research. Asteroid composition helps us determine how the solar system was born and has evolved, says Associate Professor of Planetary Astrophysics Mikael Granvik.
The probability of an asteroid collision is small
The paths of large asteroids travelling close to Earth have been carefully charted, which makes them unlikely to spring any dangerous surprises on us.
– A close encounter with Apophis, an asteroid orbiting the Sun, is expected in 2029. At this point, the asteroid, which is roughly 300 metres in diameter, will be visible from Earth with the naked eye. It will not collide with Earth, but will pass communications satellites in Earth’s orbit close by. However, the satellites should be safe, says Granvik.
– Having said that, Earth’s gravity may disturb the rotational characteristics and even the orbits of asteroid.
Next time, Apophis will pass by near Earth in 2036. However, it would take several full orbits to migrate to a collision course with Earth.
– Apophis poses no danger in this decade or the next, but we will have to see where it ends up in the 2060s and 2070s. With the appropriate methods, we can prepare for this, Granvik says.
There are more small than large asteroids, which is why they also collide more frequently with Earth. Moreover, the location and trajectory of smaller bodies is more difficult to survey. However, smaller objects usually disintegrate in the atmosphere, as happened in 2013 in Chelyabinsk, Russia, where the near-total destruction of an asteroid roughly 20 metres in diameter in Earth’s atmosphere was witnessed.
– Smaller asteroids 5–10 metres in diameter can also cause damage, especially if they collide with Earth’s atmosphere above a large city. Among other things, the blast wave can break windows, as in Chelyabinsk.
Since the threat would be primarily financial in nature, there has been discussion on how to take even smaller asteroids into consideration. The composition of asteroids also affects their impact on Earth.
– Asteroids composed of iron with one-metre diameters reach the surface of Earth intact.
Composition affects the properties of asteroids
The properties of asteroids can vary considerably with the substances they are made of and their porousness.
– Were you to investigate the composition of Earth, it would, on average, correspond to that of asteroids: they contain iron, silicate, or, in practice, rock, carbon and all kinds of minerals, says Granvik.
It remains to be seen whether such raw materials can be at some point utilised from Earth.
– Getting to the point where these substances can be utilised is likely to take decades. However, in the grand scheme of things we have overcome the initial hype. Now we can draw up more realistic and grounded plans.
The composition of asteroids is primarily investigated through spectral analysis. A new observational dataset produced by the Gaia space mission, which analyses and maps out asteroids in the solar system, was announced in June.
– Telescopes and satellites can be used to track asteroids from Earth, but their actual composition can be best ascertained by going to the surface of the asteroid, Granvik says.
For now, humanity has successfully landed on three asteroids, with samples from two sent back to Earth.
On a mission to alter the orbit of an asteroid’s moon
NASA's DART spacecraft was launched on 24 November 2021, and successfully collided with Dimorphos, satellite of the asteroid Didymos, on 26 September 2022. The goal was to change the path of the satellite. The same method could also be used to change the paths of asteroids threatening Earth.
– The mission helps to determine to what degree the orbit of the satellite is reduced by the collision. It probably doesn’t work like a billiard ball, as pieces ejected from the asteroid as a result of the collision increase the impact of the collision on the satellite’s orbit, or the reaction may be otherwise unexpected. Testing eliminates uncertainties, says Granvik.
Data on the target itself can be obtained only just before the collision. The European Space Agency (ESA) will send the Hera probe to investigate the effects of the DART collision and the asteroid’s composition on site.
– Hera will be launched in 2024, and it is carrying Finnish technology: a hyperspectral imaging camera, at least, developed by the VTT Technical Research Centre of Finland.