How the sun affects the asteroids in our solar system

Artist concept of Lucy Mission. Credit: SwRI

Asteroids embody the story of the beginning of our solar system. JupiterThe Trojan asteroids, which orbit the Sun on the same path as the gas giant, are no exception. Trojans are believed to be remnants of the objects that ultimately formed our planets, and their study could offer clues as to how the solar system came into being.

Over the next 12 years, NASA’s Lucy mission will visit eight asteroids, including seven Trojans, to help answer big questions about how planets were formed and the origins of our solar system. It will take about three and a half years for the spacecraft to reach its first destination. What could Lucy find?

Like all planets, asteroids exist in the heliosphere, the vast bubble of space defined by the ranges of the wind from our Sun. Directly and indirectly, the Sun affects many aspects of existence in this pocket of the universe. These are some of the ways the Sun influences asteroids like Trojans in our solar system.

Place in space

The Sun represents 99.8% of the mass of the solar system and therefore exerts a strong gravitational force. In the case of the Trojan asteroids that Lucy will visit, their very location in space is dictated in part by the Sun’s gravity. They are grouped in two Lagrange points. These are places where the gravitational forces of two massive objects – in this case the Sun and Jupiter – are balanced in such a way that smaller objects like asteroids or satellites stay in place relative to larger bodies. The Trojans lead and follow Jupiter in its 60 ° orbit at Lagrange points L4 and L5.

This video features Lucy’s principal researcher, Hal Levison, discussing the Trojan asteroids at Lagrange Points and how the Lucy mission will chart its course to visit them. Credits: ">NasaGoddard / James Tralie Space Flight Center

Pushing asteroids (with light!)

That’s right, sunlight can move asteroids! Like the Earth and many other objects in space, asteroids rotate. At one point, the solar face of an asteroid absorbs sunlight while the dark side gives off energy in the form of heat. When the heat escapes, it creates an infinitesimal thrust, pushing the asteroid very slightly out of its path. Over millions of years, this force, known as the Yarkovsky effect, can significantly alter the trajectory of the smallest asteroids (those less than 25 miles, or about 40 kilometers in diameter).

Likewise, sunlight can also alter the rate of rotation of small asteroids. This effect, known as YORP (named after four scientists whose work contributed to the discovery), affects asteroids in different ways depending on their size, shape, and other characteristics. Sometimes YORP makes little bodies spin faster until they come apart. Other times, it can slow down their turnover rate.

Trojans are further from the Sun than the near-Earth or Main Belt asteroids we’ve studied before, and it remains to be seen how the Yarkovsky and YORP effect affect them.

Heat and cooling of asteroids

Over millions of years, the Yarkovsky effect can significantly alter the trajectory of smaller asteroids. Credit: NASA’s Goddard Space Flight Center

Shaping the surface

Just as rocks on Earth show signs of weathering, so do rocks in space, including asteroids. When rocks heat up during the day, they expand. As they cool, they contract. Over time, this fluctuation causes cracks to form. The process is called thermal fracturing. The phenomenon is more intense on objects without atmosphere, such as asteroids, where temperatures vary enormously. Therefore, even if Trojans are further from the Sun than rocks on Earth, they are likely to show more signs of thermal fracturing.

Solar wind blown asteroids

Animation shows hundreds of solar wind particles flying from the Sun to Earth.
Asteroids are beaten by the solar wind, a constant flow of particles, magnetic fields and radiation from the Sun. Credit: NASA’s Goddard Space Flight Center

The lack of atmosphere has another implication for the weathering of asteroids: asteroids are beaten by the solar wind, a constant flow of particles, magnetic fields and radiation coming from the Sun. Much of the Earth’s magnetic field protects us from this bombardment. Particles that pass through can excite molecules in Earth’s atmosphere, causing auroras. Without magnetic fields or clean atmospheres, asteroids are hit hard by the solar wind. When incoming particles hit an asteroid, they can propel matter into space, changing the fundamental chemistry of what is left.

Arline J. Mercier