How the Sun affects asteroids in our neighborhood

Credit: NASA's Goddard Space Flight Center
The story of the beginning of our solar system is told through asteroids. The Trojan asteroids of Jupiter, which orbit the Sun along the same path that the gas giant, are not an exception. These Trojans may be remnants of the objects that eventually created our planets. Their study might provide clues as to how the solar system evolved.

NASA's Lucy mission, which will be visiting eight asteroids over the next twelve years, including seven Trojans, will help answer important questions about the formation of planets and the origins our solar system. The spacecraft will take three and a quarter years to reach its destination. What could Lucy discover?

Asteroids, like all planets, exist in the Heliosphere. This vast space is defined by the reach of the Sun's wind. The Sun has an indirect and direct impact on many aspects of life within this small pocket of the universe. These are just a few ways that the Sun can influence asteroids such as the Trojans within our solar system.

Space and Place

The Sun is 99.8% of the solar systems' mass and exerts strong gravitational forces as a consequence. The Sun's gravity is a major factor in the location of the Trojan asteroids Lucy will see. They cluster at two Lagrange points. These are places where the gravitational force of two large objects, in this case Jupiter and the Sun, is balanced so that smaller objects such as satellites or asteroids stay put relative to larger ones. At Lagrange points L4 & L5, the Trojans follow Jupiter around its orbit by 60.

This video features Lucy Principal Investigator Hal Levison who discusses Trojan Asteroids at Lagrange Points as well as how the Lucy mission will plan its path to visit them. Credit: NASA's Goddard Space Flight Center/James Tralie

Pushing Asteroids Around (with light!)

Yes, asteroids can be moved by sunlight! Asteroids rotate, just like Earth and other space objects. The Sun-facing side absorbs sunlight, while the dark side releases heat as heat. The heat creates an insignificant amount of thrust that pushes the asteroid off its path. This force, known as the Yarkovsky Effect, can alter the trajectory of smaller asteroids over millions of years (those with a diameter less than 25 miles or 40 kilometers).

The rotation speed of small asteroids can be affected by sunlight. This effect is known as YORP and was named after the four scientists who discovered it. It can affect asteroids in different ways, depending on their size, shape, or other characteristics. Sometimes YORP can cause small bodies to spin more quickly until they fall apart. Sometimes, YORP can slow down their rotation rate.

The Yarkovsky Effect can alter the trajectory of small asteroids over millions of years. Credit: NASA's Goddard Space Flight Center

The Trojans are further from the Sun that the near-Earth and Main Belt asteroids, so it remains to see how the Yarkovsky effect or YORP will affect them.

The Surface: Shaping

As rocks on Earth exhibit signs of weathering and so do rocks in space. Rocks expand when they are heated up in the day. They contract as they cool. This fluctuation can lead to crack formation over time. This is known as thermal fracturing. This phenomenon is stronger on objects that don't have atmospheres like asteroids. Temperatures vary greatly. Even though Trojans are closer to the Sun than Earth's rocks, the phenomenon of thermal fracturing will likely be more evident.

The solar wind is a constant stream of particles, magnetic field, and radiation from the Sun that batters asteroids. Credit: NASA's Goddard Space Flight Center

Another consequence of asteroid weathering is the absence of atmosphere: Asteroids are battered and harmed by the solar wind. This is a constant stream of particles, magnetic field, radiation, and magnetic fields that flows from Sun. Earth's magnetic field is able to protect us from most of this bombardment. If particles do manage to get through, they can cause auroras by excitation of molecules in Earth's atmosphere. Asteroids are the main beneficiaries of the solar wind, since they don't have their own atmospheres or magnetic fields. Incoming particles can strike an asteroid and kick some material into space, altering the fundamental chemistry of what is left behind.

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