Looking Up at the Asteroids in the Neighborhood

A Tufts astronomer explains the rocky remnants from the birth of our solar system

Asteroids fly through our solar system all the time, but it’s rare for us to take notice of them. But that’s changed this week, as an asteroid passes within 1,231,184 miles of Earth on January 18. The asteroid, dubbed 7482 (1994 PC1), was first seen in 1994 and is about two-thirds of a mile wide.

One likely reason Americans are paying more attention is because many millions have watched the Netflix film Don’t Look Up, about a comet that’s headed for Earth with disastrous consequences. But this asteroid is no Hollywood plot device; it’s just a tiny part of what makes up our solar system.

The good news is that scientists in the real world are on the case. NASA’s Center for Near-Earth Object Studies is tasked with identifying all objects that come near Earth, like asteroids, to track any that might potentially collide with Earth. 

To learn more about asteroids, where they come from, and where they are going, we reached out to Danilo Marchesini, professor of physics and astronomy in the School of Arts and Sciences. Marchesini specializes in understanding how galaxies form and evolve.

Tufts Now: What exactly are asteroids?

Danilo Marchesini: Asteroids are rocky remnants from the solar system’s formation. They orbit around the sun near the ecliptic plane—the same plane within which the planets orbit the Sun. Most asteroids are located between the orbits of Mars and Jupiter, in what is called the asteroid belt.

How many asteroids are there in our solar system?

We currently know of and have cataloged several hundred thousand asteroids—and there may be more than 10 million individual asteroids. The larger asteroids have a diameter of roughly 1,000 kilometers (about 621 miles). Despite being so

“Most asteroids are located between the orbits of Mars and Jupiter, in what is called the asteroid belt,” says Danilo Marchesini. Photo: Alonso Nichols“Most asteroids are located between the orbits of Mars and Jupiter, in what is called the asteroid belt,” says Danilo Marchesini. Photo: Alonso Nichols
numerous, the total mass of all asteroids together is small, estimated lower than 0.05% of the mass of Earth.

How do astronomers find them and track them?

The first asteroid, Ceres, was discovered in 1801 by Italian astronomer Giuseppe Piazzi. Although the discovery is referred to as accidental, astronomers were looking for objects theorized to be orbiting the Sun between Mars and Jupiter.

In general, asteroids are discovered by observing the sky night after night and identifying objects that are quickly moving across the “fixed” background of distant stars. There is a whole sub-field of astronomy devoted to systematically searching for them with medium/small sized telescopes scanning the night sky. The smaller the asteroid, the fainter it is, and the harder it is to find.

Are there different types of asteroids?

Some are on the same orbit as Jupiter, both trailing and leading Jupiter—these are called Trojans. Others are located between the orbits of Mars and Earth, the Amor asteroids. The ones that cross Earth’s orbit when they are closest to the sun are called Apollo asteroids, while those that cross Earth’s orbit when they are farthest from the sun are Aten asteroids. 

The asteroid whose orbit is coming closest to Earth this month—7482 (1994 PC1)—is one of the Apollo asteroids.

What happens when asteroids collide?

Every once in a while, there may be collisions in the asteroid belt between Jupiter and Mars—or even gravitational interactions among asteroids or gravitational perturbations with Jupiter itself, which result in asteroids being ejected from the asteroid belt. The Aten and Apollo asteroids are thought to be the results of these interactions. 

What happens over the long term to asteroids?

Astronomers theorize that some of the smallest moons orbiting some of the planets in the solar system may have been asteroids caught in a planet’s gravitational field as they wandered by. Asteroids can collide with one another, and small fragments called meteoroids get ejected from the Asteroid Belt. These can produce meteors or meteorites when entering the Earth’s atmosphere. 

Given the huge number of asteroids, astronomers theorize that most of them have had interactions and collisions with other asteroids during the past 4.6 billion years of history of the solar system. But in general, the largest majority of asteroids will continue to orbit the sun. 

Taylor McNeil can be reached at taylor.mcneil@tufts.edu.

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