Studying the first-known asteroid to orbit the Sun closer than Venus

DiRAC research scientist Sarah Greenstreet recently published a paper on the orbital stability of the first-known asteroid discovered on an orbit entirely interior to the orbit of Venus. Greenstreet and colleagues were the first to predict that such asteroids should exist back in 2012.

Near-Earth objects (NEOs) on orbits that always remain closer to the Sun than the planet Venus are called Vatira-class asteroids. The “Vatira” name was first coined by Greenstreet et al. (2012) as a play on the name of the Atira-class asteroids, which follow orbits that always keep them closer to the Sun than the Earth and are named after the first-known asteroid of its type, (163693) Atira. Vatira-class asteroids are rare among the NEOs. They are predicted to be approximately 0.2% of the NEO population, with only a couple Vatiras larger than about 0.5 miles in diameter expected to exist at any given time. Vatiras reach such small orbits around the Sun by interacting with the planets through gravity as the asteroids migrate from the main asteroid belt between the orbits of Mars and Jupiter down closer to the Sun and eventually to the very small Vatira orbits, which is a process that takes millions of years. Vatiras can then stay on such orbits for tens of millions of years before they most likely collide with a planet (most often with Venus itself).

On 4 January 2020, the Zwicky Transient Facility discovered the first Vatira-class asteroid, provisionally named 2020 AV2. 2020 AV2 is roughly 1 mile in diameter and is likely one of two Vatiras of this size currently in existence. In a paper published earlier this year by Dr. Greenstreet, she studied the stability of 2020 AV2’s orbit in the Vatira region. She found that 2020 AV2 will likely remain a Vatira for the next couple hundred thousand years before it will migrate onto a larger, Earth-crossing orbit and will most likely eventually collide with Venus (the most common scenario for Vatiras).

In addition to studying the best-fit orbit for 2020 AV2 available shortly after its discovery, Dr. Greenstreet also studied thousands of similar orbits for 2020 AV2 that were all consistent with the uncertainty of 2020 AV2’s orbit given its limited number of observations at the time. She found that it is possible that 2020 AV2 has been a Vatira for millions of years instead of a couple hundred thousand years. This is possible because 2020 AV2’s current location in the Solar System is fairly sheltered from gravitational interactions with the planets, which would otherwise cause it to leave the Vatira region much more quickly. This finding is significant, because it means the orbital location of 2020 AV2 could be where more Vatiras are lurking and provides a good hunting ground for discovering more of this rare type of asteroid.

Dr. Greenstreet is an Asteroid Institute senior researcher and a research scientist with the DiRAC Institute. She joined DiRAC in January 2018 after completing a two-year postdoctoral fellowship at Las Cumbres Observatory in Santa Barbara, CA. She received her Masters in Astronomy in 2011 and her PhD in Astronomy in 2015 from the University of British Columbia in Vancouver, Canada. She also has a Bachelors in Physics with a double minor in Astronomy and Mathematics from Western Washington University in Bellingham, WA, which she earned in 2007. 

Dr. Greenstreet’s research interests involve populations of small bodies in the Solar System that undergo unique dynamical phenomena. This includes studying the long-term evolution of asteroids in the inner and outer Solar System as their orbits change over time due to gravitational interactions with the planets and determining the potential for asteroids to impact the planets. She discovered that asteroids can orbit the Sun backwards, and she has studied asteroids that can become trapped in orbits around the Sun very near the orbit of a planet where the asteroid and planet appear to “co-orbit” the Sun. Dr. Greenstreet has also used telescopic observations to help refine the orbits of newly-discovered near-Earth asteroids, determine physical properties of asteroids of interest to NASA and upcoming targets of the Goldstone and Arecibo radar telescopes, and confirm that predicted asteroids undergo the Yarkovsky effect. She has also studied the rate at which small bodies in the Solar System impact planets, creating impact craters on their surfaces, such as those observed by the New Horizons mission to the Pluto system. Dr. Greenstreet has worked with researchers at the Asteroid Institute as well as with Associate Professor and DiRAC Institute Director Dr. Mario Juric (DIRAC, Department of Astronomy) to study the threat to Earth due to asteroid impacts and by how much we would need to “nudge” an asteroid out of Earth’s way to avoid an impact.

Dr. Greenstreet’s paper on the orbital stability of the first-known Vatira-class asteroid, 2020 AV2, was featured in an article in Physics Today earlier this year.

The full research paper can be found here.

Learn more about Dr. Greenstreet’s work at

Greenstreet, S., Gladman, B., & Ngo, H., The orbital distribution of near-Earth objects inside Earth’s orbit, 2012, Icarus, 217, 355.

Greenstreet, S., Orbital dynamics of 2020 AV2: the first Vatira Asteroid, 2020, MNRAS, 493, L129.