An asteroid striking Earth is a genuine possibility. There are tens of thousands of asteroids classified as Near-Earth Asteroids (NEAs), and we’re finding around 3,000 more each year. The number of new detections will see an uptick in the next few years as better survey telescopes come online.

Now NASA has developed a new system to classify all those asteroids and better evaluate impact probabilities.

There are various classifications for objects that come close to Earth.

A Near-Earth Object (NEO) has a perihelion of less than 1.3 astronomical units (AU.) A NEO can be either a Near-Earth Asteroid (NEA) or a Near-Earth Comet (NEC). If a NEO follows an orbit that crosses Earth’s, and if the NEO is larger than 140 meters (460 ft) in diameter, it’s a Potentially Hazardous Object (PHO.) Most PHOs are asteroids, and only a few are comets.

We know of about 27,000 NEAs and just over one hundred Near-Earth Comets. Those numbers will only grow, not shrink, and NASA needed a better way to assess the impact probabilities for all those objects. To do so, they’ve developed a next-generation algorithm.

Asteroids and comets aren’t travelling through space chaotically and unpredictably. In the distant past, things were more chaotic in the young Solar System, as planets migrated and asteroids were thrown around as a result. But now, NEOs follow predictable orbits. So even though there are tens of thousands of them, the situation is manageable.

But, there’s a little chaos and unpredictability throughout nature, and asteroids are no exception. There are also some tiny uncertainties in asteroids’ positions. There’s no room for carelessness when you only have one planetary home.

NASA maintains a NEO monitoring system at the Center for Near-Earth Object Studies (CNEOS) at JPL in California. It’s integrated with NASA’s Planetary Defense Coordination Office (PDCO). On their website, CNEOS says, “The Center for Near-Earth Object Studies (CNEOS) computes high precision orbits for Near-Earth Objects (NEOs), predicts their future motions, assesses their impact hazard, and makes these results available on this website.”

When scientists compute a NEO’s orbit, their data is imperfect. Different telescopes or observatories can come up with slightly different positions for any given asteroid. And though the differences are slight, they can result in different computed orbits. So CNEOS uses an algorithm called Sentry to analyze a range of possible orbits for each asteroid. Then it determines probabilities for an impact with Earth reaching 100 years into the future.

The original Sentry was very powerful.

“The first version of Sentry was a very capable system that was in operation for almost 20 years,” said Javier Roa Vicens, who led the development of Sentry-II while working at JPL. “It was based on some very smart mathematics: In under an hour, you could reliably get the impact probability for a newly discovered asteroid over the next 100 years – an incredible feat.”

Though Sentry is a powerful system, it has its drawbacks. Its power was in computing orbits by taking into account all the gravitational influences in the inner Solar System. But it’s not only gravity that shapes an asteroid’s orbit. Some asteroids are called “special cases” because of the Yarkovsky effect.

The Yarkovsky effect is named after the Polish-Russian engineer Ivan Osipovich Yarkovsky. Yarkovsky determined that small rotating objects in space would experience daily heating and that over time, that heating could induce small changes in the object’s orbit. The Yarkovsky effect is not that consequential on short time scales. But over decades and centuries, it can add up. The original Sentry didn’t take the Yarkovsky effect into account.

An illustration explaining the Yarkovsky effect. The Yarkovsky effect can alter the orbit of smaller NEOs. Image Credit: By Graevemoore at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4314576
This illustration explains the Yarkovsky effect. The Yarkovsky effect can alter the orbit of smaller NEOs. Image Credit: By Graevemoore at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=4314576

Now NASA has implemented Sentry’s successor, Sentry-II. Sentry-II won’t suffer from the same limitation that its predecessor did.

“The fact that Sentry couldn’t
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