Sentry-II launched: Tracking NASA’s latest system for planetary defense

Shane McGlaun - Dec 7, 2021, 8:42am CST
Sentry-II launched: Tracking NASA’s latest system for planetary defense

One of the critical tasks that NASA handles for the entire world is tracking as many near-Earth asteroids (NEAs) as possible. So far, almost 28,000 NEAs have been discovered utilizing telescopes that scan the night sky. Those telescopes add about 3000 new near-Earth asteroids each year, but there are still more to discover.

Via NASA/JPL-Caltech

NASA says as it utilizes more advanced surface telescopes over the next few years, it expects rapid growth in the number of discovered NEAs. With a significant increase in the amount of NEAs expected to be discovered in the coming years, astronomers have developed a next-generation asteroid monitoring algorithm called Sentry-II. It’s designed to improve the prediction of potential impacts of the tens of thousands of asteroids that pose a potential risk to the planet.

The image above highlights just how many asteroids NASA is tracking. While at first glance, it may look like orbits of the inner solar system planets on a hazy background, each of those blue lines is an asteroid orbit. The number of asteroids creates a literal spiderweb of orbiting patterns NASA is attempting to track. NASA wants people to know that asteroids don’t fly through the solar system haphazardly. Instead, each of them has its own orbital path and are extremely predictable following known paths around the sun.

The Center for Near Earth Object Studies calculates an orbit for every known NEA with the goal of improving impact hazard assessments to support the NASA Planetary Defense Coordination Office. The Sentry-II software is replacing the original software, unsurprisingly called Sentry, used by the Center for Near Earth Object Studies to monitor impact risk since 2002.

The NASA Jet Propulsion Laboratory (JPL) manages the Center for Near Earth Object Studies. Javier Roa Vicens led the development of Sentry-II when he was working at JPL as a navigation engineer. Recently, he left JPL and went to work for SpaceX. Vicens says that Sentry was a capable system in operation for nearly two decades based on “very smart” mathematics. He says in under an hour, reliable impact probabilities for newly discovered asteroids over the next 100 years were generated.

Sentry-II is a tool able to rapidly calculate impact probabilities for all known NEAs, including some special case asteroids that the original Sentry software could not handle. Sentry-II can also report the objects posing the most risk in the Center for Near Earth Object Studies Sentry Table. The new software can calculate impact probabilities utilizing a new method making the impact monitoring system more robust. With Sentry-II, NASA can confidently assess potential impacts of asteroids as low as a few chances in 10 million.

The special case asteroids that the original Sentry software could not handle were asteroids that were affected by non-gravitational forces. The most significant of those forces is thermal caused by the heat from the sun. The original Sentry struggled taking thermal forces into account.

When asteroids spin, the dayside of the objects are heated by the sun. That heated surface would then rotate to the nightside of the asteroid and cool down, releasing infrared energy. That infrared energy created a small and continual amount of thrust on the asteroid, something known as the Yarkovsky effect. The Yarkovsky effect has very little influence on the motion of an asteroid across a short period. Still, over decades and centuries, it can significantly impact the asteroid’s orbit.

JPL navigation engineer Davide Farnocchia says that Sentry not taking that effect into account automatically was a limitation of the software. Another limitation was that as scientists came across special case asteroids such as Apophis, Bennu, and 1950 DA, researchers had to conduct complex and time-consuming manual analyses. Thankfully, with Sentry-II now active, it can automatically handle the Yarkovsky effect and special case asteroids, and manual analysis won’t be required.

Vicens says that while special case asteroids discovered represent a “very tiny fraction” of all near-Earth asteroids impact probabilities are calculated for, many more will be discovered when the NEO Surveyor mission and the Vera C. Rubin Observatory in Chile go online. Sentry-II is helping scientists to be prepared for the new capabilities offered by the systems.

Sentry-II models thousands of random points that are not limited by assumptions about how the uncertainty region might change. The uncertainty region is described as the number of potential orbits with the actual orbit lying somewhere inside the cloud of possible orbits. Sentry-II chooses random points throughout an asteroid’s entire uncertainty region, and then the algorithm determines possible orbits within the entire region of uncertainty that could impact the Earth. Sentry-II can zero in on one or more very low probability impact scenarios the original software could’ve missed because it doesn’t use calculations shaped by predetermined assumptions regarding which portion of the uncertainty region might lead to impact.

To follow along with the tracking of these asteroids with NASA, take a peek at the official @AsteroidWatch Twitter account, as run by NASA’s Joshua Handal at NASA’s Planetary Defense Coordination Office. Once this system is more fully matured, we’re crossing our fingers and hoping we’ll see additional resources for on-the-spot tracking of NEA of all sorts!

Defending the planet

The first step in defending the planet against a potentially hazardous asteroid comes in knowing the hazardous asteroid is out there. Sentry-II is designed to step in once a potentially hazardous asteroid is discovered and let us know if there is a real chance it will hit the planet. Should an asteroid be discovered that threatens life on Earth, the next step would be to prevent that impact from happening.

This is where the NASA DART mission would come in. DART is NASA’s mission to deflect an asteroid using an impactor spacecraft. Essentially, NASA wants to know if an asteroid poses a threat to life on Earth if it’s possible to crash spacecraft into the asteroid, changing its orbit enough to prevent the impact. The target for DART is Dimorphos, an asteroid that’s half a mile wide and is part of the Didymos binary system.

DART is a very large spacecraft approximately the size of a small car. However, it will be traveling at an extreme velocity of around 14,400 mph when it impacts Dimorphos. NASA hopes to shift the asteroid’s orbit enough to allow telescopes on Earth to observe the change. An Italian Space Agency CubeSat tagging along with DART called LICIACube will be deployed before DART impacts the asteroid. It will record what happens shortly after the impact from a much closer perspective.

DART launched in late November, and its travel time is about ten months to its target. While its asteroid target is a whopping 6.8 million miles away from Earth, that vast distance is still close enough for DART’s effects to be observable from Earth.


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