NASA has been watching the skies for potentially hazardous asteroids for decades, but there are some factors that the existing Sentry program couldn’t account for. The second-generation system, Sentry-II, has now gone online, allowing astronomers to calculate the orbits – and impact chances – of asteroids far more precisely.
Asteroid impacts can be catastrophic for Earth and everything that calls it home – just ask the dinosaurs. That doomsday rock is believed to have been around 10 km (6.2 miles) wide, but even a much smaller asteroid could do some serious damage. The meteor that exploded over Chelyabinsk, Russia in 2013 was estimated to be just 20 m (66 ft) wide, and it still injured almost 1,500 people. A rock measuring a few hundred meters could wipe a city off the map with an explosion many times greater than the bomb dropped on Hiroshima.
With enough warning, it might be possible to intervene and prevent this kind of Armageddon. Just a few weeks ago, NASA launched the DART mission to test the viability of crashing a spacecraft into an asteroid to change its path, a technique that could come in handy if we spot any big, Earthbound space rocks.
Of course, to pull that off we would need to know about it in advance, and so the Center for Near Earth Object Studies (CNEOS) calculates the orbit of every known near-Earth asteroid (NEA), a number that’s approaching 28,000 objects. Then it’s determined whether they pose a threat to our home planet, using JPL software called Sentry.
“The first version of Sentry was a very capable system that was in operation for almost 20 years,” says Javier Roa Vicens, a JPL engineer. “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.”
Sentry was very effective at calculating orbital paths based on how an asteroid is affected by the gravitational pull of the Sun and planets, but there were a few factors that it couldn’t account for. In the long run, these uncertainties can snowball into many possible orbits that may or may not impact Earth.
The Yarkovsky effect, for instance, is where the Sun unevenly heats the surface of an asteroid as it spins, creating thermal forces between the “day” and “night” sides of the rock that can produce thrust. Other times, asteroids that swing past Earth very closely could be nudged into different orbits by the planet’s gravity, changing the paths of their eventual return.
The first Sentry system couldn’t incorporate either of these two factors, meaning that for special case asteroids like Bennu or Apophis, astronomers would have to manually analyze their orbits, which is a complex and time-consuming process.
But Sentry-II is designed to account for things like these. This latest version uses a different algorithm that models thousands of random points within the uncertainty space of an asteroid’s orbit, then figures out which ones have a chance of striking Earth in future. This, the team says, could help find scenarios that have very low probability of impact.
“Sentry-II is a fantastic advancement in finding tiny impact probabilities for a huge range of scenarios,” says Steve Chesley, a senior research scientist at JPL who worked on both iterations of Sentry. “When the consequences of a future asteroid impact are so big, it pays to find even the smallest impact risk hiding in the data.”
A study outlining how Sentry-II works was published in the Astronomical Journal. The video below illustrates how orbits are calculated and why they may be uncertain.
Source: NASA