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NASA boosts impact risk from 'potentially hazardous' asteroid Bennu

Friday, August 13th 2021, 6:05 pm - While the chances are still extremely low, new findings from the OSIRIS-REx mission have made an impact with Bennu slightly more likely.

Potentially hazardous asteroid Bennu just became slightly more potentially dangerous.

Over a century from now, in 2135, a half-kilometre-wide asteroid named 101955 Bennu will pass between the Earth and the Moon. While there is absolutely no chance of an impact with Bennu at that time, the close encounter throws uncertainty into the predictions. As a result, there's no saying for sure that Earth will be safe from Bennu afterward.

Before NASA sent their OSIRIS-REx spacecraft to Bennu, astronomers tracked asteroid Bennu using telescopes. Their observations gave us very accurate calculations of Bennu's orbit. The relatively close passes of Bennu in 1999, 2005 and 2011 helped with this. Although it was officially classified as a "Potentially Hazardous Asteroid" (PHA), NASA could rule out any possibility of a Bennu impact for the next 100+ years.

However, the September 2135 encounter added an extra complication. The asteroid comes very close to Earth at that time. On September 24 of that year, its absolute minimum distance could be around 110,000 km from the planet's surface or less than one-third the distance to the Moon. As it flies past, it will encounter one of several 'gravitational keyholes'. As it passes through one of these tiny points in space, Earth's gravity will tug on the asteroid and alter its orbit around the Sun.

Bennu Sept 2135 keyholes - NASA/GoddardThis screenshot of an animation used during the NASA press briefing on Wednesday, August 11, 2021, shows the potential paths of Bennu during the 2135 flyby, based on the most up-to-date telescope observations, with a separate 'gravitational keyhole' for each path. Credit: NASA/Goddard

Even with the carefully plotted orbit of Bennu, the minor uncertainties made it challenging to tell which 'keyhole' it would pass through during that 2135 flyby. Thus, there was no way to know with any certainty which orbit it would end up on afterward.

This was of great concern to scientists because it introduced over 150 possible impacts, with a cumulative 1 in 2,700 impact chance between 2135 and 2300. The most likely impact would be on September 24, 2182.

Bennu Sept 2182 potential impact - NASA/GoddardThe effects of one particular 'gravitational keyhole' could have disastrous results in 2182. Credit: NASA/Goddard

With OSIRIS-REx following along with the asteroid for over two years, though, it gave scientists an unprecedented chance to closely track the influence of the Yarkovsky effect.

The Yarkovsky effect is a tiny, constant 'push' that an asteroid receives as it radiates heat into space.

Watch below: How the Sun pushes asteroids around the solar system

According to Davide Farnocchia from the Center for Near Earth Object Studies (CNEOS) at NASA's Jet Propulsion Lab, who is the lead author of a new study published this week, the force of this 'push' on Bennu is equivalent to about the weight of three grapes.

"Think about that, just three grapes," Farnocchia said in a NASA teleconference on Wednesday. "Because this acceleration is persistent, its effect builds up over time, and it becomes very significant by the time we get to 2135."

"The OSIRIS-REx data give us so much more precise information, we can test the limits of our models and calculate the future trajectory of Bennu to a very high degree of certainty through 2135," Farnocchia said in a NASA press release. "We've never modelled an asteroid's trajectory to this precision before."

Based on their study of the Yarkovsky effect on Bennu, Farnocchia and his colleagues calculated an updated cumulative probability of a Bennu impact between 2135 and 2200. Instead of 1 in 2,700, the chance is now just 1 in 1,750. The individual chance of an impact on September 24, 2182, is now 1 in 2,700.

It should be noted that these probabilities are tiny.

A 1 in 1,750 chance is equal to a 0.057 per cent impact probability, while 1 in 2,700 is equivalent to 0.037 per cent. That means there's over a 99.94 per cent chance that Bennu will miss Earth on all the potential encounters it has towards the end of the 22nd century. Specifically for September 2182, there's a 99.96 per cent chance of a miss.

During their research, Farnocchia and the others accounted for every conceivable force that could affect Bennu's orbit. They even considered whether the Touch-and-Go (TAG) maneuver that netted NASA a sample of the asteroid was enough to alter its trajectory.

Watch below: NASA's OSIRIS-REx tags asteroid Bennu

"The force exerted on Bennu's surface during the TAG event were tiny even in comparison to the effects of other small forces considered," Rich Burns, OSIRIS-REx project manager at NASA's Goddard Space Flight Center, said in the press release. "TAG did not alter Bennu's likelihood of impacting Earth."

The next steps in determining Bennu's true potential as a threat to Earth will come in 2023 and 2037.

In 2023, OSIRIS-REx will swing past Earth. On its way past, it will drop off the sample it collected from Bennu's surface back in the Fall of 2020. As scientists examine these samples in the laboratory, we will learn more about Bennu's composition. This might provide information about how we could deflect such an asteroid on an impact trajectory.

In 2037, Bennu will be making its next close (but safe) flyby past Earth. During this pass, radio telescopes can gather radar data on the asteroid. These readings will help confirm the calculations made based on OSIRIS-REx's data and give us an even better estimate of its exact path for that 2135 encounter.

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