A spacecraft to save the planet

A spacecraft to save the planet

Planetary defence

Hollywood’s answer to the daunting problem of a potential asteroid collision with the Earth in 1998 was to send a crack team of astronauts and blue-collar workers riding space shuttles with a nuclear bomb. Over twenty years later, NASA is planning to impact an asteroid with more effective means albeit with slightly less cinematic glitz.

The agency’s Double Asteroid Redirection Test (DART) mission will see a spacecraft depart Vandenberg Space Force Base on a SpaceX rocket in November and following an 11-month journey, enter into a collision course with a two-asteroid body as they are 10.3 million miles from Earth in October 2022.

Dr Andrew Rivkin of Johns Hopkins Applied Physics Laboratory and head of the investigation team explained in a talk given for the Indian Institute of Astrophysics (IIA) that the objective is to hit a 160-metre-sized asteroid moon called Dimorphos which orbits a larger asteroid called Didymos, which is about five times larger, comparable to the height of the Burj Khalifa skyscraper. They were both originally discovered in 1996. 

“The plan is to hit Dimorphos which is roughly the size of the pyramids of Giza with a spacecraft that is roughly the size of a standard vending machine,” Dr Rivkin explained.

He added that the central mission objective is to change the orbital period of the asteroid around the main body by at least 73 seconds. Dimorphos’ orbit period currently is about 12 hours. Researchers believe that even affecting this small change to the asteroid moon’s orbit could prove the viability of future planetary defence missions. The DART project involves scores of international researchers and organisations, including a scientist from IIA.

An experiment

“This testbed project is extremely important because there are many near-earth asteroids around our planet. This particular study may help us to protect it from any future disasters,” said Dr Chrisphin Karthik of the Indian Institute of Astrophysics in Bengaluru, who is part of an over-100 worldwide investigation team of scientists.

The researchers believe that there are 500 million near-earth asteroids measuring less than four metres in length, some five million asteroids measuring 25 metres, 20,000 asteroids measuring 160 metres and about 900 objects measuring 1,000 metres. An impact from large asteroids could result in global devastation and trigger the failure of crops. The team also knows of three asteroids measuring 10,000 metres. 

An asteroid of this type was famously known for ending the Cretaceous period 66 million years ago. 

“The larger asteroid impacts happen less often, perhaps once every 100 to 200 million years. As you get to smaller sized asteroids, those impacts happen more often. Even something about the size of 160 metres could leave a pretty big crater which could cause a bad problem if something that big hit, a city or near a city. We have found less than half of such objects out there, so we’re less confident about what our future is for those,” Dr Rivkin said.

Dangers of impact

Even smaller asteroids pose a danger, he added, pointing to the 2013 impact of a 20-metre-long asteroid over the Siberian town of Chelyabinsk in Siberia. “There was a big flash, there was a fireball, and then a minute or so later, there was a big sonic boom which blew out windows all throughout the town, and ended up injuring about 1,000 people,” Rickin said about the impact that carried broad impact energy equivalent to 500 kilotons of TNT. 

An asteroid about 50 metres across could create greater havoc, Dr Rivkin said, pointing to the Barringer crater in Arizona which was created 50,000 years ago. “Within 10 kilometres of that crater, there would have been a fireball. Large animals would have been killed or wounded. Within 30-24 kilometres, there would have been hurricane-force winds. If that were to happen in an inhabited area today, it could be very bad. So, planetary defence is the field of trying to deal with what’s out there,” he added.

Data from NASA shows that India has endured three such hits in the last 18 years, including a 4.6 kiloton impact in Odisha in 2003.

Ultimately more data is needed about near-earth objects, the scientists said, adding that this would potentially save lives through even the smallest interventions. “Even a five-minute warning to the residents of Chelyabinsk not to rush to windows after the 2013 impact happened would have prevented injuries,” the scientists said.

Wouldn’t it be better to set off a nuclear device in space to destroy dangerous incoming asteroids, in the mould of Michael Bay’s 1998 blockbuster Armageddon?

While Dr Rivkin acknowledged that the biggest tool humankind has is a nuclear device, its deployment would not be like the movies. “We would not try to blow up an object and disperse it, we would probably explode that device off to the side and let it melt and vaporise some of the asteroid,” he said. 

However, there are some caveats. “For example, by international law, you cannot test nuclear packages or nuclear devices in space. So there’s been a lot of interest in developing other things that could be used. The kinetic impactor method to be used by DART is one such technique. We would be using the momentum that you bring from the spacecraft and change the asteroid’s orbit,” he added.

While this is a relatively simple concept, the scientists said the pilot programme to be launched is also a test of the concept. “Every time we send an asteroid mission, we find something that we weren’t expecting before,” they said.