NASA's Double Asteroid Redirection Test spacecraft
© NASA/JHUAPL
An artist’s concept of NASA’s Double Asteroid Redirection Test spacecraft, which, if all goes well, will slam into a space rock and knock it into a different orbit.
If all goes to plan, in September 2022 a NASA spacecraft, the Double Asteroid Redirection Test mission or DART, will slam into a space rock with the equivalent energy of three tons of TNT. The goal is to nudge the orbit of its target object ever-so-slightly, a practice run to see if we could divert an asteroid from a catastrophic impact with our planet in the future.

The impact on that asteroid could produce the first meteor shower ever to result from human activities in space, according to a paper published earlier this year in The Planetary Science Journal. Observing the shower could let scientists on Earth study the composition of near-Earth asteroids. But this cloud of debris would also mark a small irony for a space mission that has a goal of helping to protect our planet.

If this small shower of space rocks reaches our planet, it will create a minuscule amount of peril for orbiting satellites. Although the risk is tiny, the study's author says, anticipating the effects of the spacecraft's operations could establish a template for future space missions to minimize their impacts on Earth and the commons of space through which it travels.

NASA plans to launch the 1,100-pound DART spacecraft in 2021. Its target is Didymos, a pair of near-Earth asteroids that travel around the sun together. DART is aiming for the smaller of the two, affectionately named Didymoon, which measures about 535 feet across and orbits the larger asteroid. The force of the impact is expected to change Didymoon's 11.92-hour orbit by about 4 minutes, a big enough change for telescopes on Earth to detect. If it succeeds, the mission might help confirm that humanity's best defense against a rogue asteroid is to bump it into another orbit away from Earth.

Didymos makes regular passes of our planet at a minimum of 4 million miles — or 16 times the Earth-moon distance — approximately every 20 years. Its next close pass is scheduled for Oct. 4, 2022, at a distance of about 6.6 million miles, just after DART is scheduled to impact on Sept. 30, making observations from Earth easier.

The impact is expected to produce between 22,000 and 220,000 pounds of centimeter-sized debris.

"There's a fair amount of material that will be ejected," said Paul Wiegert, the paper's author and an astronomy professor at the University of Western Ontario.

Most of the wreckage should be ejected at less than about 2,000 miles per hour and will follow the orbit of the asteroid, with no chance of reaching Earth for thousands of years. If some of the debris reaches more than about 13,000 miles per hour, which will depend on the structure of the asteroid and the angle of impact, it could make the relatively short jump to Earth, in as little as 15-30 days.

The amount of material that could reach Earth is modest; Dr. Wiegert estimates perhaps a few grams, resulting in only "a few to ten" meteors visible in our night sky over a few days. But that could be enough to learn more about the composition of the asteroid as the meteors disintegrate.

"When they burn out, they emit some light," said Audrey Bouvier, a planetary scientist from the University of Bayreuth in Germany. And by analyzing the spectrum of that light, Dr. Bouvier says it is possible to "establish which elements were present."

The prospect that any of this debris will damage Earth orbiting satellites is negligible. Tom Statler, the program scientist for DART at NASA, says the team's own analysis shows there is "no significant debris hazard."

But however remote the risks from the DART mission, Dr. Wiegert and other astronomers suggest that it will set an important precedent.

Aaron Boley, a planetary astronomer at the University of British Columbia, notes this would be the first time human activity on an asteroid ejects debris that reaches Earth.

"Space is big, but what we do in space can affect us," he said.

Future human activities in space, such as near-Earth asteroid mining and further planetary defense testing, could shed more material that arrives in Earth's orbit. That means the DART mission might be an opportunity to consider how human activities in deep space affect life on and around Earth.

"There's an opportunity here for a clear demonstration of astro-environmental stewardship," Dr. Boley said.

Dr. Boley suggests that changes to the DART mission could avert debris reaching Earth in that 15- to 30-day time frame and set a precedent for future asteroid activities. According to Dr. Wiegert's calculations, if the impact occurs outside of a window one week before or after the asteroid's closest approach with Earth on Oct. 4, no material would cross the planet's path this quickly.

"If it's the case that launching it two weeks later or earlier does not have any additional operational effect on the mission, then it would be worth it to set the precedent," Dr. Boley said.

Dr. Statler, however, says the timing of the impact is "dictated by orbital dynamics and communication with Earth," and the planned impact date also allows for optimal viewing by ground-based observatories, so it would not be feasible to reschedule it.

While DART poses no meaningful risk, Dr. Wiegert says future asteroid missions should take the debris issue into account, just as missions closer to Earth need to better plan for space junk they leave in orbit. "It's the first of a possibly large number of meteoroid streams we might create in the solar system that could become a hazard," he said.