Six nights of observations by two powerful telescopes confirmed that the orbit of double asteroid Didymos is perfectly aligned for NASA’s asteroid-smashing DART spacecraft to arrive in late September.

Didymos and its moon Dimorphos will make their closest approach to Earth in years in late September, passing at a distance of about 6.7 million miles (10.8 million kilometers) from the planet. During this time, on Sept. 26, the DART spacecraft will slam into the 560-foot-wide (170 meters) Dimorphos in an attempt to alter its orbit around the 0.5-mile-wide (780 m) Didymos. The experiment, the first ever attempt to change an orbit of an asteroid, might pave the way for a future planetary defense mission if an asteroid were ever to threaten Earth.

The observations, conducted in early July by the Lowell Discovery Telescope in Arizona and the Magellan Telescope in Chile, confirmed earlier orbit calculations from 2021. The new data comes as the Double Asteroid Redirection Test (DART) spacecraft is racing to the pair with plans to crash into the smaller rock, dubbed Dimorphos, to test a potential technique to deflect an asteroid that threatens Earth, which Didymos and Dimorphos do not.

“The measurements the team made in early 2021 were critical for making sure that DART arrived at the right place and the right time for its kinetic impact into Dimorphos,” Andy Rivkin, the DART investigation team co-lead at the Johns Hopkins University Applied Physics Laboratory in Maryland, said in a statement. “Confirming those measurements with new observations shows us that we don’t need any course changes and we’re already right on target.”

Asteroid Didymos in images captured by the Lowell Discovery Telescope in July 2022.

Scientists need the detailed orbital parameters of the two space rocks not just to reliably guide DART to its target. After the impact, astronomers all over the world will measure the asteroids’ orbits again, to see how the orbit of Dimorphos sped up following the collision. The alteration might be rather minute and therefore extremely precise measurements of the initial configuration are required.

“The before-and-after nature of this experiment requires exquisite knowledge of the asteroid system before we do anything to it,” Nick Moskovitz, an astronomer with Lowell Observatory in Arizona and co-lead of the July observation campaign, said in the statement. “We don’t want to, at the last minute, say, ‘Oh, here’s something we hadn’t thought about or phenomena we hadn’t considered.’ We want to be sure that any change we see is entirely due to what DART did.”

Apart from the obvious forces, such as the gravitational pulls of larger bodies, asteroid orbits can be influenced by more subtle phenomena, such as the pressure of solar radiation, the scientists said in the statement.

The orbit of Dimorphos around Didymos is expected to shorten by several minutes after the impact, as the moon moves closer to the bigger asteroid. By measuring the change with maximum precision, astronomers will be able to glean important information about Dimorphos’ structure and properties of the material it is made of.

The recent measurement campaign determined the orbital period of Dimorphos around Didymos by observing the change in brightness that takes place when one asteroid passes in front of the other. It was, however, tricky to make enough observations, as skywatching conditions at this time of the year are not favorable due to the short summer nights coinciding with the rainy season in Arizona, the researchers said. Earlier this year, the asteroids were too far away from Earth to be observable.

“It was a tricky time of year to get these observations,” Moskovitz said. “We asked for six half-nights of observation with some expectation that about half of those would be lost to weather, but we only lost one night. We got really lucky. We really have high confidence now that the asteroid system is well understood and we are set up to understand what happens after impact.”