If asteroid deflection missions aren’t conducted with the utmost care and precision, they may send the object through a “gravitational keyhole,” resulting in it boomeranging to impact the Earth at a later date, according to new research presented at this week’s EPSC-DPS2025 Joint Meeting in Helsinki.
Such missions send an artificial craft towards space rocks with enough power to push the potential dangers onto a new course, avoiding a planetary impact. However, more researchers are beginning to examine the possible outcomes of even successful missions, which may have unintended consequences.
“Even if we intentionally push an asteroid away from Earth with a space mission, we must make sure it doesn’t drift into one of these keyholes afterwards. Otherwise, we’d be facing the same impact threat again down the line,” said Rahil Makadia, a NASA Space Technology Graduate Research Opportunity Fellow at the University of Illinois at Urbana-Champaign, who is presenting the findings at the EPSC-DPS2025 meeting.
NASA’s DART Mission
In September 2022, the first successful asteroid redirection mission took place with NASA’s Double Asteroid Redirection Test, or DART for short. For the test, NASA engineers launched a spacecraft on an intercept with a small asteroid named Dimorphos, in orbit around the larger Didymos asteroid. The craft acted as a “kinetic impactor,” hitting the asteroid with enough force to change its orbit successfully and thereby prove that asteroids can be deflected from their course.
NASA is not alone in pursuing asteroid detection goals. The European Space Agency is planning a direct follow-up to DART, called Hera, for December 2026. Instead of making another impact, Hera will be tasked with journeying to Didymos and Dimorphos for an up-close investigation of the original impact to provide engineers with more precise details on the DART mission’s aftereffects.
The Keyhole Effect
Going into the DART mission, NASA had little to fear from unintended consequences. Being in orbit around Didymos, Dimorphous wasn’t likely to go off by itself anywhere, and Didymos was far too large for DART to inadvertently change its course. Yet a different asteroid orbiting the Sun on a collision course with Earth could easily be sent through a gravitational keyhole back onto a direct collision course.
The dangerous keyhole effect is when a planet’s gravity influences a small area of space around it, having enough pull to alter an asteroid’s orbit onto a collision course with the planet later. If a future mission similar to DART, but tackling an actual threat, sends its target through a gravitational keyhole, the hazard will return on the next orbit, instead of being permanently abated. Future missions will need to take this into account, identifying the spot on the asteroid where impact is least likely to push the object through a keyhole.
“If an asteroid passed through one of these keyholes, its motion through the Solar System would steer it onto a path that causes it to hit Earth in the future,” said Makadia.
Targeting an Asteroid Deflection
Precision will be key in future deflection missions in light of the keyhole effect. Every single point across an asteroid’s surface comes with a unique probability of sending the object through a keyhole. Fortunately, Makadia’s team did not end their work at identifying the issue, but developed a method for computing likely trajectories for different kinetic impact points along an asteroid’s surface.
The researchers used data from DART to develop their mapping technique, but caution that due to variable characteristics, other asteroids will have slightly different results. Features such as shape, rotation, mass, and topography will have substantial effects and must therefore be determined to produce accurate predictions.
Ideally, a reconnaissance mission to the hazardous asteroid should be conducted first, to collect the most accurate and detailed information possible, yet that may not prove possible if the danger is not identified sufficiently far in advance. However, ground-based observations could provide enough precision to calculate the most and least safe impact points.
“With these probability maps, we can push asteroids away while preventing them from returning on an impact trajectory, protecting the Earth in the long run,” said Makadia.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.