For decades, astronomers have described the impending head-on collision between our Milky Way and the neighboring Andromeda galaxy as a foregone conclusion. Early measurements of Andromeda’s motion, combined with gravitational models, suggested the two galaxies would merge in roughly five billion years, triggering spectacular bursts of star formation and fundamentally reshaping the Local Group. However, a new study that synthesizes the latest observational data from NASA’s Hubble Space Telescope and the European Space Agency’s (ESA) Gaia mission finds that a direct collision may be far less certain than previously believed.
Astronomer Till Sawala of the University of Helsinki and his international team—comprising researchers from Durham University (UK), the University of Toulouse (France) and the University of Western Australia—have used advanced computer simulations to show that there is only approximately a fifty-percent chance that the Milky Way and Andromeda will collide within the next ten billion years. Published June 2, 2025 in Nature Astronomy, their work underscores the significant uncertainties in forecasting galactic futures and calls into question models that have treated a cosmic smashup as all but inevitable.
Historical Context: From Nebula to Galactic Neighbor
The 1912 Revelation and Early Predictions
In 1912, astronomer Vesto Slipher first measured the radial velocity of the Andromeda “nebula,” leading to the realization that it was moving toward our solar system at a significant speed. Decades later, Edwin Hubble’s distance measurements confirmed that Andromeda lay far beyond the Milky Way, establishing it as a separate galaxy. Early theoretical work speculated that, given Andromeda’s approach, a collision with the Milky Way must eventually occur.
Hubble’s Proper Motion Measurements
Nearly 100 years after Slipher’s discovery, the Hubble Space Telescope enabled precise proper motion measurements of Andromeda. In 2012, Roeland van der Marel and Tony Sohn of the Space Telescope Science Institute (STScI) announced that Andromeda’s transverse velocity—its motion across the sky—was surprisingly small. Their analysis of Hubble’s five- to seven-year baseline data suggested that Andromeda was on a nearly radial path toward the Milky Way, virtually guaranteeing a merger in about 5 billion years. Combined with estimates of dark-matter masses for both galaxies, this scenario appeared as certain as “death and taxes,” borrowing a phrase from Benjamin Franklin.
New Observations: Integrating Hubble and Gaia Data
Advances in Astrometric Precision
While Hubble provided critical proper-motion data, ESA’s Gaia mission has since revolutionized astrometry by mapping the positions, distances and motions of more than one billion stars in the Milky Way and nearby galaxies. Gaia’s high-precision measurements of stellar motions in Andromeda and its satellite galaxies, combined with similar data for stars in the outer regions of the Milky Way, allow astronomers to refine estimates of relative velocities, distances and the overall mass distribution within the Local Group.
Gathering and Harmonizing Observational Inputs
Sawala’s team undertook the most comprehensive treatment of observational uncertainties to date. They incorporated 22 variables—ranging from Andromeda’s tangential velocity and radial approach speed to the masses and distribution of dark matter in both galaxies, as well as the influence of massive satellites such as the Large Magellanic Cloud (LMC) and Andromeda’s own satellite, M33. By combining Hubble’s multi-epoch proper-motion measurements with Gaia’s expansive astrometric catalog, the researchers accounted for previously neglected error sources and gained a fuller view of the system’s kinematics.
Methodology: Monte Carlo Simulations of Galactic Futures
Constructing 100,000 “Crash-Dummy” Scenarios
No single set of measurements can predict a galaxy’s trajectory with absolute certainty. Instead, Sawala and colleagues generated 100,000 Monte Carlo simulations by randomly sampling each of the 22 variables according to their observational uncertainties. Each simulated realization propagated the positions and velocities of the Milky Way, Andromeda, M33 and the LMC forward 10 billion years, under the influence of gravity and dynamical friction—drag caused by interactions between a galaxy’s dark-matter halo and other passing masses.
Accounting for Satellite Influences
Including M33 and the LMC in the simulations was critical. M33’s gravitational pull can nudge Andromeda’s orbit slightly closer to the Milky Way, increasing the chance of an encounter. Conversely, the LMC—massive enough to perturb the Milky Way’s motion—can deflect our galaxy’s path away from Andromeda. These competing effects proved instrumental in producing a more balanced set of outcomes.
Modeling Dynamical Friction and Dark-Matter Halos
The gradual decay of relative orbits depends on dynamical friction, a process by which a galaxy moving through the dark-matter halo of another slows as its gravity draws in nearby particles. Simulating dynamical friction accurately requires robust estimates of halo masses and density profiles; small changes can significantly affect whether galaxies eventually merge or drift apart indefinitely. By sampling current uncertainties in halo parameters, Sawala’s team ensured that each simulation reflected a realistic range of possible mass distributions.
Results: A 50–50 Chance of Collision
Collision Versus Long-Term Coexistence
The simulations yielded two broad categories of outcomes. In approximately 50 percent of cases, the Milky Way and Andromeda enter sufficiently close proximity (within roughly 500,000 light-years—five times the Galaxy’s current diameter) such that dynamical friction dissipates orbital energy and ultimately drives a merger. In these scenarios, the galaxies first pass each other, move outward on an elongated orbit, and then spiral back together over billions of years before coalescing.
In the remaining 50 percent of simulations, the galaxies never approach close enough for dynamical friction to overcome their relative motion. Instead, they perform a long-term gravitational “dance,” orbiting one another without merging within the 10 billion–year timeframe. In such cases, Andromeda and the Milky Way might pass within a similar distance but retain enough angular momentum to avoid coalescence—effectively surviving as separate systems for as long as the simulation extends.
A Small Chance of Early Head-On Impact
A surprising subset—around 2 percent—of the simulated realizations showed a direct, head-on collision within the next four to five billion years, echoing earlier predictions. These occurred when observational inputs aligned such that Andromeda’s tangential motion was minimal and the LMC’s deflection effect was limited, funneling the two galaxies on a near-perfect radial path. However, this outcome is substantially less likely than previously reported estimates, according to Sawala’s analysis.
Implications: Rethinking the Future of the Local Group
Uncertain Long-Term Fate
Astronomers have long treated the Milky Way–Andromeda merger as a canonical example of galaxy evolution. If the new fifty-fifty result holds, many textbooks—along with public conceptions of our cosmic future—may need revision. The study underscores that even with Hubble’s precision and Gaia’s breadth, our long-term cosmic forecasts carry large error margins. “We have the most comprehensive study of this problem today that actually folds in all the observational uncertainties,” Sawala remarked. “But the only way to improve this prediction is with even better data—another decade of Gaia, next-generation telescopes and refined dynamical models.”
Role of Satellites in Galactic Dynamics
Including M33 and the LMC revealed how satellite galaxies can critically influence the trajectories of massive hosts. M33’s additional mass tends to pull Andromeda slightly closer, while the LMC’s gravitational tug displaces the Milky Way’s center of mass away from Andromeda. “The LMC doesn’t ‘save’ us from a merger,” Sawala noted, “but it makes the direct collision a bit less likely.” Future refinements of satellite masses, or discovery of other hitherto unknown massive companions, could tilt the balance further in one direction.
Broader Lessons for Galaxy Evolution
The Milky Way–Andromeda encounter is only one of many galaxy–galaxy interactions throughout the universe. Sawala’s method—combining high-precision astrometry with robust uncertainty modeling—can be applied to other close galaxy pairs within a few megaparsecs. As astronomers study interactions such as M81 and M82 or the Antennae Galaxies, recognizing the full suite of uncertainties will lead to more nuanced predictions of merger rates and timescales across cosmic history.
Future Observations: Toward Greater Certainty
Gaia Extensions and Future Missions
Gaia is currently operating beyond its nominal five-year mission, and its upcoming data releases promise yet finer-grained astrometry for stars in both the Milky Way and Andromeda. Improved measurements of M31’s (Andromeda’s) proper motions could narrow its tangential velocity uncertainty from tens of kilometers per second to just a few, dramatically shrinking the range of possible future trajectories. Similarly, better astrometry for M33 and the LMC will yield more accurate satellite masses and orbits.
James Webb Space Telescope and Next-Generation Telescopes
NASA’s James Webb Space Telescope (JWST) and ground-based observatories such as the Extremely Large Telescope (ELT) will contribute by resolving individual stars in Andromeda’s outskirts, refining distance estimates and halo mass models. JWST’s infrared sensitivity will also enable more precise determinations of stellar populations, providing constraints on the galaxy’s star-formation history—information that indirectly informs mass distribution and dynamic modeling.
Upcoming All-Sky Surveys
The Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) will repeatedly scan the southern sky over a decade, capturing transient events and the motions of faint stars in the outer reaches of the Milky Way. LSST’s proper-motion measurements could reveal subtle deformations in the Milky Way’s halo due to its satellites, yielding further constraints on the gravitational field. Likewise, ESA’s Euclid and NASA’s Nancy Grace Roman Space Telescope will map large-scale cosmic structures, shedding light on dark-matter distribution in galaxy groups akin to the Local Group.
Reflections: Earth’s Place Amid Cosmic Uncertainty
Timescales and Human Perspective
The prospect of a Milky Way–Andromeda collision has captured popular imagination and served as a vivid illustration of galaxy evolution. Yet the new findings remind us of the vast uncertainties in cosmic forecasting—particularly when projecting events billions of years into the future. Given that the Sun itself will likely render Earth uninhabitable in approximately one billion years, and its red-giant phase will extinguish planetary life roughly five billion years from now, a galactic collision in four to five billion years—let alone one in nine to ten billion years—becomes a more academic than existential concern.
Continuing Search for Knowledge
Still, astrometric precision and improved models are essential to deepen our understanding of how galaxies grow, interact and transform. The Local Group—home to more than fifty known galaxies—serves as an astrophysical laboratory. Determining whether Andromeda and the Milky Way will annihilate each other, or continue orbiting as separate entities, yields insights into the role of dark matter, the physics of dynamical friction and the long-term fate of disk galaxies.
Public Engagement and Outreach
The iconic image of two spirals colliding has inspired countless educational materials, documentaries and public talks. By highlighting the fifty-fifty uncertainty, astronomers can demonstrate the evolving nature of science: that even widely accepted theories may be revised when new data emerges. The prospect of multiple possible futures—merger, near-miss, or prolonged orbit—invites broader conversations about our galaxy’s fate and encourages public fascination with how cosmic-scale observations translate into predictions.
Conclusion: A More Nuanced Galactic Future
The study led by Till Sawala and published in Nature Astronomy marks a milestone in our quest to understand the destiny of the Milky Way–Andromeda system. By integrating Hubble’s stellar-velocity measurements with Gaia’s astrometric data, the researchers have illuminated just how delicate our assumptions were about the inevitability of a galactic collision. Running 100,000 Monte Carlo simulations that incorporate uncertainties in Andromeda’s transverse speed, dark-matter halo properties and satellite influences, they conclude that there is roughly a fifty-percent chance of a merger within the next ten billion years.
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Even as they temper our cosmic expectations, the researchers underscore that future observations—particularly additional Gaia data, next-generation telescopes and refined satellite mass measurements—can further constrain the odds. Until then, the Milky Way’s fate remains open to two divergent possibilities: a dramatic union with Andromeda or a graceful, long-lived orbital waltz. In either scenario, the findings demonstrate the profound complexities of galactic dynamics and the power of combining multiple space-based observatories to challenge long-held certainties.
As we gaze at the night sky and contemplate our place in the cosmos, this study reminds us that—andromeda or not—astronomy is a science defined by perpetual discovery. The next decade of observations promises to clarify whether our Galaxy will one day carry on alone or emerge from a grand collision to shine anew as part of a larger, merged spiral. Until then, astronomers and the public alike can watch the skies and await the next data-driven twist in our galaxy’s story.
Credits
This article is based on research by Till Sawala et al., published in Nature Astronomy on June 2, 2025.
Hubble Space Telescope observations are managed by NASA’s Goddard Space Flight Center, with science operations conducted by the Space Telescope Science Institute. ESA’s Gaia mission is spearheaded by ESA in collaboration with the European scientific community.
