By Paul Rincon
Science reporter, BBC News, Seattle
Two nearby galaxies - long thought to be true companions of the Milky Way - may instead be drifters, passing through the cosmic neighbourhood.
Astronomers say the Magellanic Clouds may be moving too fast to be gravitationally bound to our galaxy.
If the clouds were satellites of our galaxy, the Milky Way would contain twice as much mass as is thought.
Details of the research were presented at the 209th meeting of the American Astronomical Society (AAS) in Seattle.
The Large and Small Magellanic Clouds are irregular dwarf galaxies that are visible only from the Southern Hemisphere.
Using the Hubble Space Telescope, a team of astronomers made what it says are the most precise measurements yet of the galaxies' three-dimensional velocities.
"We found that the velocities of the [Magellanic Clouds] are unexpectedly large - almost twice those previously thought," said co-researcher Nitya Kallivayalil, from the Harvard-Smithsonian Center for Astrophysics.
"Radial velocities" indicate how fast the objects are moving directly towards or away from us.
These properties are well known for the Magellanic Clouds, and relatively easy to measure.
More difficult to determine are the "proper motions" of the galaxies - their actual motion across the sky (not towards or away from the Earth).
This requires astronomers to take extraordinarily precise measurements over several years.
Both proper motion and radial velocities must be known to calculate the true 3D velocities of the galaxies.
Dr Kallivayalil and her colleagues made two sets of observations two years apart using Hubble.
They calculated precise proper motions for the Large and Small Magellanic Clouds. Then, they combined the proper motions with the radial velocities.
The researchers say there are two possible explanations for the unexpectedly high 3D velocities of the dwarf galaxies.
The Milky Way could be twice as massive as previously thought: this excess mass, pulling on the clouds, would be required to keep them gravitationally bound.
Alternatively, the Large and Small Magellanic Clouds might not be bound to our galaxy after all.
If previous calculations of the Milky Way's mass are correct, it does not possess enough mass to hold on to its companions.
In a few billion years, they will escape the clutches of our galaxy.
"The Magellanic Clouds may not be true companions of the Milky Way," Dr Kallivayalil said. "Perhaps they are travellers, just passing through our cosmic neighbourhood."
Furthermore, the team's measurements suggest the clouds may not be bound to each other.
Their velocities relative to one another are unexpectedly high. This observation may explain why the two galaxies did not merge long ago.