Spaceflight Now Home

Spaceflight Now +

Subscribe to Spaceflight Now Plus for access to our extensive video collections!
How do I sign up?
Video archive

Mars lander preview

A preview of NASA's Phoenix Mars lander mission and the science objectives to dig into the arctic plains of the Red Planet are presented here.


Phoenix animation

Project officials narrate animation of Phoenix's launch from Earth, arrival at Mars, touchdown using landing rockets and the craft's robot arm and science gear in action.


Dawn launch delay

Jim Green, director of the Planetary Science Division at NASA Headquarters, explains why the agency decided to delay launch of the Dawn asteroid probe from July to September.


Endeavour rolls to VAB

Shuttle Endeavour is transported from its hangar to the Vehicle Assembly Building for joining with a fuel tank and boosters for launch on STS-118.


Welcome home, Atlantis

The Shuttle Carrier Aircraft touches down on Kennedy Space Center's Runway 15 on July 3, delivering Atlantis back to its homeport after a two-day coast-to-coast ferry flight.

 Landing | Tow

Atlantis heads home

Nine days after landing at Edwards Air Force Base to conclude the STS-117 mission, Atlantis begins its cross-country ferry flight back to Florida.

 Taxi | Takeoff

Dawn preview movie

Learn more about the upcoming Dawn mission that will use an ion engine propulsion system to visit two of the largest objects in the asteroid belt.


Become a subscriber
More video

Satellites discover biggest collisions in the Universe
Posted: July 18, 2007

The orbiting X-ray telescopes XMM-Newton and Chandra have caught a pair of galaxy clusters merging into a giant cluster. The discovery adds to existing evidence that galaxy clusters can collide faster than previously thought.

This is a composite image of the Bullet Cluster. The optical image from the Magellan and the Hubble Space Telescope shows galaxies in orange and white in the background. Hot gas, which contains the bulk of the normal matter in the cluster, is shown by the Chandra X-ray image, which showst the hot intracluster gas (pink). Gravitational lensing, the distortion of background images by mass in the cluster, reveals the mass of the cluster is dominated by dark matter (blue). Credits: X-ray: NASA/CXC/CfA/M.Markevitch, Optical and lensing map: NASA/STScI, Magellan/U.Arizona/D.Clowe, Lensing map: ESO WFI
When individual galaxies collide and spiral into one another, they discard trails of hot gas that stretch across space, providing signposts to the mayhem. Recognising the signs of collisions between whole clusters of galaxies, however, is not as easy.

Undaunted, Renato Dupke and colleagues from the University of Michigan, Ann Arbor, have used ESA¹s XMM-Newton and NASA¹s Chandra orbiting X-ray observatories, to disentangle the puzzling galaxy cluster, Abell 576.

Previous X-ray observations had hinted that the gas was not moving uniformly across the cluster. Using the superior sensitivity and spectral resolution of XMM-Newton and Chandra¹s high spatial resolution, Dupke took readings from two locations in the cluster and saw that there was a distinct difference in the velocity of the gas. One part of the cluster seemed to be moving away from us faster than the other.

The puzzle was that the moving gas itself was cold by astronomical standards. If this gas moved at such high speeds, it should have had a temperature of more than double the measured 50 million degrees Celsius. ³The only explanation was to take the Bullet Cluster and turn it in the line of sight, such that one galaxy cluster is directly behind the other² says Dupke.

The Bullet Cluster is a much-studied pair of galaxy clusters, which have collided head on. One has passed through the other, like a bullet travelling through an apple. In the Bullet Cluster, this is happening across our line of sight, so we can clearly see the two clusters.

Dupke realised that Abell 576 is also a collision, but seen head on, so one cluster is now almost directly behind the other. The Œcold¹ clouds of gas are the cores of each cluster, which have survived the initial collision but will eventually fall back together to become one.

The data reveals that the clusters have collided at a speed of over 3300 km/s. This is interesting because there are some computer models of colliding galaxy clusters that suggest that such a high speed is impossible to reach.

Nevertheless, the Bullet Cluster is estimated to have a collision speed similar to the Abell 576 system. ³There is now a growing body of evidence that these high collision velocities are possible,² says Dupke. The job of explaining these high speeds now rests with the cosmologists.

Major cluster-cluster collisions are expected to be rare, with estimates of their frequency ranging from less than one in a thousand clusters to one in a hundred. On collision, their internal gas is thrown out of equilibrium and if unrecognised, causes underestimation of its mass by between 5 and 20 percent.

This is important because the masses of the various galaxy clusters are used to estimate the cosmological parameters that describe how the Universe expands. So, identifying colliding systems is extremely important to our understanding of the Universe.

Dupke and colleagues are already investigating a number of other clusters that also appear to be interacting.