"Where you used to be, there is a hole in the world, which I find myself constantly walking around in the daytime, and falling in at night. I miss you like hell." -Edna St. Vincent Millay

It was just a little while ago that we were all speculating wildly -- and optimistically -- about Comet ISON, as it plunged towards the Sun from its origins in the very, very distant Solar System. As its perihelion date (the moment of closest approach to the Sun) drew near, you may have noticed something interesting about photos of the comet: it's tail appeared to get longer and longer!

Image credit & copyright: All rights reserved by Joseph Brimacombe, via flickr.

If you compare this to earlier photos, you might think it had something to do with flying too close to the Sun, à la the Icarus and Daedalus myth. But it has a lot more to do with the physics of gravity than with anything melting due to heat!

Don't believe me? Take a look at this video, captured by NASA and ESA's great solar observatories, and pay particular attention to the speed of the comet.

Did you notice anything different about the speed of the comet (or what's left of it) at different times in the video? If you're looking at the same thing I am, it might appear that it moves somewhat slowly as it approaches the Sun, speeds up, and then slows down again as it moves away.

Is this a trick of the angle from which we view the comet?

Image credit: National Air and Space Museum, Smithsonian Institution.

Not a chance; that's just gravity. You see, you might be familiar with escape velocity, or the speed at which you'd need to travel at in order to escape the gravitational pull of a massive, centrally located object.

From the surface of the Earth, that's something like 25,000 miles-per-hour (40,000 km/hour).

Image credit: Pearson / Prentice Hall.

But a lesser-known application of the same physical laws and properties tells us that if you dropped an object from almost zero velocity in the vicinity of that same gravitationally massive source, it will fall towards that center-of-mass, reaching that exact escape velocity if it collides with the surface of the massive object dominating the system.

For something falling into the Sun from rest an arbitrary distance away, it would be moving at a whopping 617 km/s, or about 0.2% the speed of light, when it hits the Sun's surface. Comet ISON didn't quite get there, but it did get all the way up to 377 km/s, or about 12 times as fast as the Earth orbits the Sun. In general, the closer an object operating only under the influence of gravity gets to the center-of-mass, the faster it moves; Kepler's famous second law is a special case of this.

Image credit: Wikimedia Commons user Gonfer.

But the Sun is hardly the most massive thing we know of, and speeds well in excess of this occur naturally, for huge astrophysical systems, all the time.

Consider a typical, Milky-Way sized galaxy; for our Solar System to escape from it, we'd need to achieve a speed of about 550 km/s, and we're already some 25,000 light-years from the galactic center! That means if we dropped an object -- say a much smaller galaxy -- from an arbitrary distance away, it would be moving at about 550 km/s when it collided with us.

More spectacularly, however, two massive galaxies attract each other in the depths of space, creating a trainwreck with cosmic motions on the order of double that -- or at around 1,000 km/s -- when they finally collide!

Image credit: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Evans (University of Virginia, Charlottesville/NRAO/Stony Brook University). (Click for an insane version, if you dare!)

This is, in fact, the future fate of our own galaxy; our somewhat bigger sister, Andromeda, is moving towards us at around 40 km/s, from a gigantic distance of a little more than 2 million light years away. By time we meet in a few billion years, the two galaxies will move at a maximum speed approaching that figure -- 1,000 km/s -- relative to one another!

Butgalaxies merging like this is hardly large or unusual on the scale of the cosmos.

Much more interesting is when we have large collections of galaxies smashing into one another all at the same time! Two spectacular examples are Seyfert's Sextet,

Image credit: Hubble Legacy Archive, NASA, ESA; Processing: Judy Schmidt.

and Stephan's Quintet,

Image Credits: X-ray: NASA/CXC/CfA/E. O'Sullivan Optical: Canada-France-Hawaii-Telescope/Coelum.

both of which will have their constituent galaxies travel in speeds exceeding that value, relative to one another!

But why settle for groups of five-or-six galaxies, when we have clusters containing thousands?

The Virgo Cluster, shown here, has galaxies whizzing about at around a full 1% the speed of light relative to one another, and many of these galaxies are huge, giant ellipticals some ten times (or more!) the mass of our Milky Way!

You think that's big?

Now, imagine two of these giant clusters falling into and colliding with each other!



Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al.

Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al. Image credit: X-ray: NASA/CXC/CfA/ M.Markevitch et al.;Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al.Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.

That's what we've got for the Bullet Cluster (above), cluster Abell 520 (below),

Image credit: NASA, ESA, CFHT, CXO, M.J. Jee (University of California, Davis), and A. Mahdavi (San Francisco State University).

and cluster MACSJ0025 (at bottom), among many others. These composite images show the individual galaxies in the optical, dark matter (a proxy for total mass) in blue, and shocked, hot X-ray gas in pink (or green, for Abell 520).

Image credit: NASA, ESA, CXC, M. Bradac (University of California, Santa Barbara), and S. Allen (Stanford University).

With maximum relative speeds reaching 4,000 km/s, or around 1.4% the speed of light, these are the largest-scale, fastest-moving giant objects in the Universe, and as our telescopic reach extends farther and farther back, we're only finding progressively more impressive ones! Dark matter plays a huge role here, increasing the hefty masses of these objects by a factor of five or so over what they'd weigh if they were made of normal matter alone, and the highest speeds we see are raised by a factor of about 140% as a result!

So that's a little glimpse at the greatest cosmic trainwrecks in the Universe, at speeds you might never have imagined for something so large!