70 million years ago, scientists believe an enormous cloud of gas dubbed, Smith Cloud, was launched from the outer reaches of the Milky Way Galaxy. Today, astronomers using the Hubble Space Telescope see the cloud plunging back home at nearly 700,000 miles per hour.

Smith Cloud today

Smith Cloud, itself, isn’t special. There are countless gas clouds shuffling around the outskirts of our galaxy. But, what makes Smith Cloud special is that we know all about its trajectory. In the early 1960s, Gail Smith, an astronomy student, spotted the cloud by detecting radio waves emitted by hydrogen. Since then, astronomers have taken countless measurements of the gigantic blob of gas.

We know it’s 11,000 light-years long by 2,500 light-years across. I know, it’s hard to wrap your head around those dimensions. Let’s look at it another way. If we could see Smith Cloud in the night’s sky, it would have an apparent diameter 30 times the size of the full moon.

Smith Cloud moon

“The cloud is an example of how the galaxy is changing with time,” explained team leader Andrew Fox of the Space Telescope Science Institute in Baltimore, Maryland. “It’s telling us that the Milky Way is a bubbling, very active place where gas can be thrown out of one part of the disk and then return back down into another.”

By looking at Smith Cloud, Fox and other astronomers can get an idea of how active he disks of other galaxies are.

How the Hubble Space Telescope figured out where Smith Cloud came from

Fox and his colleagues say Smith Cloud is a runaway from the Milky Way Galaxy, but how did they figure it out? As Neil Degrasse Tyson says, it’s all about the star stuff. More specifically, what kind of star stuff is inside Smith Cloud.

How Scientists Measured the Universe’s New Expansion Rate

For a while, astronomers believed Smith Cloud might have been a failed, starless galaxy. Or, just a random blob of gas coming towards the Milky Way from the vastness of intergalactic space. For these two scenarios to work, the gas cloud would be made up of mostly hydrogen and helium.

The Hubble Space Telescope helped Fox and his team figure out the chemical makeup of Smith Cloud. How? By looking behind it. Hubble observed three active galaxies billions of light-years beyond the cloud. Then, Hubble’s Cosmic Origins Spectrograph measured how the light filtered through the cloud.

Smith Cloud sulfur

Sulfur ended up being the dead giveaway. “By measuring sulfur, you can learn how enriched in sulfur atoms the cloud is compared to the sun,” said Fox. Put simply, the presence of sulfur would mean Smith Cloud was enriched by material from stars. If all astronomers found was hydrogen, that would indicate the cloud was either a failed galaxy or came from outside the Milky Way.

The levels of sulfur in Smith Cloud match levels seen in Milky Way’s outer disk.

That settles that. Smith Cloud is coming back home. It’s screaming back towards the Milky Way at 700,000 miles per hour, but astronomers estimate it’s still about 30 million years away. What happens when it collides with the Milky Way? Scientists are predicting an incredible burst of star formation of up to 2 million suns.

Smith Cloud future

Hubble’s Cosmic Origins Spectrograph

Also called COS, here’s what it looks like.

Hubble COS

Did you know it didn’t launch with the Hubble Space Telescope? It was installed during the fifth and final space shuttle mission to the telescope in May 2009. Hubble’s Wide Field Camera 3 also made the trip up and was installed.

Pointing the Hubble Telescope - When Will It Finally Pass the Torch

COS isn’t responsible for capturing the breathtaking images the Hubble is known for. But, it plays a pivotal role in helping scientists understand how galaxies, stars and even planets form and evolve.


There are still many lingering questions about Smith Cloud. Mainly, what happened? Why did the cloud get ejected in the first place? If some cataclysmic event caused the ejection, how did the cloud hold together? As with most things in astronomy, answers often lead to more questions.

Image credits: NASA/ESA

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