If all goes according to some very intricate planning, the first fireworks celebrating Independence Day will actually happen late night on Sunday the 3rd, at about 11 PM our time. It will happen about 83 million miles away, and you won’t be able to see it unless you have access to a telescope. But it will be big!

Here’s what’s up.

We all know that there are objects in the solar system besides our familiar sun and planets. For example, we have the asteroids, b-zillions of them littering the place. Most are between Mars and Jupiter, but some pass nearby and pose real threats to us.

But we also have our comet visitors, those long hairs from outer space. Far from being the portents of doom that they were to many of our ancestors we now know some things about them – but not everything, by far.

We know from where they probably come, we know how the tails form, we know basically what they are made of – well… kinda sorta. Their full chemistry and how they hold together remain mysteries. And knowing these objects well, these fossil remnants of the creation of the solar system, can help us understand how the whole place was put together more than 4.6 billion years ago.

But to know more about comet innards we have to throw something at it and break it. It’s a common thing in science, this sort of invasive surgery. Physicists throw subatomic particles at each other to see what they are made of; geologists crack open geodes to reveal the beauty within. Now astronomers will throw one fast moving projectile at a comet, Comet 9P/Tempel 1, to reveal the secrets within.

Twenty-four hours before the fireworks begin, NASA’s Deep Impact spacecraft, launched last January and headed for Temple 1 at a breakneck speed, will toss off a washer/dryer-sized projectile towards the 8-mile-across comet. This tiny projectile, weighing in at 820 pounds, will be traveling so fast, 23,000 miles an hour, about 6 miles per second, that when it hits it will be like lighting up 5 tons of TNT. No small explosion that.

The engineers working on it estimate that this tiny projectile will punch a crater into the comet bigger than a football field and about 50 meters deep.

Meanwhile, the main spacecraft will zip by the comet as it happens, recording the whole event from a safe distance.

So what? How can slamming something into a comet tell us its composition?

By studying the impact, scientists can estimate how hard the surface is. Is the comet layered through like earth, with a crust and core? Or is it a thrown together mess? Seeing the size of the impact and the depth of the crater will help these investigators figure that one out.

How far the blast debris (the “ejecta”) gets thrown depends on the gravity of the comet. So observing where the ejecta settle can help estimate density and possibly what stuff is deep down inside.

Analyzing the spectra of the light reflected by all the fireworks will help astronomers figure out what compounds make up the surface of old Temple 1.

And of course the images, close-up and finely detailed, will help determine perhaps how she’s put together at the surface.

But understand that no comets will be harmed in this experiment. One engineer compares the collision to a gnat striking the windshield of a Boeing 747. Only one fatality there.

We on the West Coast might – key word: might – be able to see it when it happens. It’s always a tough call predicting what might happen – there are so many variables – but some scientists predict that the impact might be bright enough to be seen with a backyard telescope. That’s worth breaking out the scope for backyard astronomers like myself.

If interested in the details of when, where, and how, see http://deepimpact.jpl.nasa.gov.