The spacecraft was launched to test technology that could one day be needed to deflect a dangerous asteroid off course.
NASA's Dart mission will evaluate a long-standing proposal to neutralize a massive space rock headed toward Earth.
The spacecraft will collide with an object named Dimorphos to see how much its speed and trajectory can be changed.
If a piece of cosmic debris a few hundred meters wide were to collide with our planet, it could wreak havoc across a continent.
It is the first attempt to deflect an asteroid in order to find out how to protect the Earth, although this particular asteroid does not pose a threat to us.
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“The dart will only change the orbital period of the Dimorphos by a fraction. And that's really all that's needed in case the asteroid is detected well ahead of time," says Kelly Gast of NASA's Planetary Defense Coordination Office.
On Wednesday morning, the Falcon 9 rocket carrying the Dart spacecraft lifted off from the Vandenberg Space Base in California.
Asteroids are the waste building material of the solar system, and most of them do not pose a threat to our planet.
But when the path of a space rock around the Sun crosses that of the Earth, and these two objects cross at the same time, a collision could occur.
The $325 million Dart mission will target a pair of asteroids that closely orbit each other - known as binaries.
The larger of these two objects, named Didimos, is 780 meters wide, while its smaller companion - Dimorfos - is about 160 meters wide.
An object the size of Dimorphos could explode many times more powerfully than an ordinary nuclear bomb, destroying populated areas and killing tens of thousands of people.
Asteroids with a diameter of 300 meters or more can cause destruction across the continent, while those larger than a kilometer would cause consequences on a global scale.
After Dart is launched, it will first break free from Earth's gravity and then follow its own orbit around the Sun.
It will then intercept binary asteroids when they come within 2022 million miles of Earth in September 6,7.
The dart will hit the "moon" Dimorphos at a speed of about 6,6 kilometers per second.
This should alter the object's velocity by a fraction of a millimeter per second - in turn altering its orbit around Didymos.
That's a very small change, but it could be just enough to throw the object off a collision course with Earth.
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"There are a lot more small asteroids than there are large ones, and so the most likely threat we'll ever have to face -- if we have to at all -- is probably going to be in the form of asteroids around that size," said Tom Statler, mission program scientist at NASA.
In 2005, the US Congress ordered NASA to detect and track 90 percent of near-Earth asteroids larger than 140 meters.
No known asteroid of this category poses an immediate danger to Earth, but it is estimated that only 40 percent of these objects have actually been discovered.
Dart carries a camera called Draco that will send images of the asteroid and help steer the spacecraft in the right direction to collide with Dimorphos.
About ten days before the Dart hits its target, the American spacecraft will launch a small Italian-made satellite called LiciaCube.
The smaller spacecraft will send back images of the impact, the debris cloud and the resulting crater.
The tiny change in Dimorphos' path around Didimovs will be measured by telescopes from Earth.
Tom Statler commented:
"What we really want to know is: Did we really deflect the asteroid and how effectively did we do it?"
Binary asteroids are the perfect natural laboratory for such a test.
The collision should change Dimorphos' orbit around Didymos by about 1 percent, a change that can be registered by ground-based telescopes in the weeks or months afterward.
However, if Dart struck a lone asteroid, its orbital period around the Sun would change by about 0,000006 percent, which would take many years to measure.
The binary asteroid is so small that, even to the most powerful telescopes, it appears as a point of light.
However, Dimorphos blocks some of Didymos' reflected light as it passes in front of it, and the opposite happens when the smaller object moves behind its larger companion.
"We can measure the frequency of these eclipses," explained Dart research leader Andy Rivkin, adding: "So we know that Dimorphos orbits Didymos in a period of 11 hours and 55 minutes."
After the collision, astronomers will make those measurements again.
"It's going to happen a little more frequently—maybe two every 11 hours and 45 minutes, maybe one every 11 hours and 20 minutes," says Dr. Rivkin.
She works at the Applied Physics Laboratory at Johns Hopkins University in Laurel, Maryland.
He says there is some degree of uncertainty about how Dimorphos will react to a collision, in part because its internal structure is unknown.
If Dimorphos is relatively solid inside, instead of full of empty space, it could produce a lot of debris - which will give the object extra thrust.
Dart's method of dealing with a dangerous asteroid is known as the kinetic impact technique.
However, there are other ideas, such as deflecting the asteroid more slowly over a long period of time or even detonating a nuclear bomb - an option familiar from Hollywood films such as Armageddon and The Big Bang.
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