Sunday, April 17, 2016

solar system - Why is Voyager 1 approaching Earth?



When I read JPL's mission status for Voyager 1 and Voyager 2 the distance between Earth and Voyager 1 is decreasing.


Is it right?



Answer




Voyager $1$ is headed away from the Sun at around $17$ km per second at an angle to the ecliptic of around $35$º.


The orbital velocity of the Earth is $29.8$ km per second, and multiplying by $\cos 35$º to get the component of the velocity in Voyager's direction gives a shade over $24$ km per second.


So at the point in its orbit where Earth is moving closest to the direction of Voyager's travel we are actually catching up with it at around $24 - 17 = 7$ km per second. Conversely at the point in Earth's orbit where it is moving opposite to Voyager it is moving away from us at $24 + 17 = 41$ km per second.


Voyager relative speed


(not to scale!)


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