By Jeffrey Kluger
December 21, 2015

Planets are a little like dogs: They all belong to the same general group, but they look so different you can’t always tell. That’s especially true of the gas giants in the outer solar system, like Jupiter, and the lapdogs closer in, like Mars. This year, two NASA spacecraft going to those two planets will help answer the question of how such different kinds of celestial bodies came to be.

The first ship to get to work will be Juno, the Jupiter probe, which was launched in 2011 and will arrive at its destination on July 4, 2016, swinging into orbit for a 20-month stay. While there, it will study Jupiter’s gravity and magnetic fields, the composition of its colorful atmosphere and its brilliant polar auroras. It will also seek to determine if Jupiter does or does not have a solid core.

Leaving Earth sometime in March 2016 will be the Mars InSight probe, which will land on the Red Planet in September. InSight will not be a rover; its most important experiment will be to drive a probe 16 ft. (5 m) into the Martian interior, studying heat flow and thermal history. InSight’s other instruments will measure seismic activity and the faint wobbles caused by the sun’s gravity as the planet glides through its orbits–providing more clues to Mars’ makeup and past.

Both spacecraft will meet poignant ends. InSight will operate for 728 Earth days before powering down forever. Juno will be deliberately deorbited into Jupiter’s clouds, preventing it from crashing instead on a Jovian moon and potentially contaminating it with bacteria from Earth.

Both ships will have spent their brief lives well.

Write to Jeffrey Kluger at jeffrey.kluger@time.com.

This appears in the December 28, 2015 issue of TIME.

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