62 ASTRONOMY • DECEMBER 2021
ASK ASTRO Astronomy’s experts from around the globe answer your cosmic questions.
QI
WHAT KIND OF FUEL DO
SPACECRAFT USE TO EXPLORE
THE OUTER SOLAR SYSTEM AND
BEYOND?
Larry Owen
Jamul, CaliforniaAI
Sir Isaac Newton’s first law of motion states
that an object at rest stays at rest and an object
in motion stays in motion with the same speed unless
acted upon by an outside force. And in space, the only
thing to slow down a spacecraft is gravity.
B efore a spac ec r a f t i s e ven lau nched , sc ient i st s work
out precisely where planets and other bodies will be to
determine how the gravity of these objects will affect
the trajectory of the craft. This allows them to calculate
where to aim the craft, when to launch it, and how fast
it needs to be traveling to reach its destination.
NASA then uses a two-pronged approach to fuel the
launch. The rocket’s main engines use a combinationRocket fuel
of liquid hydrogen and liquid oxygen. Hydrogen has
the lowest molecular weight of any known substance,
making it ideal for keeping the weight of a rocket rela-
tively small. When combined with liquid oxygen,
hydrogen creates the most efficient thrust of any rocket
propellant.
But depending on the weight of the rocket, an extra
boost may be needed to get off Earth. In these cases,
solid propellent, usually aluminum, is combined with
a n ex plosive , such a s a m mon iu m perch lor ate , to propel
the rocket all the way into space.
But that’s just the start of the journey for a spacecraft.
For minor course corrections, small thrusters attached
to the craft fire off small puffs of gas, such as
hydrogen peroxide. Scientists can also plan for
gravitational assists from nearby planets
for an extra boost as the craft travels to
the far reaches of our solar system.
Spacecrafts also need a reliable
supply of electricity to report back
their findings. Crafts operatingABOVE: The twin
Voyager spacecrafts
launched in 1977 are
the most distant
human-made objects.
To help propel them
on their journey, the
crafts took advantage
of our solar system’s
largest planets. NASA/
JPL-CALTECH
ABOVE RIGHT: A
SpaceX Falcon 9
rocket lifts off from
the Kennedy Space
Center in Florida. NASA/
KEVIN O’CONNELL & KENNY ALLEN
Mercury’s Caloris Basin appears
on the upper right of the planet in
this false-color image assembled
using images taken by NASA’s
MESSENGER mission. NASA/JOHNS
HOPKINS UNIVERSITY APPLIED PHYSICS
LABORATORY/CARNEGIE INSTITUTION OF
WASHINGTON