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We
are going...
NASA reserachers
and engineers cannot identify all that we will gain from
space exploration in the future; however, the return on
our investment will be great because the Vision for Space
Exploration states as one of the goals: "...to advance
U.S. scientific, security, and economic interests through
a robust space exploration program."
In the past,
NASA's human space flight and robotic exploration programs
have largely oeprated independently of each other.
However, this is quickly changing. Coordination
and integration will be an essential part of the future
of exploration.
Over the next
three decades, NASA will send robotic missions to the
Moon, Mars, the moons of Jupiter, and other planetary
bodies in the outer solar system. These robots will
serve as counterparts to human explorers by going where
humans cannot go providing an extra set of "hands
and eyes". These robotic explorers will visit
new worlds to obtain scientific data, demnonstrate technology
capabilities, identify space resources, and gather critical
information to maintaining health and productivity of
human explorers. They will also serve as testbeds
for developing and testing technologies that will eventually
carry human explorers beyond low Earth orbit.
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| When
do we go?
The distance between Mars and
Earth varies because, like all planets, they have elliptical
(oval) orbits. Earth's orbit is only slightly elliptical;
Mars's orbit is more so. Each planet also takes a different
amount of time to travel round its orbit: 365 days for
Earth and 687 Earth-days for Mars. Think of the two planets
as bicycles racing on a track. Earth is traveling faster
than Mars on the inside track, so it will periodically
catch up and overtake it. When Earth is on the point of
overtaking Mars, the two planets are lined up with the
Sun. Click
here for an interactive shockwave simulation
showing the two orbits.
When the planets
are lined up, it is called an opposition
because, as seen from Earth, Mars is opposite the Sun
in the sky. Mars oppositions occur approximately once
every 780 days (or about 2 Earth years). These are optimal
times to travel to Mars because the planets are closer
together. They are also good times to view Mars from the
Earth because all of Mars illuminated side faces us.
Mars 2003 perihelion opposition
Click to enlarge. |
Mars 2010 aphelion opposition
Click
to enlarge. |
If opposition
occurs when Mars is at its closest proximity to the Sun
(a position on its elliptical orbit called perihelion),
the distance between the two planets will be a minimum
– about 55 million kilometers. When Mars is at its
farthest point from the Sun (called aphelion),
the distance at opposition will be about 99 million kilometers.
A spacecraft to Mars can be launched around any opposition,
about once every 26 months, but the journey will be shortest
and use the least fuel around a perihelic opposition,
which occurs about once every 17 years. The next time
this will happen is 2020.
How
do we go?
We use rockets
or launch vehicles to boost robotic spacecraft
into orbit around the Earth. The spacecraft then uses
its own rocket engine to lift it out of Earth orbit and
to send it on its way to Mars. This is called the trans-Mars
injection burn or TMI.
Current robotic spacecraft use chemical fuels for propulsion
and solar or nuclear power for electrical energy. Radio
communications between the spacecraft computer and computers
on Earth help to track and guide the spacecraft.
So far the U.S.
has only sent spacecraft on one-way journeys into the
solar system. The Stardust
mission to a comet includes a sample return spacecraft.
The projected Mars Sample Return missions would also bring
spacecraft (and samples) back to Earth.
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How
long does it take?
Spacecraft
traveling through the solar system start out at
relatively fast speeds (if they are moving away
from the Sun) and slow down during flight. As they
begin to enter into the gravitational pull of the
target body, they begin to speed up again. Course
corrections during the mission generally do not
affect overall travel speeds. Many missions to the
outer planets use the slingshot approach to help
speed up. This means they use the gravitational
force of a passing planet to pull them past and
around and send them off at higher speeds. Trips
to the planet Mars would not use this method, however.
The travel times of the missions that have been
sent to Mars have varied slightly depending on where
the Earth and Mars are in their respective orbits.
Most missions have taken spacecraft from six to
twelve months to reach Mars.
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What
are the risks?
| All
space missions, including those without humans on
board, entail a variety of risks. There are risks
to the spacecraft systems caused by external forces
and internal mechanisms. Spacecraft can be affected
by cosmic and solar radiation, micrometeorite and
meteorite damage, and a vast array of problems connected
to the complexity of the engineering designs and
requirements of the spacecraft itself. Robotic spacecraft,
which are extremely complex, are composed of many
delicate systems including propulsion, communications,
guidance and payload (the experiments and instruments
onboard the spacecraft). Humans, who are not without
fault, design these spacecraft and errors do occur.
NASA has a policy of redundancy that requires backup
systems for all mechanical functions on a spacecraft.
This can help ensure the success of the mission.
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Questions
to think about:
- If you were planning a series of missions to Mars,
how would you go about planning your timeline?
- In planning missions to other planets, how can you
justify the importance of exploration versus the high
cost of sending missions that might fail?
- What kinds of measures can you take to help ensure
spacecraft safety and success?
Next... Mariner
Missions (pg. 3 of 12) |
