|Technologies of Broad Benefit: Propulsion
Propulsion technologies provide the energy to get to Mars and conduct long-term studies.
Propulsion Innovations for the Mars Exploration Rover Mission:
The Opportunity rover sits inside the fairing of this Boeing Delta II heavy rocket, awaiting its launch and seven-month journey to Mars.
Modifying a rocket to "pack a major punch"
The second Mars Exploration Rover, Opportunity, was the first NASA mission lifted on what is referred
to as a Delta II "heavy" rocket. It is a more powerful vehicle that uses larger
strap-on solid rocket motors developed for the Delta III launch vehicle. The launch of the
MER-B spacecraft was the second use of a core Delta II "Heavy" launch vehicle and the first
use of a Delta II "Heavy" launch vehicle with an upper stage.
The use of this rocket for the Mars Exploration Rover mission helped pave the way for many future
space missions that will need a bit more energy to send them on their way beyond Earth. The
MESSENGER mission to Mercury, the seventh mission selected for NASA's Discovery Program, was launched
August 3rd, 2004, on a Delta II heavy. [More on the Mars Exploration Rover launch vehicle.]
The traditional Delta II rocket (used to lift Spirit) has been used to propel many NASA missions into space and has a history of more than 40 successful launches, but some spacecraft need more energy to propel them on their way to Mars and other planets.
Reasons for the Differences
Every 26 months, Earth, Mars and the Sun align for the most efficient, least energy-consuming path between Earth and Mars.
(For advanced details, see at the JPL website Basics of Space Flight: Launch.)
This image shows the position of Earth and Mars at both launch and arrival of each rover. The green line shows the spacecraft's trajectory.
The amount of energy it took to launch each of the two Mars Exploration Rovers was slightly different because the distances
between Earth, Mars and the Sun changed in between the two rover launch periods. (Spirit's launch period fell between
May 30 and June 16, 2003. Opportunity's launch period was roughly a month later, between June 25 and July 12, 2003.)
The launch period for Spirit used close to the minimum energy to get to Mars in the 2003 opportunity, while Opportunity
needed more energy to get to Mars. The two vehicles could not be launched at the same time due to restrictions at the
launch site and the availability of supporting teams.
Improving precision navigation
Missions sent to Mars often have to conduct "trajectory correction maneuvers" to keep the
spacecraft on course throughout its 460 million kilometer (286 million mile) voyage to Mars.
With these propellant burns, navigators can change the spacecraft's velocity, move it sideways
or turn it. Navigators for Spirit put the spacecraft so close to a bull's-eye with earlier maneuvers
that mission managers chose to skip the final two optional trajectory correction maneuvers for
adjusting course before arrival at Mars. Likewise for Opportunity, only three trajectory correction
maneuvers were required to achieve its "interplanetary hole in one," into a crater that revealed
the first bedrock seen up-close on Mars.
Technologies associated with these propulsion maneuvers often involve better ways of measuring
where the spacecraft is in space so that engineers can determine the appropriate burns needed to
tweak the spacecraft's trajectory.
Like Pathfinder, the Mars Exploration Rover cruise stage had a star scanner and sun sensor that
allowed the spacecraft to know where it was in space by analyzing the position of the sun and other
stars in relation to itself. The same precision method that guided NASA's Mars Odyssey spacecraft
to within one kilometer (0.6 miles) of its target in martian orbit, was applied to the Mars
Exploration Rover mission. Navigators used traditional tracking methods like measuring the
time and shift in radio signals sent from Earth to the spacecraft.
The two traditional tracking methods, ranging and Doppler, were complemented by a newer method
called "delta differential one-way range measurement." It adds information about the location
of the spacecraft in directions perpendicular to the line of sight. Pairs of antennas at Deep
Space Network sites on two different continents simultaneously received signals from the
spacecraft, then used the same antennas to observe natural radio waves from a known celestial
reference point, such as a quasar. Successful use of this triangulation method can shave several
kilometers or miles off the amount of uncertainty in delivering the rovers to their targeted
landing sites. Opportunity landed about 24 kilometers (about 15 miles) down range from the center
of the target landing area. Spirit made it to within 10 kilometers (about 6 miles) of its intended