The rovers were named Spirit and Opportunity. The names were the winning entries in a naming contest NASA held in 2002. The winning entry came from a third-grade girl living in Scottsdale, Arizona. She was born in Russia and adopted by an American family. She chose the names to honor her feelings about America.
NASA adopted seven specific objectives for the MER missions:
- Find and sample rocks and soils that could reveal evidence of past water on the planet
- Characterize the composition of rocks, soils, and minerals near the landing sites
- Look for evidence of geological processes (such as erosion or volcanic activity) that could have shaped the Martian surface
- Use the rovers to verify data reported by the orbiters regarding Martian geology
- Probe for minerals containing iron or water or minerals known to form in water
- Analyze rocks and soils to characterize their mineral content and morphology (form and structure)
- Seek out clues about the geological history of the planet to determine whether watery conditions could have supported life
FIGURE 7.5
Spirit's mission trajectory
The Launches
Spirit launched first on June 10, 2003. Opportunity launched several weeks later on July 7, 2003. The launch dates were chosen to put the spacecraft in flight near the time of Mars' perihelic opposition.
Both spacecraft were launched atop Delta II rockets from Cape Canaveral Air Force Station in Florida. Figure 7.4 shows a drawing of a Rover spacecraft being released by its rocket to make the journey to Mars.
The Flight Trajectories
The flight trajectories for the MERs were chosen to take advantage of the perihelic opposition configuration of the Sun, Earth, and Mars.
Figure 7.5 shows the flight trajectory for the Spirit spacecraft. The Sun is pictured at the middle of the diagram.
FIGURE 7.6
Opportunity's mission trajectory
Earth's orbit is the innermost circle, while Mars' orbit is the outer circle. Both planets travel in a counterclockwise direction around the Sun. The figure shows the position of the Earth and Mars at the time of Spirit's launch and its arrival at Mars.
The Earth's path around the Sun is almost a perfect circle. Mars' path is more elliptical. The Sun does not sit at the center of the ellipse, but is offset to one side. The Earth travels around the Sun in 365.25 days. It takes Mars nearly twice that long to make an orbit (687 days). This is why Mars did not move as far around the Sun as the Earth during Spirit's journey.
Figure 7.6 shows the flight trajectory for the Opportunity spacecraft. Both spacecraft were subjected to occasional flight maneuvers along the way to keep them on their path to intercept Mars in January 2004.
Notice that the MER missions took place when the paths of Mars and the Earth were relatively close to each other. The spacecraft would have taken longer and had to travel farther and use more fuel if scientists had timed them to occur when the orbital paths of Mars and Earth were farther apart.
Landing on Mars
Figure 7.7 shows the various parts of the spacecraft that traveled to Mars. Each rover was nestled inside a landing vehicle protected by an aeroshell connected to the cruise
FIGURE 7.7
Mars Exploration Rover flight system
stage of the spacecraft. The cruise stage contained fuel tanks, solar panels, and the propulsion system for trajectory corrections during flight. The aeroshell included two parts, a back shell and a heat shield. The back shell carried a deceleration instrument to ensure that the parachute was deployed at the right altitude above the Martian surface. It
FIGURE 7.8
Mars Exploration Rover entry, descent, and landing
also had some small rockets to stabilize the spacecraft as it fell. The heat shield protected the lander/rover package from the heat generated by entering the Martian atmosphere.
The stages of entry, descent, and landing are shown in Figure 7.8. At twenty-one minutes before landing (L-21 min) the cruise stage separated from the rest of the spacecraft. Fifteen minutes later the spacecraft entered the atmosphere about seventy-four miles above the surface. The parachute deployed at an altitude of five miles when the craft was traveling nearly 300 miles per hour. Seconds later the heat shield was jettisoned away. Eight seconds before hitting the ground the spacecraft deployed its air bags to cushion its impact with the ground. Retro-rockets were fired to slow its descent. Three seconds later the parachute line was cut. The spacecraft ball bounced and rolled until it finally came to a stop. About an hour after landing the airbags were deflated and retracted so the lander could open its petal and release the rover.
On January 4, 2004, the Spirit MER landed on Mars. It was just after 8:30 p.m. at the mission control center in California. The landing site was in a crater named Gusev Crater in honor of the Russian astronomer Matvei Gusev (1826–1866). The crater is about 100 miles in diameter and lies at the end of a long valley known as Ma'adim Vallis. This translates as Mars Valley, because Ma'adim is the Hebrew word for Mars. Major valleys on the Red Planet are named for the word Mars in different Earth languages.
On January 25, 2004, Opportunity set down near Mars' equator in an area called Meridiani Planum. Planum means plateau or high plains. The Meridiani Planum is considered the site of Mars zero longitude. This is the longitude arbitrarily selected by astrogeologists to be the prime meridian for the rest of the planet. Opportunity's landing site was nearly half way around Mars from Gusev Crater.
FIGURE 7.9
Mars Exploration Rover
Both landing sites were chosen for their very flat terrain. Gusev Crater is of interest to scientists, because they believe it could be a dried up lakebed. The Meridiani Planum is thought to contain a layer of hematite beneath the surface. Hematite is a grey iron ore mineral similar to red rust that on Earth usually only forms in a wet environment. Both landing sites were considered prime locations to look for evidence of ancient water.
Roving Spirit and Opportunity
The components of an MER rover are labeled in Figure 7.9. The rovers are just over five feet long. The panoramic cameras sit about five feet above the ground atop a mast.
Each rover weighed about 380 pounds on Earth and carried a package of science instruments called an Athena science payload. Each payload includes two survey instruments, three instruments for close-up investigation of rocks, and a tool for scraping off the outer layer of rocks.
The rovers were designed to operate independently of their landers. Each rover carries its own telecommunications equipment, camera, and computer. The electronic equipment received power from batteries that were repeatedly recharged by solar arrays. It was late summer on Mars when the rovers began their mission. Scientists expected that power generation would continue for at least 90 sols (or 92 Earth days) before the arrays became too dust-coated to harness solar power. A seasonal change (from summer to autumn) was also expected to limit the effectiveness of the solar arrays as time went by. The Rovers were designed to move at a top speed of two inches per second. An average speed of 0.4 inches per second was expected when a rover was traveling over rougher terrain.
Water and Blueberries
On March 2, 2004, NASA scientists announced that the Opportunity rover had uncovered strong evidence that its landing area Meridiani Planum was "soaking wet" in the past.
The claim was based on examination of the chemical composition and structure of rocks found in an outcrop in the area. The rocks contained minerals, such as sulfate salts, known to form in watery areas on Earth. The rocks also had niches in which crystals appear to have grown in the past. These empty niches are called vugs, and are a strong indicator that the rocks sat in water for some time. Finally, there are round particles embedded in the rock that are about the size of ball bearings. Scientists have nicknamed them blueberries. The way that the "blueberries" are embedded in the rocks hints that water acted against the rocks in the past.
Mission Costs
The total cost of the MER missions was estimated at $825 million. Each spacecraft cost about $325 million to develop, build, and equip with scientific instruments. Another $100 million was spent launching the spacecraft. About $75 million was devoted to operations and science costs.
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