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
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's 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. Earth's orbit is the innermost circle, while Mars's 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's 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 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 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
FIGURE 7.4 Mars Exploration Rover spacecraft released by Delta II rocket
On January 4, 2004, the Spirit MER landed on Mars. It was just after 8:30 PM 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 (1826–66). 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's equator in an area called Meridiani Planum. Planum means plateau or high plains. The Meridiani Planum is considered the site of zero longitude on Mars. 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.
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 gray 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.
FIGURE 7.5 Spirit's mission trajectory
Roving Spirit and Opportunity
The components of an MER 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 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.
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 taper off after about ninety sols (or ninety-two Earth days) and eventually stop as the arrays became too dust-coated to harness solar power. However, scientists were pleasantly surprised when dust devils kept sweeping by the rovers and blowing the dust off the arrays. These periodic cleanings have allowed the rovers to keep operating for much longer than expected.
FIGURE 7.6 Opportunity's mission trajectory
The rovers completed their prime missions in April 2004. Since that time they have investigated dozens of additional sites. In May 2005 the Opportunity rover became stuck in a small sand dune when its wheels sank into soft sand and could not gain traction. Scientists worked for nearly five weeks to maneuver the rover back onto more solid ground. In September 2005 NASA reported that the Spirit rover had reached the summit of a Martian hill nearly 350 feet higher than where the rover landed. The hill was tentatively named "Husband Hill" in honor of Rick Husband, the commander of the doomed space shuttle Columbia. Scientists used the panoramic pictures captured from this vantage point to map out future exploration routes for the rover.
As of 2006 the rovers were still operating after a full Martian year (687 Earth days) in service. Each rover had traveled more than three miles from its original landing site. NASA scientists believe the rovers will operate indefinitely as long as their solar arrays continue to be cleaned by Mars's dust devils. Thus far, the planet has not experienced a global-wide dust storm during the MER missions. However, such storms do occur on Mars and could render the rovers inoperable.
The Name Game
Only the IAU has the authority to assign official names to planetary features. Major features, such as mountains, valleys, and large craters, have already been
FIGURE 7.7 Mars Exploration Rover flight system
FIGURE 7.8 Mars Exploration Rover entry, descent, and landing
Most of the names are picked arbitrarily by whatever scientist first views an incoming image. Features are named after people, places, sailing ships, or other things the scientist fancies. The Opportunity rover landed within a tiny crater dubbed "Eagle Crater" in honor of the Apollo 11 spacecraft that carried the first men to Earth's moon. When the Spirit rover landed in January 2004 it captured images of seven hilltops about two miles in the distance. Scientists dubbed them the "Columbia Hills" in honor of the seven shuttle Columbia astronauts that perished during 2003. Each hill was named after one of the astronauts. NASA hopes that the IAU will choose to make these names official.
Water and Blueberries
On March 2, 2004, NASA scientists announced that the Opportunity rover had uncovered strong evidence that its landing area Meridiani Planum had been "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 iron-rich composition of the "blueberries" and the way they are embedded in the rocks hints that water acted against the rocks in the past.
In November 2005 NASA published a series of reports in Earth and Planetary Science Letters detailing the latest findings from the MER rover Opportunity. Scientists believe that ancient conditions in the Meridiani Planum region were "strongly acidic, oxidizing, and sometimes wet." These harsh conditions are considered
FIGURE 7.9 Mars Exploration Rover
Mission Costs
The total cost of the MER missions has been 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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