Mars, the fourth planet from the Sun, has always attracted the attention of mankind. The first records of it appeared 3,500 years ago in Babylon. The ancient Egyptians and Greeks built a simple mathematical model predicting its movement, and the Romans named it after the god of war because of its characteristic red color. In 1610, Galileo Galilei was the first to observe its surface through a telescope, and subsequent research in the 18th and 19th centuries made it possible to compile a fairly detailed map of the planet.
But a huge leap in research came with humanity's conquest of space: since the 1960s, the US and the USSR have sent more than one automatic interplanetary station to Mars. Today we will talk about one of the most iconic missions to study Mars — Mars Pathfinder.
What happened before the Mars Pathfinder mission
On October 4, 1957, the USSR launched the first artificial Earth satellite with the telling name Sputnik-1. Two years later, on December 10, 1959, Government Decree No. 1388-618 “On the Development of Space Exploration” was issued, which, among other things, prescribed the organization of research into the planets of the Solar System closest to Earth — Venus and Mars.
The 1M program was responsible for flights to the red planet. As part of this program, OKB-1 developed automatic interplanetary stations (AIS) that were to reach Mars, enter orbit, and explore the planet in as much detail as possible: take pictures of its surface, study the ionosphere and magnetosphere, and try to detect living organisms.
The first spacecraft was Mars 1960A (NASA calls it Marsnik-1, a combination of the words Mars and Sputnik) weighing 640 kg, which had various sensors on board: a magnetometer, a GCL recorder, a spectral reflectometer, a radiometer, and a camera. The data would be sent using a radio transmitter operating in the decimeter range. A four-stage 8K78 Molniya carrier rocket was responsible for launching it into orbit.
Development took two years. The launch took place on October 10, 1960, and was unsuccessful: exactly five minutes into the flight, vibration in the second stage damaged the gyroscope, causing the rocket to deviate from its course. Ten seconds later, the engines shut down in an emergency: the rocket had reached an altitude of 120 km.
Four days later, on October 14, 1960, the second spacecraft, Mars 1960B, was launched. It suffered the same fate — it managed to climb to an altitude of 120 km, but fell due to a liquid oxygen leak. To understand, if everything had gone well, the AMS would have reached Mars' orbit on May 15, 1961, after six months of flight.
The next milestone in the attempt to reach Mars was the Mars program, which developed the second generation (2MV) of Soviet AMS. On October 24, 1962, the Mars 1962A spacecraft (NASA calls it “Sputnik-22”), weighing 893 kg, also failed to reach its destination: the last fourth stage of the 8K78 rocket exploded. It is noteworthy that the accident caused panic in the US Department of Defense: the launch took place at the height of the Caribbean crisis, and the debris from the AMS was mistaken for an intercontinental ballistic missile. But everything turned out fine.
The next launch of the Mars-1 spacecraft on November 4, 1962, was successful — this is one of the reasons why it was assigned a full serial number. The spacecraft became the first in the history of the AMS to successfully enter the calculated trajectory to Mars. However, a day later, a leak was discovered in one of the valves responsible for the spacecraft's nitrogen-based orientation system. When the entire gas supply was depleted, Mars 1 continued its uncontrolled flight, making it impossible to enter the planet's orbit.
At the same time, the spacecraft continued to transmit data: every two days during the first month, and then every five days. A total of 61 communication sessions with Earth were established, during which valuable scientific data was transmitted, including measurements of cosmic radiation, radiation background, meteor stream density, and much more. On March 21, 1963, contact with AMS was finally lost — at that moment, the distance to Earth was 106 million km, which was a record for communication range.
In 1962, NASA joined the race to explore Mars. As part of the Mariner program, the Mariner 3 spacecraft was developed — the first two spacecraft were intended for the exploration of Venus. AMS weighed significantly less than the Soviet stations — 261 kg — and was a modified Ranger lunar probe. On board were a television camera, a magnetometer, and detectors for cosmic rays, Van Allen belt particles, and cosmic dust.
The Atlas-Agena two-stage launch vehicle was responsible for bringing the spacecraft onto its calculated trajectory. The launch took place on November 5, 1964, but was unsuccessful — the fairing did not separate after passing through the layers of the atmosphere. As a result, the spacecraft's speed did not match the calculated speed, and the solar panels did not open, meaning that power was not supplied to any of the systems. Nine hours later, contact with the spacecraft was lost.
On November 28, 1964, an identical spacecraft, Mariner 4, was launched. It went completely according to plan, as the engineers had managed to refine the fairing separation mechanism: the spacecraft successfully entered its calculated trajectory. The star Canopus, the second brightest after Sirius, was used as a reference point for course correction. After 7.5 months of flight, on July 14-15, 1965, Mariner 4 entered Mars' flyby orbit: while circling the planet, it took 21 photographs and transmitted them to Earth from a distance of 216 million km.
After that, the spacecraft began to move away from Mars, continuing to transmit important scientific data. On December 21, 1967, contact with the spacecraft was lost. In addition to the images, Mariner 4 allowed scientists to assess the high level of radiation and confirmed the absence of the planet's magnetic field. This mission was the first successful one in history.
After that, the US launched two more spacecraft: Mariner 6 flew over Mars on July 31, 1969, and Mariner 7 on August 4. Both spacecraft took 143 images of significantly better quality than Mariner 4, from a closer distance above the surface — about 3,500 km versus 9,800 km. In addition, they studied the gas composition of the planet's atmosphere using spectroscopic methods and determined the surface temperature from infrared radiation measurements.
After the Mariner 8 spacecraft crashed during launch on May 9, 1971, NASA launched Mariner 9 on May 30 of the same year — and this spacecraft not only flew around Mars, but also became its artificial satellite. It carried 63 kg of the most advanced instruments on board: wide- and narrow-angle cameras, an infrared radiometer, and ultraviolet and infrared interferometric spectrometers.
On November 14, 1971, it entered Mars orbit and began its scientific mission. On February 14, 1972, NASA announced the completion of the program, and communication with the AMS ceased on October 27, 1972. In just 349 days, the spacecraft transmitted 7,329 images, which made it possible to map 85% of the surface of Mars.
The USSR tried not to fall behind. After the failures of the Zond-2 spacecraft in 1964, Mars 1969A, Mars 1969B, and Mars 1971C (the number corresponds to the year of launch), the fourth generation of USSR space program to conquer the red planet, Mars-2 and Mars-3, made successful flights. The development was carried out by the Lavochkin Research and Production Association.
The spacecraft had a unique navigation system — course corrections during flight were made automatically without commands from Earth. There were many measuring instruments on board: an infrared radiometer, a photometer for determining the concentration of water vapor, a sensor for detecting hydrogen in the upper layers of the atmosphere (Lyman-alpha line), and so on.
But the main feature of the spacecraft was that they consisted of two modules: one was designed to operate in orbit, and the other was a landing module. They also carried the first Mars rovers in history, PrOP-M, short for “Passability Estimating Vehicle for Mars,” weighing 4.5 kg. They were to be powered by the landing module via a 15-meter cable and move across the surface using original walking robots, independently determining the distance to obstacles. On board, there was a penetrometer and a gamma-ray densitometer for studying the Martian soil.
Mars 2 was launched on May 19, 1971, and Mars 3 a week later. Both spacecraft entered orbit, but problems arose during the descent of the landing module. In the first case, the module entered the Martian atmosphere on November 27, 1971, but due to an incorrect entry angle, it failed to slow down and crashed into the surface at a speed of about 6 km/s. The second module landed successfully and even began transmitting images, but after 15 seconds, transmission ceased due to a severe dust storm. From December 1971 to March 1972, the orbital modules transmitted about 60 images, as well as data on temperature, atmospheric composition, and pressure. On August 22, both spacecraft were shut down after they ran out of nitrogen, which was used for orientation.
In 1973, the USSR went even further and launched a more ambitious project—the simultaneous operation of four AMSs to study the red planet. The scientific objectives were similar to those of previous expeditions. Two spacecraft, Mars-4 and Mars-5, were designed to be placed in orbit, while Mars-6 and Mars-7 were designed to land on the surface. All spacecraft were successfully launched in July and August 1973 using four-stage Proton-K carrier rockets.
But there were some setbacks:
- Mars-4 reached Mars on February 10, 1974, and even got within 2,000 km of it, but one of the braking engines failed to ignite, causing the spacecraft to fly past the planet, managing to transmit several images in the process.
- Mars 5 entered orbit on February 12, 1974. However, due to the depressurization of the compartment with scientific instruments, it became clear that the spacecraft would only be able to operate for a couple of weeks. On February 28, it stopped communicating, having managed to transmit 40 images of the planet.
- Mars 6, like Mars 7, consisted of two parts: a flyby module and a descent module. On board were a panoramic telephotometer, sensors for temperature, pressure, density, and so on. The Mars 6 landing module separated on March 12, 1974, at an altitude of 48,000 km and landed successfully, but due to damage, it only transmitted data for a few minutes.
- Mars 7 flew past Mars on March 9, 1974, but due to errors in the calculation of the systems, the landing module separated earlier and flew past the planet's surface.
NASA decided to use the same concept as the USSR and launched the Viking project. The goal was to send two AMSs to Mars, each consisting of an orbital module and a landing module.
The Viking-1 and Viking-2 spacecraft weighed 3,500 kg, including 1,400 kg of fuel. The orbital module carried two cameras that could take pictures with a resolution of 40 meters at an altitude of 1,500 km, as well as spectrometers for analyzing the atmosphere and surface temperatures. The landing module contained many more sensors, including a seismometer, a mass spectrometer for soil, and a gas chromatograph.
Viking 1 and Viking 2 were launched by Titan 3E carrier rockets on August 20 and September 9, 1975, and entered orbit on June 19 and August 7, 1976, respectively. On July 20 and September 3, the landing modules separated: Viking 1 landed in the west of the Chryse Planitia, and Viking 2 in the Utopia Planitia, in both cases without incident.
Both spacecraft operated for more than one year. But the Viking 1 landing module operated the longest, until November 11, 1982, or 6 years and 116 days. However, communication was lost solely due to an operator error when updating the module's software.
During this time, the spacecraft transmitted more than 16,000 photographs, conducted meteorological and seismic measurements, analyzed the composition of the soil (for example, confirming that the composition of Mars is almost identical to some meteorites found on Earth), and, in general, allowed us to learn much more about Mars than all previous expeditions. In addition, during the descent, scientists tested the effect of gravitational time dilation (the Shapiro effect) and confirmed that the concepts of general relativity are correct.
In 1989, the USSR launched another spacecraft, Phobos-2, after the unsuccessful flight of Phobos-1. For 57 days, it received data from one of Mars' moons, but during an attempt to descend to the surface of Phobos, the communication systems shut down and the spacecraft was lost.
After the collapse of the USSR, Roscosmos decided to repeat the mission of the Phobos spacecraft, using Soviet developments but trying to take into account design flaws. This led to the Mars 96 project, a collaboration with the US and European countries. It was the heaviest AMS ever built, weighing 6,800 kg at launch and carrying 500 kg of scientific equipment, four small landing modules, and two autonomous stations. However, the launch on November 16, 1996, was unsuccessful: due to a programming error, the spacecraft did not reach its intended trajectory to Mars, made three revolutions around the Earth, and burned up in the dense layers of the atmosphere.
The US began to consider the financial side of the issue: they had spent a colossal $1.06 billion on the Viking program, which was successful in all respects, but more than $7 billion by today's standards. This was especially true after the failed Mars Observer mission, when a spacecraft worth $817 million was lost on its approach to Mars.
How the Mars Pathfinder mission was prepared
In 1992, Daniel Goldin took over as head of NASA and announced a concept designed to significantly reduce costs. It was called Faster, Better, Cheaper (FCB) and consisted of creating missions that were not global in scope, such as Viking or Apollo, involving unlimited financial resources, but rather using a different approach to development.
To do this, simpler missions were created to solve more specific tasks and launched more frequently. For example, for the hypothetical goal of “studying Mars for five years,” it is necessary to develop a heavier spacecraft, packed with a large set of scientific equipment and designed for long-term operation. It is more difficult to launch it on a given trajectory, and the risks due to high cost or failure are an order of magnitude higher.
Another approach is to “test a certain hypothesis and verify a certain fact.” For example, in the case of Mars, this would involve testing the technology for landing the module and the technical aspects of the rover's movement in conditions of dust storms and high radiation. And the module should be tested not for years, but for a couple of weeks, say. Did it work? Great, let's move on to the next project. Didn't work? Let's figure it out. But in doing so, we lost 10 times less money.
The program implementing the FCB concept was called Discovery — it was supposed to be a chain of projects with tight deadlines and limited funding. One of these projects was to be Mars Pathfinder. The following basic tasks had to be completed:
- Tight development deadlines — no more than three years. To achieve this, involve as few third-party contractors as possible and focus on proven technologies.
- Low cost — no more than $270 million for everything, including launch by carrier rocket and scientific work. According to preliminary estimates, the Mars rover had to cost no more than $25 million and the landing module no more than $150 million.
- Testing a new landing method — using a special aerodynamic shell with a heat shield and additional airbags that would serve as shock absorbers and slow down the fall together with the parachute.
- Use of a robotic Mars rover — lightweight and autonomous, capable of making decisions without communication with Earth. Moreover, such devices had already been tested in lunar programs, for example, by the Soviet Union 30 years earlier.
- Checking telemetry — after landing, the landing module was supposed to turn into a real weather station: tracking data on temperature, pressure, and wind, as well as recording what was happening with a special stereoscopic camera. And all this was supposed to consume as little energy as possible, taking into account the power supply from solar panels and batteries.
And, of course, the standard collection of scientific data, such as studying soil composition, atmospheric phenomena, and so on.
While the module itself was originally called Mars Pathfinder, the rover was given a different name — Sojourner. It was chosen through a competition among 3,500 schoolchildren. The winner was 12-year-old Valerie Ambrose from Connecticut, who suggested naming the rover after Sojourner Truth, a black woman who fought for women's and African American rights in the 19th century. Anthony Spear from NASA's Jet Propulsion Laboratory (JPL) was appointed project manager.
Let's take a quick look at what several NASA departments have managed to create in three years of work.
Sojourner Mars Rover
The device, which was supposed to roam the surface of Mars, was very compact. Weighing about 15 kg, it could easily fit on a kitchen table: it was 65 cm long, 48 cm wide, and 30 cm high.
Power was provided by 0.22 square meters of solar panels with GaAs/Ge cells. They had an efficiency of about 18% and generated up to 15 W at peak. Engineers tested them in climate chambers to ensure that they could withstand low temperatures down to -140 degrees Celsius. A 150 Wh lithium-thionyl chloride battery was used to ensure uninterrupted operation. Looking ahead, the low temperature had a negative effect on its capacity — after about 40 days, Sojourner could only move during the day.
The rover moved using six electric motors mounted on each of its spring-loaded aluminum wheels, which had a diameter of 12.7 cm. Four wheels were used for turning, which was sufficient for basic maneuvers. The wheels had teeth and a stainless steel coating so that Sojourner could overcome small obstacles by clinging to the Martian soil. Engineers conducted a series of tests to understand how well the wheels would withstand movement on Martian soil and be subject to abrasion.
The electronics were controlled by an Intel 80C85 processor and had 64 KB of memory. The board was housed inside a heated enclosure filled with aerogel for component sealing and thermal insulation. All this allowed the temperature to remain above -40 degrees at all times.
A radio modem with a transmission speed of 9600 baud was used to communicate with Pathfinder — communication was provided up to half a kilometer, although in fact the rover did not travel more than 10 meters. Two Kodak KAI-0371 monochrome cameras with CCD matrices and zinc selenide lenses were installed at the front, and a KAI-037M color camera was installed at the rear. The equipment did not provide the highest resolution of 768 x 484 pixels, but within the limited budget and mission objectives, this was quite sufficient. Obstacle detection was provided by five lasers located between the front cameras.
The rover was controlled from Earth using RCS software: operator Brian Cooper, wearing 3D glasses, tracked the movement from the cameras and gave commands on where to move. Of course, due to the great distance between Earth and Mars, everything happened with a delay of several minutes.
Among the sensors on board was an alpha proton X-ray spectrometer (APXS) located at the rear of the rover. It was activated by a special robotic arm that made contact with the ground or rocks. Curium-244 with a half-life of 18 years was used as the source of alpha radiation in the APXS.
Pathfinder landing module
The device was a tetrahedron 1.5 meters high, 2.64 meters in diameter, and weighing 894 kg, including the fuel and equipment needed for descent. After landing on Mars, its weight was no more than 370 kg.
Power on the surface was provided by solar panels with a total area of 2.8 square meters, which unfolded like supports, generating up to 1200 W of power — this was enough to power all the electronics and charge the silver-zinc batteries.
A special stereoscopic camera with two lenses mounted 15 cm apart, called the Imager for Mars Pathfinder (IMP), was located on an extendable support with a height of up to 1.5 meters. Twelve filters were used on each lens for observation, which, in certain combinations, allowed the camera to adapt to different situations (for example, for shooting the ground or the sky at different times of the day) and obtain high-quality images. The camera could rotate freely through 360 degrees.
Two antennas were used for communication: one with low gain and one with high gain, operating in the X-band at a frequency of 8.43 GHz and having two degrees of freedom for precise orientation to Earth. The NASA Deep Space Network was responsible for receiving the signal.
The parameters of the Martian atmosphere were monitored using the ASI/MET system, which consisted of a set of three sensors for wind speed, temperature, and pressure, located on a retractable rod about one meter high. They were used both during entry into the atmosphere and for stationary operation on the surface.
The landing module's electronics were based on a radiation-hardened IBM RAD6000 processor with 128 MB of RAM and 6 MB of EEPROM memory. VxWorks served as the operating system.
A problem arose with the system. When the device was already operating on Mars, at some point the computer began to reboot. The reason was unclear at first. However, after three weeks, NASA programmers finally figured out what was going on. With a large flow of information (video signals, sensor readings, and control commands from Earth) transmitted over a single data bus, at some point a priority inversion occurred — the watchdog timer was triggered and reset the system. Once the cause was clear, it took just a couple of hours to upgrade the code. The situation is described in more detail in this article.
Separately, it is worth noting the large series of tests conducted before the launch and related to the study of the landing and entry into the atmosphere process. Engineers spent more than a month testing the heat shield, parachute, and system of four airbags: they selected the right material and pressure and observed how the device would behave when falling onto the Martian surface at a given speed. The decision was made to use a two-layer vectran fabric — a compromise between strength and excess weight.
How the Mars Pathfinder mission went
After all the tests and preparations, the engineers were confident that the spacecraft could accomplish the mission's objectives within the budget.
It was necessary to select a landing site that was sufficiently safe, with relatively flat terrain, but also offered something to explore. NASA analyzed hundreds of photographs taken by the Viking orbital stations and found the best location—the Ares Valley. The proposed landing site contained numerous small rocks that were of scientific interest—according to scientists, they were formed as a result of extensive flooding that occurred on Mars in the past.
The launch from Cape Canaveral took place on December 4, 1996, using the time-tested Delta II launch vehicle, which was relatively inexpensive. Incidentally, a month earlier, another spacecraft was launched — Mars Global Surveyor, the first “swallow” of the Discovery program. But we will talk about this spacecraft another time.
The flight lasted seven months without any incidents — only four trajectory corrections were needed to reach the target point on July 4, 1997.
Mars Pathfinder entered the Martian atmosphere on a hyperbolic trajectory at a speed of about 6.1 km/s — just over four minutes remained before landing. Here's what happened next:
- deceleration due to the heat shield to 370 m/s;
- deployment of an 11-meter-diameter parachute;
- detachment of the shield;
- lowering of the landing module on a 20-meter cable;
- inflation of four airbags surrounding the landing module;
- activation of three solid-fuel rocket engines for additional deceleration of the module to 28 m/s at an altitude of about 98 meters;
- detachment of the module with a two-second delay and its fall to the surface at a speed of about 14 m/s;
- several bounces thanks to the shock-absorbing properties of the airbags.
As soon as the landing module stopped rolling, the airbags deflated, and the module flipped itself into an upright position and unfolded three “petals” with solar panels. As a result, the module ended up 20 km from the estimated landing point, but overall, the innovative descent algorithm worked perfectly — one of the mission's goals had already been achieved. Immediately after landing, the Pathfinder module was renamed the Carl Sagan Memorial Station, as the great popularizer of space had died two weeks after launch.
The landing took place at night, and it was necessary to wait until sunrise for the module to send the first data about the landing process to Earth. During this time, several photographs were taken with the IMP, and several meteorological measurements were made.
First, a low-gain antenna was turned on to transmit measurements, and a few hours later, a high-gain antenna was turned on to transmit images. In one of the images, NASA engineers saw that there was a small problem — one of the airbags had not completely deflated. This could have prevented the long-awaited descent of Sojourner. The engineers resorted to a trick: they pulled in one of the “petals” and flattened it with a winch attached to the cushion.
Based on the photographs received, the researchers drew up a plan for the rover's work within a day: using the APXS, they needed to analyze several dozen rocks around the module. Each rock was named after a cartoon character. For example, the first “victim” was a 40-centimeter rock named Barnacle Bill, after Popeye's enemy. It took about 10 hours to study it, and the results showed that the rock was completely identical in composition to Earth's andesites and was probably of volcanic origin.
Over the next two and a half months, the rover examined 14 more rocks (such as Yogi and Scooby-Doo) around the landing module and traveled 104 meters, always staying within 10 meters of the module. During this time, engineers conducted several scheduled tests:
- obstacle recognition systems;
- wheel traction and abrasion;
- camera performance in different conditions;
- communication stability between the rover and the module, as well as with Earth.
It is remarkable that NASA initially expected the rover to operate for no more than a week and the module for about a month. However, the system performed excellently: radiation and wind did not have a significant impact on the electronics. The module operated until September 27, 1997, when communication with it was finally lost.
A possible reason could be the failure of the batteries, which had undergone too many charge-discharge cycles. The heat generated by the batteries kept the probe's electronics at an acceptable temperature above -40 degrees. When they stopped working, the communication module may have failed due to overcooling. But we are unlikely to ever know the true cause.
Mission results
During its operation, the rover transmitted 550 images and used the APXS to measure 15 rocks and 10 soil samples. The lander took a total of 16,500 images with the IMP and performed 8.5 million meteorological measurements. Here are some of the most important scientific achievements described in the December 5, 1997 issue of Science magazine:
- The rocks contain a lot of silica (SiO2). This significantly distinguishes them from meteorites that have fallen to Earth.
- The shape of some rocks indicates that they may have been exposed to prolonged exposure to water several billion years ago.
- Analysis of the chemical composition of the soil from the Viking and Pathfinder devices was very similar, although the sampling sites were hundreds of kilometers apart. This means that the soil is fairly homogeneous.
- By evaluating radio signal delays, it was possible to more accurately determine the planet's orbital period and estimate the diameter of its core.
- Analysis of the dust showed that many particles have magnetic properties due to the presence of maghemite.
- Comparing images from cameras and meteorological data made it possible to hypothesize the mechanism of formation of so-called “dust devils.”
- The color of the soil in some places was similar to that of iron oxyhydroxide, confirming the theory of a warmer and wetter climate in the past.
And much more — more detailed results can be found at the link.
But most importantly, NASA proved the correctness of its FCB approach. The mission cost only $265 million, which was many times cheaper than Viking. In addition, it was possible to test an innovative landing method and confirm that rovers can operate on the surface of the red planet.
Another factor that contributed to the popularity of the Mars Pathfinder mission was the rapid spread of the internet in the late 1990s. NASA realized that this tool should be used as actively as possible: to share high-quality photos after additional computer processing. And they succeeded in drawing attention to the exploration of Mars. For example, photographs from the spacecraft appear at the beginning of the TV series Star Trek: Enterprise. And, of course, Mark Watney uses it to establish communication with Earth.
The Mars Pathfinder mission, during which a Mars rover was successfully used for the first time, provided the basis for other similar projects to explore Mars: Spirit, Opportunity, Curiosity, and many others.