Temperature

Rover Temperature Controls

Like the human body, the Mars Exploration Rover cannot function well under excessively hot or cold temperatures. In order to survive during all of the various mission phases, the rover's "vital organs" must not exceed extreme temperatures of -40° Celsius to +40° Celsius (-40° Fahrenheit to 104° Fahrenheit).

The rover's essentials, such as the batteries, electronics, and computer, which are basically the rover's heart and brains, stay safe inside a Warm Electronics Box (WEB), commonly called the "rover body." Heaters are packed inside the rover body, and like a warm coat, the WEB walls help keep heat in when the night temperatures on Mars can drop to -96° Celsius (-140° Fahrenheit). Just as an athlete sweats to release heat after an intense workout, the rover's body can also release excess heat through its radiators, similar to ones used in car engines.

Methods used to keep the rover at the right temperature:

To help prevent heat from escaping out of the rover body and cold air from coming inside during landing operations, the outside of the rover body is painted with a gold coating. Using the same technique that many jewelers use, engineers lightly "sputter" or spray on the gold metal. The gold coating helps reduce energy that is radiated (energy spread outward) from the rover body.

The highly reflective gold finish effectively isolates the rover body from emitting heat energy out to the cold, sky temperature. This technique is similar to that used in a thermos bottle, where the reflective coating on the inner vacuum bottle helps keep coffee hot by minimizing radiation heat transfer across the bottle.

The rover is also kept warm by a special layer of insulation, called solid silica aerogel, which prevents heat from escaping outside of the rover body walls. Aerogel traps heat inside the rover body. It is a unique silicon-based substance nicknamed "solid smoke" because it is 99.8% air. Aerogel is one thousand times less dense than glass, so it is extraordinarily lightweight, which makes it much cheaper and easier to launch and fly to Mars.

Aerogel is a powerful material. Not only can it block heat from leaving the Mars Exploration Rover body, but it's the same material used to trap "cosmic bullets" for the Stardust spacecraft that flew through a comet's tail in January of 2004, just as the rovers were reaching Mars.

The current plan is to heat the rover by a combination of:

  • excess heat coming from the electronics (like heat coming off the top of a television set or an oven)
  • eight Radioisotope Heater Units (RHUs), which are constant 1-watt heaters that generate heat through the decay of a low-grade isotope)
  • electrical heaters

Electrical power is a precious commodity on Mars, and even more so during the Mars night where the rover relies solely on the batteries for power. RHUs help conserve nighttime battery power. While an electrical heater can be turned on or off, RHUs continuously provide about 1-watt of heat. Without the use of RHUs, the rover would be unable to meet its landed mission lifetime of 90 Mars days, due to the increased power demand on the batteries. In fact, the life expectancy of the rover would fall short of its requirement by nearly 70 Mars days.

Thermostats

During the day, the rover actually might need to release heat if any excess heat is generated by the rover electronics or electrical heaters. Just as a light bulb gives off heat or a car engine produces excess heat that can be felt through the hood, the rover can run the risk of working too hard and overheating.

The rover has a "thermal switches" that turn the heaters on and off, depending on the "open" and "closed" set-points of the thermostats, which are similar to common thermostats used in homes. The thermal switches would automatically activate or deactivate heaters to maintain electronics and battery temperatures above -40° Celsius (-40° Fahrenheit), even during the martian night, while the rover is "asleep."

The batteries inside the rover body need individual attention because, like a beating heart, they ultimately keep the rover alive. The baseline plan is to pack the batteries with six RHUs, survival heaters, and two heat switches that each connect to separate radiators mounted on the walls of the rover body.

Heat switch

The heat switch is another autonomous device that moderates a heat transfer path in order to maintain a specific battery temperature. If the batteries start to cool below -20° Celsius (-4° Fahrenheit), the heaters turn on. If the temperature of the batteries approaches 20° Celsius (68° Fahrenheit), the switch increases the heat transfer path to the radiators so that the excess heat is convected and radiated away to the martian environment. (Convection is heat transfer between sources that have different densities.) When the excess heat becomes necessary to maintain battery temperatures (that is, when daytime turns to nighttime), the switch decreases the heat transfer path to the radiators in order to retain heat. The radiators are similar to the heat fins on the back of a stereo amplifier, whereby a relatively larger surface is in contact with the environment to reject heat by convection (and heat radiation in the case of the Rover).

The "Russian doll" configuration of the spacecraft (where the rover is at the center of many different surrounding structures) makes it difficult to get rid of the excess heat from the rover electronics. During cruise, the rover is nestled inside the lander structure, which, in turn is surrounded by the aeroshell (the structure that protects the lander and rover from the fierce aerodynamic heating during Mars entry). The aeroshell is further attached to the cruise stage, which carries the propulsion and power system for the journey from launch to Mars entry. Because the rover acts as the brains at the heart of the spacecraft, a large amount of power (and heat) is generated within the rover body, or Warm Electronics Box, during cruise. Hence, a mechanically pumped fluid system known as the Heat Rejection System (HRS) was developed.

The heat rejection system is comprised of a pump on the cruise stage and tubing that snakes across the cruise stage, down the lander, and into the rover to pick up the heat (including the battery RHUs, and radiators on the cruise stage) and release the heat out into space. The pump is capable of shuttling 150 watts of rover waste heat. Its working fluid is CFC-12, similar to freon that is used in older automobile air conditioners. This fluid is maintained between -7° and 0° Celsius (19° Fahrenheit and 32° Fahrenheit) throughout cruise, which keeps the electronics and batteries at similarly tight temperature levels.

The cruise stage contains the propulsion system. The propellant lines and tanks and thrusters are maintained at appropriate temperatures by classic approaches: thermal blankets known as multi-layer insulation and thermostatically controlled heaters.

The aeroshell and lander also have critical equipment that must be temperature-controlled. They also use thermal blankets and heaters. In some cases, the heaters are used only to condition (that is, "warm-up") critical equipment such as airbags, gas generators, rocket assisted descent rocket motors, and the transverse impulse rocket system prior to Mars entry.

Why the rover needs temperature control systems on Mars

Many of these methods are very important to making sure the rover doesn't "freeze to death" in the cold of deep space or on Mars. Many people often assume that Mars is hot, but it is farther away from the sun and has a much thinner atmosphere than Earth, so any heat it does get during the day dissipates at night. In fact, the ground temperatures at the rover landing sites swing up during the day and down again during the night, varying by up to 113 degrees Celsius (or 235 degrees Fahrenheit) per Mars day. That's quite a temperature swing, when you consider that Earth temperatures typically vary by tens of degrees on average between night and day.

At the landing sites, an expected daytime high on the ground might be around 22° Celsius (71° Fahrenheit). An expected nighttime lows might be -99° Celsius (-146° Fahrenheit). Atmospheric temperatures, by contrast, can vary up to 83° Celsius (181° Fahrenheit). An atmospheric daytime high might be -3° Celsius (26° Fahrenheit), while a nighttime low might be -96° Celsius (-140° Fahrenheit).