Unlike the satellites that are kept in orbit around the Earth, many spacecraft are sent far out into space, which are usually called space probes. These spacecraft help us to understand other bodies in our solar system, including planets, moons and asteroids, and even what lies beyond it.
One type of probe is a fly-by space probe that travels, or “flies”, past objects in the solar system but do not typically orbit around them, unless to gain momentum for further space travel. These missions must design their spacecraft to collect information as they pass by these celestial bodies, like planets and moons. The advantage of fly-by space probes is that they can pass by several different bodies as they travel outward through space. Much of our knowledge about our solar system’s planets and their moons come from fly-by missions. NASA’s Mariner program completed the first ever fly-bys of Mercury, Venus and Mars. In fact, Mariner 2 was the first spacecraft to have flown by another planet. In 1977, the agency launched two space probes as part of the Voyager program that looked to take advantage of a favourable alignment of the solar system’s outer planets. Voyager 2 is the only spacecraft to fly past Uranus or Neptune, and with its accompanying probe Voyager 1, also completed fly-bys of Saturn, Jupiter and other moons. Voyager 1 is also the furthest man-made object from Earth and is now believed to have left our solar system altogether. The first flyby of Pluto was completed by NASA’s New Horizons probe in 2015.
This false-color image of Pluto was taken by the New Horizons spacecraft during its fly-by of the dwarf planet in 2015 (NASA).
If a space probe sent to explore the solar system does not fly by planets and moons that will cross its path, the spacecraft may instead be designed to orbit around them (called “orbiters”) or to land (called “landers”). A project that included both an orbiter and a lander was the Cassini–Huygens mission, which was a cooperative effort between the American, European and Italian space agencies. The Cassini orbiter space probe travelled with the Huygens lander through space for six years to reach Saturn. Once there, Huygens successfully landed on the planet’s moon Titan. This was first landing on a moon other than our own and is still the furthest landing conducted from Earth to date. Meanwhile, the Cassini orbiter circled the planet Saturn for thirteen years, collecting valuable data before ending its mission. Of its many successes, the project is recognized for discovering seven new moons of Saturn, taking unmatched images of the planet and for flying the orbiter through the planet’s rings.
This image of Saturn's north polar hexagon was taken by the Cassini Spacecraft in 2012 (NASA/JPL-Caltech/Space Science Institute).
Not all orbiting and landing space probes are designed to orbit moons and planets. The European Space Agency performed the first landing on a comet when the Philae lander touched down on comet “67P” in 2014. Philae was accompanied by the orbiter probe Rosetta, which circled the comet to harvest data for analysis. Other space probes have been dedicated to studying and collecting samples from asteroids. To do this, the space probe must orbit the asteroid before extracting the material to bring back to Earth for further study. One of these missions is NASA’s OSIRIS-REx mission (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) that intends to return a sample from the “Bennu” asteroid in 2023.
Unlike these stationary landers that stay in the location where they land and usually only send us information for a short period of time, rovers are designed to survive for longer periods and to travel in order to collect data. These robots can navigate through rocky and sloped terrain, analyse soil content, take pictures of the landscape and more - all on their own. This technology began in the mid 1960’s when NASA delivered seven robotic spacecraft to the moon as part of the Surveyor program that are still there today but are no longer working. These landings, of which five were successful, helped engineers test the technology of safely landing robotic spacecraft on other bodies. More recently, today’s space enthusiasts are widely fascinated by the rovers that have operated on Mars. Four robots have operated successfully on Mars so far, all as part of NASA projects. The Opportunity (launched in 2003) and Curiosity (launched in 2011) rovers remain active today. They hope to determine whether life ever arose on Mars, to learn about the planet’s climate and geology, and to prepare for future human exploration on Mars.
Communications with these rovers and their instruments takes significant time to travel to Earth so the rovers are highly independent. They are able to use the technology in their many cameras to scan their surroundings to find the best travel routes to desired locations. For example, NASA’s Curiosity rover can image its surroundings to plan a path to a particular feature (such as a rock) up to 50 meters away, while avoiding large obstacles along the way. When rocks are unavoidable, these rovers can rely on a suspension system to move over them and a 6-wheel drive system. The rovers can withstand a tilt of up to 45 degrees without tipping over. When communication to Earth does take place, the rovers can transmit directly to Earth or they can send data to the spacecraft that are orbiting Mars, which is then transmitted to Earth.
To find out if Mars is or has been a host life, several technical instruments are used. For example, Curiosity uses an instrument called the dynamic albedo of neutrons. When cosmic rays hit the Marian soil, this instrument detects neutrons that escape. This instrument measures the energy of these escaped neutrons because of hydrogen is present, it will slow them down. This is a valuable element to look for because it could mean there is a possible presence of water. Another useful instrument is the alpha particle x-ray spectrometer that measures what chemical elements, and how much of them, are present in rocks and soils on the Martian surface. To do this, the instrument exposes the soil to x-rays and alpha particles, and then measured the energy that is returned back to it. The rover is also equipped with cameras that capture images and videos of meteorological activities (such as clouds and wind), soil properties, and landscape processes.
New Mars rover projects are currently being developed by various agencies. This includes the joint Russian and European initiative ExoMars, which plans to deliver a rover dedicated entirely to the search for signs of life in the year 2020. To do this, the rover will collect samples up to two meters below the surface with a drill, which can then be analysed with the instruments on-board. For example, an instrument called an infrared spectrometer will study the minerals in drill samples. These samples can be crushed into a fine powder to study their chemistry and what they are made of. The rover will use solar panels to make the needed energy to operate and it will rely on heaters and batteries to keep warm overnight. To prepare for the rover’s arrival, a research and communications satellite was placed into an orbit around Mars in 2016.
Also planned for launch in 2020 is NASA's Mars 2020 rover as part of the Mars Exploration program. The new rover’s hardware will be largely copied from the successful Curiosity rover, but it will also be upgrading its navigation and imaging capabilities. The rover will use ground-penetrating radar to study how water, rock and ice is spread below the surface. To assist with the landing of the rover, the mission will also use a range trigger, which will use speed and location information to decide on its own when to deploy the main parachute for landing. NASA’s new rover also intends to conduct on-site sample measurements and hopes to one day return these samples to Earth to study them further. In April 2018, ESA and NASA together signed a statement of intent to explore new concepts for missions that would bring samples from Mars back to Earth (which has yet to be done).
A self-portrait of NASA's Curiosity rover taken in 2015.