People have always wanted to explore beyond Earth into the mysteries of the sky and outer space. We have always been curious about the stars and the planets we see at night. As technology advanced in the 20th century we finally sent space probes and rockets into space. We then started exploring our solar system and the rest of the universe for a better understanding. The first space probe (which is a spacecraft that gathers information from space) was launched in 1957 by the Soviet Union.¹ Hundreds of other space probes have given us a fairly good picture of our solar system and the rest of the universe. In 1961, the first astronaut, a Soviet, reached space. Eight years later the first astronaut, an American, landed on the moon. The space race between the Americans and the Soviets was the beginning of space exploration. Many different methods have additionally led to discoveries of planets outside of our solar system known as exoplanets. A couple of planetary systems have been discovered resembling our own such as TRAPPIST-1. Elon Musk has founded SpaceX intending to inhabit a second planet, Mars, in the future. Humans have always wanted to expand and need a Planet B due to problems that may plague Earth such as climate change, pandemics, and asteroids.
Space probes are fundamental elements of space exploration. Space probes are any type of unmanned spacecraft that travels through space to collect information. There are three types of space probes (interplanetary, orbiters, and landers).² Interplanetary space probes are spacecrafts that fly-by planets and other materials (e.g. Voyager 2 that flew by Saturn and continued on its path). The next type are orbiters which orbit celestial bodies and map the surface (e.g. Mariner 9 Mars Orbiter which was the first orbiter and took photos of Mars and discovered the biggest volcano in our Solar System). The last and most important space probe type are landers which land on the surface of celestial bodies and study the surface (e.g. Pathfinder Mars Rover). Landers can additionally bring back samples of the surface. Space probes have given us a plethora of information on planets in our solar system, exoplanets and other celestial bodies (like stars, asteroids, and meteors).
The Beginning of Space Probes
The first-ever space probe was the Sputnik 1 made by the former Soviet Union (which launched on October 4th, 1957). Then, on January 31st, 1958, the USA sent their space probe, Explorer 1, into outer space. These space probes studied Earth and gave us a feeling of what it’s like in space. This marked the beginning of the Space Race between the USA and the Soviet Union. Mariner 2 was the first space probe to study another planet. It was launched on December 14th, 1962 and flew past Venus (and gave us information that it’s very hot). The first space probe that took a photo of another planet was Mariner 4 which launched on July 14th, 1965 and took photos of Mars’ moon-like surface.¹
Voyager Space Probes
The most famous space probes are the Voyager Space Probes. The two space probes were launched in 1977 and went on different paths. They flew past Jupiter, Saturn, Uranus and Neptune (giving us information about them). They are the two space probes that have traveled the furthest into outer space. Originally, they aimed to last for five years and only study Jupiter and Saturn however they are still working and have already lasted 43 years. In 2012, Voyager 1 exited the heliosphere (a bubble of particles and magnetic fields created by the Sun) and reached interstellar space. Later, in 2018, Voyager 2 exited the heliosphere. An instrument that measures solar wind confirmed that on November 5th, 2018 Voyager 2 had passed the heliopause (where hot star wind meets cold interstellar medium). They are currently 18 billion kilometers away from Earth and it takes information about 16.5 hours from the space probes to travel to Earth. The probes are powered by heat from the decay of radioactive material. Unfortunately, the power output decreases by four Watts per year and therefore the cameras have been turned off to save power. They communicate using NASA’s Deep Space Network (in California, Spain, and Australia).³ The Voyager Space Probes are currently researching interstellar space which we thought was nearly impossible with the technology from the 1970s.
Current and Future Missions
Many space agencies such as NASA and ESA are investing lots of money and research for current and future space probes to gather even more information and hopefully resulting in groundbreaking discoveries. Specifically, a lot is being invested for Mars Research to inhabit it one day. Some of the most important current space probes are Voyager 1 and 2, and the Spitzer Space Telescope (see section Transit). The orbiter from which we have the most information about Mars is the Mars Odyssey which launched in April 2001. It stills collects information about the climate and geology of the planet.⁴ Another important current space probe is the NEOWISE Space Telescope which was launched in December 2009 by NASA. The Space Telescope was launched to identify potentially dangerous asteroids and comets heading towards Earth.⁴ OSIRIS-REx is another big mission by NASA which was launched in September 2016 and is expected to return in September 2023. It is supposed to return a sample from asteroid Bennu which researchers hope will give us crucial information about the early Solar System and hazards and resources of near-Earth objects.⁵ Recently the NASA Mars Perseverance Rover was launched into space on July 30th. It is expected to land on the Mar’s Jezero Crater (place favorable for microbial life on Mars) in February 2021 and will be able to drill to examine rocks and soil and will gather important information about the planet.⁶
The most important future space probes for exoplanet exploration are the James Webb Space Telescope (2021 launch date) and the WFIRST Telescope (2025 launch date) (see section Direct Imaging). A future Mars exploration mission is the unpiloted mission to Mars by SpaceX expected to launch in 2022. Its goal is to confirm water sources, identify hazards and set initial power, mining and life support infrastructure for future missions.⁷ The LISA (Laser Interferometer Space Antenna) is three satellites expected to launch in 2034. They will detect and measure gravitational waves which will locate black holes.⁵
Exoplanets are all planets outside of our solar system that are orbiting around a star. They are very tough to be seen by our telescopes due to the glare of their suns. Researchers say that there are more than a trillion planets in our galaxy. As of September 19th, 2020 there have been 4277 confirmed exoplanets discovered: 1449 Neptune-like (like Neptune or Uranus), 1338 Gas Giants (like Saturn or Jupiter), 1332 Super-Earth (bigger than Earth and lighter than Gas Giants), 162 Terrestrial (like Mercury, Venus, Earth or Mars) and 6 unknown. Additionally, there are 5,431 NASA Candidate Planets which are yet to be confirmed.⁸ These planets have been discovered in many different ways and there are many notable discoveries.
Methods of Discovering Exoplanets
The first exoplanet, 51 Pegasi B, was discovered in 1995. 51 Pegasi B is a star-hugging gas giant about half of the size of Jupiter which completes an orbit around its star in just four days.⁸ It was discovered due to the wobble of its star using a method called “Radial Velocity”.
The first method of discovering exoplanets is called “Radial Velocity”. Orbiting planets cause the star to wobble due to gravitational tugs. This method measures the wavelengths of starlight which would be squeezed and stretched as the star moves closer and further away. This method is responsible currently for 19.2% of exoplanet discoveries. The takeaway of this method is that it usually only finds big planets like Jupiter because they cause notable “wobbles”. The astronomers Paul Butler and Geoff Marcy from the San Francisco State University found around seventy of the first one hundred exoplanets using this method.⁸
The method responsible for most discoveries (76.1%) is called “Transit”. In 2009, the Kepler Space Telescope was sent into deep space for four years and had its focus fixed at a small patch of sky with more than 150,000 stars .⁹ This telescope measured the amount of light received from each star and a small dip would confirm that a planet has passed in front of it during orbit. Some telescopes like the Hubble Space Telescope and NASA Spitzer Space Telescope used this method and could even get some information about the constituents in the atmosphere because different gases and chemicals absorb different wavelengths and therefore certain parts of the light spectrum are missing.⁸
One of the newest methods of finding exoplanets is called “Direct Imaging” (accounts for 1.2% of discoveries). This method sends objects into space which block some of the starlight and lessen the glare, therefore making it possible to take clearer images of an exoplanet. The most notable future telescopes that will use this method are the James Webb Space Telescope (2021 launch date) and the WFIRST Telescope (2025 launch date).⁸
TRAPPIST-1 Planetary System
The most interesting system of exoplanets discovered is the “TRAPPIST-1” which is 39 light-years away. The system was discovered in February 2017 and consists of seven Earth-sized planets that orbit a cool dwarf star. A Belgian scientist from the University of Liege discovered the system with his SPECULOOS Project (which was launched in 2001).¹⁰ At least the six inner exoplanets have rocky surfaces. Three of the planets (TRAPPIST-1e, f, & g) are in “goldilocks” orbit which means they are in the habitable zone around the star where the temperature is just right for liquid water to exist.¹⁰ The form of water which is on the planets depends on the distance away from the star (which is only 9% of the size of our sun). The closest planets probably have water vapor and the ones further out might have liquid water or water frozen as ice.¹¹ The planets are so closely packed together that if you stood on one of them you would see the others closer than the Moon to the Earth. Also, they all orbit their sun closer than Mercury does to our sun. All of them are “tidally locked” which means that one side of a planet always faces the ultracool star (TRAPPIST-1).¹² NASA’s Spitzer and Kepler Space Telescopes have gathered data about the density (gathered by “transit time variations” which is due to the planets slightly changing each other’s orbit times), mass, radius and other parameters of these exoplanets.
In summary, one can say that space exploration will stay and become an even more relevant topic in the following decades. Space exploration is possible by using space probes which are unmanned spacecraft that travel through space to collect information. The Space Race between the USA and Russia which lasted from 1955 to 1975 rapidly started space research and exploration. The space probes from the 20th century that are now gathering information from interstellar space are Voyager 1 and 2. Before exiting the heliosphere they gave us information about Jupiter, Saturn, Uranus, and Neptune while in orbit. Space agencies are investing more than ever in research. The most notable future space probes are the James Webb Space Telescope (which will be used for the “direct imaging” technique for exoplanets), the Mars Perseverance Rover and the LISA (which will locate black holes). Exoplanets are all planets out of our solar system that are in orbit around a star. As of September 19th, 2020, there have been 4277 discovered exoplanets. Three of the main techniques used to discover exoplanets are the “radial velocity” technique, the “transit” technique and the “direct imaging” technique. The “radial velocity” technique observes a star’s wobble due to the gravitational tugs caused by the exoplanets. The “transit” technique detects exoplanets by observing a dip in the starlight measured. The newest and future of exoplanet discoveries is “direct imaging” which sends space probes into space to create a block of the starlight and to easily take pictures of planets that are usually in the glare of the star. The most similar exoplanet system to our solar system is the TRAPPIST-1 Planetary system. The system is located 39 light-years away and contains seven Earth-sized exoplanets of which three could contain liquid water. One can conclude that space exploration is the future of astronomy. Some recommendations I would make are that our main goal should be to inhabit Mars as soon as possible and to protect the Earth from dangers such as meteors and comets. Space agencies should also plan to send landers and other space probes to habitable planets outside our solar system.
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