* Interplanetary probes

In the early days of the Space Age, soon after the first artifact in space (the Soviet Sputnik 1, in 1957), the first living being (the dog Laika), the first discovery in outer space (the Van Allen radiation belts in 1958), the next logical step seemed to be trying to make something reach the Moon.

The first object built by humans which managed to leave once and for all the Earth's gravity (the first "artificial planet") was the Lunik 1, in 1959. It was launched towards the Moon (it missed the target) but relayed important details about the environment between the Earth and its natural satellite. Shortly thereafter, the Soviets launched a sister spacecraft, the Lunik 2, and yes this made a perfect trip to shatter itself against the lunar surface. For the first time humans could touch, albeit indirectly, another celestial body. To the scientific and historical importance was united the political importance of the achievement, demonstrated by the fact that an important part of the small payload of the spacecraft was Soviet medals and badges: we were in the Cold War. Even more spectacular results came through the advanced Lunik 3, when it photographed for the first time the far side of the Moon, something never before seen by humans.

In the early 60's the U. S. were about to begin a much more ambitious program. The goal for the end of the decade was nothing less than putting a man on the Moon and bring him safely back to Earth: it was the Apollo program. To achieve it, it was imperative to have a previous knowledge of the Moon. Three robotics projects emerged: the Ranger, basically free-fall spacecraft, which would film with TV cameras the last few minutes before the fatal crash; the Surveyor, landing craft that would test equipment and techniques and would analyze lunar soil to see whether it was possible or not the landing of large crewed spacecraft; and the Lunar Orbiter, that would make Moon maps much more detailed than previous ones in order to choose the best landing sites. All these steps were carried out with great success.

The Soviets continued their programs of spacecraft to the Moon, and sometimes even stood in front of the U. S., for example with the first successful soft landing (Lunik 9) and the first artificial satellite around another world (Lunik 10), but in generally failed to keep pace with the U. S.. The latter put six large crewed ships with a dozen astronauts between 1969 and 1972 in the lunar soil, finishing the work begun by Ranger. The Soviets had to settle for far less spectacular results, but they also have their importance, e.g. the first (and until now only) samples from another world brought by remote control (Lunik 16), and the first remote-control cars to roll on an alien surface (Lunokhod).

The true era of planetary exploration began in 1962, with the launch of the U .S. Mariner 2 toward the nearest neighbor, Venus. This spacecraft confirmed the suspicion that the surface of Venus is hotter than a kitchen oven, and not least, the robotic spacecraft managed to survive several months in the hostile interplanetary environment and transmit information to Earth from many millions of kilometers away. A more advanced brother, equipped with cameras, showed that there are no gaps between the clouds, and then it came the time to study them with a descent spacecraft. The Soviet Venera 4 became the first object constructed by humans to enter the atmosphere of another planet, but it did not got to the floor because it was overwhelmed by the amazing atmospheric pressure of Venus. 

Meanwhile, the new spacecraft of the Mariner series were headed the other way, toward Mars. The first pictures showed a uninteresting surface that some people thought similar to the Moon's surface, but the revolution came in 1971, with Mariner 9. First artificial satellite from Earth around another planet, it studied Mars continuously sending more than 7000 photographs, in which the first evidence that Mars has not always been a desert appeared: dry riverbeds. Other impressive findings were the largest canyon in the Solar System (now called Mariner Valley) and the highest mountains: volcanoes up to 26 km high. It also showed that the small moons of Mars are probably captured asteroids.

In the early years of the 70's travels to still further afield began: Mercury and, beyond the main belt of asteroids, Jupiter and Saturn. The Pioneer 10 and 11 are also the first spacecraft that, after a preliminary reconnaissance of these gas giants, continued traveling through space to leave the Solar System. They are the first interstellar spacecraft launched by Humankind. As a signature of its builders, they carry gold-plated aluminum plaques with a drawing of a man and a woman, a special code that is expected to be understood by any intelligent being and a map of the position of the Sun and Earth in the Galaxy with the time when the spacecraft were launched.

This first phase of the use of spacecraft is characterized by the large number of launches (in part because of the space race during the Cold War) but also by the large number of failures, because the technology was still in its infancy. After the arrival of humans to the Moon, by virtue of the technological breakthroughs achieved, especially in the area of electronics and the size of the rocket, a second generation of interplanetary probes appears: big, heavy, equipped with payloads of very refined instruments, with extremely high budgets and with missions lasting up to decades. Again the first are launched to the closest neighbor, Venus. The Soviet Venera series continues to be refined and manage landings with transmissions of images and other important data from the surface, over the years 1970s and 1980s. The U. S. launch missions for the detailed study of the atmosphere, and using radar, make the first high-quality maps of the permanently overcast surface. The height of sophistication of this technique comes at the end of the 80s, with the probe Magellan with a radar employing the technique of Synthetic Aperture Radar (more expensive than the spacecraft itself) that makes images equivalent to high-quality photographs.

The mission that symbolizes the establishment of this new era is undoubtedly the Viking, to Mars. The U. S. pass for the first time the barrier of 1000 million dollars for a non-crewed mission with spacecraft so advanced that a thousand people are needed to monitor the operation. Fed by even nuclear batteries, they perform experiments for years since 1976, both from orbit and from the ground of the Red Planet. Undoubtedly the most ambitious experiments are those seeking signs of life in the Martian soil. The results of these biological experiments, appear contradictory and nothing conclusive.

The most successful interplanetary probe in History is from this era. Shortly after Viking, the Voyager probes are launched, a reduced version of a project to take advantage of a rare conjunction of planets to perform a "Grand Tour" by the outer planets. With the most modern and sophisticated with regard to sensors and computers the twin spacecraft study in unprecedented detail Jupiter and Saturn (Voyager 1 and 2), and also Uranus and Neptune (Voyager 2), not only re-writing entire scientific books when passing by these distant worlds while filling entire shelves of new information: virtually everything we know about that part of the Solar System, we ought it to Voyager, which working much longer than originally foreseen, continue until today sending information while leaving the Solar System towards the stars. Gold records with songs, messages and images from Earth are traveling with them, in the hope of being found and deciphered someday by some civilization that we can not even imagine. 

The last descendants of these spacecraft are the Galileo, managed by the United States, launched in 1989 to study for at least 2 years (which in the end were almost 8 years) the system of Jupiter, and the Cassini-Huygens, an international project that, with a price of 3000 million dollars is also the most expensive and sophisticated in History. Launched in 1997, arrived at Saturn in 2004 and studied this system over 4 years, already renewed.

In more recent times comes a new vision about space exploration. On the one hand, new countries (especially those in Western Europe and Japan) desire to explore interplanetary space, in an affordable way of course. Moreover, the United States of America, the only surviving superpower, is seeking to reduce in the new missions the investment risks. Rather than build and launch a single spacecraft of thousands of millions of dollars with 20 different scientific instruments, it is deemed wiser to launch four spacecraft with 5 very specific instruments, at a lower price and with much less risk of losing the entire mission if any failure happens in a critical system.

An interest in new objects in the Solar System also surge. Missions like the encounters with comet Halley in 1986, which had as main protagonist the Giotto probe of the European Space Agency, show that small spacecraft and international cooperation instead of confrontation also get spectacular results. Currently, missions to study the Sun, smaller bodies such as comets and asteroids and a renewed interest in Mars follow this philosophy.

Despite increasingly reduced budgets, the role of interplanetary probes as research tools are still irreplaceable. In the new decades of the 21st century, we expect a more active participation of new explorer nations, systematic missions to the more scientifically interesting objects, and travels for the first time to places that have not yet been explored, such as the trans-Neptunian Pluto, in the depths of the System Solar.


CHRONOLOGY OF PLANETARY EXPLORATION

THE PIONEERING MISSIONS TO THE MOON
Lunik 1 - 1959 - Moon - Flyby
Lunik 2 - 1959 - Moon - Impact
Lunik 3 - 1959 - Moon - Flyby
Ranger 7 - 1964 - Moon - Impact
Ranger 8 - 1965 - Moon - Impact
Ranger 9 - 1965 - Moon - Impact
Lunik 9 - 1966 - Moon - Lander
Lunik 13 - 1966 - Moon - Lander
Lunik 10 - 1966 - Moon - Orbiter
Lunik 11 - 1966 - Moon - Orbiter
Lunik 12 - 1966 - Moon - Orbiter
Lunik 14 - 1968 - Moon - Orbiter
Surveyor 1 - 1966 - Moon - Lander
Surveyor 3 - 1967 - Moon - Lander
Surveyor 5 - 1967 - Moon - Lander
Surveyor 6 - 1967 - Moon - Lander
Surveyor 7 - 1968 - Moon - Lander
Lunik 16 - 1970 - Moon - Samples
Lunik 20 - 1972 - Moon - Samples
Lunik 24 - 1976 - Moon - Samples
Lunik 17 / Lunokhod 1 - 1970 - Moon - Lander / Rover
Lunik 21 / Lunokhod 2 - 1973 - Moon - Lander / Rover

OTHER PIONEERING MISSIONS TO THE MOON
Pioneer 4 - 1959 - Moon - Flyby
Zond 3 - 1965 - Moon - Flyby
Explorer 35 (IMP-E) - 1967 - Moon - Orbiter
Zond 5 - 1968 - Moon - Circumlunar and return 
Zond 6 - 1968 - Moon - Circumlunar and return
Lunik 15 - 1969 - Moon - Samples (Failed, orbiter)
Zond 7 - 1969 - Moon - Circumlunar and return
Zond 8 - 1970 - Moon - Circumlunar and return
Lunik 19 - 1971 - Moon - Orbiter
Explorer 49 (RAE-B) - 1973 - Moon - Orbiter / Radio Astronomy
Lunik 22 - 1974 - Moon - Orbiter

THE PIONEERING INTERPLANETARY MISSIONS
Mariner 2 - 1962 - Venus - Flyby
Mariner 5 - 1967 - Venus - Flyby
Venera 4 - 1967 - Venus - Descent
Venera 5 - 1969 - Venus - Descent
Venera 6 - 1969 - Venus - Descent
Mariner 4 - 1964 - Mars - Flyby
Mariner 6 - 1969 - Mars - Flyby
Mariner 7 - 1969 - Mars - Flyby
Mariner 9 - 1971 - Mars - Orbiter
Mariner 10 - 1973 - Venus and Mercury - Flyby

OTHER PIONEERING INTERPLANETARY MISSIONS
Pioneer 5 - 1960 - Interplanetary Environment
Mars 1 - 1962 - Mars - Flyby with failure
Zond 1 - 1964 - Interplanetary Environment
Venera 3 - 1965 - Venus - Descent with failure
Pioneer 6 - 1965 - Interplanetary Environment
Pioneer 7 - 1966 - Interplanetary Environment
Pioneer 8 - 1967 - Interplanetary Environment
Pioneer 9 - 1968 - Interplanetary Environment
Venera 7 - 1970 - Venus - Descent and landing
Mars 2 - 1971 - Mars - Orbiter / Lander (Failed)
Mars 3 - 1971 - Mars - Orbiter / Lander with failure
Venera 8 - 1972 - Venus - Landing
Mars 4 - 1973 - Mars - Orbiter (Failed, flyby)
Mars 5 - 1973 - Mars - Orbiter
Mars 6 - 1973 - Mars - Lander (Failed)
Mars 7 - 1973 - Mars - Lander (Failed, flyby)
Pioneer 10 - 1972 - Jupiter - Flyby
Pioneer 11 - 1973 - Jupiter and Saturn - Flyby

THE BIG PROBES
Venera 9 - 1975 - Venus - Orbiter / Lander
Venera 10 - 1975 - Venus - Orbiter / Lander
Pioneer Venus 1 - 1978 - Venus - Orbiter
Pioneer Venus 2 - 1978 - Venus - Descent, multiple probe
Magellan - 1989 - Venus - Orbiter
Viking Orbiter 1 / Viking Lander 1 - 1975 - Mars - Orbiter / Lander
Viking Orbiter 2 / Viking Lander 2 - 1975 - Mars - Orbiter / Lander
Voyager 2 - 1977 - Jupiter, Saturn, Uranus and Neptune - Flyby
Voyager 1 - 1977 - Jupiter and Saturn - Flyby
Galileo - 1989 - Jupiter - Orbiter / Descent
Cassini / Huygens - 1997 - Saturn - Orbiter / Descent

OTHER BIG PROBES
Venera 11 - 1978 - Venus - Flyby / Lander
Venera 12 - 1978 - Venus - Flyby / Lander
Venera 13 - 1981 - Venus - Flyby / Lander
Venera 14 - 1981 - Venus - Flyby / Lander
Venera 15 - 1983 - Venus - Orbiter
Venera 16 - 1983 - Venus - Orbiter
Vega 1 - 1984 - Venus and comet 1P/Halley - Flyby / Lander / Balloons and flyby
Vega 2 - 1984 - Venus and comet 1P/Halley - Flyby / Lander / Balloons and flyby
Phobos 1 - 1988 - Mars - Orbiter (Failed) / Landing on Phobos (Failed)
Phobos 2 - 1988 - Mars - Orbiter / Landing on Phobos (Failed)

THE NEW EXPLORERS
ICE (ISEE-3) - 1978 - Solar research and comets 21P/Giacobini-Zinner and 1P/Halley - Flyby
Sakigake - 1985 - Comet 1P/Halley - Flyby
Giotto -1985 - Comets 1P/Halley and 26P/Grigg-Skjellerup - Flyby
Suisei - 1985 - Comet 1P/Halley - Flyby
SOHO - 1995 - Solar research
NEAR-Shoemaker - 1996 - Asteroid (433) Eros - Orbiter, landing
Mars Global Surveyor - 1996 - Mars - Orbiter
Mars Pathfinder / Sojourner - 1996 - Mars - Lander / Rover
Nozomi - 1998 - Mars - Orbiter (Failed, flyby)
Lunar Prospector - 1998 - Moon - Orbiter
Deep Space 1 - 1998 - Asteroid (9969) Braille and comet 19P/Borrelly - Flyby
Stardust - 1999 - Comet 81P/Wild 2 - Samples

OTHER NEW EXPLORERS
Helios 1 - 1974 - Solar research
Helios 2 - 1974 - Solar research
Hiten - 1990 - Moon - Flyby / Orbiter
WIND - 1994 - Solar research
Clementine - 1994 - Moon - Orbiter 
ACE - 1997 - Solar research

21st CENTURY
Genesis - 2001 - Solar Wind - Samples
2001 Mars Odyssey - 2001 - Mars - Orbiter
Hayabusa - 2003 - Asteroid (25143) Itokawa - Landing and samples
SMART 1 - 2003 - Moon - Orbiter
Spirit - 2003 - Mars - Rover
Opportunity - 2003 - Mars - Rover
Mars Express / Beagle 2 - 2003 - Mars - Orbiter / Lander (Failed)
Rosetta / Philae - 2004 - Comet 67P/Churyumov-Gerasimenko - Orbiter / Lander
Deep Impact - 2005 - Comet 9P/Tempel 1 and 103P/Hartley 2 - Meeting / Impact
MESSENGER - 2004 - Mercury - Orbiter
Mars Reconnaissance Orbiter - 2005 - Mars - Orbiter
Venus Express - 2005 - Venus - Orbiter
New Horizons - 2006 - Pluto - Flyby
Phoenix - 2007 - Mars - Lander
Kaguya / Ouna / Okina - 2007 - Moon - Constellation of orbiters
Dawn - 2007 - Asteroid (4) Vesta and (1) Ceres - Orbiter
Chang'e 1 - 2007 - Moon - Orbiter
Chandrayaan 1 / MIP - 2008 - Moon - Orbiter / Impact
Kepler - 2009 - Extrasolar research
Lunar Reconnaisance Orbiter - 2009 - Moon - Orbiter
LCROSS - 2009 - Moon - Impact
Akatsuki - 2010 - Venus - Orbiter (Failed, flyby)
Chang'e 2 - 2010 - Moon - Orbiter
Juno - 2011 - Jupiter - Orbiter
GRAIL - 2011 - Moon - Orbiter
Curiosity - 2011 - Mars - Rover
LADEE - 2013 - Moon - Orbiter
Mangalyaan - 2013 - Mars - Orbiter
MAVEN - 2013 - Mars - Orbiter
Chang'e 3 / Yutu - 2013 - Moon - Lander / Rover
Chang'e 5 T-1 - 2014 - Moon - Circumlunar and return / Orbiter
Hayabusa 2 - 2014 - Asteroid (162 173) Ryugu - Sample return
Trace Gas Orbiter / Schiaparelli - 2016 - Mars - Orbiter / Lander
OSIRIS-REx - 2016 - Asteroid (101 955) Bennu - Sample return


A. L.

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From a lecture given at USP, originally on 11 November 2000. Originally published by USP, from a document dated 26 January 2001. Photo: The most successful spacecraft in History were probably the twin Voyager 1 and Voyager 2, the findings of which rewrote entire books about the Solar System. Credit: Courtesy NASA / JPL-Caltech.

A scientific, very respectful and well-thought reply to the popular question "Do you believe in UFOs?"  This book evolved as a reply to one of the most frequent questions that I used to hear from the public when I was working in an astronomical observatory: "Do you believe in UFOs?". That seems an odd question to ask to scientists, but after researching conscientiously for about a full year, I discovered, to my surprise, that mainstream Science has a few things to say about the topic.  This book is not about conspiracy theory, "NASA is hiding the truth", or much less, that flying saucers have already landed on the lawn of the White House. Rather, it is a book about what is the most rational reply that a scientist, or in my case, a science writer, can offer when people insist on asking that question.  Of course, "Do you believe in UFOs?" is, understandable, one of the most popular questions that common people ask (even if silently, to themselves) when they raise their eyes and look at the stars. So it has to be treated respectfully, and why not, given a well-thought reply.

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