Now that we are at the end of 2015, I figured it was time to look back over this year’s material on Drew Ex Machina and see which of the 71 essays I posted during this site’s first full calendar year online proved to be most popular. In addition to satisfying my curiosity, this exercise is an attempt to figure out what kinds of articles my readers prefer so that I can focus my attention on similar topics over the coming months when possible. What follows is a review of the Top Ten most popular articles on this site during 2015 based on a raw tally of their page views. If you are interested in reading any of these articles, they can be accessed by clicking on the titles or the feature images in the reviews that follow.
Comparison of recent Kepler discoveries. (NASA/Kepler)
#10 Habitable Planet Reality Check: 8 New Habitable Zone Planets
This past year proved to be a banner year for the discovery of planets in or near the habitable zones of their stars. One of the first posts in 2015 reviewed the announcement made on January 6 by Guillermo Torres (Harvard-Smithsonian Center for Astrophysics) of eight new extrasolar planets found orbiting in the habitable zone discovered by NASA’s Kepler mission. In a marked departure from many earlier announcements, the discovery paper by Torres et al. had a fairly cautious tone about the nature of their discoveries taking into account not only the uncertainties in the properties of their finds, but also the probability that these new exoplanets were rocky planets like the Earth instead of volatile-rich mini-Neptunes with poor prospects of being habitable in the conventional sense. While my review found that most of the eight planets were unlikely to be habitable, Kepler 442b seems to have good prospects given its size and position comfortably inside even more conservative definitions of the habitable zone (for more results from NASA’s ongoing Kepler mission, see the Kepler Mission page)
Atlas 20D with its Able upper stages before the launch of Pioneer P-3 on November 26, 1959. (USAF)
#9 NASA’s Forgotten Lunar Program
For those of us growing up during the heyday of the Apollo program, it was well known that NASA’s Ranger, Surveyor and Lunar Orbiter programs performed the initial reconnaissance of the Moon before the manned missions were launched. Less well known, especially to later generations of space enthusiasts, were NASA’s earlier lunar missions which were not as successful. This article, ranked #9 for 2015, looks at NASA’s Pioneer lunar orbiter missions launched in 1959 and 1960 using a stop gap rocket known as the Atlas-Able. Originally started by the USAF, NASA inherited the Pioneer series of spacecraft designed and built by TRW’s Space Technology Laboratory after it was founded in October 1958. While NASA was working to define its first homegrown lunar spacecraft, Ranger (see “The Prototype That Conquered the Solar System”), they attempted three times to launch the highly advanced Pioneer “paddle wheel” space probes to beat the Soviet Union into lunar orbit. Unfortunately, all these attempts failed due to problems with the Atlas-Able rocket which was quickly replaced by the more capable (and, eventually, more reliable) Atlas-Agena.
The explosion of the Space Shuttle Challenger. (NASA)
#8 A Brief History of Launch Aborts
One of the most dangerous phases in any space mission is launch. With rockets assembled from thousands of precisely engineered components filled with hundred of tons of volatile propellants, even the slightest problem can end in disaster. It is for this reason that all crewed spacecraft have launch abort options for most if not all phases of ascent into space. One class of launch abort involves the “pad abort” where the launch vehicle’s engines shutdown before leaving the pad. The first known example of a manned mission pad abort took place a half century ago during the Gemini 6 launch attempt on December 12, 1965 (see “Rendezvous in Orbit: The Gemini 6 Launch Abort”). This article on launch aborts, originally posted in November 2014 (and ranked #19 for most read posts in 2014), instead looks at the only two launch aborts where the crews left the pad but failed to achieve orbit. In both cases, the crews involved in the launch aborts of “Soyuz 18A” in 1975 and “Soyuz T-10A” in 1983 survived because of the various safeguards built into their Soyuz spacecraft and launch vehicle.
A painting by space artist Davis Paul Meltzer showing the sequence of events of a Gemini docking with an Agena in orbit. (NASA)
#7 The Unflown Mission of Gemini 6
Some of the most important skills required for the Apollo Moon program, not to mention the space programs that followed, were first developed by NASA a half a century ago during their Gemini missions of 1965 and 1966. Among these skills vital to spaceflight today are rendezvous and docking of two spacecraft in orbit. Originally NASA planned to accomplish this for the first time during the Gemini 6 mission to be launch on October 25, 1965 whose details are present in this article. Unfortunately, the original Gemini 6 mission was scrubbed less than a half an hour before launch when its Agena target vehicle failed to achieve orbit. Without a replacement target available, NASA officials quickly devised an alternate mission plan where the previously scheduled Gemini 7 spacecraft, to be launched on its long-duration mission on December 4, 1965, would serve as a rendezvous target for Gemini 6 with no docking to be attempted (see “Rendezvous in Space: The Launch of Gemini 7”). Unfortunately, the first attempt to get the new “Gemini 6A” mission off the ground failed with the first ever pad abort of a crewed space mission (see “Rendezvous in Space: The Gemini 6 Launch Abort”). Gemini 6 finally made it into orbit on December 15 and successfully performed the first rendezvous opening the door to the first docking schedule to take place in early 1966 (see “Rendezvous in Space: Gemini 6 and 7”).
An artist’s rendition of what sunset might look like on Gliese 667Cc. The host star, Gliese 667C, orbits a distant stellar binary (Gliese 667AB), seen in the background. (ESO/L. Calçada)
#6 Habitable Planet Reality Check: GJ 667C
It is said that if something seems to good to be true, then it probably isn’t. This certainly seems to be the case with the nearby star called GJ 667C which was the subject of this article first posted in September 2014 and ranked #6 for 2015 (as well as #22 in 2014). Based on an analysis of years of precision radial velocity measurements acquired at three observatories scattered across the globe, it was claimed that as many as seven Earth to super-Earth size planets orbited this red dwarf star with three of them supposedly being potentially habitable. Unfortunately, independent analyses of these data cast doubts on this interpretation almost immediately. Finally, a detailed analysis of all the data by Paul Robertson and Suvrath Mahadevan (Pennsylvania State University) found that there was clear evidence for only two planets orbiting GJ 667C with some doubts about the potential habitability of one of them designated GJ 667Cc based on other work. The analysis of Robertson and Mahadevan found that time varying magnetic activity on the surface of GJ 667C modulated by its 105-day rotation period and sampled irregularly in time was likely responsible for the other apparently periodic signals in the radial velocity data. Robertson, Mahadevan and others on their team had found similar issues with the claimed habitable worlds orbiting GJ 581 and Kapteyn’s Star (see “Habitable Planet Reality Check: GJ 581d” and “Kapteyn b: Has Another Habitable Planet ‘Disappeared’?”). The issues with these finds along with those of the unconfirmed α Centauri Bb (see “The Discovery of Alpha Centauri Bb: Three Years Later”) hints that we may be approaching the practical detection limits of the radial velocity method for finding small extrasolar planets.
Artist conception of a red dwarf. (NASA)
#5 The Real Wolf 359
For many fans of science fiction like myself, the mention of Wolf 359 immediately brings to mind the famous battle between the Borg and the Federation in the Star Trek universe. But what is Wolf 359 really like? This post on Drew Ex Machina reviews the latest scientific findings relating to the real Wolf 359 including the results from searches for orbiting planets and brown dwarfs. Located only 7.8 light years away, Wolf 359 is the fifth closest star system to our own and is a fairly typical red dwarf. Although nothing like that described in the numerous science fiction stories set in this system has been found, it continues to be a target of study by the astronomical community (for more posts about our neighbors, see the Nearby Stars page).
A model of a 4V-1 Venera lander on the surface of Venus. (TASS)
#4 Venera 9 and 10 to Venus
For long time space exploration observers like myself, the crowning achievement of the old Soviet planetary exploration program was the Venera series of missions to Venus in the 1970s and 1980s. During this period, the Soviet Union managed to land ten spacecraft on the surface of Venus and place four spacecraft into orbit including a pair that made the first high-resolution radar maps of the Venusian surface from orbit. Probably the most memorable of these missions, and the subject of the #4 most popular post of 2015, were those of Venera 9 and 10 which reached Venus in late October 1975. Although they had been preceded by the successful landings of Venera 7 and 8, these were the first Venera missions to use the much larger and more capable second-generation planetary spacecraft designed at the Soviet NPO Lavochkin design bureau. These ingeniously designed landers were able to return much new data about the atmosphere and surface of Venus as well as return the first images from the surface of another planet. The carriers for these landers were then able to enter orbit around Venus to make the first long term close up observations of our sister planet. These planetary missions were clearly the most successful flown by the Soviet Union to date and marked the beginning of a decade of Soviet dominance in the exploration of Venus.
Comparison of Earth and Neptune to the same scale. (NASA)
#3 Habitable Planet Reality Check: Terrestrial Planet Size Limit
Even though the primary mission of NASA’s Kepler spacecraft has ended, the analysis of its huge database continues. One of the many outstanding questions that this analysis has begun to address for the first time is the nature of the transition from rocky terrestrial planets like Earth to volatile-rich planets like Neptune. While planets in this size range do not exist in our Solar System, astronomers have been finding and characterizing the properties of dozens of such worlds orbiting stars observed by the Kepler mission. Considering the importance of this topic in assessing the potential habitability of extrasolar planets larger than the Earth, it is no wonder that this article, originally posted on Drew Ex Machina in July 2014, ranked #5 in 2014 as well.
This article summarizes the work in this area by astronomer Leslie Rogers (Hubble Fellow at Caltech) which now shows that planets with radii larger than about 1½ times that of the Earth (corresponding to masses greater than 4 to 6 times that of the Earth) are unlikely to be terrestrial planets but are probably mini-Neptunes instead. One of the consequences of this continuing work is that the overwhelming majority of extrasolar “super-Earths” that enthusiasts had touted in recent years as being potentially habitable are in fact not even terrestrial planets, never mind potentially habitable. The impact of this new finding was illustrated in my review of the currently known potentially habitable exoplanets published in Centauri Dreams in January 2015 (see “A Review of the Best Habitable Planet Candidates”). Work in the refining the mass-radius relationship of planets larger than Earth but smaller than Neptune continues (see “The Composition of Super-Earths” and “A Mass-Radius Relationship for ‘Sub-Neptunes’”)
A view of Proxima Centauri (circled) with the much brighter stars Alpha and Beta Centauri visible at the top center and top right, respectively.
#2 The Search for Planets Around Proxima Centauri
The small red dwarf star Proxima Centauri is the closest star known to our Solar System narrowly beating out the pair of Sun-like stars, α Centauri A and B, that appear to be associated with it (see “The Search for Planets Around Alpha Centauri” and “The Search for Planets Around Alpha Centauri – II”). Because of its closeness and small size, it has been a target for the search of planetary companions for decades. This post, ranked #2 for 2015, summarizes the latest findings of the various searches for extrasolar planets orbiting Proxima Centauri. Although nothing has been found to date, these searches have eliminated the possibility that our neighbor harbors any Jupiter-size planet in distant orbits or even Neptune-size planets in orbits with periods less than 40 days. These findings still leave open the possibility that Proxima Centauri has Mars to super-Earth size planets including potentially rocky planets orbiting inside the habitable zone of our neighbor.
Engines used in the Saturn launch vehicles (NASA)
#1 A History of American Rocket Engine Development
The most popular post on Drew Ex Machina in 2015 was also the most popular in 2014 which is not too surprising. With the ongoing issues of the Russian-built RD-180 engines used by ULA’s Atlas V and efforts to develop engines for new families of American launch vehicles, the state of rocket engine development in the US has been a subject of much interest recently. This essay recounts the origins of the most important large rocket engines of the early Space Age during the 1950s and 1960s. After the development of the Space Shuttle SSME in the 1970s (also known today as the RS-25), most rocket engines used on American launch vehicles had been just incrementally upgraded and modernized versions of the earliest rocket engine designs. Only two totally new engines have been developed in the US over the past couple of decades and actually flown: the RS-68 used on ULA’s Delta IV and the Merlin-series engine for the Falcon built by SpaceX. Given that it has taken decades of bad government policy, a long string of competition-killing corporate mergers and a series of bad business decisions to get us into the situation we find ourselves today, it will likely take a while for today’s issues to be resolved and for American rocket engine technology development to flourish once again.
I would like to take this opportunity to thank the almost 66,000 people around the globe who have taken time to read these Top Ten as well as the many other posts on Drew Ex Machina during 2015. The many comments and feedback on this site as well as in other forums have also been greatly appreciated. I intend to continue posting interesting essays on space-related topics during 2016 and hope that all of you continue reading and enjoying them.
Happy New Year! Drew LePage
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“The Top Ten Posts of 2014”, Drew Ex Machina, December 31, 2014 [Post]