18 April 2015

Doing Spaceflight History the Hard Way

The moon and Command and Service Module Odyssey (right) as viewed from the Lunar Module Aquarius during Apollo 13. The mission was NASA's third lunar landing attempt and the first Apollo mission aimed at a site selected for its likely contribution to the scientific understanding of the moon. Image credit: NASA
For many people, spaceflight history is really space nostalgia. I have believed this to be true for a long time, and the recent anniversary of Apollo 13 seems to me to confirm this view.

Before I go any further, let me say that I like to celebrate spaceflight. I am a fan of Yuri's Night and belong to a group of long-time space geek pals who without fail exchange greetings on "The Day" (the anniversary of the Apollo 11 moon landing). Spaceflight has been a central part of my life since Apollo 8 orbited the moon when I was six years old.

Spaceflight nostalgia is a residue of early spaceflight propaganda. We are used to celebrating our space achievements, not delving into them. Celebrating the first steps in our endless migration through the cosmos is good and proper, but it is not enough. Only by understanding our space achievements - their origins, the influences that shaped them, their impact on spaceflight today - can we truly appreciate them and give them the consideration that is their due.

About 15 years ago I was involved in a NASA-sponsored project to write a scholarly history of the Shuttle-Mir Program akin to the early classic NASA histories. Those meaty tomes chronicled Mercury, Gemini, Apollo, and Skylab. Without those books and the document collections historians amassed to write them, much of the history of those pioneering programs would have been lost. Because they exist, one can, if one desires, dig deeper and learn what made our early space projects tick.

When it became obvious that the people behind the project really wanted a "celebratory" (to use their term) picture book with as little analysis as possible, and that nothing I could say would change that, I quit. By then I had accumulated 11 cartons of primary source documents. Those I handed over; I have no idea what became of them. I do know that the book eventually published did not use them. It constituted a missed opportunity to carefully record for posterity a significant space project.

I like to say that "I do space history the hard way." That's a tongue-in-cheek way of saying that I approach it like a scholar. I am not content to describe a space project in isolation. I nearly always seek to include plenty of social, economic, political, scientific, and technical context. I want to show the bruises and bumps and scars. These often make the victories all the more impressive, though on rare occasions they point up hidden failings and threaten to knock heroes from their pedestals.

Nor am I content to focus on missions and programs that successfully ran the gauntlet and resulted in something being launched into space. I want to know about the many proposals for space missions and programs that didn't make it and, crucially, try to learn why they did not. In my view, they tell the real story of spaceflight.

Least of all am I content to focus only on missions. Many space writers merely rehash the same old heroic tales, adorned, perhaps, with a recent quote from one of the original participants. That is the laziest way to write spaceflight history. I would argue that it really isn't spaceflight history at all. It is space nostalgia through and through.

When I approach Apollo 13, or any mission or program, I look for something new to write about. I dig in the archives for something that indicates a broader context. For example, I wrote about the little-known backup plan that would have taken effect had the Apollo 13 Lunar Module malfunctioned on the way to the moon. The mission would have entered lunar orbit and performed science and landing site selection photography. If its descent engine still functioned, the Lunar Module might have been used to carry out orbital plane changes, enabling close-up photography of otherwise inaccessible lunar surface targets.

I also wrote about an Apollo 13-inspired study of piloted planetary mission aborts. The study, performed mere weeks after Apollo 13 returned safely to Earth, found that a quick return to Earth after a major planetary mission malfunction was impossible unless the abort took place immediately after the spacecraft departed Earth orbit. Beyond that point, return to Earth would need weeks or, more often, months. Adding enough propellants to meaningfully extend the "quick-return" abort period could easily double the mass of the spacecraft, which meant in turn a near-doubling of the number of costly assembly launches.

This kind of historical writing shines a new light on Apollo 13. It shows that engineers recognized the many opportunities that existed for failure during Apollo missions and that they sought to put in place useful alternate mission plans. It also shows how Apollo 13 led to new planetary mission risk analysis (I think of it as the engineering equivalent of soul-searching).

I could go on about other problems associated with space nostalgia - the devaluation of spaceflight archives and primary source documents, the creation of comfortable myths that hide important lessons, the sometimes tragic failure to give credit where credit is due in favor of promoting established myths, the neglect of crucial stepping-stone missions in favor of dramatic culmination missions (why does no one celebrate Apollo 9?), and the general non-recognition of spaceflight history as a legitimate field of study in academia - but I think that I have made my point. "The hard way" is really the only way if we truly care about spaceflight past, present, and future.

04 April 2015

The Seventh Planet: A Gravity-Assist Tour of the Uranian System (2003)

2006: The Uranian equator is turned nearly edge on to the Sun. Image credit: HST/NASA/ESA/L. Sromovsky
The four largest and most massive satellites of Jupiter are, in order out from the planet, Io, Europa, Ganymede, and Callisto. Io and Europa form a pair of roughly the same size, as do Ganymede and Callisto. Io has a diameter of 3636 kilometers, while Europa, the smallest of the four, is 3138 kilometers in diameter. Ganymede, the largest moon in the Solar System, measures 5262 kilometers across. Callisto, Jupiter's outermost large moon, is 4810 kilometers in diameter.

The presence of four large, massive moons enabled the Galileo spacecraft to carry out a complex tour of the Jupiter system between December 1995 and September 2003. Over the course of 35 revolutions around the giant planet, Galileo used gravity-assist flybys of the four moons to change its orbit.

By contrast, Saturn and Neptune each have only one large, massive moon. Saturn's moon Titan, the second-largest moon in the Solar System, measures 5152 kilometers in diameter, while Neptune's moon Triton is just 2706 kilometers across. The Cassini Saturn Orbiter, at this writing exploring the Saturn system, must rely on Titan for most of its gravity assists, which means that it must rely more often than did Galileo on its finite supply of rocket propellants to make orbital changes. A Neptune orbiter, with only Triton available for significant gravity assists, would face a similar challenge.

The four largest and most massive moons of Uranus are puny compared with Io, Europa, Ganymede, Callisto, Titan, and Triton. Titania, the largest, measures just 1578 kilometers in diameter. The others are: Ariel (1158 kilometers across), innermost of the four moons; Umbriel, 1169 kilometers wide; and Oberon, (1522 kilometers), outermost of the four. Titania orbits between Umbriel and Oberon.

To scale: Voyager 2 images of the five largest moons of Uranus. From left to right in order out from the planet they are Miranda, Ariel, Umbriel, Titania, and Oberon. Image credit: NASA
Though often derided as small and dull, the reality is that the Uranian satellites are little known. Voyager 2, the only spacecraft to visit Uranus, imaged no more than 40% of any Uranian moon as it flew through the system in January 1986. Furthermore, the Cassini Saturn tour has revealed that even small outer Solar System satellites can be surprising: Enceladus, for example, just 505 kilometers wide and by all rights cold and dead, is hot enough inside that it blasts salty water into space from parallel cracks ("tiger stripes") at its south pole at more than 2000 kilometers per hour.

In a paper published in the Journal of Spacecraft and Rockets shortly before Galileo concluded its Jupiter satellite tour, Andrew Heaton of NASA Marshall Space Flight Center and James Longuski of Purdue University demonstrated that the Uranus system could support a complex Galileo-style tour. This was, they acknowledged, "contrary to intuition. . .because the Uranian satellites are much less massive than those of Jupiter."

A Galileo-style tour would be possible, they explained, because "the key to a significant gravity assist is not the absolute size of the satellite, but the ratio of its mass to its primary, and the mass ratios of the Uranian satellites to Uranus are similar to those of the Jovian satellites to Jupiter." Titania and Oberon form a large outer pair similar to Ganymede and Callisto, they noted, while Ariel and Umbriel form a small inner pair equivalent to Io and Europa. The "Uranian system is nearly a smaller replica of the Jovian system," Heaton and Longuski wrote.

To perform their calculations, they relied on "Tisserand graphs" developed at Purdue University in the late 1990s. Their mathematical tool was named for 19th-century mathematician Felix Tisserand, who had calculated the effects of planetary gravity on the motion of comets. Tisserand followed in the footsteps of Anders Johan Lexell, who in the early 1770s had sought to explain the sudden appearance and subsequent disappearance of a previously unknown comet. In 1770, Comet Lexell flew past the Earth at a distance of 2.3 million kilometers.

A previous post detailed how, in the early 1960s, Michael Minovitch used his own graphs and University of California-Los Angeles and JPL computers to calculate dozens of gravity-assist trajectories. His work laid the groundwork for many planetary missions, including the Mariner 10 Venus-Mercury flybys and Voyager 2's Jupiter-Saturn-Uranus-Neptune "Grand Tour." Minovitch did not, however, calculate satellite system tours; presumably this was because in the early 1960s so little was known of outer Solar System moons.

Next in line: Uranus (upper left) as viewed by the Cassini spacecraft in Saturn orbit. Image credit: NASA
Heaton and Longuski described a three-phase, 811-day Uranian system tour. After launch from Earth in March 2008 and a gravity-assist fly-by of Jupiter in September 2009, the Uranus tour spacecraft would fire its main rocket engine to capture into an elliptical Uranus orbit on Valentine's Day in 2018. This would mark the start of the first Uranus tour phase, which would be devoted to matching the plane of the Uranian equator, ring system, and moon orbits.

Uranus is tipped on its side relative to the other planets in the Solar System, and its moons have equatorial orbits. Heaton and Longuski wrote that the Uranian system would appear edge-on to the Sun in 2007, then would tilt gradually until the planet and its moons pointed their north poles at the Sun in 2028.  

The Uranus tour spacecraft would capture into an initial orbit tilted 13.6° relative to the planet's equator and system plane. It would fly past Titania in May 2019 at a distance of 316 kilometers, allowing the largest Uranian satellite to "crank" its orbital plane. A total of nine similar Titania flybys over 261 days would place the spacecraft into the same plane as the Uranian equator, rings, and moons.

The second phase of the Uranus tour, the energy-reduction phase, would see the spacecraft reduce the size of its orbit, thus shortening its orbital period, while at the same time conducting a thorough exploration of the four largest Uranian moons. This would begin 287 days after the spacecraft captured into Uranus orbit with a flyby of Oberon at a distance of 414-kilometers and would proceed through eight Ariel flybys, five Umbriel flybys, three Titania flybys, and four additional Oberon flybys over the course of the next 395 days. 

The spacecraft would pass nearest any world in the Uranian system during this phase. At the start of its 14th revolution about Uranus, almost exactly one Earth year (364.3 days) after arriving at the planet, it would pass just 54 kilometers over Umbriel's icy landscapes.

Miranda's south polar region in 1986: a mosaic of images from Voyager 2. Image credit: NASA
Heaton and Longuski did not include the enigmatic moon Miranda on their list of close flybys because it orbits close to Uranus and, with a diameter of just 480 kilometers (only a little smaller than surprising Enceladus) is less than half the size of Ariel, the smallest moon they employed for gravity assists. Close proximity to Uranus and low mass would mean that Miranda's gravity could contribute little to shaping the Uranus tour. 

Miranda has some of the most intriguing known surface features on the Uranian satellites - for example, Verona Rupes, a five-kilometer-high fault scarp that begins near the edge of the lighted area visible to Voyager 2. Presumably the Uranus tour spacecraft would image Miranda whenever its tour route took it relatively close by.


The third and final phase of the tour would commence 691 days after Uranus arrival with a 151-kilometer Umbriel flyby. The somewhat arbitrary goal of the third phase would be to place the Uranus tour spacecraft into orbit around Ariel. Through three additional Umbriel flybys and four Titania flybys over 120 days the spacecraft would nearly match Ariel's orbit about Uranus, reducing its maximum velocity relative to its target to slightly less than one kilometer per second. The Uranus tour spacecraft would then briefly fire its rocket motor to slip into orbit about Ariel.


Source

"Feasibility of a Galileo-Style Tour of the Uranian Satellites," A. Heaton and J. Longuski, Journal of Spacecraft and Rockets, Volume 40, Number 4, July-August 2003, pp. 591-596.

More Information

The Challenge of the Planets, Part Three: Gravity