Spacebag, Volume I
Black Holes, Mars Missions, and Very Fast Spacecraft
On Twitter and Facebook, I asked for your space and spaceflight related questions, and got a whole bunch. Let's dive right in.
@venkersteell: Do black holes move, or are they stationary?
Black holes move, much like stars and planets do. As with stars and planets, it's important to ask: moving relative to what? When we consider the traditional model of our solar system, with the Earth and other planets and small objects orbiting around the sun, we typically imagine (or illustrate) the sun as stationary in space. But of course, it isn't. The sun, along with all the other stars in the Milky Way galaxy, revolves around the center of the galaxy at a velocity of about 220 km/s, completing one orbit of the center of the galaxy every 240 million years, and dragging all of its orbiting children with it. Any black hole that is a part of the Milky Way galaxy also orbits in this fashion, and black holes in other galaxies orbit their galactic center.
A notable exception is the centers of galaxies themselves. It is now suspected that most galaxies contain a supermassive black hole at their center, and our galaxy is no exception, harboring Sagittarius A* at its core. Sagittarius A* is believed to be 4.1 million times more massive than the sun, with a radius of perhaps 45 AU (astronomical unit; 1 AU is the distance from the Sun to the Earth). Galactic center black holes come in even more gargantuan flavors: the black hole at the center of NGC 4889 is estimated to be 21 billion solar masses.
But these central black holes are also moving through space; they and the galaxies that surround them are all receding from one another via the continuing expansion of space. They are also meandering around a bit, deviating from the paths they would take if their motion was attributable to cosmic expansion alone -- this wandering is called peculiar motion. For example, the Laniakea Supercluster, of which the Milky Way and about 100,000 other galaxies are a part, has a peculiar motion in the direction of the nearby (relatively) Shapley Supercluster. Many galaxies and their central black holes have peculiar motion in some random direction or other.
There is a third possibility to consider. There are certain cases in which interactions between three or more orbiting bodies can result in one body being flung out of the system at high velocity. This can mean planets that have escaped their parent stars and orbit the center of the galaxy directly, or stars that have achieved escape velocity from their galaxy and are on their way to, or already moving through, the immense void of intergalactic space. There is no reason (that I'm aware of) why smaller black holes can't also undergo this interaction. Such a "rogue" black hole would be moving relative to other stars as well as the galaxy it is traveling through or has escaped.
@achillesheald: In your mind, what engineering problem poses the biggest obstacle to a manned Mars mission?
There's no shortage of choices here. There are dozens of components of a crewed Mars mission where we'll need to substantially advance our knowledge and technology before we can safely and feasibly accomplish one.
Living in space is a tempting choice. The International Space Station has given us an ongoing opportunity to improve human comfort and adaptation to living in a microgravity environment for long durations. Two astronauts will begin a one year mission aboard the ISS this month with the goal of better understanding the impact of living in space on human physiology. The problem is that right now we have no surefire solutions to issues like bone density loss and eyesight degradation that occur in astronauts who spend significant time in microgravity. And after 200 or more days in space on the outbound leg of a Mars mission, the crew does not return to the comfort and safety of Earth to recuperate, but instead has to accomplish a complex, high-energy descent and landing, and eke out a living in Mars' hostile environment while completing their scientific objectives. Space's toll on the human body could easily sink a mission.
But is this really an engineering problem, or just the inevitable consequence of some over-evolved primates bumping up against the hard limits of their biology? There may be an engineering solution: simulating gravity with a centrifuge. By placing the habitat portion of the spacecraft in a rotating torus, the astronauts can be made to experience "gravity" via centripetal force. This is an enormous engineering challenge, and even if it is overcome, it would be infeasible to create a centrifuge that could simulate the full 1 G of gravity that humans experience on Earth. It's critical, then, to determine whether 0.5 Gs or less is enough simulated gravity to prevent the health problems that plague long-duration missions. This entails testing, which could easily push a crewed Mars mission into the 2040s or later.
Another obstacle is EDL -- entry, descent, and landing upon the missions arrival at Mars. This might seem pessimistic, since we've already had several successful EDL phases with robotic craft like Pathfinder, Spirit and Opportunity, and the Curiosity rover. But I don't think it's as simple as just taking those efforts and scaling them up.
Mars Curiosity Rover: An Inside Account From Curiosity's Chief Engineer by Rob Manning is a great account of the engineering efforts behind landing the van-sized Curiosity rover on Mars. But Manning also describes a 2004 panel he chaired, the Human Planetary Landing Systems Panel, which comprised astronauts and engineers and put considerable thought into the technical challenges of landing a crewed spacecraft on Mars. Manning points out that, taking into account all the supplies needed to support human activity on Mars, a crewed mission could require landing up to 70 metric tons of materials at a precise location on the planet -- a much heartier challenge than landing relatively small robotic probes.
Unlike landing on the Moon, a Mars lander needs a heat shield to protect against entry into the atmosphere, and, as previous Mars landers have done, will need to jettison it to reduce landing weight once it's no longer needed. For a craft large enough to support a crewed landing, this heat shield would be enormous, and impact with the surface at high velocity after jettison could easily destroy any prearranged base camp. Moreover, Manning's panel conceded that parachutes capable of decelerating a supersonic entry vehicle had only ever been tested at diameters of 26 meters, and the physics of parachutes mean that designing and implementing ones large enough to decelerate a large crewed craft may be a dubious proposition at best.
Manning describes landing on Mars as "annoyingly in between" the procedures for landing on the Moon (slowing the craft with rocket propulsion) and landing on the Earth (slowing the craft with parachutes):
[Mars] has too much atmosphere to land as we do on the Moon and not enough to land as we do on Earth [...] That's the problem we face when designing our Mars landers. It's the reason our machines are such Rube Goldbergesque contraptions and why seven minutes of entry, descent, and landing are so terrifying. We have to combine all of the tricks we use to land on Earth (heat shields, parachutes) with the techniques we use to land on the Moon (retrorockets, airbags), among many others.
We've only recently mastered pulling off this blend of techniques for robotic probes and rovers. Accomplishing it for much, much larger crewed spacecraft is vastly more complicated than simply making larger components.
So it's tough for me to choose between these two issues, although the latter might fall more under the category of engineering obstacle. Launching from Earth and from Mars shouldn't require breaking any new ground, as we've gotten very good at launch vehicles. Getting all of the mission's mass on a trans-Mars trajectory may require launching and inserting it in multiple runs, or docking together components in Earth orbit first, but we have some experience with those techniques. Surviving in a deleterious environment en route, and landing on a large body with an unhelpfully thin atmosphere pose much bigger stumbling blocks.
@Phrozen: Pick one: Mars Direct or One Way to Mars or Constellation or SLS-Mars?
For those who may not be familiar, let's just quickly define these at the outset:
Mars Direct is a proposal dating back to the early 1990s by NASA engineer Robert Zubrin, who runs the Mars Society. It aims to combine simple engineering techniques and well-developed science to achieve a crewed mars mission as quickly, simply, and cheaply as possible.
One Way to Mars or Mars One is a Dutch project that I'll discuss in a bit more detail later on.
Constellation was a Bush-era NASA program that had the goal of initiating beyond-Earth orbit operations (i.e. the Moon and Mars) just as the Shuttle program was winding down. Constellation planned for a crew launch vehicle that used a modified Shuttle solid rocket booster (SRB) as the first stage, and for a separate launch of all the necessary cargo for a deep space mission using a larger rocket that combined elements of the Shuttle and Delta IV launch vehicles. The two would then rendezvous in space and proceed to the destination.
Constellation was canceled by President Obama in late 2010, and in 2011 the Space Launch System (SLS) was announced as its replacement. Like Constellation, SLS is a beyond Earth orbit (BEO) system, and utilizes modified elements from the Shuttle program. The launch vehicle uses two modified Shuttle SRBs as boosters (with plans to replace these with more advanced boosters down the line, possibly liquid fueled), as well as 4 RS-25 Space Shuttle Main Engines on the core stage. The initial version (block 0) is planned to put 70 metric tons into low Earth orbit, while the end goal is a version that can put 130 metric tons in LEO (by way of comparison, the Saturn V rocket put 118 metric tons in LEO). Both Constellation and SLS use the Orion Multi-Purpose Crew Vehicle, which had its first test flight in December, for transporting astronauts.
NASA produced animation depicting the launch of the 70 metric ton SLS configuration, showing ascent, separation of the SRBs, and separation of the upper stage and Orion crew module.
Mars Direct seems to win just on maturity, here. If we resolved right now that we absolutely had to put people on Mars as quickly as possible, the resulting mission would end up looking a lot like Mars Direct. This is because people have been thinking about and revising Mars Direct for nearly twenty years now. Both of NASA's BEO programs, Constellation and SLS, have borrowed a bit from Mars Direct. The Constellation cargo launch vehicle Ares V, for example, looked quite a bit like some imaginings of the Mars Direct launch vehicle. NASA's baseline design for a Mars mission is based on an examination and modification of the Mars Direct model.
The plan solves some of the problems mentioned above using fairly basic and established science in clever ways. The problem of mass is solved using uncrewed missions that first land supplies and the ascent and return vehicle on Mars. An automated fuel plant in the return vehicle begins manufacturing fuel using an onboard supply of hydrogen and carbon dioxide from the Martian atmosphere, via a very simple chemical reaction. The human element doesn't depart for Mars until the necessary return fuel is manufactured. The issue of microgravity is solved with a simple tether. Instead of a large centrifuge, the crew habitat is attached to one end of a tether, with the propulsion module attached to the other end, and the whole assembly is spun up, as illustrated in this slide from a demonstration deck about the Mars Direct approach:
This method simulates gravity via the centripetal force without having to overcome the engineering challenge of building one continuous spinning structure.
Gregory Benford's The Martian Race is a good realistic depiction of an implementation of Mars Direct. Benford plays up the thriftiness of the plan, portraying it as a sort of crassly commercialized profit-grab that uses Mars Direct's cheapness to its advantage. I think, though, that the way Mars Direct leverages canny engineering techniques to make for a relatively simple mission renders it worthy of consideration at any price.
Any crewed Mars mission is unlikely to happen before 2035. SLS, as far as I know, hasn't been paired with any mission architecture that's as specific and developed as Mars Direct, and it may be that NASA would lean toward less adventurous methods than in-situ propellant manufacturing and tether-based gravity systems. But the big ideas and principles of Mars Direct can be adapted for suitable hardware, and that's perhaps what we should hope for. SLS and the inflatable habitat modules made by Bigelow Aerospace could be employed in a Mars Direct-type mission, and both of those are under active development. In a nightmare scenario where SLS gets axed by Congress, Mars Direct could rely on SpaceX's Falcon Heavy as the Earth launch vehicle, though it would take more launches, as the Falcon Heavy "only" lifts about 53 tons to LEO.
So assuming Elon Musk's massive, ambitious Mars Colonial Transport project doesn't quite live up to planning, a NASA modification of Mars Direct to work with the Space Launch System may be the best bet. One more book recommendation: The Martian by Andy Weir, though it focuses primarily on a man's survival on Mars, depicts the Mars "Semi-Direct" modification. It will be released as a film starring Matt Damon later this year.
Greg P. (via Facebook): I'd be curious to your opinion of the Mars One project, a bit more specificity on the plan's details, and/or your thoughts on manned deeper space missions.
The notable difference between the above projects and the Netherlands based Mars One project is that the latter intends to put humans on Mars and leave them there. Mars One aims to land 4 humans on Mars by 2024, and have a colony of 20 people up and running by 2035. The project has laid out plenty in terms of grand designs and fundraising strategies, but little in the way of specific technical plans for travel to Mars, landing, and settlement. Thus far the most that is known is that some uncrewed precursor missions will be launched in the late 2010s for proof of conept, and that SpaceX launchers and upper stages will be relied on for implementation.
Mars One garnered plenty of publicity by taking applications worldwide for the first batch of would-be colonists, even going so far as to plan for significant fundraising from the proceeds of a reality show that depicted the selection, training, and eventual mission performance of the first astronauts to participate. Last month, however, the production company slated to create the show withdrew from their relationship with Mars One.
More importantly, an independent study on the feasibility of the Mars One plan found substantial problems with their plans for settlement and sustainability. The study found that, contrary to the mantra of cost-effectiveness promoted by the Mars One, the launch vehicles alone would cost $4.5 billion (the price of 15 Falcon Heavy launches), and most of the technology that would be relied on for colony sustainability would not be close to practical readiness by the time mandated by the Mars One schedule. In particular, the degree of in-situ resource usage required (far more than that of the simple fuel production of Mars Direct) is far beyond our current abilities, and by the time colonists were on Mars, spare parts alone would make up over 60% of the mass transported to the Martian surface.
In short, Mars One is, at present, a lot of rhetoric with very little in the way of practical underpinning. There is plenty of doubt as to whether Mars One will ever even launch any element of its mission. I think it's safe to say that a NASA or SpaceX run Mars mission has a much greater chance of getting off of the ground at all, and certainly a better chance of getting underway before 2040.
As for other crewed missions to deep space, that's a question left open by our singular obsession with Mars. Where to go after accomplishing that mission hasn't been meaningfully addressed. The inner solar system is probably out of the question. Venus's incredibly dense, blazing hot atmosphere makes a crewed landing more or less out of the question, as does Mercury's proximity to the Sun. Jupiter and its ample system of moons is an obvious target. In particular, Europa's suspected subsurface ocean would be an ideal subject for scientific research, and it would be helpful to have a human crew to handle drilling through the thick crust of ice to get at it. But Jupiter's magnetosphere creates massive belts of radiation that render most of its moons a lethal environment for astronauts; dramatic advancements in workable space-based radiation shielding would be necessary before a long duration crewed mission would be possible. It's not even clear at this point that our robotic probes can withstand that environment.
Then there's Saturn, which also hosts an ensemble of scientifically interesting moons. Any crewed mission to that system would have the benefit of a decade of science performed by the Cassini spacecraft, which has mapped and evaluated nearly all of its significant natural satellites. This includes Titan, with its very thick atmosphere and hydrocarbon seas; it is perhaps the most promising candidate for hosting life beyond Earth in our solar system. But Saturn is very far away. It's twice as far from the Sun as Jupiter is, and a low energy transfer orbit to Saturn would take five years, just on the outbound leg. I have a hunch that improvements in radiation shielding will be the first domino to fall, and that a crewed Jupiter mission, perhaps to Europa, would win out against the logistics of supporting a Saturn mission that's twice as far away, with twice the time lag in communications, and round trip transit times approaching a decade or more. Either way, we're talking about the end of the 21st century at the earliest, most likely.
Anthony R. (via Facebook): How fast is the fastest spacecraft mankind has made?
There is a short and simple answer to this, and there is a longer, more nuanced one. The short answer is that the Helios II probe, on its closest approach to the Sun, attained a velocity of 70.2 km/s, making it the fastest human-made spacecraft in history. 70.2 km/s is, to use a scientific term, bookin'. That's New York to Los Angeles in 55 seconds, or from the Earth to the Moon in about the length of a feature film.
Helios II achieved this velocity via simple orbital dynamics. Take a look at its orbit around the Sun, as well as that of its twin, Helios I:
As you can see, Helios II's orbit was a very eccentric heliocentric (non-circular and sun-centered) orbit. It's aphelion, or furthest point from the Sun, just about brushed up against Earth's orbit, and it's perihelion, or closest point to the Sun, dipped inside the orbit of Mercury. At perihelion, in fact, Helios II was the closest human made object to the Sun. Recall Kepler's second law for a moment:
A line joining a planet and the Sun sweeps out equal areas during equal intervals of time
This applies not only to planets and Suns but to all bodies orbiting other bodies. It basically means that, in a non-circular orbit, the orbiting body will travel faster the closer it gets to its parent body, and will travel slower the further away it gets. This effect becomes more exaggerated the more eccentric your orbit becomes. When you combine this with the knowledge that higher velocities are required to orbit more massive bodies, it makes perfect sense that an object orbiting the a massive body like the Sun in a very eccentric orbit will be traveling very fast at perihelion. Solar Probe Plus, scheduled for launch in 2018, will get very close to the Sun, achieving a velocity of around 200 km/s at its perihelion of around 4 million miles.
Meanwhile, Voyager 1, now in interstellar space, putters along at a comparative snail's pace of 17 km/s. And yet it took a lot more change in velocity -- a lot more delta-v -- to get it on its current trajectory than Helios II required. This is because velocity is only one component of an orbit's energy. It represents the orbiting object's kinetic energy, but there is also potential energy to consider -- the energy the orbiting body possesses due to its altitude, or distance from the parent body. Voyager 1 has achieved escape velocity from the Sun, and consequently is orbiting the center of the galaxy. So while relative to the Sun it's only traveling at 17 km/s, it has a much higher specific orbital energy than Helios II or Solar Probe Plus, which have not and will not escape orbit of the Sun. Helios II holds the record for the highest heliocentric velocity, but velocity is only part of the picture for orbits.
@JNisula: How close are we to contact with #aliens?
I don't have a direct way to answer this question, but I think it's fair to say that our first contact with other intelligent life is much more likely to be a result of us observing some sign of their existence (a radio signal, say, or some detectable impact they have on their star or stellar neighborhood) rather than them cruising through our solar system on a lark. This is where the pesky Fermi Paradox comes in, the confounding question of why we haven't been able to detect signs of intelligent life in a universe that should, by sheer probability, be teeming with it.
It's taken Homo Sapiens Sapiens around 200,000 years to get to our current level of advancement, in a solar system that's been around for about 4.5 billion years. Our universe, on the other hand, is 13.8 billion years old. That's a massive expanse of time in which other intelligent civilizations could have formed and surpassed our current state, in any of the 1022 (a 10 with 22 zeroes after it) other potential solar systems in the observable universe. Even using conservative estimates for sun-like stars, sun-like stars with orbiting planets, sun-like stars with orbiting planets that have the right conditions for life, and so forth, we're still forced to conclude that there is plenty of intelligent life out there. And the logical course for the very old and advanced civilizations is to harness all of the energy from their star (a Type II civilization), and then, perhaps their galaxy (Type III). These efforts would probably be obvious to any of us humans observing the cosmos.
And so far, we've seen nothing. What gives? There's a lot of potential answers, some lonely, some reassuring, some downright terrifying. Rather than recount them all here, I'll simply strenuously recommend this excellent post from Tim Urban at Wait But Why that dives into all of the possibilities. Seriously, read it. Then come back.
Or maybe we have seen something. Read up on the Wow! signal if you never have. It got its name from the SETI investigator who saw its signature in a printout from the Big Ear Observatory where it was detected in 1977, who circled it and wrote "Wow!" in the margin.
Jerry Ehman did this because the finding had all of the hallmarks of the type of signal SETI was looking for -- emanating from outside the solar system, with signs of intelligent authorship. At its peak, the signal was 30 times louder than the background noise of space. Its frequency was almost exactly at the hydrogen resonance line, a frequency that makes sense when trying to signal other intelligent beings because its significance should be readily obvious to any race of beings that can do basic chemistry. It lasted the entire 72 seconds that one horn of the Big Ear antenna could observe it for, making it unlikely to be some errant blip. It's never been detected since, and not for lack of trying, but we've not been able to dismiss the finding with some terrestrial explanation either.
@GoogTheGoog What foreign enemy should we say is going to blow us up if we don't fund NASA properly and do meaningful manned explorations again?
China.
Michael B. (via Facebook): Is John W. Young actually the greatest astronaut in U.S. history?
Quite possibly. John Young flew in the Gemini, Apollo, and Space Shuttle programs. He learned about the Space Shuttle's approval while standing on the Moon during Apollo 16, and then flew the first shuttle mission 9 years later. He was the first astronaut to land the Space Shuttle, taking manual control for certain S-turns and a bit earlier than nominal for the approach because hey, why not? He's noted for an incredibly dry brand of humor that went right over the heads of most of his astronaut colleagues. His STS-1 co-pilot, Bob Crippen, said in Bold They Rise that Young was one of the funniest men he'd ever known.
It'd be criminal, though, not to at least mention Story Musgrave, if only for his cool name and incredible list of academic credentials, which I will paste here from Wikipedia:
[Musgrave] received a BS degree in mathematics and statistics from Syracuse University in 1958, an MBA degree in operations analysis and computer programming from the University of California, Los Angeles in 1959, a BA degree in chemistry from Marietta College in 1960, an M.D. degree from Columbia University College of Physicians and Surgeons in 1964, an MS in physiology and biophysics from the University of Kentucky in 1966 and a MA in literature from the University of Houston–Clear Lake in 1987.
@bxe1234: If you could be an ancillary character from Apollo 13, who would you be? Ancillary is < Deke Slayton.
EECOM John Aaron, because of his role in developing the power-up procedures for the command module prior to Apollo 13's reentry, but mainly because of this ridiculous moment from the prior Apollo mission:
@scottbails13: What do you think about this new theory that there was no "big bang" and that the universe is, indeed, infinite?
I'm no cosmologist, but I do know that any assertion that looks to topple concepts as well-tested as dark matter and the Big Bang should be treated with ample skepticism until they provide a whole lot of observational confirmation. This latest model comes from this paper, which stems from an attempt to reconcile some aspects of quantum mechanics and general relativity, a task that has vexed the physics world for some time now. Better to let astrophysicist Brian Oberlein explain it:
The big bang is often presented as some kind of explosion from an initial point, but actually the big bang model simply posits that the universe was extremely hot and dense when the universe was young. The model makes certain predictions, such as the existence of a thermal cosmic background, that the universe is expanding, the abundance of elements, etc. All of these have matched observation with great precision. The big bang is a robust scientific theory that isn’t going away, and this new paper does nothing to question its legitimacy.
[...]
What the authors show is that their modified Raychaudhuri model eliminates the initial singularity of the big bang. It also predicts a cosmological constant, which is a proposed mechanism for dark energy. Their model is really basic, but this first result shows that this type of approach could work. The catch is that by eliminating the singularity, the model predicts that the universe had no beginning. It existed forever as a kind of quantum potential before “collapsing” into the hot dense state we call the big bang. Unfortunately many articles confuse “no singularity” with “no big bang.”
I think I'll wrap up the Spacebag here, sorry if you sent a question and I didn't get to it. Feel free to tweet or email me questions and I'll answer them in the next one.
*Image credits: Tethered gravity for Mars Direct -- Brian Christie Design; Helios probe orbits -- NASA; Wow! signal printout -- Wikimedia Commons.