FOR FEBRUARY 2008
National Space Society
New York City Chapter
††† At the 4 December 2008 Seminar some members were shocked at one of
the handouts. It was the output from an orbit computation program to illustrate Hohmann transfers from Earth to Mars. In addition, members were freaked out by some items in my article on Hohmann transfers in other handout.
††† To briefly review what took place, we noted that a spacecraft on a Hohmann transfer (also path or trajectory or orbit) would take about eight months to reach Mars. This is in the order of the travel time actually experienced by previous trips to Mars.
††† There is a bit of difference between a robot craft -- like all the ones so far sent to Mars -- and a human spaceship. All of the previous flights to Mars were one-way trips. The craft did not return. A human ship has to return to Earth with a safe and sound crew.
††† The return Hohmann orbit from Mars to Earth is a reflection of the Earth-to-Mars orbit, taking eight months to traverse. It is the other half of the spacecraft's heliocentric orbit, completed after the crew's dwell time at Mars.
††† Many spacefarers assume that they have to plan for an 18-month voyage: 8 out, 8 back, and 2 for operations at Mars. This is maybe feasible by stretching our experience from Apollo and ISS.
††† After reading my Hohmann article, and others of similar theme, they found that a Hohmann orbit requires a specific alignment of Mars and Earth. After riding to Mars at the first alignment, we must wait at Mars for the second lineup.
††† As life will treat us, at Mars we have to wait some TWO YEARS before heading back home! The return alignment simply doesn't come until then. The demands on the crew are more like a FOUR YEAR journey.
This is absolutely impossible with any existing or reasonably expandible capability of human space travel. We can not go to Mars within 20 years
††† By speeding up the travel, we gain more leeway in when to go and come. But this requires rockets of far greater performance than any we are even thinking of, let alone actually designing and building. If you're following the woes of Mars Science Laboratory, you get some idea of the conniptions involved to speed up a flight to Mars.
††† Mission planners are right to say that the trips to Mars will not in any way be like Apollo flights, where the round trip is completed in a couple weeks. It'll be more like a World War II soldier's tour of duty. I'll NOT be home for Christmas! Keep the home fires burning, with a FOREST of wood.
††† NASA Administrator Griffin a few months ago put out a possible training mission for Mars travelers. They would endure a half-year stay on ISS to simulate the zero-g of the ride to Mars, a half-year stay on the Moon to simulate the low gravity of Mars, and a second half-year stay at ISS for the zero-g of the return ride from Mars.
††† The journey must be accomplished with only the original provisions that the astronauts took with them at the start of the trip. The mission would take 1-1/2 years, already more exposure to outer space than the time on the Moon by ALL of the Apollo teams.
††† If there's any trouble, the Moon is only a few days away and a rescue is possible. This is utterly not so for a Mars journey. Once you're on the Hohmann path, you're going to Mars, dead or alive, in one piece or many.
††† The other aspect of the Hohmann flight is the immense amount of fuel needed. A Hohmann path is the most energy efficient way to transfer between Earth and Mars because all rocket firings are tangential to the orbits of the two bodies. They adjust the speed of the spacecraft to leave the orbit of Earth and slow it down on arrival at Mars. Similar tangential firings are used to get back from Mars to Earth.
††† The savings come from not having to push the rocket against solar gravity. The tradeoff is the length of the trip. It is a slow boat to Mars. Yet even in the ideal Hohmann trajectory, the quantity of fuel needed is humongous.
††† To keep things ideal here, I assume that only minor rocket use is needed at Mars, like to keep the craft in orbit. No landing and liftoff are considered.
††† Here's the orbit computation handout. The left panel has parms for Earth's orbit, assumed circular for simplicity of dialog at the Seminar. The middle panel has the orbit specs for Mars, circular and coplanar with Earth's. The 'burns' are the rocket firings to leave Earth and arrive at Mars. The two are added to give a total change of speed of 5-1/2Km/s.
††† This change of velocity is the 'delta-V' you hear about in trajectory planning. 'delta' is the math term for 'change of' or 'increment of'. Why do we add them as positive and not net them? If we speed up going from Earth and slow down at Mars, shouldn't we partly cancel out the change of speed?
††† Take a closer look at how a rocket works. After any impulse from its motor, the rocket continues in a ballistic path indefinitely. Its trajectory is governed by the ambient gravity field it passes thru.† Only another impulse from its motor can change its speed. That is, fuel for the rocket is burned to speed up AND to slow down a rocket.
††† I said 'change of speed'. This also means 'change of direction', because in rocketry, speed is really a vector with both amount and orientation. A rocket must burn fuel to steer, even if its speed is the same, as indicated by a steady reading on its celerimeter.
††† Thus, for any alteration of speed, any delta-V, fuel is spent.† That's why we numericly add all of the planned delta-Vs in a flight, whether plus or minus, and do not net them out. In fact, engineers can cite a rocket's fuel capacity as the total delta-V that the fuel can provide. The mission planner treats this delta-V as a stockpile from which he doles out small delta-Vs for each firing along the trip.
††† A rocket cannot refuel along the journey. There will be NO filling bases between Earth and Mars for a hell of a long time. ALL of the fuel needed for the ENTIRE trip must be loaded onto the rocket at the start of its trip. Fill a bit short and you kiss the mission good-bye.
††† When I say 'fuel' I mean both the combustile material, like liquid hydrogen or kerosene, and the oxidant, liquid oxygen. NO OTHER PLACE in the solar system has free oxygen, like in Earth's air, to support combustion.
††† Maybe that's a good thing. Imagine if Titan, unknown to us, had a mix of methane and oxygen in its air. The two coexisted quietly since the creation of the moon, some 4 billion years ago. The Huygens lander drops into this air and turns on its instruments. A teeny spark jumps out from an electrical switch or mechanical joint. You would have seen the fireball with bare eye from Earth.
††† Thus, a rocket has a triple burden. They spend fuel for every maneuver, including slowing and steering. They must carry with them all of the fuel needed to complete the trip. And they must bring along all of the oxygen needed to burn the combustile.
††† We now look at the right panel of the handout. This gives the mass of fuel required for the Hohmann path in the instant example. I assumed the Mars ship has a departure mass of 100 tons. The ship is hoisted into low Earth orbit by the Ares-5 booster, which falls away.† Only the 100-ton ship goes to Mars.
††† In order to change speed the rocket throws mass out of its nozzle.† This comes from the gases generated in burning the fuel. By choosing appropriate chemicals to make the combustile (the oxidant is typically liquid oxygen) we can get various amounts of mass generated per second and shoot it out of the nozzle. The reaction force, a-la Newton, pushes the rocket in the opposite direction.
††† To change speed, such as to slow the ship at Mars, the rocket is fired in the forward direction so its reaction force opposes the forward motion. If the ship has only one motor, it must turn end-for-end to aim the nozzle forward. This can be done with gyros or reaction flywheels, avoiding thruster jets that require extra fuel expense..
†† †A measure of the 'strength' of the combustile is the 'specific impulse', SI. This is the force produced when a kilogram per second of it is burned. The value determined assumes a typical rocket motor and operation procedure. It is not an innate property of the chemical.
††† There are some chemicals with high SI, but they are too dangerous, expensive and corrosive (they eat thru the tanks!). The '300' in the panel is a typical value for rocket fuels of today.
††† One point to understand about SI. It is calibrated for EARTH GRAVITY. That's probably because in the early years of rocketry, before the Space Age, fuel was made and tested like other chemicals in labs and shops on Earth. The procedure was simply carried forward with little inkling that the fuel would be used in other gravity regimes.
††† It is trivial to convert the Earth-based SI to a general value for zero-G, but this is not normally done. Think of the SI value as just a number, the higher being the better.
††† As you burn the fuel, its mass is discarded from the rocket nozzle and the rocket speed is altered. For the first burn, to leave Earth, the change is a speedup. The speed ramps up until it reaches the desired final value. Then the rocket is turned off. The time duration of the burn accumulates a total mass of fuel spent. Similar reasoning applies to the slowdown at Mars.
††† Look again at the figures in that right panel. For the Hohmann orbit of this particular example, the ship would consume EIGHTY-FIVE TONS of its initial 100-ton mass! That's how mush fuel has to be tossed out the nozzle to accomplish the two delta-Vs for this flight!!
††† That leaves ONLY FIFTEEN TONS for the useful mass of the ship!† That's for tanks, structure, frame, furniture, fixtures, safety system, pastime services, crew's body mass, spacesuits, batteries, life support, supplies, tools, food & water, waste system, pipes & pumps, rocket motor, electronics, radio & radar, EVERY THING.
††† Did you forget that the example here is for a ONE-WAY trip? Now, let's be sensible. 15 tons is hefty mass for a robotic craft, maybe with multiple landers and daughter satellites, For a human flight, the ship has to return with an intact crew, so the fuel penalty is all that much greater.
††† Spacefarers have a tough go at realizing that chemical rockets aren't going to improve much in the future and there is nothing what so ever else to push a human spacecraft to Mars. There are solar sails, ion drives, electrostatic impulse, and all that. They do work for minuscule size and mass of craft and each has its own flaws.
††† There was in the 1960s only ONE OTHER technically workable means under development for propelling a human spacecraft. That was the nuclear impulse rocket. It would have tossed out kiloton-yield atom bombs to explode against a pushplate behind the rocket! This project was cancelled by the prohibition of nuclear explosions in space.
††† So you see that human space travel is hardly like the Star Trek fiction, where the pilot stomps on the gas pedal and away he flies to Mars in a week. It sure looks like for the next few decades we must accept that even a round-&-back human flyby of Mars is one hell of a dream. It may in the end be that the only way a human can fly to Mars is in a memorial tube of his cremation ashes.
††† Let's stay on Earth and look a little father than Mars. Saturn is coming into the evening sky in the east. His rings are a tiny bit more open than in January but still pretty flattened out. Your telescope friend may be watching the planet more closely now that the hour is more convenient.
††† If he's new to astronomy, he's going bonkers over the ringed planet having no rings! After all, the ring edgeon phase comes only every 15 years, a span exceeding the career of most astronomers.
††† I myself watched THREE ring crossings in my career: 1966, 1980, and 1995. All occurred with Saturn in the night sky AND ALL WERE TRIPLE CROSSINGS! Tricky geometry allowed Earth to see the rings at zero inclination THREE TIMES within a few months.
††† Usually we get a single crossing from one side of the rings to the other. A veteran astronomer, like me, may be jaded! Aren't ALL ring crossings triple crossings?
††† I give here these three triple crossings and the 2009 single crossing event. The dates are in New York time. I include for context the superior conjunctions and oppositions associated with the crossings.
~See top table on left~
††† 'elong' is the distance of Saturn east or west from the Sun along the ecliptic. For all the events of 1966-1980-1995 the planet was in a dark sky away from the Sun. I and other astronomers avida mente watched the events on every instance, weather permitting..
††† Amazingly to me, during the edgeon apparitions, my small scopes COULD DISCERN THE RINGS! They were the thinnest of lines, like a thread thru the planet's globe! The blackout period, with no rings at all, was only a week on either side of the geometric edgeon date.
††† See that for 2009, the crossing takes place a few days before superior conjunction! Saturn is in strong twilight; the event is not observable from the ground in ordinary telescopes! That's why you BETTER get your views of Saturn NOW and into the summer, even tho the rings are still a bit open. If you wait until next year, the rings are opening up again. The wait for the next crossing could be beyond your remaining lifespan.
††† The ring crossing is an integral part of the International Year of Astronomy. This is a special effort during 2009 to present the culture and science of the heavens to the world. It comes on the 400th anniversary of Galileo's first exploration of the stars with his telescope.
††† For home astronomy, the effort is managed thru the JPL Night Sky
Network, for which a massive stock of physical material is available to help in your promotion. NYSkies Astronomy Inc is a member, for New York City, of NSN. Being that the NSS-NYC Chapter meets with NYSkies at the Seminars, it, too, can benefit from the IYA support. You took part in the discussion and demonstration we had about IYA at the
January 1st Seminar meeting
††† Showing Saturn to the public also is part of your normal spacefaring promotion, like for explaining the Cassini mission, It is among the few mature and grownup ways space enthusiasts can participate in the space program. They can draw on some really wonderful tuition materials like posters, handouts, props and videos.† What's more, they earn positive and specific thanks for their work with JPL promotion of their clubs, pins for their members, spacefaring tools to keep for future use.
††† Another IYA project is monitoring epsilon Aurigae (EPP-sih-lonn aw-REE-gigh). I hope you're practicing to recognize this star in the sky as I urged you to in previous SpaceWalks. This star has a spectacular event every TWENTY-SEVEN YEARS! Most astronomers see only ONE instance of this event!!
††† This star is an eclipsing binary star whose orbit is edgeon to our line of sight. The companion star crosses in front of the main star once in 27.1 years, the orbit period of this pair. The incredible feature is that the eclipse takes TWENTY MONTHS to complete! that's from the first contact of the two stars, the first dimming of light, thru total coverage, to final contact, when light returns to normal.
††† There are many models for the star, of which the present one is a huge thin disc of opaque dust is the comes. It swirls around something, a star?, hidden in the middle. It's so huge that it just takes 20 months to get across the main star. The eclipse is not completely total because the disc comes across the central part of the main star, leaving its top and bottom still open to view. So the star dims but does not back out.
††† The show begins in August or September 2009 and continues into 2011. The reason I urge you to get acquainted with epsilon NOW is that the eclipse occurs when it is in the same part of the sky in the same months as now.
††† Spacefaring comes in big time for both Saturn and epsilon. The two are under study now, with major build up of work later, by space observatories. Both are examined in various parts of the spectrum, those not received on the ground and felt only from outer space. Both bodies suffer from white out when the Sun comes near them from the ground. From space they can be examined pretty much at any date, regardless of where the Sun is.
††† Hence, you can point out in the sky positive ways by which we now benefit from space projects. You enhance your spacefaring advocacy in a meaningful way. More over, you do this with out looking like you're shilling for some space company or agency.
††† This is especially important to most spacefarers, who grumble and gripe that no one pays attention to them. People think we're all Buck Rogers or Star Trek freaks with no substantial value to society. This is an exaggeration, of course, but I do hear such complaints.
††† There is an old new comet in the sky. It's a bit dim for easy spotting and is in the late night sky. When your telescope buddy shows you Saturn, ask for a look at comet Lulin (LOO-linn) or comet 2007-N3.† Lulin, a Chinese home astronomer, found it back in July 2007 as a real faint smudge on his photographs. It lingered for over a year in the depths of the solar system, not drawing much attention. In late fall it migrated nearer to the Sun, brightened, and sprouted multiple tails. It was still faint and lost in morning twilight. Now it moved into dark sky, still growing brighter.
††† It's mow in binocular range during February but you do need a detail chart to find it. Give your astronomy friend these elements and ephemeris. He'll know how to read them. Remind him that the dates are for 0h UT which, proudly you elaborate, is 19h EST on the PREVIOUS date. He'll likely key the elements into his computer planetarium program and plot the comet's path on a starchart.
~See StarChart on left #2~
††† 'Earth' and 'Sun' are the distances of Lulin from each body with the Earth's orbit radius as unit, the 'astronomical unit' or AU.. For February 1st (in Greenwich!) the comet is 0n.941 AU from Earth and 1.257 AU from the Sun. To get linear measure, multiply these by the actual Earth orbit radius of, rounded, 150 million kilometers.
††† Lulin is gradually creeping closer to Earth, within about Ĺ AU by mid month. At the same time it's receding gradually from Sun, to about 1-1/2 AU by end month.
††† Lulin passes just south of Saturn on the evening of February 23 and just south of Regulus (REH-guh-luss) on the 27th. There's no telling how prominent it'll be then but it'll be easy to look for.
††† I warn that we have no credible means of accurately foretelling the prominence of a comet. We, as one example you surely can recall, missed predicting the explosion of comet Holmes in fall of 2007. We looked up on one night and, WOW!, what happened?!?!. Don't be surprised if, when your scope friend shows you comet Lulin, it's nothing bt a misty splat against the stars.
††† If the comet fizzles out or the weather smothers it, we got, oh, no!, an other Pleiades (PLAY-a-deez) occultation!! On the evening of February 3rd, the Moon slides across the northwest corner of the group according as the timetable here.
~See table on left #3~
'cusp' os te angle along the edge of the Moon where the star touches. It's measured in degrees from the closer of the north or south cusp of the Moon. A '+' value means the contact is on the dark side; '-', bright.
††† 'pos' is the angle of contact on the edge of the Moon counted counterclockwise from north. This is suited for certain telescopes that turn on axes aligned N-S and E-W on the sky..
††† The chart here shows the scene at 20:00, before Taygeta (TIGH-jeh-ta) is covered. You must orient it to match the stars in the sky. You can line up the diameter of the Moon joining the two cusps. The Moon slides toward the left-upperleft in the chart, hitting only stars in the northwest part of the Pleiades.
††† Besides the brighter stars I mention for Pleiades occultations, you'll see many other dim stars. The moon passes over them, too. I don't give predictions for them to keep the table short and to let you focus on the main stars, like thru binoculars.
††† Don't neglect Venus! Itís in the west, shifting to there from southwest in January. Get frequent looks thru your telescope friend's lens. The simulated views here show how Venus starts as a fat crescent and shrinks to a thin crescent while swelling about 1-1/2 times in angular size. The terminator in most small telescopes will be a sharp
curve, like the outer edge of the planet. The fuzzy terminator in the graphic is an artifact of the computer modeling. Also in small scopes, the planet will be smooth all over with no texture on it.
††† February is really the last good month to inspect Venus. As March gets going, Venus shoots almost straight downward toward the Sun, rounds her inferior conjunction, and leaves the evening sky.
††† This 2008-2009 apparition of Venus is a short one, only three months for the casual spacewalker. that's because in summer and fall of 2008 Venus hugged the horizon and hided behind skyline until quite Thanksgiving. Her convention with the Moon and Jupiter was her real
emergence into public awareness for this go around.
††† Our panorama chart is drawn for 2000 February 14 20h EST. We look
east with zenith at top center. That's Saturn low in the East, the only substantial star in that sector of the sky. Higher up in the east is Regulus. High in east are Castor and Pollux (KASS-torr, PO-luks). Pollux is a planetary star, with specs here.
~See last table on left #4~
††† Pollux is the brightest of planetary stars, just nosing out its close rival Fomalhaut (FOH-al-hawt). The planet runs in a nearly circular orbit, It was suspected since 1993 but confirmed in July 2006.
††† You can point to Pollux and blurb out 'See that there star? It got a planet around it, maybe with its own people looking back at us!' As a spacefarer you can then explain about NASA's Kepler spaceprobe, now at Cape Canaveral for fitting into its rocket for a summer launch. No, it's not examining Pollux or other stars in the winter-spring sky.† It's concentrating on certain stars in the summer sky. The principles are the same: Go and find those planets!
††† I marked the location of comet Lulin for February 23rd in New York when it is next to Saturn and on the 27th next to Regulus. The places are approximate due to the rapid motion of the comet, discernible within a couple hours. By pure chance, Lulin is drifting west, upward at chart time, almost exactly along the ecliptic.
††† With binoculars, look for the star cluster M44 or the Beehive.† It's about Ĺ between Pollux and Regulus. If the air be clear (and likely cold!) and the Moon is out of the sky, you may see the Beehive as a tiny round cloud. A few people I met over the years claim they can see the individual stars. These folk had extra keen vision. Normal eyes will see only the soft glowing patch but any binoculars will burst it into stars.
††† Now is a good month to say 'hi!' to the Big Dipper in the northeast balancing on its handle. For us in the City the Dipper is hidden low in the north for the late fall and early winter months in evening.
††† With binoculars look 1/3 from Denebola (deh-neh-BOH-la( and Alkaid (al-KAYD), you'll come across an other star cluster. This one, Coma (KOH-ma), is so close and spread out that it's its own constellation.† It takes a bit of better vision to see its separate stars by bare eye, but binoculars bring them easily. Its main stars make a wishbone
pattern. Behind Coma's stars, is the Coma luster of galaxies. Your
telescope friend may show you one or two of these.
New York Space Society
Chapter of the National Space Society