Walkabout the Galaxy: Our Newest Episode Yet

Check out episode 5 of Walkabout the Galaxy and subscribe to our podcast. In this episode, guaranteed to be the newest one until the next episode, Josh, Addie and Tracy discuss the original Wiener, getting sliced into tiny pieces, and then there’s a bit about the LADEE mission to the Moon as well.



Return to the Interwebs

For a variety of reasons, most of which are either too depressing or too boring to recount, this blog has been stagnant (and even unaccessible) for a while. I’ve got a new hosting service now, and to celebrate here are a couple of movies I’ve created with the UCF Center for Distributed Learning and two amazing students, Tracy Becker and Meghan Keough. This one explains the geometry of lunar and solar eclipses, and this one explains the orbit and phases of the Moon. More to come!

Cassini Resumes Observations of Saturn’s Rings

After spending more than a year orbiting Saturn in the planet’s equatorial plane, the Cassini spacecraft has embarked on the “IN-1” series of inclined orbits that will give us excellent views of Saturn’s rings. The observations I analyze are stellar occultations in which we measure the brightness of a star as the rings pass in front of it. Our first IN-1 ring stellar occs are coming up June 28-29. Our occultations in the Cassini Solstice Mission (that runs through the planet’s northern summer solstice) are all unique in some particular geometric or scientific aspect. In some, the path of the star relative to the ring particles will slow to a relative crawl, allowing us to sample the structure of the rings at the scale of individual ring particles (~ 1 meter). We will also be observing Sirius, the brightest star in the sky, jointly with the Cassini Visual and Infrared Mapping Spectrometer (VIMS). It is one of the few stars that both VIMS and the instrument I work with (UVIS) can see since we look at very different wavelengths. This will enable us to confirm small, unusual features that we discover as well as provide information on the population of the smallest particles in the rings. In the meantime, there are excellent images of the rings and the rest of the Saturn system available here, including my famous ultraviolet image of the rings in the Cassini Images Hall of Fame.

The 2012 Nonsense

It is an occupational hazard of an astronomer to be asked about the supposed catastrophe(s) in the year 2012. Usually people want to know about the effects of the alignment of the planets in that year. Sometimes, prompted by the movie 2012, they mention neutrinos. Even if they don’t think the world will end, they are surprised when I tell them that there is no planetary alignment in 2012, and uncomprehending when I point out that it would not make a bit of difference if there were.

Here is a movie of where the 8 major planets are from 2000 through 2050 (click the link to open and play the movie; use the controls to scroll through and pause on any year, displayed at lower left).
Motion of the Planets from 2000 to 2050
Notice how closely spaced the four inner planets (Mercury, Venus, Earth, and Mars) are compared to the outer four. Notice also the frenetic pace of the inner planets. Clearly it’s not too rare for Earth and its terrestrial neighbors to be roughly lined up simply because they are all orbiting the Sun relatively quickly. And just as clearly, with its 165-year orbital period, Neptune (the triangle on the right side of the movie) is not very frequently in line with the rest of the planets. There is a much more impressive alignment of the planets, in fact, in 2010 than in 2012.

And just what are the implications of a planetary alignment? For the outer planets, it means skygazers will have a nice view of several planets each night, as they will all be up in the night sky at about the same time. That’s about it. If we tally up the ways planets could interact with us here on Earth, we come to a pretty short list:
– they reflect sunlight toward the Earth
– they have a gravitational effect on the Earth
Their magnetic fields do not extend to the Earth which is, anyway, enclosed in its own relatively strong magnetic field. The amount of sunlight coming to us from the planets is obviously puny and generally less than many stars.

For their gravitational influence, we can do a simple comparison. The gravitational acceleration exerted by an object on you or me is proportional to the mass of that object divided by the square of the distance between you or me and that object. So, for Jupiter, the most massive (and relatively nearby) planet, the gravitational acceleration you feel due to Jupiter’s presence is proportional to Jupiter’s mass (1.9 times 10^27 kg) divided by 3.9 times 10^25 meters squared. That gives us 48 in our units (where we are not worrying about the universal constant of gravitation since it will drop out when we make our comparison). Let’s compare that to, say, the acceleration you feel due to the gravitational influence of your spouse. If your spouse or significant other is a rather svelte 110 pounds (50 kg) and is sleeping 1 meter away from, then the gravitational acceleration you feel from that person is 50 divided by 1 squared, or 50, roughly the same value as the entire planet Jupiter. To pick a more dramatic example, when you stand next to your car, it is exerting a far greater influence on you (about 40 times greater) than all of the planets in the solar system. But then, you knew that, didn’t you?

The two astronomical objects that do produce a noticeable gravitational effect down here on the surface of the Earth are, not surprisingly, the Moon and the Sun. And when they line up it does have a measurable impact: the so-called “spring tides” or “full Moon tide” and “new Moon tide”. The gentle rising of the ocean up the beach every six hours or so is due to the tidal force of the Moon, and tides are simply due to the difference in the Moon’s gravitational pull across the body of the Earth. The Sun, although more massive, has a somewhat smaller effect on tides than the Moon because it is so much further away. But when the Sun and Moon are aligned (at full Moon and new Moon), their tidal effects combine and ocean tides are higher than usual. In the units we computed gravitational acceleration above, where Jupiter and the person standing next to you both rate about a 50, the Moon’s effect is about 460,000 and the Sun’s is 9 billion. The Earth does go around the Sun after all. (Tidal force depends on the derivative of gravity and so gets weaker with distance faster than gravity, hence the weaker tidal influence of the Sun than the Moon.)

So catastrophes in 2012 are likely to be restricted to homebound, terrestrial causes (hurricanes, earthquakes, volcanoes, famine, drought, floods, and elections). The only sky-based catastrophe possible would be the impact of a comet. A catastrophic asteroid impact in 2012 is unlikely because we have much better advance warning for asteroids than for comets due to the shapes of their orbits (comets can sneak in from the outer solar system, while we have observed most dangerous asteroids).

Astronaut Training Day 2 – Centrifuge Flights

Day 2 was all about the Phoenix centrifuge at NASTAR. After some instruction on techniques to increase blood pressure to avoid loss of vision and black out, we did a series of four flights in the morning. Because the centrifuge only accommodates one person at a time, and because there were a dozen of us, it took a while for everyone to get a ride. I was fifth to go. The four flights consisted of brief profiles of sustained acceleration along either the body’s plus X axis (into the chest) or the plus Z axis (down the spine). The latter pose problems for consciousness because +Gz makes it harder for the heart to pump blood to the brain. The Gx flights make it difficult to breathe, but are not generally likely to make one pass out, at least for the durations we were doing (about 20 seconds at a time).

I have previously had experience with two G’s on parabolic airplane flights. The first time I flew one of those flights, I oriented my body so that the two G’s were in the +z direction, and I got very sick after about a half dozen parabolas. On subsequent flights I lay flat on the floor of the plane, making those G’s in the +x direction and therefore much easier to bear. So I was concerned about our 2 Gz and 3.5 Gz flights, though they wouldn’t have the repetition of the “vomit comet” nor would they be interspersed with 0 G parabolas. On the 3.5 Gz flight I had to apply all of the body-tensing countermeasures we used because I started to get a bit of tunnel vision. The countermeasures worked. The Gx flights, at 3 and 6 G’s, were impressive. The sensation of going up very very fast was completely convincing. At 6 Gx it was a real effort to breathe, and speech was very difficult. All in all, the flights were smooth and didn’t make me sick.

In the afternoon we did two flights simulating the acceleration profile of Virgin Galactic’s SpaceShipTwo. One was at 50% of the total acceleration, and the other was full acceleration. These profiles involved both Gx and Gz at the same time, along with a visual simulation of what we would see through the window of the spaceship. These flights really gave the impression of going somewhere FAST. On the final run, I had to apply countermeasures to keep my vision as things started to go gray during the 3.8 Gz portion of the rocket burn. The peak accelerations are actually on re-entry, but they are Gx and so are easier to deal with.

Astronaut Training Day 1 – Altitude Chamber

Today we got a tour of the NASTAR center which has some impressive aircraft simulators and a gigantic centrifuge (11 ton, 25-foot arm, with bolts going 45 feet down into the bedrock and a huge mass of concrete underneath to keep it stable as it swings around). Then we had a course on the physiology of hypoxia (oxygen deprivation) and some basics on atmospheric physics before getting fitted with oxygen masks and heading for the altitude chamber. I’m not actually sure that’s the write term, but it’s a room with a dozen seats and ports for oxygen masks and can have its pressure adjusted to simulate various altitudes.

After 30 minutes of denitrogenation (breathing pure oxygen to remove nitrogen bubbles from the blood to reduce the likelihood of those bubbles expanding to painful size on ascent to high altitudes), we took our masks off and they took the chamber up to 18,000 feet. That is to say, they lowered the pressure in the room to what it is at an altitude of 18,000 feet. At that altitude, the pressure is about half what it is at sea level. So each breath delivers half the oxygen of a breath at sea level. We had some simple exercises to perform – simple math operations, some writing – to identify any degradation in mental function as we entered a hypoxic state. I noticed an increased heart rate, but no other symptoms. I have done two altitude “flights” in the past, about 10 years ago, with no noticeable effects. I could not tell if the increased heart rate was due to lack of oxygen or simple anxiety about possibly worse effects. After about 15 minutes, one member of our group passed out. By that time I was feeling a bit tired, but otherwise no overt effects of hypoxia. My simple math problems were done without error, as were the two mazes.

Suborbital Astronaut Training at NASTAR – Day 0

Today I flew to Philadelphia with my graduate student, Akbar Whizin, in preparation for a two-day course on suborbital spaceflight at the NASTAR center. With at least two companies readying commercial suborbital rockets to carry paying passengers to the lower limits of outer space, there is increased interest in the uses of these vehicles for science and education and not just high-priced sightseeing. NASA has long had a vigorous program of experimentation in suborbital sounding rockets. These new vehicles may soon find a place as laboratories for scientists and students who need quick and easy access to either the upper reaches of the atmosphere or a few precious minutes of high quality microgravity.

My own scientific interest in these vehicles lies in the study of the collisional behavior of small objects and aggregates of objects at low impact speeds. I’ve had one such experiment fly twice on the space shuttle and a similar experiment has flown several times on parabolic airplane flights. These experiments simulate in various ways the collisions that were common in the early stages of the formation of the solar system and are currently taking place in Saturn’s rings (and the rings of the other planets). It is not possible to perform experiments on these kinds of collisions without a microgravity environment. A few seconds of microgravity can be achieved in a drop tower, and 10-15 seconds of a relatively uneven low-gravity environment can be obtained on parabolic airplane flights. For many experiments a longer, more stable microgravity environment is needed.

Virgin Galactic has unveiled the first of its passenger-carrying suborbital crafts, the VSS Enterprise. Blue Origin has selected my experiment and two others to fly on a test flight of their New Shepard suborbital rocket. Other companies are developing rockets for passengers and some just for payloads. Someday soon, scientists may be flying alongside their experiments on these rockets, reacting to the performance and making real time adjustments to the operation of the experiment. And so I find myself getting ready to undergo two days of “astronaut boot camp” at the NASTAR center. Tomorrow features some hypoxia training and time in a chamber simulating high altitudes (low atmospheric pressure). Wednesday will be a full simulation of a flight on the VSS Enterprise. The final frontier awaits.

The Geysers of Enceladus

As Saturn’s south pole slips into its long winter, so does the active southern region of Enceladus, nicknamed the Tiger Stripes, bid farewell to the Sun for the next 15 years. The latest flyby of Enceladus by Cassini – the E-8 flyby – provided the most dramatic and perhaps final views of such clarity of the water vapor geysers emanating from the Tiger Stripes.

The geysers at Enceladus's south polar region.

The geysers at Enceladus's south polar region. Image: NASA/JPL/Space Science Institute.

The vapor is visible in geometries when we look back toward the Sun. In the image below, the Tiger Stripes are seen in relief making use of detailed images and a topographic map created by Paul Schenk at the Lunar and Planetary Institute.

Crevasses in the south polar region of Enceladus.

Crevasses in the south polar region of Enceladus. Image: NASA/JPL/Space Science Institute.

Researchers are still working on models to explain how such a small moon, just a few hundred miles across and therefore an object that would cool off and freeze solid shortly after formation, manages to have a reservoir of liquid water – or at least very warm ice – near its surface. If it is like the active moons of Jupiter, then flexing of the moon by tidal forces from Saturn explain the melting in Enceladus. To maintain tidal heating, Enceladus must be pushed around by gravitational interactions with nearby moons. The problem is that the tidal heating scenario for Enceladus is far less clear than it was for Io, the volcanically hyperactive moon of Jupiter. Stan Peale, a professor at UC Santa Barbara and lead author of the paper that predicted Io’s volcanoes, presented an alternative hypothesis for Enceladus at this year’s DPS meeting. Some of Saturn’s moons have co-orbital satellites: small satellite shards that share an orbit with their larger lunar siblings. Peale and co-author Rick Greenberg suggested that a collision between Enceladus and a co-orbital moon within the relatively recent past (less than 200,000 years ago) could have supplied the necessary heating to drive the geysers to the present epoch.

The LCROSS Crash

Like many, I was up bright and early last Friday morning to watch the live coverage of the LCROSS impact into the shadows of the Moon’s south polar region. While the complete fizzle of the impact probably should not have been a surprise to me, it was certainly a disappointment to countless people whose expectations had been unreasonably heightened. The purpose of the impact, of course, was not to make a cosmic fireworks display, but to determine the abundance of water ice near the surface in the permanently shadowed craters near the Moon’s south pole. Whether it met that goal will become clear in the days and weeks ahead. Science usually moves forward gradually. Eureka moments usually take some time for to confirm and validate. There are the occasional moments in space exploration, however, when something definitive happens, when there is an EVENT. Given the realities of the 24-hour news cycle, NASA usually seizes on the opportunities provided by these events (the launch of a rocket, the arrival of a spacecraft at another planet) to get some air time with the public. But they risk losing the attention of that public if they don’t learn to be more careful about managing expectations.

Saturn’s Wound-Up C Ring

Here’s an article I wrote about one of the coolest discoveries from Cassini to date. Thanks to Emily Lakdawalla at the Planetary Society for posting this.