I’ve had the mixed pleasure of spending a fair amount of time experiencing what is usually called “weightlessness”. I say it is a mixed pleasure because while the sensation of weightlessness is amazing and so different from our everyday experience of the world, I have experienced it on parabolic airplane flights which have the unhappy side effect in a segment of the population of inducing nausea and vomiting. I am in that unlucky segment. The body does adapt, and my last flight was puke-free. Other names used to describe the state of weightlessness are zero-g, no gravity, microgravity, and freefall. The latter is the only one that is truly accurate.

As an astronomer, gravity is the force that most concerns me professionally, and it is also the force that most of us have the most direct intuitive relationship with in our daily lives. And yet the relationship between gravity and freefall or “weightlessness” seems to be as elusive to most people as the sensation itself. Whether I am lecturing to a university astronomy class, speaking to a group of elementary school kids, or giving a public lecture to educated professionals, I always try to demonstrate the amazing insight of Isaac Newton about gravity: the same force that makes the Moon orbit the Earth is responsible for apples falling to the ground. While it is easy to understand those words, their implications for how the solar system works and for “weightlessness” usually remain abstract or obscure. Working against us is not just our daily experience (and, one could reasonably argue, millions of years of evolution), but also the language we use to describe gravity and its presumed absence.

Here is my standard gravity stump speech. For these purposes we do not need to stray into the exotic terrain of warped space-time and Einstein’s general relativity. Our sensation of gravity here on planet Earth comes not from the force of gravity exerted on us by the Earth, but by the competition between that force and all the stuff that gets in the way of it. If you are sitting now, you feel your weight because the chair is stopping you from falling to the floor. The actual sensation of weight I feel right now is due to pressure of a chair seat against the backs of my legs, the pressure of the floor against the bottom of my feet, and the pressure throughout my body produced by the weight of head on neck, torso on lower back, and so forth. So there are two ways to get rid of that pressure: get rid of the Earth, or get rid of the chair. If the chair beneath you were instantly snatched away, you would fall to the floor. And in that split second you would not feel the pressure of the chair on your backside. That sensation of weight would be gone, even though the Earth’s gravity is still very much present.

How about the weight of your head on your neck, etc? Galileo’s famous experiment at the tower of Pisa gives us the answer. Here again, though we may be familiar with the facts of the experiment, the implications are difficult to internalize: gravity makes everything fall at the same speed, whether it be a feather or a hammer, a head or a body. We (and centuries of thinkers between Aristotle and Galileo) have a hard time with this because air does a better job of slowing a feather than it does of slowing a hammer, so, in fact, the feather does fall slower. But if you get rid of the air (easy enough in a small lab experiment), they all fall at exactly the same rate. So when that chair is snatched away, all parts of your body will fall toward the floor at exactly the same rate. There will be no pressure of any part pushing up against any other part. And since that pressure is what we experience as weight, its absence gives us, in that brief period before slamming into the floor, “weightlessness.”

And yet we are still experiencing the Earth’s gravitational pull. In fact, in physics the term “weight” refers not to the pressure we feel from the chair, but simply the force of gravity acting on an object. Removing the chair does nothing to alter that force. It removes instead what is called the “normal force” of the chair that exactly cancels the force of gravity acting on our bodies. The rigid structure of the chair exerts an upward force on our bodies that keeps us from moving down due to the force of gravity. One might then consider the sensation we experience when the chair disappears not to be weightlessness, but normallessness.

I don’t think that will catch on.

We usually associate “weightlessness” with the image of astronauts “floating” inside a spaceship. This gives the impression of motionlessness (I’m going to see how many words I can add “lessness” to). However, it is the very large motion of these astronauts that makes them “weightless”. They are in a spaceship that is falling toward the Earth. There is no chair holding it up. And because the spaceship and the astronaut (like the hammer and the feather) fall toward the Earth at the same rate, the astronaut does not move relative to the spaceship. She appears to float inside it, yet there is nothing holding her up. Both she and the spaceship are falling freely toward the center of the Earth. Happily, they will not hit the Earth because previously, rockets accelerated the spaceship to such a high speed that by the time it has fallen the distance needed to hit the Earth, it has zipped over so much of the Earth that the curvature of the Earth has made the surface that much further away from the spaceship again. Here, then, is the similarity between the apple and the Moon that Newton recognized: the Moon is falling toward the Earth, but because of its great speed, it keeps missing the Earth.

Orbiting = Falling

An orbiting object such as the Moon or the space station is simply falling toward the Earth, but missing it.

So the only connection between space (as in “outer space”) and weightlessness is that getting above the atmosphere is the easiest way to fall for a very long time without running into something. But the exact same thing happens (for a very short time) when you snatch the chair out from under someone. So, “weightlessness” can be achieved by finding a way to fall for an extended period of time without any slowing due to air friction or, preferably, uncomfortably hard landings. Parabolic airplane flights accomplish this by flying the same path that an object falling toward the Earth would follow if there were no atmosphere. Because this is easily calculated, pilots can fly planes on such paths. While they do so, everything inside the plane follows the path than an object falling toward the Earth would follow if there were no atmosphere. So the airplane seat is falling as fast as you are, and it therefore doesn’t push up on you. Your arms are falling as fast as your shoulders, so they do not pull down on you either. You experience “weightlessness” because you are falling freely very quickly. The pilots make sure to achieve crashlessness (okay, that’s a stretch) on the flight by having the plane pull up before it heads toward the Earth too quickly. When it does, your body wants to head toward the Earth quickly, but the plane is rudely interrupting that fall and exerts a pressure against you that is much greater than normal. We thus feel heavy or excessive weight.

In fact, you are, when “weightless” accelerating at 1-g, where g is 9.8 meters per second per second. Right now, sitting on a chair in a normal terrestrial environment, your acceleration is zero-g. Weightlessness is really motion at 1-g, and not zero-g. The net force acting on us when we feel heavy is zero, while the net force acting on us when we feel weightless is equal to the local force due to gravity.

Average Rating: 4.7 out of 5 based on 234 user reviews.

I’m attending the annual Fall Meeting of the American Geophysical Union in San Francisco where more than 10, 000 scientists spanning a wide range of fields, from my own planetary science, to hydrology, geology, atmospheric science, heliophysics and more are meeting to present their latest research results. Monday I attended an associated workshop on Human-Tended Suborbital Science. The idea of this program, which has some traction at NASA, is to take advantage of the nascent private-sector suborbital launch industry. That is a boring way of describing the numerous enterprises which hope to sell you a ticket for a ride into space. Soon.

SpaceShipOne won the Ansari X Prize in 2004 by completing two trips into space (for these purposes, technically defined as 100 km in altitude) within two weeks. The $10 million prize was certainly far less than the cost to develop the rocket, but Sir Richard Branson recognized a potential tourist market and now Virgin Galactic is nearing completion of the SpaceShipTwo model which will fly 6 paying passengers at a time to suborbital space for about 5 minutes of weightlessness, a view of the curved horizon, and astronaut wings. The name of the first ship is the VSS Enterprise, warming the hearts of all us Trekkies. Branson reportedly offered Shatner a ticket on the first passenger flight, but the former Star Trek actor apparently would like to see a bit more reliability data before he takes his ride.

Monday’s workshop was a dialog between the many companies developing private suborbital launch capabilities and the scientific community that could take advantage of those flights for scientific experiments. I was there to discuss the science that could be accomplished on such flights with experimental studies of the behavior of small colliding particles in microgravity. I have done these experiments on the space shuttle (expensive, limited access) and parabolic flights (short time in reduced gravity, not a very smooth gravity environment), and 5 minutes on a suborbital rocket would certainly open up a broad new parameter space.

One of the striking things to me about the workshop is how many companies are getting close to having rockets flying. In addition to Virgin Galactic, Jeff Bezos of Amazon.com is leading the Blue Origin company’s development of a passenger-ferrying rocket, SpaceX is selling unmanned payload space, and others presented plans for smaller rockets. Whether I ever personally perform experiments on such a rocket, it will certainly be exciting to watch these projects roll out over the next couple of years, and I felt it was only appropriate for me to wear my Star Trek tie to the meeting.

Average Rating: 4.4 out of 5 based on 161 user reviews.

I completed my seventh lifetime parabolic flight on Sunday, but first flight with the Zero-G corporation. While my earlier flights on NASA’s KC-135 (now retired and replaced by a C-9) involved anywhere from 36 to 51 parabolas, Zero-G does only 15 parabolas on non-research flights. They also currently have a contract to sell flights to NASA, and I think those do the full set of 40-50 parabolas, but the flight I was on Sunday was sponsored by Space Florida for educators, and operated in pretty much the same mode as their passenger flights. The limited number of parabolas is to limit motion sickness. As someone who has gotten violently ill on the longer flights, I think this is a good idea. Paying five grand (their current ticket price) to get violently ill, even with the unique experience of weightlessness, would probably leave a lot of customers grumpy.

Their flight plan begins with one parabola simulating at martian gravity followed by two “lunars”. Parabolas are flown in groups of three followed by a couple of minutes of straight and level flight to get set up for the next set.

ZG-210 Silver Team

The Silver Team poses in front of “G-Force-One” prior to our flight on Sunday December 7 at the Space Coast Regional Airport.

I took on board one of the impact experiment chambers from my earlier “Physics of Regolith Impacts in Microgravity Experiment” (PRIME) to do a test run. The experiment basically consists of shooting a marble into a tray of sand at very low speeds in microgravity and measuring the speed and quantity of material ejected. However, because this was being flown as a commercial flight rather than a government flight, it was not possible to evacuate the test chamber. The test material floated out of the target chamber, limiting the amount of ejecta. However, this provided a fairly dramatic demonstration of the effects of air as a lubricant for granular materials and underscores the need for evacuated test chambers on future flights.

PRIME Experiment on ZG-210

I give the thumbs up after successful operation of the PRIME test.

I also tried to do a simpler experiment for classroom demonstration of equipartition of energy in a granular gas. That’s a fancy way of saying “watching different-sized marbles bounce around at different speeds”. This was compromised by the lack of foot restraints on the plane and the general chaos of floating bodies throughout the plane volume. Nevertheless, I think I got some good video.

Granular Experiment on ZG-210

My marble experiment is behaving fine, but my body won’t hold still.

Average Rating: 4.4 out of 5 based on 297 user reviews.

Sunday December 7 I’ll be flying on the Zero-G Corporation’s “G-Force-1” airplane (a modified Boeing 727) out of the Space Coast Regional Airport. The pilots of G-Force-1 fly the plane as close as possible to a perfect parabola at a constant horizontal speed and a constant negative acceleration of 9.8 meters per second squared. That is, they make the plane follow the path of a freely falling object. Because all objects, regardless of mass, fall at the same rate (remember Galileo and that famous leaning tower), I and everyone else inside the plane will be in a state of freefall for about 25 seconds per parabola. During that time we will experience the same sensation as astronauts orbiting the Earth.

At the end of each parabola, the plane must accelerate upward giving us a weight of about 1.8 times normal (or 1.8 g’s). During one parabola I’ll be testing a modified experiment on the formation of planets. In particular, I’ll be studying the effects of low-speed collisions between a large object and a collection of small particles to see how well things stick together or blow apart when gravity isn’t present to hold them together. I have done similar experiments on NASA’s version of G-Force-1 before (affectionately known as the “vomit comet”). NASA’s plane typically does 45 parabolas per flight, while Zero-G is kinder to its passengers and limits the parabolas to one simulating martian gravity, two simulating lunar gravity, and 12 zero-g parabolas. With that profile, I am confident I will avoid the upset stomach that plagued me on flights on NASA’s plane.

Average Rating: 4.6 out of 5 based on 200 user reviews.

Alan Stern resigned as NASA’s Associate Administrator of the Science Mission Directorate. In his short tenure as AA Alan had embarked on an ambitious program to overhaul how SMD operates. Speaking from the perspective of a university researcher, his changes to the Research and Analysis programs were a great improvement: faster and better communication between NASA HQ and proposers, longer terms for typical awards coupled with new “on-ramps” for young researchers, new science programs to capitalize on the new exploration initiative, and new programs for small space experiments, such as sounding rocket experiments. Of course, anytime there is something “new” without an increase in the budget means there’s going to be a cut to something “old”. Alan addressed the American Astronomical Society’s Division of Planetary Sciences meeting last October and said that in a zero-sum budget environment, his plan to get new missions and programs started was to hold the line on budget overruns on existing programs. Many high profile missions are running over their budgets. His departure suggests that he may not have had the flexibility he needed to deal with those cost overruns. Hopefully some of the changes he did manage to institute during his short tenure will persist into the new administration.

Average Rating: 4.4 out of 5 based on 232 user reviews.

Florida’s State Board of Education voted (4-3!) this week to include evolution (for the first time!) in the science curriculum of public schools. (Previously the concepts of evolution were taught in Florida, but the curriculum referred only to things such as “change over time”. My own recollecton of Honors Biology in a Florida High School are that our class had a debate on the topic of evolution. I was one of three or four on the side arguing in support of evolution, and the opposing group argued for Biblical creation. I do not recall any instruction on the matter in class at all.) The new standards were apparently headed for defeat until a so-called compromise was reached by inserting the words “the scientific theory of” before the word “evolution”. This concisely illustrates the anti-evolution advocates’ lack of understanding not only of evolution but also of what “scientific theory” means.

Average Rating: 4.6 out of 5 based on 248 user reviews.

The United States entered the space race 50 years ago today (okay, I wrote this on the anniversary, but am posting a day late) with the successful launch of the Explorer 1 spacecraft. Although Explorer 1 was a modest 30 pounds and quite simple by the standards of today’s spacecraft, it made an important scientific discovery. The spacecraft was built at JPL, where I happen to be at this moment for a Cassini Project Science Group meeting, and where there is a distinctly festive air and displays of the history of JPL’s involvement in the U.S. space program. The key instrument on Explorer 1 was a Cosmic Ray Package that was essentially a Geiger counter for detection of charged particles. The flux of charged particles was much less than expected at high altitudes (>2000 km), but equal to the expected value at lower altitudes. James Van Allen, lead instrument scientist on Explorer 1, hypothesized that the instrument was saturated by radiation from charged particles trapped in the Earth’s magnetosphere at high altitudes. The confirmation of this by Explorer 3 a few months later earned these regions the name they are still known by: the Van Allen radiation belts. Because of the high energy of the particles in these toroidal regions around the Earth, they pose a hazard for astronauts as well as spacecraft electronics. The space shuttle and space station orbit safely beneath the belts. Pics of the festivities at JPL coming shortly.

Hmm. I think the pictures are on my phone, but they didn’t translate to my computer last time I synched.

Average Rating: 4.4 out of 5 based on 293 user reviews.

These amazing video tests of chimpanzee visual recall show to my amateur eye that chimps process and retain the visual field in a way that is different than humans. Some explanation and commentary on the significance of the results can be found in this New Scientist article. As Darth Vader said, “Impressive. Most Impressive.”

Average Rating: 4.7 out of 5 based on 266 user reviews.

Apparently there’s nothing like a post on UFOs and conspiracies to boost the number of comments on my blog. In response to this post on the burden of proof, one commenter says:

If you actualy believe that there is life on other planets, get out of your dream world and stop being such an idiot!


To believe that there is life on other planets, you need to believe in evolution. By the way…… there is no proof for evolution!! All evolution is is another religion, because you have to believe it, without any physical evidence! so stop acting like it is science…because its not!!

Everything he says is incorrect, though the implication in the third sentence that religion is something one believes without physical evidence is reasonable. Of course, one does not need to accept the fact of evolution to believe that there is life on other planets. Presumably the idea is that life was created on Earth by a deity, but to believe that deity created life only on the Earth in a universe with over 100 billion galaxies containing roughly 100 billion stars each is to have a very restrictive view of that deity’s powers as well as presuming it to be something of a wasteful, or at least mischievous, creator. I am sure there are many people who believe that some god or other created life on many worlds. Whether or not they also understand evolution is irrelevant.

As for proof of evolution, I refer the reader to any biology textbook as well as many fine museums. There is so much physical evidence for evolution, that to deny the existence of the evidence reveals a willful determination to ignore reality. As for life elsewhere in the universe, my expectation that it exists is based on scientific speculation, but certainly not on physical evidence, and perhaps this is what irked the person quoted above. In fact, when I reread what I wrote I expected to find that I had written that I “believed” there was life elsewhere in the universe. Instead I wrote

I would be thrilled by the discovery of extraterrestrial intelligence, but so far there is no evidence…

In fact, I do believe that there is life elsewhere, but what I believe is irrelevant to scientific inquiry. That’s the whole point of science: to advance understanding through observation and experimentation, in short, through evidence. This is also why the word “belief” is irrelevant to the question of evolution. It is based on evidence, not belief. The evidence is also that the Earth is not unique and that life developed on it through natural processes and that the laws of physics are the same throughout the universe. It is therefore a reasonable hypothesis that life exists elsewhere, and that’s why we are looking for the physical evidence through SETI and exploration of our neighboring planets and their moons. The hypothesis will remain a hypothesis until we obtain evidence. A persistent absence of evidence would be reason to discard the hypothesis, but we are so far from being able to explore other Earthlike worlds that it will be a long time before we can reject the hypothesis that there is life elsewhere.

Average Rating: 4.7 out of 5 based on 287 user reviews.

Yesterday I attended a short speech given by former Apollo astronaut Dr. Edgar Mitchell at the University of Central Florida. Mitchell presented a scholarship to a UCF Engineering student and then gave a brief description of his career as an astronaut culminating in two 5-hour sojourns on the lunar surface as part of the Apollo 14 mission. Mitchell, with a Sc.D. from the Massachusetts Institute of Technology in Aeronautics and Astronautics, then expressed his concerns and hopes for the future of humanity: concerns that we will destroy ourselves as our technological capabilities advance faster than our sensibilities, and hopes that we will survive these dangers to continue exploration of the cosmos. They are good and noble sentiments, and I appreciate him expressing them and pointing out that when the Earth is viewed from afar, as only he and a handful of other men have done, national boundaries evaporate and the precarious and unique status of the Earth as home to all life becomes painfully clear.

In response to questions, Dr. Mitchell expressed that aliens have visited the Earth, and while he wasn’t explicit, implied that they are living among us and concealed from us by a vast cover-up. He also affirmed that he had successfully communicated through ESP while on the surface of the Moon and that quantum mechanics now explained how this could happen. And this brings me to the criticism from “Object Reporter” on my post a couple of days ago in which I expressed dismay in a new call for government-funded research into UFOs. Object Reporter says I am uninformed on the topic of UFOs and accuses me of spouting nonsense. I stand by my statement that aliens are the least probable explanation for UFO sightings. While it is physically possible for aliens from another planet to visit Earth, there is no compelling evidence that that has ever happened. And compelling evidence is required for such an extraordinary claim. It is an extraordinary claim because the amount of energy needed for interstellar travel is huge, and a visit to the Earth by aliens would represent a huge investment of resources. For them to make this investment and then hide, but hide poorly, does not make sense. They hide poorly because somehow, while they escape detection by the vast network of aircraft and spacecraft tracking systems as well as the vast majority of the population including people like me who would be thrilled to meet them, they apparently occasionally make themselves plainly visible to some casual observers. Other explanations are more likely because in the vast majority of UFO sightings, ordinary terrestrial explanations for those sightings have already been demonstrated to be the case. If one hundred UFO sightings are demonstrated to be due to weather balloons, military aircraft, meteors, ball lightning, camera flares and other mundane explanations, then it’s likely something like that is the explanation for the one hundred and first. The claim of evidence for extraterrestrials carries the burden of proof. I, happily, do not have the burden to debunk each UFO sighting, anymore than if I claim that there is an underground civilization on the Moon someone else has the burden to prove me wrong. Extraordinary claims require extraordinary evidence. I would be thrilled by the discovery of extraterrestrial intelligence, but so far there is no evidence, and the most likely evidence we’ll get would be an extraterrestrial radio signal.

The idea of a vast cover-up is also extraordinary. Forget for the moment that it is difficult to imagine a motive for a cover-up. The power of people in government is always enhanced by engendering fear in the populace, and fear is presumably one consequence of discovering aliens. NASA has as its mission to look for life elsewhere in the universe, and nothing would boost its budget more than the discovery of extraterrestrial life, even microbial in form. In short, lots of people in government have a self-interest in seeing extraterrestrial life proven, not hidden. But the biggest problem I see with the cover-up idea is how consistently miserable people are at keeping secrets. The most powerful person on the planet, the President of the United States, could not even keep a hotel break-in secret or (in a different incarnation) oral sex. The idea that, for decades, hundreds or thousands of people in the military and government could hide the existence of alien visitors defies reason. As for Dr. Mitchell’s demonstration of ESP, James Randi has a million dollars waiting for anyone who can demonstrate ESP or other paranormal phenomena. No one has succeeded, and by the way quantum mechanics, which beautifully describes the behavior of electrons and atoms, offers no explanation for ESP. Finally, in response to the comment on my previous post that “I wasn’t aware that the three doctors, half a dozen ex-military officials and a former state governor were supposed to be taken lightly…”: it is the claims of these people that aliens are on Earth I take lightly because they do not meet the burden of proof. And by the way, George W. Bush is a former state governor, and I take almost everything he says lightly. It’s the only way to avoid depression. Live long and prosper.

Average Rating: 4.8 out of 5 based on 170 user reviews.