On January 4th, 2025 at 8:28 a.m. EST, Earth will reach perihelion, the point in its orbit when it is closest to the sun. Then why is it so cold, at least in the northern hemisphere?
Winter—indeed all seasons—don’t depend on the distance from the sun. If they did, then Australia would celebrate the Christmas season with snow instead of visits to the beach. Seasons are due to the axial tilt of the earth relative to its orbit. Throughout the year, the sun appears lower or higher in the sky, and the hours of daylight are shorter or longer.
At that moment on the 4th, Earth will be 91,405,993 miles away from the sun. Six months later on July 3rd at aphelion, it will be 94,502,939 miles away. The earth’s orbit only varies by plus or minus 1.7% from an average value. Yes, it’s elliptical and not circular, but not by much!
An interesting contrast arises on Mars, whose orbit is much more elliptical, varying by 9% from the average. Combined with Mars’s axial tilt (very similar to Earth’s), this makes seasons in Mars’s southern hemisphere rather extreme. When it is summer there, Mars is at perihelion, making it warmer. And when it is winter in the south of Mars, the planet is farthest from the sun at aphelion, making it even colder.
We should note that “warm” on Mars means a maximum of 68°F (20°C) at noontime on the equator. With an extremely thin atmosphere, temperatures plummet to -100°F (-73°C) once the sun goes down.
We should be grateful for many aspects of the Earth’s orbit for our relatively mild diurnal and seasonal variations!
Rocket science at its most basic is surprisingly simple. If you throw enough stuff fast enough out the back of your rocket, it will make the rocket go forward. The more stuff you throw out and the faster you throw it, the faster your rocket will go.
If you’ve ever blown up a balloon and then let it go, you’ve seen this in action.
But this isn’t very efficient, is it? The balloon just flies around randomly. This is why rocket engines have nozzles at the rear to direct the flow of hot gases created in the engine.
So here is a puzzle for you. The SpaceX Starship uses the same Raptor engine for its first and second stages. But as this image shows, even though the engine hardware for the engines used on the first stage (on the left) is the same as for the engines used on the second stage (on the right), the nozzle is much larger for the second stage engines. Why is that? Fair warning: we are going from Rocket Science 101 to 201 in the next two paragraphs.
Take a close look at the diagram below, and focus on the dotted lines representing exhaust gases emerging from the nozzle. If those lines are not parallel to the desired direction of motion, they are not contributing 100% to that motion. Think of all the lines above the black arrow that will push the rocket down, and all the lines below that will push it up. That is wasted effort, and means the rocket is not maximally efficient. That maximum efficiency is reached only when ALL the exhaust gas is ejected in the same direction.
The velocity with which the gases exit the nozzle is a key measure of how much thrust they can impart. But the direction in which they exit is a key measure of how efficiently they move the rocket forward.
Now to Rocket Science 301.
This is the purpose of the nozzle—it allows the gases to expand before they exit. Ideally, the gases exit the nozzle at the same pressure as the surrounding atmosphere. At sea level, where the atmospheric pressure is greatest, the gases don’t need to expand much to reach that pressure, and the nozzle can be smaller for the first stage. For the second stage, which only ignites in the upper atmosphere where the atmospheric pressure is much lower, the nozzle must be larger to allow the gas to expand more to reach a lower exit pressure.
Of course, the engine will only reach its maximum efficiency at a specific pressure, and therefore at a specific altitude. The size of the nozzle is a compromise meant to operate well enough through a range of altitudes from sea level to about 40 miles high, where the first stage of Starship falls away and the second stage engines ignite. The atmospheric pressure this high up is close to zero, and the second stage engine nozzles are much larger to allow for more expansion and a lower exhaust gas exit pressure.
Screenshot from https://www.youtube.com/watch?v=yMrJl-lJrRI
For a rocket engine meant to operate only in the vacuum of space, the nozzle will be as large as possible given the constraints of weight and practicality. It can’t be infinitely large! Look at the nozzle on the Apollo spacecraft. This engine operated only in a vacuum, and the nozzle is not small relative to the size of the craft it propelled.
For most people, if they are aware of a meteor shower, it is the Perseid shower in August. It’s pretty reliable, and it takes place at a time of year when being outside in the wee hours of the morning is actually pleasant.
But meteor showers occur at predictable times throughout the year, and there are some good ones coming this month and next. They are due to a comet (or rarely, an asteroid) that warms up when approaching the sun and kicks off dust particles. Those particles spread out all along the comet’s orbit. When the Earth intercepts that stream, a meteor shower takes place.
The interception is at a specific place in Earth’s orbit, and therefore at a specific time of year. The remaining showers in 2024 and their peak times of activity are:
Leonids: November 16-17.
Geminids: December 12-13.
Ursids: December 21-22.
And the objects responsible for each are:
Leonids: Comet Temple-Tuttle
Geminids: 3200 Phaeton (one of the few asteroids responsible for a shower)
Ursids: Comet Tuttle
Meteor showers are named for the constellation from which they appear to radiate. For example, the Leonids appear to track back to the constellation of Leo.
But this is a trick of perspective. The particles entering the atmosphere are actually on parallel tracks. They appear to diverge for the same reason that parallel railroad tracks appear to do so.
The best time to see meteors is between midnight and dawn. That’s when the Earth encounters the particle stream “head on” without their having to catch up to the Earth in its orbit.
This is not a great year for any of these showers, unfortunately. The moon will be just past full for the Leonids, making it difficult to see any but the brightest meteors. For the Geminids, if you are willing to be outside around 4:30 am, the 88% full moon will have set, and you will have another 80 minutes or so before the sky begins to lighten up before dawn. The Ursids have the moon in the sky from a little before midnight and well past dawn.
Oh, well! Just as with your favorite baseball team that isn’t the Dodgers, wait ‘til next year!
In August 1977, a radio telescope operated by Ohio State University, and being used to support the search for extraterrestrial intelligence (SETI), detected a strong signal whose explanation has remained a mystery for nearly fifty years. Despite extensive searches, the signal never repeated, and nothing like it has ever been detected since. But some scientists poring over old data from the now-defunct Arecibo Telescope think they might have an answer.
What was the Wow! signal?
Ohio State’s Big Ear telescope conducted a search for extraterrestrial intelligence from 1973 to 1995. The assumption was that a strong radio signal at a specific frequency would be evidence of an intelligent origin. On the night of August 15, 1977, a very strong signal of very specific frequency was detected. A few days later, a volunteer working at the observatory to analyze the data discovered the signal on a computer printout. His circling of the signal’s intensity and his handwritten comment gave rise to the name.
Think of the printout as a graph. Each vertical column is a narrow frequency channel. Each horizontal line represents a time interval twelve seconds wide. The numbers and letters represent intensity on a scale that runs from 1 to 9, then continuing from A through the alphabet. Here is another way to look at it that is a little easier to picture.
The telescope swept the sky as it rotated with the Earth, so a source located at a precise spot would only be visible for a minute or so.
Why might this be considered a possible sign of intelligence?
For years, SETI scientists have postulated that a signal at a frequency of 1420 MHz would be a logical choice for interstellar communication. It lies in a quiet window with little interference or absorption from either galactic or terrestrial sources. That frequency is one specific to hydrogen, the most abundant element in the universe.
Any observation that cannot be replicated must remain in the “Who knows?” category. But in a preprint submitted in August, three scientists poring through data from the Arecibo Telescope found something they think might explain the Wow! signal.
A very brief and very rare source of radiation could, at least theoretically, cause a laser-like emission of radiation from a cloud of hydrogen. (Since the emission is of microwaves rather than light, the scrupulously correct term is maser.) The authors observed many signals similar to that observed in 1977, but of much lower intensity.
Here is a portion of the abstract from their submission. It has not yet been peer reviewed; the original can be found here.
The methods, frequency, and bandwidth of these observations are similar to those used to detect the Wow! Signal. However, our observations are more sensitive, have better temporal resolution, and include polarization measurements. We report the detection of narrowband signals (∆ν ≤ 10 kHz) near the hydrogen line similar to the Wow! Signal, although two orders of magnitude less intense and in multiple locations. Despite the similarities, these signals are easily identifiable as due to interstellar clouds of cold hydrogen (HI) in the galaxy. We hypothesize that the Wow! Signal was caused by sudden brightening from stimulated emission of the hydrogen line due to a strong transient radiation source, such as a magnetar flare or a soft gamma repeater (SGR). These are very rare events that depend on special conditions and alignments, where these clouds might become much brighter for seconds to minutes. The original source or the cloud might not be detectable, depending on the sensitivity of the telescope or because the maximum brightness might arrive seconds later to the observer. The precise location of the Wow! Signal might be determined by searching for transient radio sources behind the cold hydrogen clouds in the corresponding region. Our hypothesis explains all observed properties of the Wow! Signal, proposes a new source of false positives in technosignature searches, and suggests that the Wow! Signal could be the first recorded event of an astronomical maser flare in the hydrogen line.
The epigraph for this section was popularized by Carl Sagan, one of the foremost proponents of SETI. It’s a version of Occam’s Razor, in that a simpler explanation is preferred to a more complicated one. Magnetars and masers may not seem simple to you, but they are far simpler than aliens attempting to communicate!
How difficult is interstellar travel? I’m a huge fan of space travel both real and imagined, and have followed the voyages of the Starship Enterprise since its earliest manifestations. But warp engines aren’t real, and cheating the universe’s speed limit of the speed of light requires physics of the most speculative sort. What would it take, for real, to send a human to the nearest exoplanet, Proxima Centauri b, 4.22 light years away?
This artist’s impression shows a view of the surface of the planet Proxima b orbiting the red dwarf star Proxima Centauri, the closest star to the Solar System. The double star Alpha Centauri AB also appears in the image to the upper-right of Proxima itself. Proxima b is a little more massive than the Earth and orbits in the habitable zone around Proxima Centauri, where the temperature is suitable for liquid water to exist on its surface.
https://cdn.eso.org/images/screen/eso1629a.jpg
Let’s keep the physics real while giving ourselves every advantage that we can. We’ll start off by making this a solo one-way journey. One person requires less in the way of life support to keep them alive. Getting there is hard enough! Getting back would be at least twice as hard.
The fully loaded Space Shuttle Orbiter had a mass a little shy of 150 U.S. tons, much of which was payload not designed to support the crew. Let’s be generous to our brave astronaut and provide them with a 300 ton craft. They will have a greenhouse on board that will recycle carbon dioxide, help supplement oxygen, and supply fresh food. There will be perfect recycling of water and waste. Life support, let’s assume, will not be an issue. We will have adequate shielding so that the effects of cosmic radiation will be minimized to the extent possible.
And let’s not subject our crew to the deleterious effects of prolonged weightlessness. We’ll get them there as fast as possible, accelerating to the halfway point, then decelerating to arrive at our destination and stop without zooming by. Accelerating at 1g all the way, we’ll experience normal weight.
This will get us to a maximum velocity of nearly 95% c (95% of the speed of light) at the halfway point. At those velocities, relativistic effects are significant. While our trip takes a little less than six years for Mission Control on Earth, our intrepid space voyager experiences the passage of only a little more than three and a half years. Save the Oreos for the halfway flip over and arrival ceremonies.
What would it take? If we assume 100% efficient fuel that converts all its mass into the equivalent energy, our 300 ton spacecraft would have to carry more than 11,000 tons of fuel. And traveling at those incredible speeds means that shielding better be really good, not only to protect against cosmic radiation that will arrive from head on with greater force. A dust speck at these speeds can wreak devastation. Remember that a 15 gram bullet does no damage if dropped on your foot. But fired from a gun at 250 meters per second? Quite a different story.
Sending three men on a two week long voyage to our nearest celestial neighbor more than fifty years ago stretched our technology to the limit, and we haven’t repeated the feat since. Interplanetary travel is likely a couple of decades away. Interstellar travel by humans will remain in the realm of science fiction for any foreseeable future.
You can play with your own real physics based trips at this handy site.
Two problems showed up as the spacecraft approached the International Space Station (ISS) back in June.
Five of the 28 thrusters used to maneuver in orbit malfunctioned. Four were eventually brought back to functionality, and the astronauts were able to dock successfully.
There have also been slow helium leaks in the propulsion system. In the weightless environment of space, pressurized helium is used to force fuel and oxidizer into the combustion chamber of a thruster.
If these thrusters were to malfunction during a reentry attempt, the astronauts might be unable to undock, back away from the ISS, and properly orient the spacecraft so that its protective heat shield faced forward.
Why is this a bigger deal for safe reentry than for launch?
When a spacecraft is launched, its path is determined by its launch vehicle, in other words, a rocket. The rocket takes it into orbit. Upon reentry, the spacecraft is on its own, dependent on its thrusters to keep it in the correct orientation to prevent its destruction by frictional heating as it plows into an ever-thickening atmosphere.
Why did NASA overrule Boeing engineers who declared the Starliner safe for reentry?
The interests of the two organizations are obviously in conflict. Boeing’s quality control problems are well-known, and the company is desperate to demonstrate competency in the aerospace industry it dominated for so long. NASA wanted two companies providing human-rated spacecraft for ferrying astronauts to and from the ISS. It provided Boeing with more than $4 billion, and SpaceX with $2.6 billion. Whatever you may think of Elon Musk, the founder of SpaceX, that company has demonstrated its clear superiority with fewer taxpayer dollars.
There are a lot of NASA managers who remember all too well the Columbia space shuttle disaster of 2003, when seven astronauts died as the spacecraft disintegrated during reentry. The culture of NASA has shifted since then toward prioritizing the safety of crew members.
Flying into space in inherently risky. If I were a NASA astronaut, I would be grateful for today’s decision.
There are a few astronomical events that, even though they recur, are once-in-a-lifetime occasions, separated by too much time for all but the luckiest to see twice. For me, Halley’s Comet in 1986 was such an occasion. I lifted my three-year-old son to the eyepiece of my telescope so he could perhaps say, 76 years later, that he had seen the comet twice! In truth, however, he couldn’t really see it, and doesn’t remember the moment. Ah, well.
There is another such event coming up, perhaps by the end of September and very likely before the end of the year. It is a recurrent nova, T Coronae Borealis. This article does a very good job of explaining what it is and how it works.
I have a couple of things to add to that account. If you’re like me, you may read this sentence
If people observed these “recurrent novae” when they happened in the past, scientists can compare how light from the system changed in the lead-up to each outburst and look for similar behavior today.
and ask yourself just what that similar behavior might be. Measuring how the brightness of the system changes is the answer. Here is the “light curve” from the last time this nova erupted, in 1946. Brightness increases as you go up the graph.
And if seeing that the nova will lie between Arcturus and M13 doesn’t do much for you, this diagram might help. It shows the sky from about latitude 37 degrees north, at 9:30 pm on September 20. T Coronae Borealis is in the red “bulls-eye.”
Image from The Sky software, created by Neal Sumerlin
Watch the news for word of this! It should be easily visible, but will fade in a few days. And hope for clear skies!
Why does something look big? Is it because it actually IS big, or is it because it is close to us?
The closest celestial body to the Earth is our Moon. It’s an object that’s very easy to see without any visual aid, visible even in the daytime if you are observant enough. The Sun appears to be the same size as the Moon in our sky, but this is only because the much larger Sun is much farther away. Their actual sizes are much different.
What are the actual relative sizes of the Sun, its eight planets, and our Moon?
https://i.redd.it/yllhtptmu5z11.jpg
Clearly, the moon is not the largest object shown here; in fact, it is the smallest! The largest planet Jupiter is much larger, but even at its closest approach to Earth, it is more than 1500 times more distant than the Moon. At their closest approach, this is how large each of these solar system objects appear from Earth.
The only planet with a surface visible from Earth is Mars. Mercury is too small, too distant, and too close to the Sun. Venus is shrouded with clouds. The other planets have no solid surfaces. But Mars is closest to Earth, and therefore appears largest, at opposition, when it lies along a straight line connecting the Sun, Earth, and Mars.
This happens every 26 months, and next occurs on January 16, 2025.
But because Mars has an orbit that is highly elliptical, not circular, its distance from the Sun and therefore from the Earth varies a good bit. At its closest approach to the Sun, called perihelion, it is 128 million miles away; at its most distant, aphelion, 155 million miles. This opposition unfortunately occurs closer to aphelion than perihelion, and that will be the case well into the 2030s.
Still, if you can get to a telescope in mid-January under clear skies, you should still be able to see the largest dark surface markings, and the bright white polar ice caps should also be visible.
And give a thought while you’re looking to all the human-made hardware that orbits the planet, some inert and some still working, and the landers and rovers—and one helicopter—that rest on and still explore its dusty red surface.
When all you have is one example, it’s interesting to speculate but hard to draw conclusions.
How Common Are Planets?
For most of my lifetime, we only knew of one star hosting a planetary system: ours. Were the conditions that allowed Sol to host planets from Mercury to Neptune, and smaller objects from rocky remnants to distant ice balls, common? Unusual? Unique?
As we began to better understand how our own planets were formed, and were able to detect signs of incipient formation around other stars, It seemed more and more likely that at least some other stars were orbited by planets, too. This would fall in line with something called the Copernican Principle, which holds that we occupy no special or privileged spot in the universe. It’s anthropocentric, even egotistical, to assert that we are anything out of the ordinary.
Beginning about 30 years ago, we began to acquire instruments, both ground based and space based, sensitive enough to detect the presence of planets around other stars, exoplanets. At present there are more than 5,000 exoplanets whose existence has been confirmed, and thousands more “candidate” detections that require further observations. Planets are indeed common. It seems that, as one study concluded, “…stars are orbited by planets as a rule, rather than the exception”.
How Common Is Life?
The one example we have for the presence of life arose on a planet. While it’s at least conceivable that extraterrestrial life could have an entirely different chemistry from our own, it seems unlikely. After all, hydrogen, oxygen, and carbon are among the most abundant elements present in the universe. Water and carbon-based life on a planetary surface is “life as we know it,” and looking for conditions that can support that is a logical search strategy. That means searching for rocky planets orbiting their stars in the “habitable zone”, something of a Goldilocks region that is neither so hot that water boils away nor so cold that it all freezes into ice. We need liquid water, a rocky surface, and an atmosphere that can provide the pressure needed to maintain water as a liquid.
You’ll note from the diagram that larger and hotter stars will have habitable zones farther away from the star, while smaller and dimmer stars require planets to huddle close for sufficient warmth.
But of course there are other factors. The largest and hottest stars are also the shortest-lived. Our star Sol is midway between the largest and the smallest stars; it is roughly 5 billion years old, with another 5 billion to go before it exhausts its nuclear fuel. The largest stars last only 5-10 million years. That’s a long time for humans, but a mere eyeblink in the history of the universe.
No one knows exactly how or when life arose on Earth from inanimate matter, but it seems that happened about a billion years after the planet’s formation. Another three billion years passed before life moved from single cells to multicellular organisms. And the kinds of brains that can build spacecraft and telescopes capable of detecting exoplanets are very recent developments.
The immediate conclusion would seem to be that our best bet for finding worlds where not only are the conditions favorable for the emergence of life, but where those conditions existed for long stretches of geologic time. Sun-like stars and stars smaller and cooler than the sun, in other words. There are far more of these smaller stars than larger ones.
What makes a planet potentially habitable?
Once again, there are complicating factors! The habitable zone of the smaller stars is so close to the star that tidal locking of any planet orbiting there is almost certain.
With one side constantly facing the star and the other facing away into interstellar space, there would be extreme temperature differences between the two hemispheres. These stars are also quite variable in their luminosity, and subject to violent flares. Imagine the heating and cooling system in your house cycling unpredictably between Phoenix in July and Helsinki in January!
Detecting an Earth-like planet around a Sun-like star is difficult for any number of reasons, and will remain so for some years to come. And we are only barely able to determine whether a planet might be habitable, not whether it actually harbors life. Nonetheless, there are some candidates. With somewhere between 100 and 400 billion stars and at least an equal number of planets in our own Milky Way galaxy, it’s hard to imagine that they are all devoid of any form of life.
There is a long list of potentially habitable planets that you can find here. Note that exoplanets are named with the name of the star followed by b for the first planet discovered orbiting that star, c for the second, and so on.
Let me just highlight one planet and one planetary system of particular interest.
Proxima Centauri b is an Earth-size planet that orbits the star that is nearest to us, only 4.2 light years away. The star is a small and cool red dwarf, orbiting the other two larger and brighter stars of this triple system at some distance. The habitable zone in which the planet resides is very close to the star. Orbiting only 4.5 million miles away (Mercury orbits our sun at a distance of 29 million miles at its closest), it is almost certainly tidally locked. And Proxima Centauri is a flare star, emitting powerful radiation that could well strip off any atmosphere the planet might possess. Still…it’s close!
A little over a month away now! Rather than write a lengthy post, I’m just going to address the main questions people usually have.
The sun is going to be 90% obscured at my home. Is it worth traveling to the path of totality?
Yes, yes, a thousand times yes! Unless you have witnessed it, you cannot imagine what an awe-inspiring sight this is. Just as no picture of the Grand Canyon can do it proper justice, no video or photograph of a total solar eclipse can convey what it is like to gaze at a black hole in the sky where the sun shone an hour before. As one wit remarked, “Seeing a partial solar eclipse and saying you’ve seen an eclipse is like standing outside an opera house and saying you’ve seen an opera.” But unless you have already made arrangements, you’d better go a day early and plan to sleep in your car.
Where will I be able to see a total eclipse?
The best interactive map I have found is here. The times on the map are given in UT (Universal Time), which is four hours ahead of EDT (Eastern Daylight Time), five hours ahead of Central Daylight Time, etc.
Will I see the same thing anywhere within that dark shadow?
The closer you are to the center line of the path of totality, the longer the period of totality. That time is roughly four and a half minutes. Which will seem like ten seconds.
When is it safe to look directly at the sun?
During totality, the sun’s surface is completely obscured by the moon, and it is perfectly safe to look directly at it. The few seconds immediately before and immediately after totality will allow you to witness the “diamond ring” effect, as the last visible portion of the sun’s surface peeks through lunar mountain ranges at the edge of the moon’s disc.
The sun is nearing a peak of activity, which means its ghostly outer atmosphere, the corona, may be especially prominent.
https://cdn.mos.cms.futurecdn.net/Zw4XLku8Pu2XGqpfGqWEUB-1200-80.jpg.webpOther than those few seconds before and after totality, and during totality itself, you need special eclipse glasses to safely look at the partial phases.
What else can I expect before and during totality?
