On Sunday, July 20, 1969, astronauts Neil Armstrong and Buzz Aldrin landed the Apollo 11 lunar module, Eagle, on the surface of the Moon at 3:17 p.m. CDT. Later that day, Armstrong, age 38, and Aldrin, age 39, became the first human beings to walk on another world, 51 years ago this day. Fifty-one years before that, World War I was nearing its end while the 1918 flu pandemic was ramping up in its fifth month of a twenty-six month ordeal.
Armstrong and Aldrin landed at lunar latitude 0.7° N and longitude 23.5° E, in Mare Tranquillitatis (The Sea of Tranquillity). Back here on Earth, the lunar phase was waxing crescent (35% illuminated), and the Moon set that night in Dodgeville at 11:23 p.m.—just a few minutes before Armstrong and Aldrin returned to the lunar module after spending over two hours on the surface of the Moon.
Neil Armstrong stepped down onto the lunar surface at 9:56 p.m. CDT, followed by Buzz Aldrin at 10:15 p.m. After exploring the lunar surface and setting up several scientific instruments, Buzz Aldrin returned to the lunar module at 12:01 a.m., followed by Neil Armstrong at 12:09 a.m. Monday morning.
After five hours of work stowing samples and other housekeeping tasks, Armstrong and Aldrin tried to get some sleep during a scheduled seven hour rest period. However, there were no beds in the lunar module—not even any chairs! Armstrong tried to sleep on the ascent engine cover in the rear of the cabin and Aldrin tried to sleep curled up on the floor. Adding to their discomfort, both astronauts had to keep their spacesuits on. And the lunar module was noisy, bright light leaked into the cabin, and they were too excited to sleep. Aldrin got about two hours of restless sleep. Armstrong got none.
The lunar module took off at 12:54 p.m. Monday afternoon, docked with the command module piloted by Michael Collins at 4:35 p.m., and then the astronauts began their journey home.
One of the little known facts of the Apollo missions is all the high-tech “garbage” that was left behind on the lunar surface to allow the astronauts to bring back more moon rocks. All in all, over 800 lbs. of moon rocks and lunar soil were brought back to Earth during the six lunar landing missions, the last of which returned to Earth on December 19, 1972.
A few years ago, I was doing some telescope sweeping of the meridian sky around declination -6˚ when, to my surprise and delight, a 10th- or 11th-magnitude slow-moving object entered my field of view. As it slowly traversed eastward through the field, I remembered the declination I was pointed to and realized that it must be a geostationary, or at least a geosynchronous, satellite. Centering the moving object and then turning off the telescope’s clock drive confirmed my suspicions. The object was a geosynchronous satellite because it appeared to lay motionless while all the stars in the field drifted toward the west. Serendipity is the spice of life!
Satellites stationed in orbits that are always directly above the Earth’s equator and that have an orbital period of 23h 56m 04.0905s (one sidereal day) have the interesting property of remaining stationary as seen from any point on the surface of the Earth. This property of geostationary satellites, as they are called, is used to great advantage by many communications and weather satellites. There are currently at least 554 satellites in geosynchronous orbits. They are stationed all around the Earth at various longitudes.
At what altitude do geostationary satellites orbit the Earth? It is well above human-occupied spacecraft like the International Space Station which currently orbits 260 miles above the Earth’s surface. Geosynchronous orbit lies some 22,236 miles above the Earth’s equator. This is quite a ways out, as the entire Earth subtends an angle of only 17° 12′ at this distance—about the same as the angular distance between Capella (α Aur) and Elnath (β Tau).
Looking at it another way, geostationary satellites orbit at an altitude that is 2.8 Earth diameters above the equator. Since the Moon orbits at a distance that ranges between 27.4 and 31.4 Earth diameters above the Earth’s surface, geosynchronous orbit is about 1/10 of the way to the Moon.
If you have a telescope, know where to point it, and turn tracking off, you can see a geostationary satellite as a stationary point of light while the stars drift by due to the Earth’s rotation. At our latitude here in southern Wisconsin (43° N), the area where you want to search for geostationary satellites (near the meridian) is around declination -6° 37′. Remember, declination tells you how many degrees above or below the celestial equator an object is, and the numbers range from -90° to +90°, the south celestial pole and north celestial pole, respectively. The celestial equator has a declination of 0°.
For any latitude1, the declination you want to search is given by
where δgs is the declination of the geostationary satellite in degrees and ϕ is your latitude in degrees
Since most calculators don’t have the cosecant (csc) or cotangent (cot) functions, this formula can be rewritten in a slightly more complicated form as
Why aren’t the satellites right on the celestial equator (δ = 0°)? They would be if they were millions of miles away or if we were located on the Earth’s equator, but at our northern latitude trigonometric parallax causes us to see the satellites somewhat below the celestial equator, relative to the distant stars.
What if the geostationary satellite is situated east or west of your meridian? How do you calculate its declination then? As you might expect, because the range (observer-to-satellite distance) is greater the further from the meridian the satellite is, the less the parallax is, and therefore the closer the declination is to the equator, though not by a lot. The declination is also symmetric about the meridian, east and west: a geostationary satellite one hour east of the meridian will have the same declination as another geostationary satellite one hour west of the meridian.
If you know the longitude of the geostationary satellite (for example, the GOES-16 weather satellite is stationed above 75.2˚ W longitude), you can calculate its declination (and right ascension) using the following two-step process.
where h is the hour angle in degrees and Δλ = λsat − λobs , the difference between the satellite and observer longitudes, in degrees and ϕ is the latitude of the observer in degrees
To determine the right ascension of the geostationary satellite, add the value of h to your local sidereal time (the right ascension of objects on your meridian). Make sure you convert h to hours before adding it to your LST.
What if you want to calculate the geostationary declination at a particular hour angle? That is a bit trickier. I could not figure out how to manipulate the equation for h above so that Δλ = f (h,φ). Instead, I rewrote the equation as
and using h as a starting value for Δλ, substituted it into the cos Δλ expression, calculated sin Δλ, took the arcsine to get a new value of Δλ, then substituted that back into the cos Δλ expression, and iterated. Fortunately, the value of Δλ converges very fast. Once you have Δλ, you can use the two-step process we used earlier to determine the declination of the geostationary satellite for a particular hour angle.
Please note that the value of the hour angle h we use here is positive east of the meridian and negative west of the meridian. This is opposite from the normal astronomical sense.
Here is a simple SAS program illustrating how to do all these calculations using a computer.
1 For latitudes south of the equator, add 180° to get your meridian geostationary declination. The equation goes singular at the equator (φ=0°) and at the poles (φ=90° N and 90° S) since we’re dividing by sin φ = 0 at the equator and tan φ is undefined at the poles. However, as you asymptotically get closer and closer to latitude 0° (0.0001° and -0.0001°, for example) you find that the meridian geostationary declination approaches δ = 0°. Likewise, as you asymptotically approach latitude 90° N and 90° S, you’ll find that the meridian geostationary declination approaches -8°36′ and +8°36′, respectively. Of course, in both cases the geostationary satellites always remain below your horizon. How far north or south in latitude would you have to go, then, to find that geostationary satellites on your meridian are on your horizon due south or due north, respectively? Through a little algebraic manipulation of the first equation above and utilizing some simple trigonometric identities, one finds that at latitudes 81°18′ N and 81°18′ S, geostationary satellites on your meridian would be on the horizon. North or south of there, respectively, you would not be able to see them because the Earth would be in the way.
Gérard Maral, Michel Bousquet, Zhili Sun. Satellite Communications Systems: Systems, Techniques and Technology, Fifth Edition. Wiley, 2009. See section 188.8.131.52 Polar mounting.
The BepiColombo spacecraft flew by the Earth last night, the first of nine gravity-assist maneuvers it will make to slow it down so that it can go into orbit around the planet Mercury on 5 December 2025. This was the only Earth gravity assist. There will be a Venus flyby later this year and next year, and six Mercury flybys from 2021-2025.
BepiColombo passed 7,877 miles over the South Atlantic Ocean at 0425 UT on 10 April 2020 at its closest approach to Earth, and I was able to image it from my backyard observatory in Dodgeville, Wisconsin at 0600 UT at a distance (range) of 21,760 miles.
North is up and East to the left in the video frame, so BepiColombo is moving in a northwesterly direction. The two stars in the field are 3UC 145-134561 (12.2m, north) and 3UC 144-138354 (12.7m, south). The predicted equatorial coordinates (epoch of date) at 0600 UT from JPL Horizons were α = 11h 38m 03.90s, δ = -18° 08′ 25.4″. Please note when using JPL Horizons to generate ephemerides for spacecraft and minor planets passing close to the Earth that you should use the ICRF coordinates (astrometric) and not the apparent coordinates. They can be significantly different!
The integration time in the video above is 7.5 frames per second, or 0.13 second per frame. The field size is 17 x 11 arcminutes.
Here’s the video light curve of BepiColombo as it passed through the field. It was fairly constant in brightness with no obvious variability amidst the noisy measurements.
Edmund Weiss (1837-1917) and many astronomers since have called asteroids “vermin of the sky”, but on October 4, 1957 another “species” of sky vermin made its debut: artificial satellites. In the process of video recording stars for possible asteroid occultations, I frequently see satellites passing through my 17 × 11 arcminute field of view.
I’ve put together a video montage of satellites I serendipitously recorded between August 9, 2019 and December 22, 2019. Many of the satellite crossings are moving across the fields as “dashes” because of the longer integration times I need to use for some of my asteroid occultation work. A table of these events is shown below the video. The range is the distance between observer and satellite at the time of observation. North is up and east is to the left.
Satellites in higher orbits take longer to cross the field. In the next video, the originally geosynchronous satellite OPS 1570 (IMEWS-3, “Integrated Missile Early Warning System”) is barely visible until it exhibits an amazing sunglint around 3:41:22 UT.
I caught one meteor on October 6, 2019 at 9:57:43 UT. Field location was UCAC4 515-043597. The meteor was a Daytime Sextantid, as determined using the method I described previously in There’s a Meteor in My Image. The meteor even left a brief afterglow—a meteor train!
References Hughes, D. W. & Marsden, B. G. 2007, J. Astron. Hist. Heritage, 10, 21
The lunar farside would be a splendid place to do radio astronomy. First, the cacophony of the Earth would be silenced by up to 2,160 miles of rock. Second, lacking an atmosphere, a radio telescope located on the lunar surface would be able to detect radio waves at frequencies that are absorbed or reflected back into space by the Earth’s ionosphere.
Radio waves below a frequency of 10 MHz (λ ≥ 30 m) cannot pass through the ionosphere to reach the Earth’s surface. The Earth’s atmosphere is variably opaque to radio waves in the frequency range of 10 MHz to 30 MHz (λ = 10 to 30 m), depending upon conditions. The Earth’s atmosphere is mostly transparent to frequencies between 30 MHz (10 m) and 22 GHz (1.4 cm).
Not surprisingly, electromagnetic radiation of a non-terrestrial origin having wavelengths longer than 10 meters has been little studied. If we look, we might discover new types of objects and phenomena.
The best part is the lunar radio telescope wouldn’t have to be a steerable parabolic dish, but instead could be a series of dipole antennas (simple metal rods or wires) imbedded into a plastic carpet that could easily be rolled out onto the lunar surface. This type of radio telescope is “steered” (pointed) electronically through phasing of the dipole elements.
Even though the ever-increasing number of lunar satellites should be communicating at wavelengths far shorter than 10 meters, care must be taken to minimize their impact (both communication and noise emissions) upon all lunar farside radio astronomy.
The basic principle behind every high-thrust interplanetary space probe is to accelerate briefly and then coast, following an elliptical, parabolic, or mildly hyperbolic solar trajectory to your destination, using gravity assists whenever possible. But this is very slow.
Imagine, for a moment, that we have a spacecraft that is capable of a constant 1g (“one gee” = 9.8 m/s2) acceleration. Your spacecraft accelerates for the first half of the journey, and then decelerates for the second half of the journey to allow an extended visit at your destination. A constant 1g acceleration would afford human occupants the comfort of an earthlike gravitational environment where you would not be weightless except during very brief periods during the mission. Granted such a rocket ship would require a tremendous source of power, far beyond what today’s chemical rockets can deliver, but the day will come—perhaps even in our lifetimes—when probes and people will routinely travel the solar system in just a few days. Journeys to the stars, however, will be much more difficult.
The key to tomorrow’s space propulsion systems will be fusion and, later, matter-antimatter annihilation. The fusion of hydrogen into helium provides energy E = 0.008 mc2. This may not seem like much energy, but when today’s technological hurdles are overcome, fusion reactors will produce far more energy in a manner far safer than today’s fission reactors. Matter-antimatter annihilation, on the other hand, completely converts mass into energy in the amount given by Einstein’s famous equation E = mc2. You cannot get any more energy than this out of any conceivable on-board power or propulsion system. Of course, no system is perfect, so there will be some losses that will reduce the efficiency of even the best fusion or matter-antimatter propulsion system by a few percent.
How long would it take to travel from Earth to the Moon or any of the planets in our solar system under constant 1g acceleration for the first half of the journey and constant 1g deceleration during the second half of the journey? Using the equations below, you can calculate this easily.
Keep in mind that under a constant 1g acceleration, your velocity quickly becomes so great that you can assume a straight-line trajectory from point a to point b anywhere in our solar system.
Maximum velocity is reached at the halfway point (when you stop accelerating and begin decelerating) and is given by
The energy per unit mass needed for the trip (one way) is then given by
How much fuel will you need for the journey?
hydrogen fusion into helium gives: Efusion = 0.008 mfuel c2
This assumes 100% of the fuel goes into propelling the spacecraft, but of course there will be energy losses and operational energy requirements which will require a greater amount of fuel than this. Moreover, we are here calculating the amount of fuel you’ll need for each kg of payload. We would need to use calculus to determine how much additional energy will be needed to accelerate the ever changing amount of fuel as well. The journey may well be analogous to the traveler not being able to carry enough water to survive crossing the desert on foot.
Now, let’s use the equations above for a journey to the nearest stars. There are currently 58 known stars within 15 light years. The nearest is the triple star system Alpha Centauri A & B and Proxima Centauri (4.3 ly), and the farthest is LHS 292 (14.9 ly).
I predict that interstellar travel will remain impractical until we figure out a way to harness the vacuum energy of spacetime itself. If we could extract energy from the medium through which we travel, we wouldn’t need to carry fuel onboard the spacecraft.
We already do something analogous to this when we perform a gravity assist maneuver. As the illustration below shows, the spacecraft “borrows” energy by infinitesimally slowing down the much more massive Jupiter in its orbit around the Sun and transferring that energy to the tiny spacecraft so that it speeds up and changes direction. When the spacecraft leaves the gravitational sphere of influence of Jupiter, it is traveling just as fast as it did when it entered it, but now the spacecraft is farther from the Sun and moving faster than it would have otherwise.
Of course, our spacecraft will be “in the middle of nowhere” traveling through interstellar space, but what if space itself has energy we can borrow?
Edmund Weiss (1837-1917) and many astronomers since have called asteroids “vermin of the sky”, but on October 4, 1957 another “species” of sky vermin made its debut: artificial satellites. In the process of video recording stars for possible asteroid occultations, I frequently see satellites passing through my ~¼° field of view.
I’ve put together a video montage of satellites I serendipitously recorded between March 31, 2019 and July 12, 2019. Many of the satellite crossings are moving across the fields as “dashes” because of the longer integration times I need to use for some of my asteroid occultation work. A table of these events is shown below the video. The range is the distance between observer and satellite at the time of observation.
Satellites in higher orbits take longer to cross the field. When possible, I’ve included graphs of brightness as a function of time for these slower-moving satellites after each individual video and corresponding table. When you watch the videos of geostationary satellites, you are actually seeing the rotation of the Earth as the line between you and the satellite sweeps across the stars as the Earth rotates!
I caught one meteor on 4 Jan 2019 between 5:32:57 and 5:32:59 UT. Field location was UCAC4 419-017279. I’m pretty sure the meteor was a Quadrantid!
And two aircraft crossed my field: on 7 Dec 2018 1:40:05 – 1:40:13 UT (UCAC4 563-026131) and 26 Jun 2019 5:02:07 – 5:02:10 UT (UCAC4 291-144196).
And high energy particles (natural radioactivity or cosmic rays) “zing” my CCD/CMOS detector every once in a while. Here are a few examples: 5 Jan 2019 3:46:00 – 3:46:02 UT (UCAC4 473-001074); 20 Apr 2019 3:41:46 – 3:41:47 UT (UCAC4 501-062663); 30 Jun 2019 7:37:31 – 7:37:33 (UCAC4 354-179484) and 7:47:41 – 7:46:44 (TYC 6243-00130-1).
References Hughes, D. W. & Marsden, B. G. 2007, J. Astron. Hist. Heritage, 10, 21
I attended the 234th meeting of the American Astronomical Society (AAS), held in St. Louis, Missouri, June 9-13, 2019. Here are some highlights from that meeting.
Day 1 – Monday, June 10, 2019
Research Notes of the AAS is a non-peer-reviewed, indexed and secure record of works in progress, comments and clarifications, null results, or timely reports of observations in astronomy and astrophysics. RNAAS.
The Bulletin of the American Astronomical Society is the publication for science meeting abstracts, obituaries, commentary articles about the discipline, and white papers of broad interest to our community. BAAS.
We still have many unanswered questions about galaxy formation. The rate of star formation in galaxies and central black hole accretion activity was highest between 10 and 11 billion years ago. This corresponds to redshift z around 2 to 3, referred to as “cosmic high noon”. This is the ideal epoch for us to answer our questions about galaxy formation. Near-infrared spectroscopy is important to the study of galaxies during this epoch, and we are quite limited in what we can do from terrestrial observatories. Space based telescopes are needed, and the James Webb Space Telescope (JWST) will be key.
Galaxies are not closed boxes. We need to understand how inflows and outflows affect their evolution (“galactic metabolism”).
There are five international space treaties, with the Outer Space Treaty of 1967 being the first and most important. The United States has signed four of the five treaties. The Moon Agreement of 1979 which states that no entity can own any part of the Moon does not include the United States as one of the signatories.
U.S. Code 51303, adopted in 2015, identifies asteroid resource and space resource rights, and states that “A United States citizen engaged in commercial recovery of an asteroid resource or a space resource under this chapter shall be entitled to any asteroid resource or space resource obtained, including to possess, own, transport, use, and sell the asteroid resource or space resource obtained in accordance with applicable law, including the international obligations of the United States.”
So, unfortunately, U.S. law does allow a commercial entity to own an asteroid, but you have to get there first before you can claim it. The large metallic asteroid 16 Psyche is highly valuable and will probably be owned by some corporation in the not-too-distant future.
Space law often relies upon maritime law as a model.
Astronomer Vayu Gokhale from Truman State University gave an interesting iPoster Plus presentation on how he and his students are operating three automated and continuous zenithal sky brightness measurement stations using narrow-field Sky Quality Meters (SQMs) from Unihedron. Even measurements when it is cloudy are of value, as clouds reflect light pollution back towards the ground. Adding cloud type and height would allow us to make better use of cloudy-night sky brightness measurements. In a light-polluted area, the darkest place is the zenith, and clouds make the sky brighter. In an un-light-polluted area, the darkest place is the horizon, and clouds make the sky darker.
A number of precision radial velocity instruments for exoplanet discovery and characterization will begin operations soon or are already in operation: NEID, HARPS, ESPRESSO, EXPRES, and iLocater, to name a few.
Dark matter: clumps together under gravity, does not emit, reflect, or absorb electromagnetic radiation, and does not interact with normal matter in any way that causes the normal matter to emit, reflect, or absorb electromagnetic radiation. The ratio between dark matter and normal (baryonic matter) in our universe is 5.36 ± 0.05 (Planck 2018).
What is dark matter? It could be a new particle. If so, can we detect its non-gravitational interactions? It could be macroscopic objects, perhaps primordial black holes. Or, it could be a mixture of both. Another possibility is that a modification to the laws of gravitation will be needed to mimic the effects of dark matter.
How “dark” is dark matter? Does it interact at all (besides gravitationally)? Can dark matter annihilate or decay? Even if dark matter started hot, it cools down rapidly as the universe expands.
Primordial black holes could have masses ranging anywhere between 10-16 and 1010 solar masses. LIGO is possibility sensitive to colliding primordial black holes with masses in the range of a few to a few hundred solar masses. Primordial black holes are a fascinating dark matter candidate, with broad phenomenology.
The Cosmic Microwave Background (CMB) is a nearly perfect blackbody with distortions < 1 part in 10,000. What this tells us is that nothing dramatically heated or cooled photons after 2 months after the Big Bang. Anisotropies are variances in the CMB temperature, and the angular power spectrum is variance of CMB temperature as a function of angular scale. CMB anisotropies are very sensitive to the ionization history of the universe. How the universe recombined plays a key role in CMB anisotropies.
Hydrogen: not such a simple atom.
The CMB is polarized. The polarization is caused by Mie scattering of photons.
At the NASA Town Hall, we learned about current and future missions: TESS, SPHEREx, HabEx, LUVOIR, Lynx, Origins Space Telescope (OST).
The highest image rate of standard CCD and CMOS video cameras for asteroid occultation work is 30 frames (60 fields) per second, providing time resolution of 0.017 seconds per field. Adaptive optics and autoguider imaging devices often have a higher sampling rate, and such a camera could perhaps be easily modified to be used for occultation work. A time-inserter would need to be added to the camera (either on-board or GPS-based), and improvements in quantum efficiency (because of the shorter exposures) would benefit from newer imaging technologies such as a Geiger-mode avalanche photodiode (APD); or the Single-photon avalanche detector (SPAD), which are frequently used in chemistry.
Gregory Simonian, graduate student at Ohio State, presented “Double Trouble: Biases Caused by Binaries in Large Stellar Rotation datasets”. The Kepler data yielded 34,030 rotation periods through starspot variability. However, the rapid rotators are mostly binaries. In the Kepler dataset, many rapid rotators have a spin period of the stars equal to the orbital period of the binary. These eclipsing binaries, also known as photometric binaries because they are detected through changes in brightness during eclipses and transits, need to be treated separately in stellar rotation datasets.
Granulation was discovered by William Herschel in 1801 and are vertical flows in the solar photosphere on the order of 1000 m/s, and 1000 km horizontal scale. Supergranulation (Hart 1954, Leighton et al. 1962) are horizontal motions in the photosphere of 300 to 500 m/s with a horizontal scale on the order of 30,000 km.
The amplitude of oscillations in red giants increase dramatically with age.
We’ve never observed the helium flash event in a red giant star, though models predict that it must occur. It is very brief and would be difficult to detect observationally.
Brad Schaefer, Professor Emeritus at Louisiana State University, gave a talk on “Predictions for Upcoming Recurrent Nova Eruptions”. Typically, recurrent novae have about a 30% variation in eruptive timescales, so predicting the next eruption is not trivial. Due to the solar gap (when the object is too close to the Sun to observe on or near the Earth), we are obviously missing some eruptions. However, orbital period changes (O-C curve) can tell us about an eruption we missed. U Sco and T CrB are well-known examples of recurrent novae. Better monitoring of recurrent novae is needed during the pre-eruption plateau. Monitoring in the blue band is important for prediction.
I had the good fortune to talk with Brad on several occasions during the conference, and found him to be enthusiastic, knowledgeable, and engaging. Perhaps you have seen The Remarkable Science of Ancient Astronomy (The Great Courses), and he is just as articulate and energetic in real life. Among other things, we discussed how the internet is filled with misinformation, and even after an idea has been convincingly debunked, the misinformation continues to survive and multiply in cyberspace. This is a huge problem in the field of archaeoastronomy and, indeed, all fields of study. People tend to believe what they want to believe, never mind the facts.
Astrobites is a daily astrophysical-literature blog written by graduate students in astronomy around the world. The goal of Astrobites is to present one interesting paper from astro-ph per day in a brief format accessible to its target audience: undergraduate students in the physical sciences who are interested in active research.
Helioseismology can be done both from space (all) and the ground (some). Active regions on the far side of the Sun can be detected with helioseismology.
All HMI (Helioseismic and Magnetic Imager) data from the Solar Dynamics Observatory is available online.
A good approach to studying solar data is to subtract the average differential rotation at each point/region on the Sun and look at the residuals.
The Wilcox Solar Observatory has been making sun-as-a-star mean magnetic field measurements since 1975.
It is possible to infer electric currents on the Sun, but this is much more difficult than measuring magnetic fields.
Future directions in solar studies: moving from zonal averages to localized regions in our modeling, and the ability through future space missions to continuously monitor the entire surface of the Sun at every moment.
Systematic errors are nearly always larger than statistical uncertainty.
Day 2 – Tuesday, June 11, 2019
It is probably not hyperbole to state that every star in our galaxy has planets. About 1/5 of G-type stars have terrestrial planets within the habitable zone. Life is widespread throughout the universe.
Gas-grain interaction is at the core of interstellar chemistry. Interstellar ices, charged ices, surface chemistry – there is more time for interactions to occur on a dust grain than in a gas. Grain collisions are important, too.
Hot cores are transient regions surrounding massive protostars very early in their evolution. Similar regions are identified around low-mass protostars and are called corinos.
Methanol (CH3OH) is key to making simple organic molecules (SOM). Evaporating ice molecules drive rich chemistry. Dust plays a key role in the chemistry and in transporting material from the interstellar medium (ISM) to planetary systems.
The Rosetta mission detected amino acids on comet 67P/Churyumov–Gerasimenko.
JUICE (JUpiter ICy moons Explorer) is an ESA mission scheduled to launch in 2022, will enter orbit around Jupiter in October 2029 and Ganymede in 2032. It will study Europa, Ganymede, and Callisto in great detail.
The gravitational wave event GW170817 (two infalling and colliding neutron stars) was also detected as a gamma-ray burst (GRB) by the Fermi gamma-ray space telescope, which has a gamma-ray burst detector that at all times monitors the 60% of the sky that is not blocked by the Earth.
The time interval between the GW and GRB can range between tens of milliseconds up to 10 seconds.
The Milky Way galaxy circumnuclear disk is best seen at infrared wavelengths around 50 microns. Linear polarization tells us the direction of rotation. The star cluster near the MW center energizes and illuminates gas structures. Gravity dominates in this region. The role of magnetic fields in this region has been a mystery.
Pitch angle – how tightly wound the spiral arms are in a spiral galaxy.
Are spiral arms transient or long lived? They are probably long lived. There may be different mechanisms of spiral arm formation in grand design spirals compared with other types of spiral galaxies.
In studying spiral galaxies, we often deproject to face-on orientation.
The co-rotation radius is the distance from the center of a spiral galaxy beyond which the stars orbit slower than the spiral arms. Inside this radius, the stars move faster than the spiral arms.
The Sun is located near the corotation circle of the Milky Way.
The origins of supermassive black holes (SMBH) at the centers of galaxies are unclear. Were they seeded from large gas clouds, or were they built up from smaller black holes?
The black holes at the centers of spiral galaxies tend to be more massive when the spiral arm winding is tight, and less massive when the spiral arm winding is loose.
Spiral Graph is in review as a Zooniverse project and has not yet launched. Citizen scientists will trace the spiral arms of 6,000 deprojected spiral galaxies, and 15 traces will be needed for each galaxy. Spiral arm tracings will provide astronomers with intermediate mass black hole candidate galaxies.
Barred spiral galaxies are very common. 66% to 75% of spiral galaxies show evidence of a bar at near-infrared wavelengths.
Magnetic fields in the inner regions of spiral galaxies are scrambling radio emissions to some extent, but radio astronomers have ways to deal with this.
For me, the plenary lecture given by Suvrath Mahadevan, Pennsylvania State University, was the first truly outstanding presentation. His topic was “The Tools of Precision Measurement in Exoplanet Discovery: Peeking Under the Hood of the Instruments”. His discussion of the advance in radial velocity instrumentation was revelatory to me, as his starting point was Roger F. Griffin’s radial velocity spectrometer we used at Iowa State University in the 1970s and 1980s, giving us a precision of about 1 km/s. My, we have come a long way since then!
To discover our Earth from another star system in the ecliptic plane would require detecting an 8.9 cm/s velocity shift in the Sun’s motion over the course of a year.
Precision radial velocity measurement requires we look at the displacement of thousands of spectral lines using high resolution spectroscopy.
The two main techniques are 1) Simultaneous reference and 2) Self reference (iodine cell). Also, externally dispersed interferometry and heterodyne spectroscopy can be used.
Griffin 1967 ~ km/s → CORAVEL 1979 ~300 m/s → CORALIE/ELODIE 1990 ~ 5-10 m/s → HARPS 2000 ~ 1 m/s → ESPRESSO/VLT, EXPRES/DCT, NEID/KPNO, HPF/HET.
We cannot build instruments that are stable over time at 10 cm/s resolution or less.
You can track the relative change in velocity much better than absolute velocity because of the “noise” generated by stellar internal motions.
Measuring the radial velocity at red or infrared wavelengths is best for M dwarfs, and cooler stars.
High radial velocity precision will require long-term observations, and a better understanding of and mitigation for stellar activity. Many things need to be considered: telescope, atmosphere, barycentric correction (chromatic effects can lead to 1/2 m/s error), fibers, modal noise, instrument decoupled from the telescope, calibrators, optics, stability, pipeline, etc. Interdisciplinary expertise is required.
NEID will measure wavelengths of 380 – 930 nm, and have a spectral resolution of R ~ 90,000.
Pushing towards 10 cm/s requires sub-milli-Kelvin instrument stability high-quality vacuum chambers, octagonal fibers, scrambling, and excellent guiding of the stellar image on the fiber to better than 0.05 arcseconds.
Precision radial velocity instruments such as NEID and HPF weigh two tons, so at present they can only be used with ground-based telescopes.
Charge Transfer Efficiency (CTE): need CCDs with CTE > 0.999999. Other CCD issues that don’t flat field out accurately: CCD stitch boundaries, cross hatching in NIR detectors, crystalline defects, sub-pixel quantum efficiency differences. Even the act of reading out the detector introduces a noise source.
10 cm/s is within reach from a purely instrumental perspective, but almost everything has to be just right. But we need to understand stellar activity better: granulation, supergranulation, flares, oscillations, etc. We may not be able to isolate these components of stellar activity, but we will certainly learn a lot in the process.
1s time resolution is required to properly apply barycentric corrections.
Gene Byrd, University of Alabama, gave an interesting short presentation, “Two Astronomy Demos”. The first was “Stars Like Grains of Sugar”, reminiscent of Archimedes’ The Sand Reckoner. And “Phases with the Sun, Moon, and Ball”. He uses a push pin in a golf ball (the golf ball even has craters!). Morning works best for this activity. The Sun lights the golf ball and the Moon and they have the same phase—nice! Touching as well as seeing the golf ball helps students understand the phases of the Moon. Here’s a link to his paper on these two activities.
Daniel Kennefick, University of Arkansas, gave a short presentation on the 1919 eclipse expedition that provided experimental evidence (besides the correct magnitude of the perihelion precession of Mercury) that validated Einstein’s General Relativity. Stephen Hawking in his famous book A Brief History of Time mis-remembered that the 1979 re-analysis of the Eddington’s 1919 eclipse data showed that he may “fudged” the results to prove General Relativity to be correct. He did not! See Daniel Kennefick’s new book on the subject, No Shadow of a Doubt: The 1919 Eclipse That Confirmed Einstein’s Theory of Relativity.
Brad Schaefer, Louisiana State University, gave another engaging talk, presenting evidence that the Australian aborigines may have discovered the variability of the star Betelgeuse, much earlier than the oft-stated discovery by John Herschel in 1836. Betelgeuse varies in brightness between magnitude 0.0 and +1.3 quasi-periodically over a period of about 423 days. It has been shown that laypeople can detect differences in brightness as small as 0.3 magnitude with the unaided eye, and with good comparison stars (like Capella, Rigel, Procyon, Pollux, Adhara, and Bellatrix—not all of which are visible from Australia—for Betelgeuse). It is plausible that the variability of Betelgeuse may have been discovered by many peoples at many different times. The Australian aborigines passed an oral tradition through many generations that described the variability of Betelgeuse. https://ui.adsabs.harvard.edu/abs/2019AAS…23422407S/abstract.
As a longtime astronomical observer myself, I have actually never noticed the variability of Betelgeuse, but Brad has. After his presentation, I mentioned to Brad that it would be interesting to speculate what would lead early peoples to look for variability in stars in the first place, which seems to me to be a prerequisite for anyone discovering the variability of Betelgeuse. His response pointed out that all it would take is one observant individual in any society who would notice/record the variability and then point it out to others.
If you haven’t looked at the NASA/IPAC Extragalactic Database (NED) lately, you will find new content and functionality. It has been expanded a great deal, and now includes many stellar objects, because we don’t always know what is really a star and what is not. There is now a single input field where you can enter names, coordinates with search radius, etc. NED is “Google for Galaxies”.
I noticed during the 10-minute iPoster Plus sessions that there is a countdown timer displayed unobtrusively in the upper right hand corner that helps the presenter know how much time they have remaining. I think this would be a great device for anyone giving a short presentation in any venue.
Galactic archaeology is the study of the oldest stars and other structures in our galaxy to better understand how our galaxy evolved.
Day 2 ended with an evening presentation of “Cielo”, a documentary film by Alison McAlpine. Highly recommended!
I noted that “Cielo” was presented on the Documentary Channel in Canada. Too bad we do not have a channel like that here in the U.S.!
Day 3 – Wednesday, June 12, 2019
Day 3 began with what for me was the finest presentation of the entire conference: Joshua Winn, Princeton University, speaking on “Transiting Exoplanets: Past, Present, and Future”. I first became familiar with Josh Winn through watching his outstanding video course, The Search for Exoplanets: What Astronomers Know, from The Great Courses. I am currently watching his second course, Introduction to Astrophysics, also from The Great Courses. Josh is an excellent teacher, public speaker, and presenter, and it was a great pleasure to meet him at this conference.
Transits provide the richest source of information we have about exoplanets. For example, we can measure the obliquity of the star’s equator relative to the planet’s orbital plane by measuring the apparent Doppler shift of the star’s light throughout transit.
Who was the first to observe a planetary transit? Pierre Gassendi (1592-1655) was the first to observe a transit of Mercury across the Sun in November 1631. Jeremiah Horrocks (1618-1641) was the first to observe a transit of Venus across the Sun in November 1639. Christoph Scheiner (1573-1650) claimed in January 1612 that spots seen moving across the Sun were planets inside Mercury’s orbit transiting the Sun, but we know know of course that sunspots are magnetically cooled regions in the Sun’s photosphere and not orbiting objects at all. Though Scheiner was wrong about the nature of sunspots, his careful observations of them led him to become the first to measure the Sun’s equatorial rotation rate, the first to notice that the Sun rotated more slowly at higher latitudes, and the first to notice that the Sun’s equator is tilted with respect to the ecliptic, and to measure its inclination.
An exoplanet can be seen to transit its host star if the exoplanet’s orbit lies within the transit cone, an angle of 2R*/a centered on our line of sight to the star. R* is the star’s radius, and a is the semi-major axis of the planet’s orbit around the star.
Because of the geometry, we are only able to see transits of 1 out of every 215 Earth-Sun analogs.
Space is by far the best place to study transiting exoplanets.
If an exoplanet crosses a starspot, or a bright spot, on the star, you will see a “blip” in the transit light curve that looks like this:
Are solar systems like our own rare? Not at all! There are powerful selection effects at work in exoplanet transit statistics. We have discovered a lot of “hot Jupiters” because large, close-in planets are much easier to detect with their short orbital periods and larger transit cones. In actuality, only 1 out of every 200 sun-like stars have hot Jupiters.
Planet statistical properties was the main goal of the Kepler mission. Here are some noteworthy discoveries:
Kepler 89 – two planets transiting at the same time (only known example)
Kepler 36 – chaotic three-body system
Kepler 16 – first known transiting exoplanet in a circumbinary orbit
Transiting Exoplanet Survey Satellite (TESS) – Unlike Kepler, which is in an Earth-trailing heliocentric orbit, TESS is in a highly-elliptical orbit around the Earth with an apogee approximately at the distance of the Moon and a perigee of 108,000 km. TESS orbits the Earth twice during the time the Moon orbits once, a 2:1 orbital resonance with the Moon.
TESS has four 10.5 cm (4-inch) telescopes, each with a 24˚ field of view. Each TESS telescope is constantly monitoring 2300 square degrees of sky.
TESS is fundamentally about short period planets. Data is posted publicly as soon as it is calibrated. TESS has already discovered 700 planet candidates. About 1/2 to 2/3 will be true exoplanets. On average, TESS is observing stars that are about 4 magnitudes brighter than stars observed by Kepler.
The TESS Follow-Up Observing Program (TFOP) is a large working group of astronomical observers brought together to provide follow-up observations to support the TESS Mission’s primary goal of measuring the masses for 50 planets smaller than 4 Earth radii, in addition to organizing and carrying out follow-up of TESS Objects of Interest (TOIs).
HD 21749 – we already had radial velocity data going back several years for this star that hosts an exoplanet that TESS discovered
Gliese 357 – the second closest transiting exoplanet around an M dwarf, after HD 219134
TESS will tell us more about planetary systems around early-type stars.
TESS will discover other transient events, such as supernovae, novae, variable stars, etc. TESS will also make asteroseismology measurements and make photometric measurements of asteroids.
The James Webb Space Telescope (JWST) will be able to do follow-up spectroscopy of planetary atmospheres.
Upcoming exoplanet space missions: CHEOPS, PLATO, and WFIRST.
Hot Jupiter orbits should often be decaying, so this is an important area of study.
Sonification is the process of turning data into sound. For example, you could “listen” to a light curve (with harmonics, e.g. helioseismology and asteroseismology) of a year’s worth a data in just a minute or so.
Solar cycles have different lengths (11-ish years…).
Some predictions: 2019 will be the warmest year on record, 2020 will be less hot. Solar cycle 24 terminate in April 2020. Solar cycle 25 will be weaker than cycle 24. Cycle 25 will start in 2020 and will be the weakest in 300 years, the maximum (such as it is) occurring in 2025. Another informed opinion was that Cycle 25 will be comparable to Cycle 24.
Maunder minimum: 1645 – 1715
Dalton minimum: 1790 – 1820
We are currently in the midst of a modern Gleissberg minimum. It remains to be seen if it will be like the Dalton minimum or a longer “grand minimum” like the Maunder minimum.
Citizen scientists scanning Spitzer Space Telescope images in the Zooniverse Milky Way Project, have discovered over 6,000 “yellow balls”. The round features are not actually yellow, they just appear that way in the infrared Spitzer image color mapping.
Yellow balls (YBs) are sites of 8 solar mass or more star formation, surrounded by ionized hydrogen (H II) gas. YBs thus reveal massive young stars and their birth clouds.
Antlia 2 is a low-surface-brightness (“dark”) dwarf galaxy that crashed into our Milky Way galaxy. Evidence for this collision comes from “galactoseismology” which is the study of ripples in the Milky Way’s disk.
The Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and the Sagittarius Dwarf Galaxy have all affected our Milky Way Galaxy, but galactoseismology has shown that there must be another perturber that has affected the Milky Way. Antlia 2, discovered in November 2018 from data collected by the Gaia spacecraft, appears to be that perturber.
Gaia Data Release 2 (DR2) indicates that the Antlia 2 dwarf galaxy is about 420,000 ly distant, and it is similar in extent to the LMC. It is an ultra-diffuse “giant” dwarf galaxy whose stars average two magnitudes fainter than the LMC. Antlia 2 is located 11˚ from the galactic plane and has a mass around 1010 solar masses.
A question that is outstanding is what is the density of dark matter in Antlia 2? In the future, Antlia 2 may well be an excellent place to probe the nature of dark matter.
Gravity drives the formation of cosmic structure, dark energy slows it down.
Stars are “noise” for observational cosmologists.
“Precision” cosmology needs accuracy also.
The Vera Rubin telescope (Large Synoptic Survey Telescope) in Chile will begin full operations in 2022, collecting 20 TB of data each night!
We have a “galaxy bias” – we need to learn much more about the relation between galaxy populations and matter distribution.
Might there be an irregular asymmetric cycle underlying the regular 22-year sunspot cycle? The dominant period associated with this asymmetry is around 35 to 50 years.
The relationship between differential rotation and constant effective temperature of the Sun: the Sun has strong differential rotation along radial lines, and there is little variation of solar intensity with latitude.
Solar filaments (solar prominences) lie between positive and negative magnetic polarity regions.
Alfvén’s theorem: in a fluid with infinite electric conductivity, the magnetic field is frozen into the fluid and has to move along with it.
Some additional solar terms and concepts to look up and study: field line helicity, filament channels, kinetic energy equation, Lorentz force, magnetic energy equation, magnetic flux, magnetic helicity, magnetohydrodynamics (MHD), meridional flow, polarity inversion lines, relative helicity, sheared arcade, solar dynamo.
We would like to be able to predict solar eruptions before they happen.
Magnetic helicity is injected by surface motions.
It accumulates at polarity inversion lines.
It is removed by coronal mass ejections.
Day 4 – Thursday, June 13, 2019
Cahokia (our name for it today) was the largest city north of Mexico 1,000 years ago. It was located at the confluence of the Mississippi, Missouri, and Illinois Rivers. At its height from 1050 – 1200 A.D., Cahokia city covered 6 square miles and had 10,000 to 20,000 people. Cahokia was a walled city. Some lived inside the walls, and others lived outside the walls.
Around 120 mounds were built at greater Cahokia; 70 are currently protected. Platform mounds had buildings on top, and some mounds were used for burial and other uses.
Monks mound is the largest prehistoric earthwork in the Americas. Mound 72 has an appalling history.
Woodhenge – controversial claim that it had an astronomical purpose. Look up Brad Schaefer’s discussion, “Case studies of three of the most famous claimed archaeoastronomical alignments in North America”.
Cahokia’s demise was probably caused by many factors, including depletion of resources and prolonged drought. We do not know who the descendents of the Cahokia people are. It is possible that they died out completely.
The Greeks borrowed many constellations from the Babylonians.
One Sky, Many Astronomies
The neutron skin of a lead nucleus (208Pb) is a useful miniature analog for a neutron star.
Infalling binary neutron stars, such as GW 170817, undergo tidal deformation.
Minisatellite: 100-180 kg
Microsatellite: 10-100 kg
Nanosatellite: 1-10 kg
Picosatellite: 0.01-1 kg
Femtosatellite: 0.001-0.01 kg
CubeSats are a class of nanosatellites that use a standard size and form factor. The standard CubeSat size uses a “one unit” or “1U” measuring 10 × 10 × 10 cm and is extendable to larger sizes, e.g. 1.5, 2, 3, 6, and even 12U.
The final plenary lecture and the final session of the conference was a truly outstanding presentation by James W. Head III, Brown University, “The Apollo Lunar Exploration Program: Scientific Impact and the Road Ahead”. Head is a geologist who trained the Apollo astronauts for their Moon missions between 1969 and 1972.
During the early years of the space program, the United States was behind the Soviet Union in space technology and accomplishments. The N1 rocket was even going to deliver one or two Soviet cosmonauts to lunar orbit so they could land on the Moon.
Early in his presidency, John F. Kennedy attempted to engage the Soviet Union in space cooperation.
Chris Kraft’s book, Flight: My Life in Mission Control is recommended.
The Apollo astronauts (test pilots) were highly motivated students.
The United States flew 21 robotic precursor missions to the Moon in the eight years before Apollo 11. Rangers 1-9 were the first attempts, but 1 through 6 were failures and we couldn’t even hit the Moon.
Head recommends the recent documentary, Apollo 11, but called First Man Hollywood fiction, saying, “That is not the Neil Armstrong I knew.”
The Apollo 11 lunar samples showed us that the lunar maria (Mare Tranquillitatis) has an age of 3.7 Gyr and has a high titanium abundance.
The Apollo 12 lunar excursion module (LEM) landed about 600 ft. from the Surveyor 3 probe in Oceanus Procellarum, and samples from that mission were used to determine the age of that lunar maria as 3.2 Gyr.
Scientists worked shoulder to shoulder with the engineers during the Apollo program, contributing greatly to its success.
Apollo 11 landed at lunar latitude 0.6˚N, Apollo 12 at 3.0˚S, Apollo 14 at 3.6˚S, and Apollo 15 at 26.1˚N. Higher latitude landings required a plane change and a more complex operation to return the LEM to the Command Module.
The lunar rover was first used on Apollo 15, and allowed the astronauts to travel up to 7 km from the LEM. Head said that Dave Scott did remarkable geological investigations on this mission. He discovered and returned green glass samples, and in 2011 it was determined that there is water inside those beads. Scott also told a little fib to Mission Control to buy him enough time to pick up a rock that turned out to be very important, the “seat belt basalt”.
In speaking about Apollo 16, Head called John Young “one of the smartest astronauts in the Apollo program”.
Harrison Schmitt, Apollo 17, was the only professional geologist to go to the Moon, and he discovered the famous “orange soil”. This is the mission where the astronauts repaired a damaged fender on the lunar rover using duct tape and geological maps to keep them from getting covered in dust while traveling in the rover.
When asked about the newly discovered large mass under the lunar surface, Head replied that it is probably uplifted mantle material rather than an impactor mass underneath the surface.
Radiometric dating of the Apollo lunar samples have errors of about ± 5%.
One of the reasons the Moon’s albedo is low is that space weather has darkened the surface.
The South Pole-Aitken basin is a key landing site for future exploration. In general, both polar regions are of great interest.
Smaller objects like the Moon and Mars cooled efficiently after their formation because of their high surface area to volume ratio.
We do not yet know if early Mars was warm and wet, or cold and icy with warming episodes. The latter is more likely if our solar system had a faint young sun.
Venus has been resurfaced in the past 0.5 Gyr, and there is no evidence of plate tectonics. The first ~80% of the history of Venus is unknown. Venus probably had an ocean and tectonic activity early on, perhaps even plate tectonics. Venus may have undergone a density inversion which exchanged massive amounts of material between the crust and mantle. 80% of the surface of Venus today is covered by lava flows.
A mention was made that a new journal of Planetary Science (in addition to Icarus, presumably) will be coming from the AAS soon.
I attend a lot of meetings and lectures (both for astronomy and SAS), and I find that I am one of the few people in attendance who write down any notes. Granted, a few are typing at their devices, but one never knows if they are multitasking instead. For those that don’t take any notes, I wonder, how do they really remember much of what they heard days or weeks later without having written down a few keywords and phrases and then reviewing them soon after? I did see a writer from Astronomy Magazine at one of the press conferences writing notes in a notebook as I do. I believe it was Jake Parks.
Anyone who knows me very well knows that I love traveling by train. To attend the AAS meeting, I took a Van Galder bus from Madison to Chicago, and then Amtrak from Chicago to St. Louis. Pretty convenient that the AAS meeting was held at the Union Station Hotel, just a few blocks from Amtrak’s Gateway Station. It is a fine hotel with a lot of history, and has an excellent on-site restaurant. I highly recommend this hotel as a place to stay and as a conference venue.
The bus and train ride to and fro afforded me a great opportunity to catch up on some reading. Here are a few things worth sharing.
astrometry.net – you can upload your astronomical image and get back an image with all the objects in the image astrometrically annotated. Wow!
16 Psyche, the most massive metal-rich asteroid, is the destination for a NASA orbiter mission that is scheduled to launch in 2022 and arrive at Psyche in 2026. See my note about 16 Psyche in the AAS notes above.
The lowest hourly meteor rate for the northern hemisphere occurs at the end of March right after the vernal equinox.
A tremendous, dynamic web-based lunar map is the Lunar Reconnaissance Orbiter Camera (LROC) Quickmap, quickmap.lroc.asu.edu.
I read with great interest Dr. Ken Wishaw’s article on pp. 34-38 in the July 2019 issue of Sky & Telescope, “Red Light Field Test”. Orange or amber light is probably better that red light for seeing well in the dark while preserving your night vision. You can read his full report here. Also, see my article “Yellow LED Astronomy Flashlights” here.
Jupiter and Saturn will have a spectacular conjunction next year. As evening twilight fades on Monday, December 21, 2020, the two planets will be just 1/10th of a degree apart, low in the southwestern sky.
The light curve of a stellar occultation by a minor planet (asteroid or TNO) resembles a square well if the object has no atmosphere (or one so thin that it cannot be detected, given the sampling rate and S/N), and the effects of Fresnel diffraction and the star’s angular diameter are negligible.
Astronomer Margaret Burbidge, who turns 100 on August 12, 2019, refused the AAS Annie Jump Cannon Award in 1972, stating in her rejection letter that “it is high time that discrimination in favor of, as well as against, women in professional life be removed, and a prize restricted to women is in this category.” In 1976, Margaret Burbidge became the first woman president of the AAS, and in 1978 she announced that the AAS would no longer hold meetings in the states that had not ratified the Equal Rights Amendment (ERA).
During the days following the conference when I was writing this report, I received the happy news from both the AAS and Sky & Telescope that AAS was the winning bidder of S&T during a bankruptcy auction of its parent company, F+W Media. I believe that this partnership between the AAS and Sky & Telescope will benefit both AAS members and S&T readers immensely. Peter Tyson, Editor in Chief of Sky & Telescope, stated in the mutual press release, “It feels like S&T is finally landing where it belongs.” I couldn’t agree more!
The first five Iridium satellites were launched on May 5, 1997, and by 2002 there were 66 operational satellites, providing consistent global satellite phone coverage. These satellites have the interesting property that their antenna panels sometimes reflect sunlight down to the Earth’s surface, causing what came to be known as “Iridium flares”, delighting terrestrial observers—myself included. During an Iridium flare event, the satellite suddenly appears and gradually brightens and then dims to invisibility as it moves slowly across a section of sky over several seconds. Many of these events reach negative magnitude, with some getting as bright as magnitude -9.5.
The next generation of Iridium satellites began launching in 2017, but these satellites are constructed in such a way that they do not produce flares. Gradually, the original Iridium satellites are de-orbiting (or being de-orbited), so eventually there will be no more Iridium flares.
The Iridium flares haven’t been much of a nuisance to astronomers because the number of events per night for a given observer have been in the single digits.
But now we’re facing too much of a good thing. The first volley of 60 Starlink satellites was launched on May 24, with 12,000 expected to be in orbit by 2028. These satellites will provide broadband internet service to the entire planet. Though the Starlink satellites aren’t expected to produce spectacular flares like the first generation of the Iridium satellites, they do reflect sunlight as any satellite does, and the sheer number of them in relatively low Earth orbit is sure to cause a lot of headaches for astronomers and stargazers throughout the world.
I estimate that about 468 of the 12,000 satellites will be above your horizon at any given moment, but how many of them will be visible will depend on their altitude (both in terms of distance above the Earth’s surface and degrees above the horizon), and where they are relative to the Earth’s shadow cone (they have to be illuminated by sunlight to be seen).
And Starlink will not be the only swarm of global broadband internet satellites, as other companies and countries plan to fly their own satellite constellations.
This situation illustrates yet another reason why we need a binding set of international laws that apply to all nations and are enforced by a global authority. The sooner we have this the better, as our survival may depend upon it. How else can we effectively confront anthropogenic climate change and the precipitous decline in biodiversity?
As for these swarms of satellites, two requirements are needed now to minimize their impact on astronomy:
Build the satellites with minimally reflective materials and finishes
Fly one internationally-managed robust constellation of global broadband internet satellites, and require competing companies and nations to utilize them, similar to the co-location often required for terrestrial communication towers
I’d like to close this piece with a few questions. Will future “stargazers” go out to watch all the satellites and generally ignore the real stars and constellations because they are too “boring”? Will professional astronomers increasingly have to move their operations off the Earth’s surface to the far side of the Moon and beyond? Will we continue to devalue the natural world and immerse ourselves ever more deeply into our human-invented virtual environments?