Astronomy, Black Hole, Cosmology, Dark Matter, NASA, Planets

Ten amazing space discoveries in 2024.

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The Early Universe is Running out of Supermassive Black holes.

As the Webb Space Telescope continues to find supermassive blackholes (SMBH) in the time after the Dark Ages, there has been a significant down turn in their masses. Now the most common SMBHs earlier than one billion years ABB are about 4 million solar masses – about the same mass as Sgr A* in our Milky Way.  At 700 million years ABB, Webb found a SMBH with 40 million solar masses. GN-z11 at 420 million years ABB has an estimated mass of 2 million suns. LID-568 (See NASA artwork above) has a mass of 10 million suns at an age 1.5 billion years ABB. ZS7 consists of two merging SMBHs each with a mass of about 50 million suns at an age of about 740 million years ABB. So, Webb is now giving us a glimpse of black hole mergers and rapid growth long before we reach the billion-sun masses of todays SMBHs.

Cosmic Gravity Wave Background

Teams of scientists worldwide have reported the discovery of the “low pitch hum” of these cosmic ripples flowing through the Milky Way. The detected signal is compelling evidence and consistent with theoretical expectations of gravity wave pulses from millions of distant binary hole mergers, where these black holes are of the SMBH variety. The artwork above is provided by NASA. [UC Berkeley News]

DESI survey of 6 million galaxies validates Big Bang

Researchers used the Dark Energy Spectroscopic Instrument (DESI) to map how nearly 6 million galaxies cluster across 11 billion years of cosmic history as shown in the image above (Credit: D. Schlegel/Berkeley Lab using data from DESI). Their observations line up with what Einstein’s theory of general relativity predicts. Looking at galaxies and how they cluster across time reveals the growth of cosmic structure, which lets DESI test theories of modified gravity – an alternative explanation for our universe’s accelerating expansion. DESI researchers found that the way galaxies cluster is consistent with our standard model of gravity and the predictions from Einstein’s theory of general relativity. There is even a suggestion in the data that Dark Energy is weakening as the universe ages over the last 11 billion years. This has huge implications for modeling the future of the universe. [News from Berkeley Lab]

Supersymmetry searches still come up empty-handed

Before the beginning of the Large Hadron Collider data taking, supersymmetry (SUSY)  was seen as a single answer to many unresolved open questions of the Standard Model. The LHC ATLAS research program has first quickly excluded most of the simplest SUSY configurations, then moved to a detailed work targeting many signatures, not necessarily favored by a theoretical prejudice. The lack of an identified SUSY signal so far at the massive ATLAS detector shown above (Credit: ATLAS Experiment © 2022 CERN) is certainly a disappointing and possibly somewhat surprising outcome to many scientists. A lot of theoretical effort into String theory and the search for a quantum theory of gravity hinges on going beyond the so-called Standard Model, and supersymmetry is a key mathematical ingredient to many of these simpler extensions. [LHC-ATLAS Consortium]

Dark Matter searches still find no candidate particles

After 40 years of searching for dark matter candidate particles, the currently most popular assumption for the nature of DM still is that of a (new) particle, even though the jury is not entirely out on whether the present observations of DM are due to a particle (or wave-like behavior at very low masses) or due to our limited understanding of the gravitational force at large scales. The figure above shows the current list of candidate particles being considered (Credit: CERN/G. Bertone and T. M. P. Tait) Despite its success, the Standard Model of particle physics (SM) in its present form (6 quarks, 6 leptons, 1 Higgs boson, plus the 12 quanta for the three non-gravity forces) is not able to offer an explanation for dark matter. It offers no known particle that can play that role. The LHC experiments, meanwhile, have by now completed and published all their main DM search analyses for the Run-2 data taken before 2016. No evidence as yet has been found for signals of the production of dark matter or dark sector particles. Dark matter, as a particle representing some 25% of all gravitating ‘stuff’ in the universe, remains one of the biggest puzzles in physics today.

Origin of the solar wind discovered

After several decades of theoretical speculation, solar physicists are now certain that they have discovered how our sun produces the interplanetary wind of matter that streams out of its corona at speeds of over 200 km/s. In 2024, the ESA-led Solar Orbiter spacecraft made the first ever connection between measurements of the solar wind around a spacecraft to high-resolution images of the Sun’s surface at a close distance. The spacecraft passed through the magnetic field connected to the edge of a coronal hole complex. This let the team watch the way the solar wind changed its speed – from fast to slow or vice versa – and other properties, confirming that they were looking at the correct region. In the end, they got a perfect combination of both types of features together. The image above (Credit:ESA & NASA/Solar Orbiter/EUI Team; acknowledgement: Lakshmi Pradeep Chitta, Max Planck Institute for Solar System Research) taken by the ESA/NASA Solar Orbiter spacecraft shows a ‘coronal hole’ near the Sun’s south pole. Subsequent analysis revealed many tiny jets of plasma being released into the corona and solar wind during the observation. 

The origin of the springtime, dinosaur-killer asteroid

According to a recent article published in Nature magazine, the object that smashed into Earth and kick-started the extinction that wiped out almost all dinosaurs 66 million years ago was an asteroid that originally formed beyond the orbit of Jupiter, according to geochemical evidence from the impact site in Chicxulub, Mexico. Comparisons between the chemical record left behind by the strike 66 million years ago and known meteorite samples suggest that the Cretaceous asteroid was a carbonaceous chondrite. This type of asteroid is one of the oldest known, having formed billions of years ago in the early solar system. As these chondrites can only come from asteroids found beyond Jupiter, it suggests that the asteroid must have had its origins there too. Some of the chondritic spherules got into the gills of dying fish, fossils of which have been used to reveal that  the asteroid impacted during the springtime in the northern hemisphere. This is possible to know based on where the lines of growth in the fish’s bones stop, which can be read somewhat like rings in a tree trunk.

Current round-up of fireball detections worldwide

The most recent world map of detected fireballs from 1988 to 2024 detected with a variety of sensors (optical, infrasound, etc) reveals that fireballs delivering less than 30 kilotons-equivalent TNT upon atmospheric detonation are uniformly spread around Earth’s surface. In 2019 it was determined that the Geostationary Lightning Mapper (GLM) instruments on GOES weather satellites can detect fireballs and bolides. This largely removes much of the observer-bias from the detections irrespective of geographic latitude. The bright red dot is the 2013 Chelyabinsk Meteor fireball and impact. (Credit: NASA/CNEOS/JPL)

NASA spacecraft detects subterranean Martian water

Using seismic activity to probe the interior of Mars, geophysicists have found evidence for a large underground reservoir of liquid water — enough to fill oceans on the planet’s surface. The data from NASA’s Insight lander (2018-2022) allowed the scientists to estimate that the amount of groundwater could cover the entire planet to a depth of between 1 and 2 kilometers, or about a mile. While that’s good news for those tracking the fate of water on the planet after its oceans disappeared more than 3 billion years ago, the reservoir won’t be of much use to anyone trying to tap into it to supply a future Mars colony. It’s located in tiny cracks and pores in rock in the middle of the Martian crust, between 11.5 and 20 kilometers (7 to 13 miles) below the surface. Even on Earth, drilling that deep would be a challenge. [UC Berkeley News].

Organized magnetic fields in Sgr A* black hole accretion disk

A new image from the Event Horizon Telescope (EHT) collaboration has uncovered strong and organized magnetic fields spiraling from the edge of the supermassive black hole Sagittarius A* (Sgr A*). Seen in polarized light for the first time, this new view of the monster lurking at the heart of the Milky Way Galaxy has revealed a magnetic field structure strikingly similar to that of the black hole at the center of the M87 galaxy, suggesting that strong magnetic fields may be common to all black holes. This similarity also hints toward a hidden jet in Sgr A*.Scientists unveiled the first image of Sgr A*— which is approximately 27,000 light-years away from Earth— in 2022, revealing that while the Milky Way’s supermassive black hole is more than a thousand times smaller and less massive than M87’s, it looks remarkably similar. This made scientists wonder whether the two shared common traits outside of their looks. To find out, the team decided to study Sgr A* in polarized light. Previous studies of light around M87* revealed that the magnetic fields around the black hole giant allowed it to launch powerful jets of material back into the surrounding environment. Building on this work, the new images have revealed that the same may be true for Sgr A*. [Credit EHT Collaboration]

Astronomy, Moon

When will the Proxigean Tides arrive between 2025-2030?

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The Proxigean Tide, also called a King Tide, occurs when the Moon is at its closest point in its orbit to the Earth (perigee) and in its New or Full Moon phase as shown in this NOAA SciJinks figure. They also coincide with ‘supermoon’ full moons. At this time, which happens about once every 18 months, the moon’s tidal effect on Earth is maximum. Tidal gravitational forces vary as the third power of the distance between Earth and Moon, so even a small difference in distance can translate into a big effect. The orbit of the moon varies from a distance of 356,500 to 406,700 kilometers with an average distance near 380,000 kilometers. The variation between the maximum and minimum distances results in tidal force changes of a factor of 1.2 times the average tidal forces. The times when this will happen often coincide with major coastal flooding events.

The perigee/apogee and lunar phase calculator at Fourmilab provides a quick means of predicting when these tides will occur. The closest perigee and most distant apogee of the year are marked with “++
” if closer in time to full Moon or “–” if closer to new Moon. Other close-to-maximum apogees and perigees are flagged with a single character, again indicating the nearer phase. Following the flags is the interval between the moment of perigee or apogee and the closest new or full phase; extrema cluster on the shorter intervals, with a smaller bias toward months surrounding the Earth’s perihelion in early January. “F” indicates the perigee or apogee is closer to full Moon, and “N” that new Moon is closer. The sign indicates whether the perigee or apogee is before (“−”) or after (“+”) the indicated phase, followed by the interval in days and hours. Scan for plus signs to find opportunities where the Moon is full close to perigee. The most extreme events based on the time between perigee and Full/New being less than 5 hours:

Date            Perigee      Phase

3/10/2024        356,893      New
10/17/2024       357,172      Full
4/27/2025        357,118      New
11/5/2025        356,832      Full
6/14/2026        357,195      New
12/24/2026       356,649      Full
8/2/2027         357,361      New
2/10/2028        356,677      Full
9/18/2028        357,047      New
3/30/2029        356,664      Full
11/5/2029        356,899      New
3/17/2030        357,017      Full
12/24/2030       356,924      New

New Moon is pretty bad because both the Sun and the Moon are on the same side of the Earth, and with the Moon near its closest point to the Earth, the tide- making potential is highest. The date for the smallest perigee distance is December 24, 2026 when the Full ‘Super Moon’ will have its largest angular diameter of 2009 arc-seconds or 0.56 degrees.

Astronomy, Physics, Sun

How much energy does the Sun produce in one hour?

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This image was taken from the International Space Station and displays the most important feature of the sun for life on Earth: Its light and heat!

The Sun is a spectral type G2 V dwarf star that emits 3.8 x 1033 ergs/sec or 3.8 x 1026 watts of electromagnetic power from gamma ray to radio wavelengths, with most of the energy emitted in the visible light spectrum between 400 nanometers and 800 nanometers. This is illustrated by the spectrum provided by Nick84 [CC BY-SA 3.0(link is external)], via Wikimedia Commons.

This is the spectrum seen at Earth’s surface where molecules of water vapor and carbon dioxide obscure some of the radiation to form the various dips in solar intensity. The common measure of solar brightness is called Irradiance. It represents the amount of energy (watts) that pass through a 1-meter2 surface facing the sun, and measured over a 1 nanometer bandwidth. Earth is located 150 million km from the sun, so if you surround the sun with a spherical surface with this radius, the surface area is A = 4piD2 or 2,8×1023 meters2. If we divide the solar luminosity by A we get 3.8×1026 watts/2.8×1023 m2 = 1,344 watts/m2 at the top of Earth’s atmosphere. Most of this is emitted in the spectrum between 300 to 900 nm for a 600 nm bandwidth, so the average irradiance over this spectral window is 1,344/600nm = 2.2 watts/m2/nm., which more or less matches the vertical axis of the above plot.

In one hour, or 3600 seconds, the sun produces 3.8×1026 joules/sec x 3600 sec = 1.4 x 1030 Joules of energy or 3.8 x 1023 kilowatt-hours.

Since E = mc2, and c = 3×108 m/s, in 1 hour the sun looses (1.4 x 1030 ergs)/(9 x 1016) = 1.5 x 1013 kilograms or 15 billion metric tons of mass each hour. It’s been doing this for about 4.5 billion years! So its mass loss over this time (3.9×1013 hours) = 5.9×1026 kg . But the sun’s mass is 2×1030 kg, so it has only lost about 0.0003 or 0.05% of its mass so far.

Space Travel

How long does it take to get to each of the planets?

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It is almost impossible to find an accurate illustration of the scale of the solar system because nearly all fail to treat the orbit spacings correctly. A clever approach is to create scaled models that span entire cities and place markers in public places for each of the planets. An example of this is the NASA/JPL Scale Model shown here. Once you understand the distances involved, the magnitude of the challenge is made much clearer.

In my book ‘Interplanetary Travel: An Astronomer’s guide‘ I discuss current and planned technologies for interplanetary travel. The bottom line is that it depends a lot on the particular trajectory that you take. Usually, the trajectories are in the form of a ‘great arc’ that gracefully connects a launch time at Earth with a destination point. These arcs are usually many times longer that the straight-line distance between the two planets at a particular moment in time. For reduced-cost travel, astrodynamicists often rely on gravity assists ‘Slingshot Orbits’ from the inner planets to reach Jupiter, and by Jupiter to reach more distant worlds. These loop-de-loops add years of extra travel time to a mission. Let’s assume for our calculations that we just take the simplest direct approach and use the minimum ‘opposition’ distance between Earth and a planet.

The table below gives you some sense for how long it takes to get to each planet at different speeds.

The Space Shuttle, of course, can’t leave Earth orbit but its speed is typical of manned spacecraft. The Galileo spacecraft, which explored Jupiter traveled twice as fast. These travel times are based upon using a big chemical rocket on Earth to blast the spacecraft into the right trajectory. But there is another technology that has been used for several decades.

Ion rocket motors get their speed by being constantly accelerated 24-hours a day for many months, and two versions of this technology are given for a low-power and high-power ion engine. Ssatellites orbiting earth often use ion engine technology for station-keeping. NASA has also used ion engine technology on two interplanetary spacecraft: Deep Space 1 and Dawn. Both spacecraft used very low thrust engines operating for thousands of days to get the spacecraft to asteroids (Dawn visited Ceres and Vesta; DS1: 9969 Braille) and comets (Borrelly) for study.

Finally, and at least on paper, solar sails can reach speeds of nearly that of the solar wind (500 km/sec),. Engineers are even now beginning to space-test this technology.

As you can see from the table, we are currently stuck in the mode of travel where it takes nearly 10 years to get to Pluto. Perhaps in another hundred years, this travel time will be reduced to a year or less… assuming Humanity feels a compelling economic need to continue this kind of exploration.

Method=ShuttleGalileoIon AIon BSolar Sail
Speed =28,000mph54,000mph65,000mph650,000mph200,000mph
Mercury52d27d22d2.2d7.3d
Venus100d52d43d4.3d14d
Mars210d109d90d9d29d
Jupiter1.9yr1yr303d31d100d
Saturn3.6yr1.8yr1.5yr55d179d
Uranus7.3yr3.8yr3.1yr113d1yr
Neptune11.4yr5.9yr4.9yr179d1.6yr
Pluto15.1yr7.8yr6.5yr238d2.1yr

Assumptions: Ion Drive using a constant thrust of A) 0.1 pounds B) 1 pound with turnaround deceleration added. Two years acceleration to reach top speed. Solar Sail estimated speed 450 km/sec or one million mph)

Another way to gauge the maximum speed of a spacecraft is by the exhaust speed of its engines. Engine exhaust speed is related to an engineering parameter called the Specific Impulse. SI is the exhaust speed divided by the acceleration of gravity at Earth’s surface. For example, chemical rockets have SI=250 seconds and so their maximum exhaust speeds are 250 x 9.8 m/sec2 = 2.4 km/sec. SInce no rocket payload can travel faster than its exhaust speed, we can compare planetary transit times in terms of the SI of the rocket technology.

In this table, I assume that the rocket continuously accelerates from Earth until it reaches half its destination distance, then turns around and decelerates for the second half of the trip. The relevant equations are

        distance = 1/2 acceleration x Time ^2
        speed = acceleration x time

With: distance in meters, time in seconds, speed in m/sec and acceleration in meters/sec^2
Destinationa=0.05a=0.15a=0.2
PlanetTime-ATime-BTime-CMax-SI
DaysDaysDaysSeconds
Mars2414834000
Jupiter804625110000
Saturn1136636160000
Uranus1669653230000
Neptune21412468300000
Pluto21412468300000

So if we could design a ship, call it System A, that produced a constant acceleration of a = 0.05 meters/sec^2, we could get to Mars in just 24 days. Similarly for System C with an acceleration of 0.2 meters/sec^2 the Mars trip takes only a week! For engineers,we can also estimate for System C that the Mars configuration would need SI=34,000 seconds to get us there in 8 days. If we wanted to get to Pluto with the same acceleration, it turns out that accelerating for half the 68-day trip would get you to a maximum speed of about 2900 km/sec which sets the limit to our exhaust speed and leads to a maximum SI of about 300,000 seconds! These SI estimates are far larger than the chemical rockets provide of 300 seconds, and so we have to look to entirely different technologies to make these travel times possible.

Sadly, it doesn’t matter if we drag along huge fuel tanks to run our chemical rockets. They will never provide the high exhaust speeds we need to carry both our fuel and payload to our destinations. We have to use technologies that enormously increase the exhaust speeds themselves.

Space Travel

What material is used to make spacecraft?

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So far as an astronomer understands the ‘witchcraft’ of spacecraft design, very light weight and durable alloys of titanium, aluminum and magnesium are commonly used. This picture of the ISS shows many different materials that have to be made space-worthy to survive the extreme conditions of temperature and pressure. (Credit: NASA: STS-132).

Aluminum – Most commonly used conventional material (used for hydrazine and nitrous oxide propellant tanks). Low density, good specific strength. Weldable, easily workable (can be extruded, cast, machined etc). Cheap and widely available. Doesn’t have a high absolute strength and has a low melting point (933 K).

Magnesium – Higher stiffness, good specific strength. Less workable than aluminium. Is chemically active and requires a surface coating (thus making is more expensive to produce).

Titanium – Melting point 1933 F. Light weight with high specific strength. Stiff than aluminium (but not as stiff as steel) Corrosion resistant. High temperature capability. Are more brittle (less ductile) than aluminium/steel. Lower availability, less workable than aluminium (6 times more expensive than stainless steel). Used for pressure tanks, fuels tanks, high speed vehicle skins.

Ferrous Alloys like Stainless Steel – Have high strength. High rigidity and hardness Corrosion resistant. High temperature resistance (1200K). Cheap Many applications in spacecraft despite high density (screws, bolts are all mostly steel).

Austentic Steels – Non-magnetic. No brittle transition temperature. Weldable, easily machined. Cheap and widely available. Susceptible to hydrogen embrittlement (hydrogen adsorbed into the lattice make the alloy brittle). Used in propulsion and cryogenic systems.

Beryllium – Stiffest naturally occurring material (beryllium metal doesn’t occur naturally but its compounds do). Low density, high specific strength High temperature tolerance Expensive and difficult to work Toxic (corrosive to tissue and carcinogenic) Low atomic number and transparent to X-rays Pure metal has been used to make rocket nozzles.

Inconel’ (An alloy of Ni and Co) – High temperature applications such as heat shields and rocket nozzles. High density (>steel, 8200 km m-3).

Aluminium-lithium – Similar strength to aluminium but several percent lighter.

Titanium-aluminide – Brittle, but lightweight and high temperature resistant.

In the near future we may use combinations of plastics and various hybrid materials such as metal-matrix composites that will greatly reduce the weight of a spacecraft, and greatly reduce launch costs. In fact, carbon fiber technology has already been used to replace many spacecraft components, except for the outer spacecraft bulkhead itself. Bulkheads are still made from titanium, aluminum or other conventional metals and alloys because of the tremendous thermal and pressure demands. The B-2 Stealth Bomber used carbon fiber materials in its wings, but the forces it would be required to take and the thermal loading are quite small compared to a spacecraft launched from the ground, or re-entering the atmosphere from space.

Fiber-reinforced materials such as carbon, aramid and glass composites have the highest strength and stiffness-to-weight ratios among engineering materials. For demanding applications such as spacecraft, aerospace and high-speed machinery, such properties make for a very efficient and high-performance system. Carbon fiber composites, for example, are five times stiffer than steel for the same weight allowing for much lighter structures for the same level of performance. In addition, carbon and aramid composites have close to zero coefficients of thermal expansion, making them essential in the design of ultra-precise optical benches and dimensionally stable antennas. Some carbon fibers have the highest thermal conductivities among all materials allowing them to be incorporated as heat dissipating elements in electronic and spacecraft applications.

Even carbon nanotubes and fibers are being developed with spacecraft and rocket technology in mind. So, we have a lot to look forward to in this exciting arena. Every pound saved is a major reduction in launch cost to the tune of $5000 per pound. This means that if you replace a pound of aluminum with a pound of some exotic new alloy, you have almost $5000 per pound in new allow cost to play with because aluminum counts for only one percent of the $5000!

Physics, Space Travel

How long would it take to get to the nearest star?

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In my book ‘Interstellar Travel:An astronomer’s guide‘ I discuss some of the modern and planned developments in fast space travel. For the most part, it’s all about speed!

Using the kinds of chemical-based propulsion systems we have today, and taking advantage of a ‘gravitational slingshot’ from Jupiter, we could probably get up to 150,000 miles per hour. The Galileo probe managed to get to about 106,000 miles per hour (18 km/sec) and currently holds the record for the fastest speed ever achieved by an artificial body. Alpha Centauri is about 4.3 light years from Earth or 40 trillion kilometers. At this speed it would take 72,000 years to get there, not including slowing down to enter the system.

Rocket designers have been studying ion propulsion since the 1950s, and mention of the technology often turns up in works of science fiction. Ion propulsion was featured in a September 1968 episode of Star Trek called “Spock’s Brain,” in which invaders steal Spock’s brain and flee in an ion-powered spacecraft. The same technology is used intermittently for altitude control aboard 11 Hughes-built communications satellites in geosynchronous orbit 22,300 miles (35,885 kilometers) above Earth. The above photo shows the NASA NEXT ion engine firing at peak power during 2009 testing at NASA’s Glenn Research Center(Image: NASA).

Deep Space 1 launched in 1998, was the first spacecraft to use ions as a primary means of propulsion. Instead of the fiery thrust produced by typical rockets, an ion engine emits only an eerie blue glow as electrically charged atoms of xenon are pushed out of the engine. Xenon is the same gas found in photo flash bulbs and lighthouse search lamps. Acceleration with patience In the engine, each xenon atom is stripped of an electron, leaving an electrically charged particle called an ion. Those ions are then jolted by electricity that is produced by the probe’s solar panels and accelerated at high speeds as they shoot out from the engine. That produces thrust for the probe. The ions travel out into space at 68,000 miles (109,430 kilometers) per hour. But Deep Space 1 doesn’t move that fast in the other direction because it is much heavier than the ions. Its cruising speed is closer to 33,000 miles (53,100 kilometers) per hour.

The thrust itself is amazingly light — about the force felt by a sheet of paper on the palm of your hand. It takes four days to go from zero to 60 (miles per hour). But once ion propulsion gets going, nothing compares to its acceleration. Over the long haul, it can deliver 10 times as much thrust-per-pound of fuel as more traditional rockets. Each day the thrust adds 15 to 20 miles (25 to 32 kilometers) per hour to the spacecraft’s speed. By the end of Deep Space 1’s mission, the ion engine will have changed its speed by 6,800 miles (11,000 kilometers) per hour.

Using current ion drive technology, with 10 times DS-1s acceleration of 25 km/hr per day, you could reach 1/2 the speed of light (540 million km/hr) in about 4900 years. For our journey to Alpha Centauri, you would accelerate for half the trip, turn around and decelerate for the second half. A bit of math shows what you get for travel time:

Distance to turn around = 2.1 light years or 20 trillion km 10x DHS acceleration is 250 km/hr/day or 8×10-7 km/sec/sec. At this acceleration, distance = 1/2 acceleration x Time2 so T = 228 years. You then turn the ship around and decelerate for another 228 years for a total trip time of 456 years! What is interesting is that this same ion rocket technology will let us travel to Mars in 270 days, so this isn’t really pushing the technology envelope very hard! If we could get to Mars in 30 days, then the same technology would get us to Alpha Centauri in about 50 years or a single human lifetime!

If you wanted to make the trip to Alpha Centauri in, say, 30 years, you would reach the half-way distance of 2.1 light years in 15 years. An acceleration rate, A, of 17 kms/sec per day would be needed. This is about 1000 times faster than Deep Space 1. At this pace, you would be traveling as fast as the solar wind (500 km/sec) in about one month, and would pass the orbit of Pluto in 100 days traveling at a speed of 1700 km/sec.

Nuclear rocket technology can make the trip even faster because the exhaust speeds can approach 20% the speed of light or higher. At these speeds, a trip to Alpha Centauri would take less than 25 years!

Astronomy, Earth, NASA, Physics

How fast does a rocket have to travel to leave the atmosphere?

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The GRACE Follow-On spacecraft launched into orbit in May 2018. Credit: NASA/Bill Ingalls

The orbit insertion velocity near the Earth’s surface is practically the same as the Earth escape velocity of 11.2 kilometers per second, or 25,805 miles per hour.

Even at an altitude of 200 kilometers, a rocket is still inside the outer reaches of the Earth’s atmosphere. To really leave the atmosphere it probably has to get to a distance of 27,000 miles near the ‘geosynchronous’ limit. There is no sharp boundary to Earth’s atmosphere. It just decreases steadily in density until it eventually matches the density of the interplanetary medium. Here is an image that shows the extent of the geocorona, whose density is so low that it has no effect on spacecraft but qualifies as part of Earth’s atmosphere nonetheless. It was taken by NASA astronaut John Young using an ultraviolet camera on Apollo 16.

Astronomy, Nebula, Physics, Space Travel, Stars

At what speed does the interstellar medium become lethal to high speed flight?

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The dilute interstellar medium permeates space at a density of about one hydrogen atom per cubic centimeter. This image shows an all-sky map of this hydrogen observed by the Wisconsin H-Alpha Mapper (WHAM) Northern Sky Survey (Haffner, L. M. et al, 2003, Astrophysical Journal Supplement, 149, 405).. The Wisconsin H-Alpha Mapper is funded by the (US) National Science Foundation.

We do not really know what the interstellar medium looks like at the human-scale. If it is just stray hydrogen atoms you will just experience a head-on flow of ‘cosmic rays’ that will collide with your spacecraft and probably generate secondary radiation in the skin of your ship. This can be annoying, but it can be shielded so long as the particles are not ultra-relativistic. At spacecraft speeds of 50-90% the speed of light, these particles are not likely to be a real problem. At speeds just below the speed of light, the particles are ultra-relativistic and would generate a very large x-ray and gamma-ray background in the skin of your ship.

As it turns out, our solar system is inside a region called the Local Bubble where the density of hydrogen atoms is about 100 times lower that in the general interstellar medium. This Bubble, produced by an ancient supernova, extends about 300 light years from the Sun but has an irregular shape. There are thousands of stars within this region which is enough to keep us very busy exploring safely. Here is one version of this region by astronomers at the Harvard-Smithsonian Center for Astrophysics.

Interstellar space also contains a few microscopic dust grains (micron-sized is common) in a region about a few meters on a side. At their expected densities you are probably in for a rough ride, but it really depends on your speed. The space shuttle, encountering flecks of paint traveling at 28,000 mph (about 6 miles/second or 0.005 percent the speed of light) is pitted and pierced by these fast moving particles, but dust grains have masses a thousand times smaller than the smallest paint fleck, so at 0.005 percent light speed, they will not be a problem.

At 50 percent the speed of light which is the minimum for interstellar travel you will cover enough distance in a short amount of time, that your likelihood of encountering a large interstellar dust grain becomes significant. Only one such impact would be enough to cause severe spacecraft damage given the kinetic energy involved.

A large dust grain might have a mass of a few milligrams. Traveling at 50% the speed of light, its kinetic energy is given non-relativistically by 1/2 mv^2 so E = .5 (0.001 grams) x (0.5 x 3 x 10^10 cm/sec) = 1.1 x 10^17 ergs. This, equals the kinetic energy of a 10 gram bullet traveling at a speed of 1500 kilometers per second, or the energy of a 100 pound person traveling at 13 miles per second! The point is that at these speeds, even a dust grain would explode like a pinpoint bomb, forming an intense fireball that would melt through the skin like a hot poker melts a block of cheese.

The dust grains at interstellar speeds become lethal interstellar ‘BB shots’ pummeling your spacecraft like rain. They puncture your ship, exploding in a brief fireball at the instant of contact.

Your likelihood of encountering a deadly dust grain is simply dependent on the volume of space your spacecraft sweeps out. The speed at which you do this only determines how often you will encounter the dust grain in your journey. At 10,000 times the space shuttle’s speed, the collision vaporizes the particles and a fair depth of the spacecraft bulkhead along the path of travel.

But the situation could well be worse than this if the interstellar medium contains lots of ice globules from ancient comets and other things we cannot begin to detect in interstellar space. These impacts even at 0.1c would be fatal…we just don’t know what the ‘size spectrum’ of matter is between interstellar ‘micron-sized’ dust grains, and small stars, in interstellar space.

My gut feeling is that interstellar space is rather filthy, and this would make interstellar, relativistic travel, not only technically difficult but impossible to boot! Safe speeds for current technology would be only slightly higher than space shuttle speeds especially if interstellar space contains chunks of comet ice.

This is an issue that no one in the science fiction world has even bothered to explore! The only possible exception is in Star Trek where the Enterprise is equipped with a forward-directed ‘Brussard Deflector’ (that big blue dish just below the main saucer) which is supposed to sweep away particles before they arrive at the ship. This is very dubious technology because hydrogen atoms are not the main problems a ship like that would have to worry about, especially traveling inside a planetary system at sub-light speeds. It’s dust grains!

Physics, Space Travel

What happens when a human is exposed to a vacuum?

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According to popular science fiction accounts, on a space station orbiting Saturn, a man inside a punctured spacesuit swells to monstrous proportions and explodes. On Mars, the eyes of a man exposed to the near-vacuum of the martian atmosphere, pop out of his head and dangle by their optic nerves on the sides of his face. En route to Jupiter on the Discovery spacecraft, Astronaut Dave Bowman space walks for 15 seconds with no helmet, and in no apparent pain, succeeds in reentering the Discovery through an open hatch. Fortunately, only in science fiction stories do humans ever come into direct contact with the vacuum of space, but these contacts are often portrayed as having horrific consequences.

To experience the vacuum is to die, but not quite in the gristly manner portrayed in the popular movies Total Recall and Outland. The truth of the matter seems to be closer to what Stanley Kubrik had in mind in 2001: A Space Odyssey.

According to the McGraw/Hill Encyclopedia of Space, when animals are subjected to explosive decompression to a vacuum-like state, they do not suddenly balloon-up or have their eyes pop out of their heads. It is, in fact, virtually impossible to compress or expand organic tissues in this way.

Instead, death arises from the response of the free gasses trapped within the tissues. When the ambient pressure falls below 47 millimeters of mercury, about 1/20 the atmospheric pressure at sea level, the water inside all tissues passes into a vapor state beginning at the skin surface. This causes the collapse of surface cells and the loss of huge amounts of body heat via evaporation. After 15 seconds, mental confusion sets-in, and after 20 seconds you become unconscious. You can survive this for about 80 seconds if a pressure higher than about 47 millimeters of mercury is then reestablished.

There have been instances of accidental exposure to a hard vacuum during space suit tests in vacuum chambers, and by pilots flying military aircraft at 100,000 feet. The experience was not fatal, or even exceptionally uncomfortable, for the typically 10 to 15 seconds or so that it was experienced.

The decompression incident on Kittinger’s balloon jump is discussed further in Shayler’s Disasters and Accidents in Manned Spaceflight: [When Kittinger reached his peak altitude] “his right hand was twice the normal size… He tried to release some of his equipment prior to landing, but was not able to as his right hand was still in great pain. He hit the ground 13 min. 45 sec. after leaving Excelsior. Three hours after landing his swollen hand and his circulation were back to normal.”

Astronomy, Earth

What would happen if a large object hit the Earth?

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About 50,000 years ago, an object about 150-meters across traveling at 45,000 mph created the Barringer Crater in Arizona that is 1,500-meters across. (Credit:Wikipedia-NASA). It was a 10-megaton explosion.

An asteroid for which there is some possibility of a collision with Earth at a future date and which is above a certain size is classified as a potentially hazardous asteroid (PHA). Specifically, all asteroids that come closer to Earth than 0.05 AU or about 8 million km, and diameters of at least 100 meters (330 to 490 ft) are considered PHAs. By December 2024, astronomers at the IAU Minor Planets Center had cataloged about 2,437 PHAs that presented a possible hazard to Earth including impacts. About 153 of these are believed to be larger than one kilometer in diameter.

The graph below shows the cumulative number of PHAs detected since 1999 and you can see that the pace of finding new ones has decreased significantly. This means that we have discovered virtually all of these objects by the present time. Even so, if just one of these rare undiscovered birds were to strike earth it would be catastrophic, so we need 100% to be discovered. This figure is provided by the Vera Rubin Observatory (LSST) website.

The largest known PHA is (53319) 1999 JM8 with a diameter of ~7 km, but it is not currently at risk of any impacts. The asteroid Ida, located in the asteroid belt outside the orbit of Mars is shown in the image below and measures 60km x 25km by 18km gives you some idea of what these huge rocks look like!

The smaller an asteroid, the more numerous they are, is the general rule of thumb for our solar system. According to the best estimates, objects 3 meters across impact the Earth every year and deliver about 2 kilotons of TNT of energy. Objects 100 meters across collide with the Earth every few hundred years and deliver about 2 Megatons of TNT equivalent. A 1 kilometer-sized object impacts the Earth every million years or so and delivers about 100,000 Megatons of TNT.

Now, the good news is that the Earth’s atmosphere shields us from objects that are initially below about 100 meters in size because they break-up and evaporate before reaching the ground. Still, the famous Tunguska Event in 1908 was a 50 meter stony meteor, which evaporated about 20 kilometers above the Earth, yet still flattened trees in a 30 kilometer area. Its yield was about 10 Megatons of TNT, and the frequency tables predict that such strikes should happen every 100 years or so. In 2013 we got lucky again!

On February 15, 2013 a once-a-century, 20-meter asteroid literally ‘came out of nowhere’ and exploded over the town of Chelyabinsk in Russia with 500 kilotons of energy, approximately 30 times the yield of the nuclear bomb over Hiroshima. There were many photos of this event taken but nearly all are copyright-protected and licensed. Here is a set of links to some representative images. Just Google ‘Chelyabinsk Meteor’ and you will find many more.

SciTechDaily.com….[Link Here].

RIA NOVOSTI/Science Photo Library – [Link Here]

Over 1,500 people were injured or hospitalized for cuts from flying glass. Had this event happened over New York City, over 100,000 people might have been hospitalized and most of the window glass from sky scrapers would be laying in the streets. Needless to say, the consequences of asteroidal impacts depend on the size of the asteroid. Here is a table of some possible consequences:

Size          Yield       Crater             Effect
            (Megatons)     (km)
..........................................................
75 m          100        1.5          Land impacts  destroy area
                                          the size of Washington

160 m         500          3.0        Destroys large urban areas

350 m        5000          6.0         Destroys area the size of a
                                       small state.         

700 m        15,000         12         Land impact destroys 
                                         a small country

1.7 km      200,000         30         Severe climate effects. 
                                         Tsunamis destroy coastal 
                                         communities.
3.0 km      1 million       60         Large nations destroyed, 
                                         Widespread fires.

7.0 km      50 million      125        Mass extinction, 
                                         long term climate change. 

16 km       200 million      250       Large mass extinction.

For the ocean impacts of objects about 300 meters across, the tsunami tidal waves produce more damage than an equivalent impact on the land. Had the Tunguska Event happened over a populated city, the damage would have been equal to a major earthquake exceeding 7.0 with perhaps thousands of people killed by the atmospheric concussion wave, which would flatten poorly designed buildings and cause fires just below the impact. Fortunately, humans occupy so little of the surface of Earth that although these impacts happen about once every century or so, in the past no one has been around to see them.

Ocean impacts of bodies in the 700 meter range would produce major tidal waves that would just reach the shores of many continents. In the 1 -2 kilometer range, these tsunamis would be 300 feet high and travel 20 or more kilometers inland putting at risk about 100 million people or 10 percent of the world population. Such an impact would be known several days in advance by direct detection by NORAD so the question is whether enough people could make it to safety. Of course when they return to their coastal homes and cities, most of these dwellings would be severely damaged or washed away by the tremendous return tide!

I am not even going to mention collisions with larger asteroids which could occur every few million years or so. There would be enough devastation caused by the more frequent ‘super Tunguska’ events to keep us busy!