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 night of August 16, 2019 UT, I was hoping to be the first person to record an occultation of a star by the asteroid 10373 MacRobert, named after Sky & Telescope senior editor Alan MacRobert. Alas, it was not to be, but I did receive a celestial consolation prize (or is that a constellation prize?) just as rare: a meteor! Here it is:
Kappa Cygnid recorded on 16 Aug 2019, 3:51:21.540 – 3:51:21.807 UT (0.267s, field size ~ 15′)
In the caption above, you’ll note that I stated this was a Kappa Cygnid meteor. How did I determine that?
The first step is to determine the direction the meteor traveled through the image. Since I have an equatorially-mounted telescope, north is always up and east is to the left, just like in the real sky. Using Bill Gray’s remarkable Guide planetarium software, which I always use when imaging at the telescope, I identified two stars (and their coordinates) very close to the path of the meteor across the field. The meteor flashed through the field so quickly that I am not able to determine whether the meteor was traveling from NNE to SSW or vice versa. But since I was imaging in Sagittarius, south of all the radiants active on that date, it is most likely that the meteor was traveling NNE to SSW. But, of course, it could have been a sporadic meteor coming from any direction, though as you will see, I think I can convincingly rule out that scenario.
The two stars very close to the meteor’s path were:
The right ascensions and declinations above are epoch of date.
Now, if this meteor came from a particular radiant, a great circle from the meteor shower radiant to either of the two stars (or the midpoint along the line connecting them) should be in the same direction as the direction between the two stars crossed by the meteor.
Meteor shower radiants drift from night to night as the Earth passes through the meteor stream due to its orbital motion around the Sun. We must find the radiant position for each meteor shower that was active on August 16, 2019 UT for that date.
Antihelion, South δ Aquariid, α Capricornid, and Piscis Austrinid meteor shower radiant drift Source: International Meteor Organization, https://www.imo.net/files/meteor-shower/cal2019.pdf Perseid meteor shower radiant drift Source: International Meteor Organization, https://www.imo.net/files/meteor-shower/cal2020.pdfKappa Cygnid meteor shower radiant drift Source: International Meteor Organization, https://www.imo.net/files/meteor-shower/cal2020.pdf
Looking at Table 6, Radiant positions during the year in α and δ, on p. 25 of the International Meteor Organization’s 2019 Meteor Shower Calendar, edited by edited Jürgen Rendtel, we find that four major meteor showers were active on August 16: the Antihelion source, which is active throughout the year (ANT), the Kappa Cygnids (KCG), the Perseids (PER), and the South Delta Aquariids (SDA). Though right ascension and declination for these radiants (presumably epoch of date) are not given specifically for August 16, we can interpolate the values given for August 15 and 20. Note that the right ascensions are given in degrees rather than in traditional hours, minutes, and seconds of time.
We are now ready to plug all these numbers into a SAS program I wrote that should help us identify the likely source of the meteor in the image.
The results show us that the Kappa Cygnids are the likely source of the meteor in the image, with a radiant that is located towards the NNE (15.8˚) from the “pointer stars” in our image, at a bearing that is just 3.7˚ different from their orientation.
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.
