Positions of the Planets on any Date

Question:  I am making a sculpture to offer to my dad with planets position at 4 of july of 1930, his day of birth. Can you help me and tell  where i can find it?  – Sérgio

Answer:  There is a nice solar system simulator which will display the positions of the planets, as viewed from above the solar system, on any date.  I ran this simulator for July 4, 1930, shown below.

Jeff Mangum

Solar System on July 4, 1930

Solar System on July 4, 1930

 

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How Do We Keep Our Space Probes From Contaminating Other Planets?

Question:  What are the chances of interplanetary space crafts, probes  and rovers carrying the air-borne bacteria and viruses and dispersing them in the target planet since they are made and assembled in earth? (Especially the rovers which landed on Mars and the Huygens probe which landed on Titan, since they are pristine planets devoid of any life forms and bacteria & viruses known survive in very extreme conditions contaminating those planets via probes).  – Vinod

Answer:  Scientists have been concerned about this issue since the early days of space exploration.  There is a committee of scientists, called the Committee on Space Research (COSPAR), that develops recommendations for avoiding interplanetary contamination by space probes.  The aim of the current regulations is to keep the number of micro-organisms on an interplanetary space probe low enough so that the probability of contamination of the target body to a level no greater than 1 in 10,000.  The details of the rules governing the potential contamination by interplanetary space probes is described very nicely in the Wikipedia article on Planetary Protection.

Jeff Mangum

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Why is the Earth’s Orbit Around the Sun Elliptical?

Question:  Why is the Earth’s revolution around the sun elliptical rather than a perfect circle? I feel like if there is the same amount of inertia and gravity acting on the Earth at all times, the revolution would end up being a circle, or at least closer to a circle than what it actually is. (Plese answer in the simplest form; I’m only in middle school)  – Cheri

Answer:  In fact, a circular orbit is just a special case of an elliptical orbit.  Elliptical orbits are stable, possessing the same amount of total energy over the orbit as circular orbits.  This is due, for example, to the fact that when the Earth is closer to the Sun in its elliptical orbit it orbits faster, while when it is further away it orbits slower, averaging to a value equivalent to that of a circular orbit.  Elliptical orbits are much more general and allow for a wider range of initial conditions which existed when a planet/star system forms, thus making them a more probable solution for the properties of a planet’s orbital characteristics.  If you want to see a nice historical account of how Johannes Kepler discovered the existence of elliptical orbits in the motion of the planets, see the How Stuff Works video on elliptical orbits.

Jeff Mangum

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The Earth-Moon Spin-Orbit Resonance

Question:  For being a chunk of Earth’s primordial matter the Moon is quite an amazing stellar feat…it orbits synchronously about the earth with an axis spin velocity 1/100 that of Earth and counterclockwise orbit equal in period as that of Earth making  it the only orbiting moon that always shows the same face to the Earth . . . due to supposedly chaotic celestial forces . . . how was that possible . . .after countless space achievement man has been able to emulate that millions of years after the fact. . .but they eventually decay and crash back into the planet . . .talk about amazing . . .that’s downright incredible . . .or is it ? . . .  – Ramon

Answer: The fact that the Moon and the Earth always show the same face to each other is one example of a “gravitational tidal lock”.  Orbits of objects about other objects tend to seek-out a lowest energy state.  One such type of low-energy state is for the orbit and rotation of a moon and planet to be in “resonance”.  The Earth-Moon system is a 1:1 resonance.  The planet Mercury is in a 3:2 spin-orbit resonance with the Sun.  This is a well understood phenomenon, and really just a by-product of Newton’s laws of gravity.

Jeff Mangum

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Radio Frequency Recordings from the Planets

Question: Hello I am an experimental musician from Houston Texas. I have been searching for radio frequency recordings of the planets. Do you know if these exist? I have my mind set on attempting to compose a composition based on the earth, moon, and sun rotation. My idea is for the song to be 24 minutes in length and use the frequencies from all three to correlate with the moon and sun changing position over my home town of Houston TX. I will also be using sound samples from the cities day and night life to program the rhythm for the composition, hoping to create the intensity and mood from these noise samples used. Any insight or point in the right direction to find recordings that might work for this idea would be much appreciated. Thank you for time.  – Ryan

Answer: There are some recordings of radio emission from planets and other objects in the Solar System that I am aware of.  Several that I could find online are:

I am sure that there are additional recordings from space probes that you could find with some more searching on the internet.

Jeff Mangum

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Solar Radiation Effects on Mars Astronauts

Question:  I have been watching a programme on the television about the preparations for a manned mission to Mars. As Mars has a very thin atmosphere the men who land on the planet will be subjected to “solar radiation”. How dangerous will this radiation be to the astronauts and will it restrict the amount of time that they can actually spend exploring the Martian surface?  – David

Answer:  In fact, this question has been analyzed quite extensively by David Chappell in his article on Radiation and the Human Mars Mission.  This analysis results in two estimates that differ by a factor of about 25, so this is a rather difficult thing to quantify.  What this study does conclude, though, is that measures will be need to shield astronauts from radiation exposure both while in-transit to Mars and when they arrive.  Actual measurements of the Mars surface radiation environment are currently being made by the RAD (Radiation Assessment Detector) on board the Curiosity rover.  These measurements will provide a clearer picture as to what measures need to be taken to allow astronauts to travel to and work on Mars.

Jeff Mangum

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Why do the Planets in our Solar System Orbit the Sun Counter-Clockwise?

Question: The planets in our solar system are orbiting the Sun counter clockwise, why? Do the laws of physics dictate that all planet orbit their respective stars counter clockwise or is it possible to have a solar system where the planets are in a clockwise motion around their star?  – David

Answer: Most of the objects in our solar system, including the Sun, planets, and asteroids, all rotate counter-clockwise.  This is due to the initial conditions in the cloud of gas and dust from which our solar system formed.  As this gas and dust cloud began to collapse it also began to rotate.  That rotation just happened to be in a counter-clockwise direction.  There is nothing special about a counter-clockwise rotation, though.  We could easily have found ourselves living in a solar system which was rotating clockwise about our Sun, if that was the initial state of rotation of the gas and dust cloud from which our solar system formed.  Note, though, that there are two oddballs in our solar system that do not rotate in the same way as the rest of the planets.  Uranus rotates about an axis that is nearly parallel with its orbital plane (i.e. on its side), while Venus rotates about its axis in a clockwise direction.  These oddities are thought to be caused by events, such as collisions, which occurred during the formation of the solar system.

Jeff Mangum

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What Happens to the Planets When the Sun Becomes a Red Giant and White Dwarf?

Question: I’ve been watching a very interesting series of television programmes in which Professor Brian Cox has been delving into Astronomy. One programme focused on the life cycle of stars. I understand from the programme that as our Sun enters its “giant red” stage the inner planets will be engulfed and destroyed but what will happen to the outer planets? As the Sun goes through its death throes and shrink down in size will it have enough “gravitational pull” to hold what is left of the solar system in their respective orbits or will the outer planets wander off into interstellar space?  – David

Answer: There have been a couple of recent studies of just these questions.  When the Sun exhausts it hydrogen fuel and enters its Red Giant phase it will expand to roughly 100 times its present size.  This will make the distance from the Sun to Jupiter shrink from 765 million to roughly 500 million kilometers.  At this shortened distance Jupiter’s surface will be heated beyond 1000 K, but it will apparently survive.  Once the Red Giant phase is complete the Sun will evolve into a White Dwarf, during which it will lose about half of its mass.  As the Sun loses mass the radii of the orbits of the remaining planets, from Jupiter outward, will increase, but still remain in orbit around a now lighter Sun.

Jeff Mangum

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Objects that Collide with Earth

Question: Hypothetical question: We hear about what would happen when the Earth is struck by comets and asteroids; the extinction of the dinosaurs for example or the end of life on our planet for us humans. In its infancy the Earth was struck by another protoplanet and survived the impact that formed our Moon. How large would the object have to be to cause the Earth to disintegrate having been struck by it?  – David

Answer: First, lets set the scale of impact events.  The object that collided with the Earth about 65 million years ago which resulted in the extinction of the dinosaurs was about 10 km is diameter.  Clearly an impact of this magnitude would lead to mass extinction of most species on Earth.  Also, as you pointed out, the most likely theory which explains the formation of our Moon involves a collision with a Mars-sized object early in the formation of the solar system.  Mars is about 3500 km in diameter.  Compare this to the diameter of the Earth, which is about 12,700 km.  So, an object which is anywhere from 5,000 to 10,000 km in diameter contains enough energy to seriously damage, and possibly even destroy, the Earth.

Jeff Mangum

 

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What Happens as the Moon Moves Away from the Earth?

Question(s): The Earth’s moon is moving away from Earth by a few centimeters a year.  Will it break free from Earth’s gravitational influence before our Sun turns into a red giant and fries the inner planets of our solar system?
Purely a hypothetical question; if the Moon was to break away from the gravitational pull of Earth now, would it be drawn into its own orbit around the Sun or get drawn into the Sun or get flung out of the solar system all together.  – David

Answer(s): Laser ranging measurements of the change in the distance from the Earth to the Moon tell us that the Moon is moving away from the Earth at a rate of about 3.78 cm per year.  Calculations of the evolution of the Earth/Moon system tell us that with this rate of separation that in about 15 billion years the Moon will stop moving away from the Earth.  Now, our Sun is expected to enter its Red Giant phase in about 6 to 7 billion years.  So, the answer to your first question is that the Sun will engulf the inner planets as its outer layers expand during its Red Giant phase before the Moon will stop moving away from the Earth.

As for your second question, the fate of the Moon after being extracted from its orbit around the Earth depends upon exactly what the mechanism was that caused it to be removed from the Earth/Moon system.  In general, though, I suspect that if the Moon broke away from the Earth it would end-up drawn into the Sun.

Jeff Mangum

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