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Founded January 1970

2022-2023: XLI

Sunrise: 6:37 a.m.

Sunset: 4:15 p.m.

Civil twilight ends: 4:46 p.m.

Sun's host constellation: Libra the Scales

Moon phase: Last Quarter

Moonrise: 11:17 p.m.

Moonset: 1:32 p.m. (11/17/2022)

Julian date: 2459900.21

"I went to the hardware store and bought some used paint. It was in the shape of a house." -Steven Wright

THE DAILY ASTRONOMER

Wednesday, November 16, 2022

Orbital Pandoras

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THIS IS YOUR BRAIN ON ASTRONOMY.
ANY QUESTIONS?

Do you have questions about astronomy?  Please send them to the "Daily Astronomer."  We promise to do our best to answer these questions in a timely fashion.

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We return to Pandora's Jar today and find two questions pertaining to Earth's orbit.  The first refers to the effect of other planets. The second pertains to the possibility of moving Earth  away from the Sun before it becomes too hot and bloated.

DO PLANETS AFFECT EARTH'S ORBIT?  -F.N.
Scarcely at all. Theoretically, every massive object exerts a gravitational influence on every other massive object. The extent of that influence is proportional to the mass of the object and inversely proportional to the square of the separation distance between the objects. [Example: Double the distance between two asteroids and the gravitational force between them reduces to a quarter of its original amount.] We refer to this relation as Newton’s Universal Law of Gravitation.

[The Universal Law of Gravitation equation. F = gravitational force; G = gravitational constant 6.67 X 10^-11 m^3 kg^-1 s^-2; m1 = mass of one object; m2 = mass of second object; r = separation distance. Included in Newton’s Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy) published in 1687.]

However, compared to the Sun, the masses of the planets are extremely small and their distances -which are constantly changing- are vast. For instance, the Sun is 1047 times more massive than Jupiter, the most massive planet. (Also, Jupiter’s minimum distance from Earth approximately equals 366 million miles, compared to Earth’s average distance from the Sun of 93 million miles.) Even when closest, Jupiter’s gravitational force on Earth is only 0.0062% that of the Sun’s. For comparison, the Moon’s force is 0.55%

That having been said, the perturbations induced on planetary orbits by other planets are measurable. For instance, Brirtish astronomer John Couch Adams (1819–1892) and French astronomer Urbain LeVerrier (1811–1877) independently discovered Neptune in 1846 by observing the then-unaccountable perturbations in the orbit of Uranus. They both correctly concluded that a more distant planet was responsible for the shifts.

So, the other planets do affect the orbit, albeit slightly.

WOULD WE BE ABLE TO MOVE EARTH BEFORE THE SUN BECOMES A RED GIANT?   -  J.T.

First, I should mention that we would need to “move” Earth prior to the stage at which the Sun expands to the red giant stage. This transformation is due to occur in 5–6 billion years, when the Sun exhausts its core hydrogen reserves. However, the Sun’s luminosity (energy output per second) is slowly increasing as a consequence of the thermonuclear core reactions. Astronomers estimate that the Sun’s luminosity increases by 6% every billion years. Consequently, Earth will be rendered uninhabitable in approximately 1.1 billion years.

In order to keep Earth habitable, we’d have to move to the Sun within 1.1 billion years and continuously shift its position away from the Sun because the habitable zone, the region in which temperatures are conducive to life’s survival, would also expand away from it. By the time the Sun becomes a red giant, the habitable zone will have extended all the way out to the orbits of Jupiter and Saturn. (Jupiter’s average heliocentric distance is 483 million miles; Saturn’s mean distance is 914 million miles.) See graphic below.

The changing “Habitable Zone.” Earth is presently located within Earth’s habitable zone, hence our continued existence on it. However, as the Sun’s luminosity increases, this zone will expand away from the Sun. Earth will be rendered uninhabitable in about 1.1 billion years. By the time the Sun expands to the red giant stage, the habitable zone will be hundreds of millions of miles farther away: in the region where Jupiter and Saturn revolve around the Sun. Image: Astronomy Magazine

A few physicists have actually considered the possibility of shifting Earth to protect it from the evolving Sun. One can well imagine that shifting Earth safely constitutes one of the most challenging astro-engineering problems. Perhaps the most feasible option discussed so far would be to direct comets and asteroids with widths exceeding 100 kilometers around Earth and then to either Jupiter or Saturn. The “pull” caused by these repeated revolutions, though quite small, could cumulatively direct Earth gradually but inexorably away from the Sun. Of course, this solution poses many problems, namely, maintaining control of these bodies to ensure that none of them eventually crashes onto Earth. Considering that the impact of a 10-km wide asteroid ended the Cretaceous period, the consequences of a 100-km wide asteroid -taking into account the 1:10 ratio between the impacting body’s diameter and that of the produced crater- could put an end to Earth life itself.

Fortunately, we have plenty of time to contemplate the matter. That last statement presumes that our species will even persist for 1.1 billion years -4,400 times longer than the present duration of homo sapiens. If humans are still extant by this time, presumably they would have devised a solution to either shift Earth away from the Sun or to actually move to another star system altogether. While the latter option would be the logistically easier of the two, who knows what technology will develop in the intervening time. After all, humans are known for their problem-solving and, admittedly, problem-creating, capacities.