Thirty-five years ago NASA launched two space-probes within 16 days of each other. Voyager 1 and Voyager 2 were sent into space to gather information about some of the more distant planets in our solar system. With those original mis­sions now completed, the probes continue on through space and are on the verge of discovering more interest­ing information about interstellar space. Many of these space missions conjure up debate concerning the worth and wisdom of such costly endeavors. Regardless of what motives man may have in such investigations, we can consider these voyages and the data that is gathered in a positive light; for by these means we grow in our understanding of the glory of God as displayed in the stars and planets of the heavens and of the “powers” He has placed within the creation.

As interesting as the data may be that is gathered about space and the distant planets, I am also amazed at how the probes even got to those planets. How the probes arrived at the planets is in itself a fascinating story that teaches us marvelous things about God’s work in His creation.

Newton’s Law of Universal Gravitation

In the previous article (see April 1, 2013, p. 295) we noticed that Brahe and Kepler gathered detailed data about the motions of the planets. Kepler’s three laws describing the motion of the planets were later explained by Newton’s Law of Universal Gravitation. Newton taught that all objects exert a force of gravity on other objects, causing them to change their mo­tion. These gravitational forces explained why planets orbit the sun. As folklore has it, Newton saw an apple fall from a tree to the ground. Newton reasoned that the apple must fall to the ground for the same reason that the earth “falls” towards the sun. The sun, a very massive star, exerts a large gravitational force on all the planets, including Earth, which causes them to “fall” to­wards the sun, or “orbit” the sun. (Planets “orbit” rather than “crash” into the sun because of another truth that Newton discovered: that an object in motion will remain in motion in a “straight line” unless acted upon by an outside force. So, if the sun would disappear to­day, then besides all the other havoc that would result, Earth would no longer travel in a “circle-like” orbit, but would travel off into distant space in the straight path that it was trying to go). Newton correctly surmised that if Earth can “fall” towards the sun because of the sun’s gravitational pull, so then the apple can “fall” to­wards the earth because of Earth’s gravitational pull. So Newton correctly explained the force of Earth’s gravity, which keeps all objects on the earth, as well as the force that keeps all planets in their orderly orbits.

It was Newton’s discovery of the Law of Universal Gravitation that has proven to be essential for man to send rockets into space in order to investigate other planets.

Newton correctly explained that the strength of an object’s gravitational force is dependent on two factors: the mass of that body and the distance between the center of that body and another object. As I sit at my desk to write this article, I (because I have mass) exert a gravitational force on the desk. And since the desk also has mass, it exerts a gravitational force on me. These gravitational forces, however, turn out to be very, very weak in strength. As I work at my desk, I do not “fall” towards the desk as the apple “falls” towards Earth. And as I walk to school each day, birds and mailboxes and other objects do not “fall” towards me. I and my desk, for example, just do not have very much mass. Our masses are relatively tiny (insignificant—less than dust in the balance) compared to that of the mass of Earth or of the sun. Consequently, our gravitational forces are very, very weak—so weak that they cannot overcome frictional forces, for example, that keep ob­jects from moving towards us.

However, there are examples in the creation where the gravitational forces are significant enough that there are noticeable phenomena. Consider the Earth-Moon system. Earth’s mass is very large, and therefore it exerts a large gravitational pull on the moon. This gravitational pull keeps the moon orbit­ing the earth, much like the sun’s gravitational pull keeps the earth orbiting the sun. But the moon is also a very massive object, and in turn also exerts a gravitational pull on the earth. Its gravitational pull on Earth doesn’t move the planet in quite the same way as the earth makes the moon orbit the earth, but the moon’s gravitational pull is responsible for a daily occurrence on Earth. Each day the tide water rises and falls around the coastal regions of the globe. The moon’s gravitational pull on the earth is responsible for the daily tides. In simple terms, we might say that all the ocean water is pulled towards the moon. When the moon is in one location it results in a high tide for Puget Sound, WA, for example. Later that day, when the moon is in a different location, a low tide results. So, God uses what we commonly call the Law of Universal Gravitation to cause the tides to rise and fall each day around the globe.

The “Inverse Square” Law

Not only is mass a factor that determines the strength of an object’s gravitational force, but also the distance between that object and another determines the strength of the gravitational force. Newton discov­ered that the strength of the gravitational force and the distance between two objects is related according to the Inverse Square Law. Suppose we consider the Earth-Moon system again. If the moon were twice as far away as it currently is, the gravitational pull the earth would have on the moon (and that the moon would have on the Earth) would be one-fourth as strong (take the number 2—because twice as far away; square it; and the answer is 4. The inverse of 4 is ¼. Thus, the gravitational pull of an object twice as far away is ¼). Three times the distance apart, and the gravitational pull would be one-ninth as strong. Any change in the distance between two objects will dramatically change the strength of the gravitational force. Interestingly, God created many relationships in the creation that fol­low this same inverse square law—perhaps something to consider for another article.


It was this understanding of the Law of Universal Gravitation, as well as an understanding of the motion and orbits of the planets, that was critical in the Voy­ager missions. How does one get the probes to the dis­tant planets of Jupiter, Saturn, Uranus, and Neptune? Perhaps surprisingly, this was not accomplished with large rocket boosters full of extra fuel or by any remote navigation. The ability to navigate the probes, once in space, would come from the gravitational forces of the planets themselves and a use of their precise orbital paths (which Kepler and others so carefully observed and recorded).

The goal of the Voyager missions was to use fuel only to propel the space-probes into space along a particular path so that they would intersect with the orbits of the planets. According to Newton’s Law of Universal Gravitation, as the spacecraft neared a planet, it would experience a stronger gravitational pull from the planet. This tug from the planet on the fast-moving spacecraft would change the direction and the speed of the spacecraft. This is called a gravity-assist. This change in the spacecraft’s path would allow it to navigate to its next destination—the next nearest planet.

But was it possible to get a gravity-assist from more than just Jupiter? Could the timing ever be right so that a spacecraft could get multiple gravity-assists in order to navigate towards the farthest planets? The correct timing to do such a thing is called an opportunity. An opportunity—the correct timing to send a spacecraft to pass close by a particular planet in order to obtain a gravity-assist—is available only every so often. And the opportunity to visit four outer-planets (Jupiter, Saturn, Uranus, and Neptune)—all by the same spacecraft using only gravity-assists for navigation is very rare! Because of the alignment and orbits of the planets, such an opportunity happens only once every 175 years! The most recent opportunity was in 1977. NASA took that opportunity to launch Voyager 2 and Voyager 1 (in that order) in the late-summer of 1977. By 1979, both probes were observing Jupiter and by 1981 both had observed Saturn. Having been launched at different dates, they received different gravity-assists, and therefore took different paths through space. Voy­ager 2 continued on a path to intersect Uranus (1985) and Neptune (1989). Today they both continue on beyond the planets, sending communication to Earth about interstellar space.

The Heavens Declare the Glory of God: Order; Providence

Man’s accomplishments with the Voyager missions may seem impressive, but they were possible only because of the very orderly creation that God created and continues to uphold. God gave to all the planets their particular motion and paths. And God continues to govern and direct the paths of the planets each day. By the word of God’s power all things were called into being. By that same word of God’s power all things continue to exist and move. God rules all things in the same way day by day, moment by moment. It is by that orderly and consistent rule that we observe an orderly creation. The data that Copernicus, Brahe, Kepler, and Galileo collected was consistent because God ordained the paths of the planets. What took the best and brightest of human minds and centuries to discover, God simply created and continues to uphold by the word of His power.

May we always be far more impressed with the wondrous work God has done in creation and in our salvation than in man’s endeavors and feats with the creation. When we consider the heavens, the works of God’s hands, we exclaim with Eliphaz, “Is not God in the height of heaven? And behold the height of the stars, how high they are?” (Job 22:12). We stand in awe of how great and majestic God is who called the vast creation into existence and continues to uphold it. Truly we marvel at the work of creation—that God made all things with such beauty and with such an or­der and interrelatedness! Yet, as wondrous as the work of creation is, all the more the work of salvation. In ref­erence to our regeneration, Canons, Heads 3/4, Article 12 makes the point clearly. “And this is the regeneration so highly celebrated in Scripture and denominated a new creation: . . . but it is evidently a supernatural work, most powerful, and at the same time most delightful, astonishing, mysterious, and ineffable; not inferior in efficacy to creation or the resurrection from the dead.” In light of this, we humbly bow in reverence before our Creator and Savior.

Not only must we learn to look at the creation to see God’s wondrous works and praise Him for them, but we must also see in them a comforting reminder that God governs all things. The movement of the stars and the orbits of the planets are no exception. They too move at His command. The very God who directs the movement of the planets and created them with just the right mass to create a specific gravity that affects the other planets is the same God who directs all things for our good. May we never doubt His power and goodness. Truly we are a speck of dust compared to all of the creation; and yet, God cares for us each day. Humbling! How very humbling! God is great and we are less than nothing (Is. 40:17); and yet, God faithfully provides for us all that we need, for body and for soul.

In fact, God specifically uses the ordinances of the stars (and planets) to remind us of His covenant faith­fulness. Having just comforted His people by assuring them that He makes an everlasting covenant with them, God illustrates the enduring nature of the covenant He establishes by stating that as man cannot change the ordinances He made for the heavens, so man can­not change His covenant. “Thus saith the Lord which giveth the sun for a light by day, and the ordinances of the moon and of the stars for a light by night, which divideth the sea when the waves thereof roar; The Lord of hosts is his name: If those ordinances depart from before me, saith the Lord, then the seed of Israel also shall cease from being a nation before me forever” (Jer. 31:35-36). Despite all our failures and unfaithfulness, God is faithful! Let us praise Him always!