Teachable Moments | December 18, 2018
Then There Were Two: Voyager 2 Reaches Interstellar Space
In the News
The Voyager 2 spacecraft, launched in 1977, has reached interstellar space, a region beyond the heliosphere – the protective bubble of particles and magnetic fields created by the Sun – where the only other human-made object is its twin, Voyager 1.
The achievement means new opportunities for scientists to study this mysterious region. And for educators, it’s a chance to get students exploring the scale and anatomy of our solar system, plus the engineering and math required for such an epic journey.
How They Did It
Launched just 16 days apart, Voyager 1 and Voyager 2 were designed to take advantage of a rare alignment of the outer planets that only occurs once every 176 years. Their trajectory took them by the outer planets, where they captured never-before-seen images. They were also able to steal a little momentum from Jupiter and Saturn that helped send them on a path toward interstellar space. This “gravity assist” gave the spacecraft a velocity boost without expending any fuel. Though both spacecraft were destined for interstellar space, they followed slightly different trajectories.
An illustration of the trajectories of Voyager 1 and Voyager 2. Image credit: NASA/JPL-Caltech | + Expand image
Voyager 1 followed a path that enabled it to fly by Jupiter in 1979, discovering the gas giant’s rings. It continued on for a 1980 close encounter with Saturn’s moon Titan before a gravity assist from Saturn hurled it above the plane of the solar system and out toward interstellar space. After Voyager 2 visited Jupiter in 1979 and Saturn in 1981, it continued on to encounter Uranus in 1986, where it obtained another assist. Its last planetary visit before heading out of the solar system was Neptune in 1989, where the gas giant’s gravity sent the probe in a southward direction toward interstellar space. Since the end of its prime mission at Neptune, Voyager 2 has been using its onboard instruments to continue sensing the environment around it, communicating data back to scientists on Earth. It was this data that scientists used to determine Voyager 2 had entered interstellar space.
How We Know
Interstellar space, the region between the stars, is beyond the influence of the solar wind, charged particles emanating from the Sun, and before the influence of the stellar wind of another star. One hint that Voyager 2 was nearing interstellar space came in late August when the Cosmic Ray Subsystem, an instrument that measures cosmic rays coming from the Sun and galactic cosmic rays coming from outside our solar system, measured an increase in galactic cosmic rays hitting the spacecraft. Then on November 5, the instrument detected a sharp decrease in high energy particles from the Sun. That downward trend continued over the following weeks.
The data from the cosmic ray instrument provided strong evidence that Voyager 2 had entered interstellar space because its twin had returned similar data when it crossed the boundary of the heliosheath. But the most compelling evidence came from its Plasma Science Experiment – an instrument that had stopped working on Voyager 1 in 1980. Until recently, the space surrounding Voyager 2 was filled mostly with plasma flowing out from our Sun. This outflow, called the solar wind, creates a bubble, the heliosphere, that envelopes all the planets in our solar system. Voyager 2’s Plasma Science Experiment can detect the speed, density, temperature, pressure and flux of that solar wind. On the same day that the spacecraft’s cosmic ray instrument detected a steep decline in the number of solar energetic particles, the plasma science instrument observed a decline in the speed of the solar wind. Since that date, the plasma instrument has observed no solar wind flow in the environment around Voyager 2, which makes mission scientists confident the probe has entered interstellar space.
This animated graph shows data returned from Voyager 2's cosmic ray and plasma science instruments, which provided the evidence that the spacecraft had entered interstellar space. Image credit: NASA/JPL-Caltech/GSFC | + Expand image
Though the spacecraft have left the heliosphere, Voyager 1 and Voyager 2 have not yet left the solar system, and won't be leaving anytime soon. The boundary of the solar system is considered to be beyond the outer edge of the Oort Cloud, a collection of small objects that are still under the influence of the Sun's gravity. The width of the Oort Cloud is not known precisely, but it is estimated to begin at about 1,000 astronomical units from the Sun and extend to about 100,000 AU. (One astronomical unit, or AU, is the distance from the Sun to Earth.) It will take about 300 years for Voyager 2 to reach the inner edge of the Oort Cloud and possibly 30,000 years to fly beyond it. By that time, both Voyager spacecraft will be completely out of the hydrazine fuel used to point them toward Earth (to send and receive data) and their power sources will have decayed beyond their usable lifetime.
Why It’s Important
Since the Voyager spacecraft launched more than 40 years ago, no other NASA missions have encountered as many planets (some of which had never been visited) and continued making science observations from such great distances. Other spacecraft, such as New Horizons and Pioneer 10 and 11, will eventually make it to interstellar space, but we will have no data from them to confirm their arrival or explore the region because their instruments already have or will have shut off by then.
Interstellar space is a region that’s still mysterious because until 2012, when Voyager 1 arrived there, no spacecraft had visited it. Now, data from Voyager 2 will help add to scientists’ growing understanding of the region. Scientists are hoping to continue using Voyager 2’s plasma science instrument to study the properties of the ionized gases, or plasma, that exist in the interstellar medium by making direct measurements of the plasma density and temperature. This new data may shed more light on the evolution of our solar neighborhood and will most certainly provide a window into the exciting unexplored region of interstellar space, improving our understanding of space and our place in it.
As power wanes on Voyager 2, scientists will have to make tough choices about which instruments to keep turned on. Further complicating the situation is the freezing cold temperature at which the spacecraft is currently operating – perilously close to the freezing point of its hydrazine fuel. But for as long as both Voyager spacecraft are able to maintain power and communication, we will continue to learn about the uncharted territory of interstellar space.
Teach It
Use these standards-aligned lessons and related activities to get students doing math and science with a real-world (and space!) connection.
-
Solar System Bead Activity
Students create a scale model of the solar system using beads and string.
Grades 1-6
Time 30 mins - 1 hr
-
Catching a Whisper from Space
Students kinesthetically model the mathematics of how NASA communicates with spacecraft.
Grades 4-12
Time 1-2 hrs
-
Solar System Scroll
Students predict the scale of our solar system and the distance between planets, then check their answers using fractions.
Grades 5-8
Time < 30 mins
-
*NEW* Modeling the Structure of the Solar System
Students will learn about the structure of the solar system and be able to identify analogous regions in a dynamic, 2-D kitchen-sink model.
Grades 6-12
Time 1-2 hrs
-
Hear Here: A 'Pi in the Sky' Math Challenge
Students use the mathematical constant pi to determine what fraction of a signal from Voyager 1 – the most distant spacecraft – reaches Earth.
Grades 10-12
Time < 30 mins
Explore More
- News Release: “NASA’s Voyager 2 Probe Enters Interstellar Space” – Dec. 10, 2018
- News Release: “NASA Spacecraft Embarks on Historic Journey Into Interstellar Space” – September 12, 2013
- Voyager Mission
- Voyager Images
- Voyager 2: Interstellar, by the Numbers
- Commemorative Voyager Posters
TAGS: Teachers, Educators, Science, Engineering, Technology, Solar System, Voyager, Spacecraft, Educator Resources, Lessons, Activities
Teachable Moments | August 29, 2017
The Farthest Operating Spacecraft, Voyagers 1 and 2, Still Exploring 40 Years Later
In the News
This year marks the 40th anniversary of the launch of the world’s farthest and longest-lived spacecraft, NASA’s Voyager 1 and 2. Four decades ago, they embarked on an ambitious mission to explore the giant outer planets, the two outermost of which had never been visited. And since completing their flybys of Jupiter, Saturn, Uranus and Neptune in 1989, they have been journeying toward the farthest reaches of our solar system – where no spacecraft has been before. These two intrepid spacecraft continue to return data to NASA daily, offering a window into the mysterious outer realms of our solar system and beyond.
How They Did It
The Voyager spacecraft were launched during a very short window that took advantage of a unique alignment of the four giant outer planets – one that would not occur again for another 176 years. (Try this lesson in calculating launch windows to get an idea of how it was done.) Launching at this point in time enabled the spacecraft to fly by all four planets in a single journey, returning never-before-seen, close-up images and scientific data from Jupiter, Saturn, Uranus and Neptune that greatly contributed to our current understanding of these planets and the solar system.
Why It’s Important
These images of Jupiter, Saturn, Uranus and Neptune (clockwise from top) were taken by Voyager 1 and 2 as the spacecraft journeyed through the solar system. See a gallery of images that Voyager took on the Voyager website. Credit: NASA/JPL-Caltech
In addition to shaping our understanding of the outer planets, the Voyager spacecraft are helping us learn more about the space beyond the planets – the outer region of our solar system. After completing their “grand tour” of the outer planets, the Voyagers continued on an extended mission to the outer solar system. They are now more than 10 billion miles from Earth, exploring the boundary region between our planetary system and what’s called interstellar space.
The beginning of interstellar space is where the constant flow of material from the Sun and its magnetic field stop influencing the surroundings. Most of the Sun’s influence is contained within the heliosphere, a bubble created by the Sun and limited by forces in interstellar space. (Note that the heliosphere doesn’t actually look like a sphere when it travels through space; it’s more of a blunt sphere with a tail.) The outer edge of the heliosphere, before interstellar space, is a boundary region called the heliopause. The heliopause is the outermost boundary of the solar wind, a stream of electrically charged atoms, composed primarily of ionized hydrogen, that stream outward from the Sun. Our planetary system lies inside the bubble of the heliosphere, bordered by the heliopause and surrounded by interstellar space.
Though we’ve learned a lot about the heliopause thanks to the Voyager spacecraft, its thickness and variation are still key unanswered questions in space physics. As the Voyagers continue their journey, scientists hope to learn more about the location and properties of the heliopause.
From their unique vantage points – Voyager 1 in the northern hemisphere and Voyager 2 in the southern hemisphere – the spacecraft have already detected differences and asymmetries in the solar wind termination shock, where the wind abruptly slows as it approaches the heliopause. For example, Voyager 2 crossed the termination shock at a distance of about 83.7 AU in the southern hemisphere. (One AU, or astronomical unit, is equal to 150 kilometers (93 million miles), the distance between Earth and the Sun.) That’s about 10 AU closer to the Sun than where Voyager 1 crossed the shock in the north. As shown in this diagram, Voyager 1 traveled through the compressed “nose” of the termination shock and Voyager 2 is expected to travel through the flank of the termination shock.
With four remaining powered instruments on Voyager 1 and five remaining powered instruments on Voyager 2, the two spacecraft continue to collect science data comparing their two distinct locations at the far reaches of the solar system.
In August 2012, Voyager 1 detected a dramatic increase in galactic cosmic rays (as shown in this animated chart). The increase, which has continued to the current peak, was associated with the spacecraft's crossing into interstellar space. Credit: NASA/JPL-Caltech
Since it launched from Earth in 1977, Voyager 1 has been using an instrument to measure high-energy, dangerous particles traveling through space called galactic cosmic rays. While studying the interaction between the bubble of the heliosphere and interstellar space, Voyager 1 revealed that the heliosphere is functioning as a radiation shield, protecting our planetary system from most of these galactic cosmic rays. So in August 2012, when Voyager 1 detected a dramatic increase in the rays, which has continued to the current peak, it was associated with the spacecraft’s crossing into interstellar space.
Meanwhile, Voyager 2 – which is still in the heliosheath, the outermost layer of the heliosphere between the shock and the heliopause – is using its solar wind instrument to measure the directional change of solar wind particles there. Within the next few years, Voyager 2 is also expected to cross into interstellar space, providing us with even more detailed data about this mysterious region.
In another 10 years, we expect one or both Voyagers to cruise outward into a more pristine region of interstellar space, returning data to inform our hypotheses about the concentration of galactic particles and the characteristics of interstellar wind.
Even with 40 years of space flight behind them, the Voyagers are expected to continue returning valuable data until about 2025. Communications will be maintained until the spacecraft’s nuclear power sources can no longer supply enough electrical energy to power critical functions. Until then, there’s still much to learn about the boundary of our heliosphere and what lies beyond in the space between the stars.
Teach It
Use these standards-aligned lessons and related activities to get students doing math and science with a real-world (and space!) connection.
- Hear Here - Students use the mathematical constant pi and information about the current location of Voyager 1 to learn about the faint data-filled signal being returned to Earth.
- Solar System Bead Activity – Students calculate and construct a scale model of solar system distances using beads and string.
- Catching a Whisper from Space – Students kinesthetically model the mathematics of how NASA communicates with spacecraft.
Explore More
- Voyager Mission
- Voyager Images
- Voyager Golden Record
- The Sounds of Interstellar Space
- Voyager Senses Sun's Tsunami Wave in Interstellar Medium
- Commemorative Voyager Posters
TAGS: Voyager, Farthest, Golden Record, STEM, Teachable Moments, Science, Engineering, Solar System, Interstellar Space, Heliopause, Heliosphere, Heliosheath, Termination Shock, Stars, Heliophysics