An artistic representation of our home galaxy, the Milky Way, highlighting the vastness of space we aim to traverse in interstellar travel.
NASA/JPL-Caltech/R. Hurt (SSC/Caltech)
Humankind has embarked on its first ambitious steps into interstellar space – the immense expanse between stars. While journeys to distant stars remain in the realm of science fiction for human travelers, significant strides are being made. NASA’s Voyager 1 and Voyager 2 probes have already crossed this frontier, venturing into the unknown. These pioneering missions, along with ongoing research and technological advancements, are laying the groundwork for future Interstellar Space Travel.
Although sending humans across the cosmic void to other star systems is still a distant dream, scientists and engineers are diligently working to develop the necessary skills and technologies. Our understanding of interstellar space is constantly evolving, and each discovery brings us closer to potentially traversing these vast distances.
Here are 10 key insights we’ve gained about interstellar space travel, highlighting the challenges and wonders of venturing beyond our solar system.
1. Defining Interstellar Space: More Than Just Empty Space
Interstellar space isn’t simply the void between stars; it’s a specific region defined by the boundaries of stellar influences. More precisely, it’s the area residing between our Sun’s heliosphere and the astrospheres of other stars.
Our heliosphere is a colossal bubble sculpted by plasma – a superheated gas of charged particles – continuously ejected from the Sun. This outward flow is known as the solar wind. This protective bubble envelops our Sun and extends far beyond the orbits of all the planets. To reach interstellar space, both Voyager spacecraft had to journey over 11 billion miles (17 billion kilometers) from the Sun, pushing past the heliosphere’s edge. The heliosphere itself is in constant motion, traveling through interstellar space as our Sun orbits the Milky Way’s galactic center, creating a bow wave akin to a ship cutting through water.
Voyager retro posters available for download, celebrating the probes’ groundbreaking journey into interstellar space.
NASA/JPL-Caltech
2. The Immense Timescales of Interstellar Journeys
Forget warp drive – for now, interstellar space travel is a marathon, not a sprint. Voyager 1, the first spacecraft to achieve this feat, was approximately 122 Astronomical Units (AU) from the Sun (Earth is 1 AU from the Sun), equivalent to about 11 billion miles (18 billion kilometers), when it finally exited the heliosphere and entered interstellar space. Launched from Earth in 1977, Voyager 1 officially reached interstellar space in 2012 – a journey spanning 35 years. It’s worth noting that Voyager 1 didn’t take a direct route; it embarked on a “grand tour” of Jupiter and Saturn first. Voyager 2, traveling at a slightly slower pace and also touring Uranus and Neptune, took 41 years to reach interstellar space. These timelines underscore the vast distances involved and the patience required for interstellar exploration.
Captured by Voyager 1 on February 14, 1990, this image is part of the first “family portrait” of our solar system from beyond Neptune, including the iconic “Pale Blue Dot” image of Earth.
NASA/JPL-Caltech
3. Visualizing the Void: The Lack of Interstellar Photography
Don’t expect stunning interstellar landscape photos from the Voyager probes. After Voyager 1 captured images for the “Solar System Family Portrait” in 1990, including the famous “Pale Blue Dot” image of Earth, its cameras were deactivated. This was done to conserve precious power and computer memory for the ongoing interstellar mission. Furthermore, the original camera software is no longer available, and the ground-based computers capable of interpreting it are obsolete. Decades of exposure to extreme cold in space may also have compromised the cameras’ functionality. Even if reactivation were possible, interstellar space wouldn’t offer dramatically different views from what Voyager captured in 1990, primarily just a different perspective on the distant stars.
Captured by NASA’s Voyager 1 spacecraft, these are the eerie sounds of interstellar space, detected by its plasma wave instrument.
NASA/JPL-Caltech
4. The Sounds of Silence? Interstellar Space Acoustics
Interstellar space is often described as a near-perfect vacuum. In such an environment, sound waves, which require a medium like air to travel, cannot propagate in the way we perceive them. However, Voyager’s instruments possess sensitivities far exceeding human hearing. They are capable of “listening” to different types of waves that do traverse the interstellar medium. And what they detected was, in a sense, a form of “music” to the ears of scientists.
Don Gurnett, the principal investigator for Voyager 1’s Plasma Wave Science instrument, presented an audio recording of plasma wave data at a press conference in 2013. These sounds provided definitive evidence that Voyager 1 had indeed crossed the heliosphere’s boundary and entered interstellar space. Technically, the plasma wave instrument doesn’t detect sound in the conventional sense. It senses waves in the plasma generated by solar eruptions, known as coronal mass ejections. These waves interact with the interstellar medium, allowing Voyager to detect them both inside and outside the heliosphere. While these plasma waves are far too faint for human ears to perceive directly due to the sparse nature of interstellar plasma, some of these waves fall within the audible frequency range. By amplifying these signals, scientists made them audible, offering a unique “soundscape” of interstellar space.
A deep space image showing the interstellar object ‘Oumuamua at the center, with star trails created as telescopes tracked the fast-moving object.
ESO/K. Meech et al.
5. ‘Oumuamua: An Interstellar Visitor to Our Solar System
In late 2017, an extraordinary object sped through our solar system, capturing the attention of astronomers worldwide. Its trajectory was unmistakably interstellar, indicating it originated from beyond our solar system. This marked the first confirmed detection of an object from another star system visiting our own.
Scientists named it ‘Oumuamua, a Hawaiian term signifying “visitor from afar arriving first.” The exact nature of ‘Oumuamua remains somewhat enigmatic. As the first of its kind detected and observed only briefly at a distance, drawing definitive conclusions proved challenging. Nevertheless, it was clear that ‘Oumuamua was substantial in size, traveling at high velocity, and exhibiting a tumbling motion as it traversed our solar system.
Estimates suggest ‘Oumuamua was approximately half a mile (800 meters) in length. Astronomers had never encountered a natural object with such extreme proportions within our solar system before. The last observation recorded it traveling away from the Sun at around 196,000 mph (87.3 kilometers per second), on its trajectory back into interstellar space. By January 2018, ‘Oumuamua had faded beyond the reach of even the most powerful telescopes.
An artist’s depiction of the wake created by our solar system as it moves through interstellar space, with Voyager probes positioned outside the heliosphere.
NASA/JPL-Caltech
6. Pioneering the Interstellar Frontier: Voyager and Beyond
Only two spacecraft have successfully reached interstellar space: Voyager 1, which crossed the boundary in August 2012, and its twin, Voyager 2, which followed suit on November 5, 2018. These missions represent humanity’s first physical presence in interstellar space.
NASA’s New Horizons probe, renowned for its exploration of Pluto and the Kuiper Belt Object Arrokoth, is also on a trajectory towards interstellar space, heading generally towards the constellation Sagittarius. While not specifically designed for interstellar exploration, it will eventually join the Voyager probes in traversing the space between stars.
NASA’s Pioneer 10 and Pioneer 11 spacecraft, though no longer operational, are also silently drifting into interstellar space, acting as ghostly ambassadors. Pioneer 10 is directed towards the red giant star Aldebaran in the constellation Taurus, while Pioneer 11 is heading towards the galactic center in the direction of Sagittarius.
Voyager 2 launching on a Titan/Centaur rocket from Florida in 1977, embarking on its historic interstellar journey.
NASA/JPL-Caltech
7. Escape Velocity: The Key to Interstellar Departure
While numerous spacecraft have been launched beyond Earth’s orbit, only a select few are destined to leave our solar system. The reason? Most missions are designed for specific objectives within our solar system, such as planetary flybys, orbits, or landings.
Reaching interstellar space necessitates launching a probe into a precise trajectory with sufficient velocity to overcome the Sun’s gravitational pull. This requires powerful rockets to impart the necessary escape velocity.
The Voyager missions ingeniously utilized a rare planetary alignment that occurs approximately every 176 years. They employed gravity assists, using the gravitational forces of Jupiter, Saturn, Uranus, and Neptune to “slingshot” themselves from one planet to the next, gaining velocity without requiring massive propulsion systems. These gravity assists provided the crucial speed boosts needed to escape the Sun’s gravitational influence and embark on their interstellar journeys.
Animated graph showing cosmic ray increase and solar particle decrease as Voyager 2 enters interstellar space.
Animated data graph from Voyager 2 showing cosmic ray and plasma science instrument readings confirming entry into interstellar space.
NASA/JPL-Caltech/GSFC
8. Long-Lived Explorers: Voyager’s Enduring Legacy
Launched just 16 days apart in 1977, Voyager 2 was launched first, but Voyager 1 followed a faster trajectory. They hold the distinction of being the longest continuously operating spacecraft in history. Collectively, they have explored all the gas giant planets of our solar system, revolutionizing our understanding of these distant worlds.
Despite being in interstellar space, the Voyager probes haven’t truly exited the solar system’s full extent. The solar system’s boundary is considered to be beyond the Oort Cloud, a vast, spherical region of icy bodies still gravitationally bound to the Sun. Most comets that enter the inner solar system originate from the Oort Cloud. It could take the Voyager probes approximately 300 years to even reach the inner edge of the Oort Cloud, highlighting the solar system’s immense scale.
The Voyager Golden Record, a phonograph record containing sounds, images, and messages from Earth, intended for any intelligent extraterrestrial life encountered.
NASA/JPL-Caltech
9. Voyager’s Interstellar Future: Drifting Ambassadors
Eventually, the Voyager probes will pass by other stars on their immense journeys. Voyager 1 is currently traveling at about 3.5 AU per year, moving 35 degrees north of the ecliptic plane, generally towards the solar apex – the direction of the Sun’s motion relative to nearby stars. Voyager 1 is heading towards the constellation Ophiuchus. In the distant future, around the year 40,272 CE (over 38,000 years from now), Voyager 1 will pass within 1.7 light-years of a faint star in the constellation Ursa Minor (the Little Bear) named Gliese 445.
Voyager 2 is traveling at approximately 3.1 AU per year towards the constellations Sagittarius and Pavo. In about 40,000 years, Voyager 2 will come within roughly 1.7 light-years of Ross 248, a small star in the constellation Andromeda. Beyond these encounters, the Voyager spacecraft are destined to orbit within the Milky Way galaxy indefinitely, serving as silent ambassadors from Earth, possibly for eternity. Each probe carries a Golden Record containing sounds, images, and messages from Earth, a time capsule for any civilization that might encounter them in the distant future.
Launched in 2008, the Interstellar Boundary Explorer (IBEX) satellite studies the heliosphere’s outer boundary from Earth orbit.
NASA’s Goddard Space Flight Center
10. Beyond Voyager: Future Interstellar Exploration
Currently, NASA has no active missions planned to send new spacecraft directly to interstellar space. However, researchers are actively exploring various innovative concepts and technologies for future interstellar probes, considering both feasibility and scientific value. In the meantime, NASA operates two satellites dedicated to studying interstellar space from locations relatively close to Earth.
The Interstellar Boundary Explorer (IBEX) is a small satellite already in Earth orbit. IBEX utilizes specialized instruments to map the boundary of interstellar space for the first time, providing valuable data about this critical region. NASA is also preparing to launch the Interstellar Mapping and Acceleration Probe (IMAP) in 2025. IMAP will be positioned approximately 1 million miles (1.6 million kilometers) from Earth, towards the Sun, at the first Lagrange point (L1). IMAP will further enhance our understanding of the heliosphere’s boundary and its interaction with interstellar space, paving the way for future interstellar space travel.
