Humanities Journey to Interstellar Space

Seemingly, since man first looked into the night, we have always yearned to explore the cosmos. But, despite hundreds of thousands of years of evolution, from our earliest ancestors to the present era, we have only recently begun to venture “far” away from our home. 

At the time of the release of this article, the furthest man-made object, Voyager 1, is roughly 24.3 billion kilometers (15.1 billion miles) or 162.7 Astronomical Units (AU) from Earth. Voyager 1 was launched back in 1977 and after some 35 years of coasting through the solar system, it finally entered "interstellar space" after it ventured beyond the Sun's heliopause. The heliopause is the theoretical boundary for the Sun's heliosphere. In essence, the heliosphere is the region in space under the influence of the magnetic field and solar wind of the host star(s). Thus, any spacecraft that crosses the heliopause, traveling into the interstellar medium, has officially entered interstellar space. 

While humanity technically reached interstellar space in 2012, very little is known about the region just beyond the heliopause. This has to do with the fact that the Voyager probes were intended to be outer planet explorers, not interstellar ones. While the journey to interstellar space was an added benefit, it wasn’t the primary objective. By the time the Voyagers reached the edge of the heliosphere, it had been over 40 years since they first left home. At this point in their life span, their RTGs were outputting far less power, meaning most of the science equipment onboard was shut down to ensure the spacecraft could stay in contact with Earth. Thus, very little is known about the outer heliosphere, the heliopause, and the region just beyond this boundary. 

 That is where the John Hopkins University of Applied Physics Laboratory (JHUAPL) proposed interstellar probe comes into play. This mission proposal intends to push humanity further into the cosmos than ever before.

Science Objectives

Of course, any deep space mission needs a spacecraft and despite being nameless at the time of posting, John Hopkin's proposed interstellar probe is quite the spacecraft. The probe would have two main science priorities, one would be the study of the spectral line of hydrogen (or other electron atoms). While the other portion of the scientific payload would be in charge of both visible and infrared imagery of flyby targets.

As mentioned above, little is known about the heliopause and what lies beyond, despite the Voyagers passing the invisible boundary between our solar system and the interstellar medium about 8 years ago. Understanding this region will immensely help scientists better understand how a star's magnetic field and solar winds interact with interstellar space. This will also help man piece together how the Universe as a whole works. 

With this information in mind, you can kind of guess what science would be conducted and its purpose. Below are some of the main and potential goals of this proposed mission.

✦ How the heliosphere interacts with both the Sun and Interstellar medium

✦ Uncover the nature of the Interstellar Medium

✦ Viewing the Sun from beyond the heliopause

These objectives are the bread and butter of what this mission will do. The main objectives all have to do with adding to information discovered by the Voyager probes. There also is a potential science benefit depending on when the mission is given the green light. Outbound Gas Giant/Dwarf planet flybys could be a secondary point of scientific discovery.

Here is the split of science the interstellar probe would conduct:

Hardware Specs of the Probe

Of course, this interstellar voyager needs to beam back its findings to Earth. That is where its 5m in diameter radio antenna comes into play. This powerful radio transmitter allows for maximum data rates between 365-2592bps. Such a wide data rate discrepancy is mainly caused by distance, but other factors can slow it down. Still, this is an immensely high data rate at the absurd distances the probe intends to go. For comparison, New Horizon has a data rate maximum of 800bps and Voyager only 160bps. Not only will the spacecraft have better data rate maximums, but it can achieve these rates at further distances. However, it is important to note that these numbers account for the next generation Very Large Array (ngVLA). If the probe has to use the Deep Space Network (DSN) instead, it would likely achieve much smaller data rates, as ngVLA has more sensitive radio dishes.

To make sure both the science can be collected and sent back home, the probe needs a fair bit of power. This power would be supplied by two 16-module NextGen RTGs with a shunt regulation unit. The Shunt unit ensures a stable voltage output from the unstable power input. This architecture is similar to that seen on New Horizon and the Parker Solar Probe, so it is not an unproven technology.

In all, the probe weighs about 860kgs (1900lbs) at launch and has an estimated cost of 1.7 billion USD; with an additional 230 million USD every decade for operational costs.

Launch hardware

While the science objectives and probe hardware are just one part of the puzzle; escaping the Sun's gravity is no easy feat and while it has been done, it takes IMMENSE power to achieve. That is why the team at Hopkins envisions this interstellar explorer to be flung out of our solar system on board an SLS Block 2, with an Atlas V Centaur third stage and a Northrop Grumman STAR 48BV fourth stage. 

This pairing would be absolutely monstrous, especially considering the small liftoff mass of the probe. SLS Block 2 is planned to be capable of lifting up to 130 metric tons to LEO, the interstellar probe, weighing in at 890kgs is only 0.65% of that. Of course, once you account for the weight of the third and fourth stages it isn't so absurd (~19%), but this really highlights just how much power is needed for such a mission.

Trajectory and launch dates

While all this delta-v isn't needed to escape the solar system necessarily, it allows for a higher exit speed. This means the heliopause and interstellar medium can be reached earlier in the mission. The earlier the probe reaches the outer parts of the solar system, the healthier the RTGs powering the probe are. 

Due to the mission's uncertainty, the team at Johns Hopkins has to prepare for many possible trajectories. As it stands those working on this mission proposal assume the launch would occur within the years of 2036-2041. The baseline trajectory using a Jupiter gravity assist would launch sometime in the year 2036, but this can differ depending on when the probe is actually ready to launch. Because launch windows are crucial and aerospace is full of delays, secondary trajectories in the following years are also accounted for. As mentioned above, science gained from outer planet flybys could be an added benefit depending on when the probe begins its voyage. 

Timeline of events

If the interstellar traveler launches sometime within the year 2036 it will reach 100 AU in less than 15 years. For comparison, it took Voyager 1 about 30 years to reach the 100 AU mark.

Like most space missions redundancy is planned from the get-go, so other launch dates have their estimated time frames accounted for. 

Closing remarks

To wrap everything up, the John Hopkins interstellar probe is an amazing mission proposal that would do a lot of good for very little cost. In my opinion, it truly would be one of man's greatest feats as it would be the first, true interstellar mission. If this mission comes to fruition, we could genuinely see another "pale blue dot" level photo, as the probe faces its cameras toward Sol and snaps a photo of our star. Ignoring the potential flyby of Kuiper Belt Objects, the additional science collected at the Jovian system and the core mission in itself has immense value. 

And while the Voyagers that came before it were the first to step beyond our solar system, the interstellar probe would truly mark the beginning of man's interstellar era.

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