We've seen quite a few probes examining the Solar System in the recent years, but beyond Voyager delivering new measurements every couple months now, I haven't heard about any of the probes underway towards "out beyond" the Solar System.

Are there any successors to the Voyagers currently, aiming towards neighbor stars or just "out of the system" in general? If so, could you summarize their equipment and missions?

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    $\begingroup$ Note that the Voyagers were never intended to probe "out beyond" the Solar System. In fact, they weren't even intended to explore past Saturn. The fact that Voyager 2 also netted Uranus and Neptune was a huge bonus in an extended mission. The fact that both Voyagers continued after that to measure the solar plasma was an amazing additional bonus. The fact that one of the Voyagers is now measuring the interstellar plasma is an incredible bonus! Planning a mission to repeat that as its primary mission would be hugely expensive. $\endgroup$
    – Mark Adler
    Commented Dec 8, 2017 at 18:24

2 Answers 2


New Horizons has examined Pluto on 14 July 2015 and Ultima Thule on 1 January 2019. It may continue to explore a third Kuiper Belt object, but it's not aiming to measure the interstellar medium, let alone nearby stars; it's too slow for that.

It didn't go into orbit in the Pluto-Charon system; there's no atmosphere thick enough for aerobraking, and it doesn't have the propulsive capability to slow down. It made a flyby on 14 July 2015.

After Pluto and the Ultima Thule flyby, it is flying on and ultimately, in the 2040s or so, leave the solar system. It's slower than the Voyager spacecrafts and will no longer be operational when it enters the interstellar medium.


There are no active interstellar missions. JHUAPL is investigating what such a mission would look like. This presentation goes into a bit more detail.

As part of the studies for the next Heliophysics Decadal Survey JHUAPL is studying a probe to get to 1,000 AU within 50 years. ...

The study tries to answer these questions:

  • Focus on the time frame in the next Decadal: 2023 – 2032: Can we fly then?
  • Assess the science: Does the case remain compelling?
  • Approach for Decadal: Assess technical readiness for a launch NO LATER THAN 2030

Conclude this effort in nine months, reporting back to NASA by February 2019

A weight target for the probe has been set:

The probe will weight 300-800 kg of which 40 kg will be instruments. By comparison New Horizons is a 478 kg with 30.4 kg of instruments, Parker Solar Probe 685 kg with 50 kg of instruments, while Pioneer 10/11 and Ulysses were lighter.

Various propulsion options and gravity assists are examined.

In-Space Propulsion Does Not Solve the Problem – Underpowered for the Mass. SEP, RTG with ion engine, nuclear electric system studied.

The only other option is a really big rocket, SLS combined with gravity assist. Even then, it'd be difficult. Goal is a speed of 20 AU/year. a STAR 48 upper stage gets you 4 km/s, you need 14 km/s during the approach to the Sun at a distance of 4 solar radii. 10 km/s at 2 solar radii.


The Oberth-Kuiper maneuver requires close approach to the Sun, which requires heat shielding which adds weight.

The current mission plan is for an SLS launch to Jupiter, reverse gravity assist to reduce speed, plunge into the Sun at 4 solar radii (Parker Solar Probe will reach 8 solar radii) and use an Oberth maneuver to achieve 8 AU per year or twice the heliospheric escape speed of Voyager 1. Since they will be in the area, they are also proposing that a Kuiper Belt target is also target for a fast flyby.

Ulysses used an IUS plus a PAM-D for 20 tons worth of upper stages to get the delta-V needed here. STAR-48 is about 2 tons. SLS is not big enough to get 20 tons to the required orbit.


  • $\begingroup$ 8 AU per year doesn't fit with 1000AU in 50 years. Anyone know what bridges the gap? $\endgroup$ Commented Dec 6, 2018 at 18:41
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    $\begingroup$ @SteveLinton The 1000 AU probe was supposed to launch with a nuclear reactor and an ion drive, and accelerate to about 106 km/s over 10 years, for about 22 AU/year. $\endgroup$
    – Snoopy
    Commented Dec 6, 2018 at 18:45
  • $\begingroup$ 8 AU/y is the highest speed they found among a bunch of options studied. 20 AU/y is the goal. $\endgroup$
    – Hobbes
    Commented Dec 6, 2018 at 18:51
  • $\begingroup$ Just as a thought experiment, suppose you used the cargo variant of the BFS/SpaceX StarShip and were willing to refuel completely in elliptical Earth orbit (so you're probably talking 10+ launches from Earth in total) Then you need 3.1 km/s to get to Jupiter, which suggests you could deliver a payload there of 200-300 tons. That should be enough for a heat-shielded multi-stage solid booster for the solar gravity-well maneuver and a reasonable sized probe. You've thrown away your cargo Starship, probably into Jupiter or the Sun, so this is NOT a cheap mission, but 20AU/year might be on $\endgroup$ Commented Dec 7, 2018 at 12:06
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    $\begingroup$ For reference, 1000 AU is 0.0158 light years or 5.77 light days. $\endgroup$ Commented Jan 4, 2019 at 0:42

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