Why don't we build an interstellar specific mission spacecraft?

Question

Voyager 1 and 2 go at ~60k km/h, It took them approx. 40 years to leave the heliosphere and get into the interstellar space.

The Parker Solar Probe is intend to reach 600k km/h. With this velocity, reaching the heliopause (123AU - 18 billion kms) would take about 3 years, after accelerating to full speed.

It's something achievable, considering the propulsion, communication system and detector technologies we have today we can learn a lot about the interstellar space, while the spacecraft is fully functional.

Considering the statement that "We realized the interstellar space is very much different than we assumed" when Voyager 1 left the heliosphere , why don't we carry such a mission?

Thanks.

Answer 1

The Parker Solar Probe is intend to reach 600k km/h. With this velocity, reaching the heliopause (123AU - 18 billion kms) would take about 3 years, after accelerating to full speed.

The Parker Solar Probe will achieve this incredibly high velocity by making it go into a highly elliptical orbit. It will do this with gravitational assists from Venus, and it will do this seven times. Each encounter with Venus is designed to make the Parker Solar Probe go even slower at aphelion. After the seventh encounter, the Parker Solar Probe will be going about 1/3 of Venus's orbital velocity at aphelion. This slowness at Venus's orbital altitude is what will result in the Parker Solar Probe's incredibly high velocity at perihelion.

That said, a small velocity change at perihelion could make the Parker Solar Probe escape the solar system were it were equipped with thrusters to do so. At its closest approach, the Parker Solar Probe will be going at 5.969 km/s (21488 km/h) slower than escape velocity from the Sun. A 6 km/s impulsive burn performed at perihelion would put the Parker Solar Probe on a slow escape trajectory, with a $v_\infty$ (velocity after escaping the solar system) of about 3.5 km/s. This is slower than Voyager 1. But thanks to the Oberth effect, a small increase in the magnitude of that impulsive burn would result in a much faster $v_\infty$. For example, a 7 km/s impulsive burn performed at perihelion would make the Parker Solar Probe be on a trajectory with a $v_\infty$ of 20 km/s, faster than Voyager 1.

An altered approach could result in an even higher velocity on escape from the solar system: use Jupiter instead of Venus. Suppose multiple gravity assists from Jupiter are used to make a future probe dive as close to the Sun as the Parker Space Probe is planned to get. Instead of lacking 5.969 km/s to escape the pull of the Sun, this future probe would be within 867 m/s from escaping the solar system at perihelion. Instead of a $v_\infty$ of 20 km/s, a 7 km/s impulsive burn at perihelion with this altered approach would result in a $v_\infty$ of almost 50 km/s. The Oberth effect can yield highly nonintuitive results.

Answer 2

We don't have the capability to send a probe away from the sun at that high a speed.

Parker Solar Probe reaches 690,000 km/h at its closest approach to the sun, at 6.9 million km distance -- about 1/20 the Earth's distance to the Sun. It gets that fast only because it's falling towards the sun; after it passes the sun, it slows down all the way to the high end of its orbit, reaching maximum altitude at something like the radius of Venus's orbit around the sun before beginning to fall back.

It would be possible to make an interstellar probe smaller than the Voyagers, using a very large booster, using more modern instruments that build on what we learned from the Voyager missions. Such a spacecraft could go faster than the Voyagers and eventually overtake them, but it wouldn't be orders of magnitude faster. So far there's no budget for that kind of project.