The fastest spacecraft ever launched was the Parker Solar Probe, which reached speeds of about 700,000 kilometers per hour (430,000 miles per hour). However, this probe is designed for solar observation and not for interplanetary travel. For a more realistic comparison, let's consider the New Horizons probe, which was launched in 2006 and reached speeds of about 58,536 kilometers per hour (36,373 miles per hour) after its gravity assist from Jupiter.

Given that the distance between Earth and Neptune varies between 4.3 billion and 4.6 billion kilometers (2.7 billion and 2.9 billion miles) depending on their positions in their orbits, we can calculate the time it would take to travel that distance at the speed of the New Horizons probe:

4.3 billion km / 58,536 km/h = 73,454 hours

Converting this to days:

73,454 hours / 24 hours per day = 3,060 days

This is a rough estimate and assumes a direct path between Earth and Neptune without any gravity assists or other factors that might affect the travel time. In reality, the travel time would likely be longer due to the need for trajectory corrections and the use of gravity assists to save fuel and increase speed. Additionally, this estimate is based on the technology and knowledge available up to September 2021 and does not account for any advancements or new spacecraft developed between then and 2023.
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