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Comment by droithomme | Hacker News Reader

Interesting about the 1000 ft tall ice shards. The article mentions a possible orbiter mission to Pluto. Is it even possible? The spacecraft would have to either massively decelerate at the end of the mission, meaning take a huge amount of fuel with itself, or would have to travel much slower, with the mission possibly taking decades to arrive.

In 2017 some astrophysicists met to discuss a possible orbiter mission:

https://www.space.com/36697-pluto-orbiter-mission-after-new-...

They've also discussed a Charon lander, but seem to be now preferring an orbiter with a complex dual-object orbit.

In 2017 they decided to firm up the plan so they can make a formal proposal at a 2020 meeting where NASA decides what it wants to do over the next decade. So we likely will hear a lot about how this would work next year.

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"Travelling much slower" isn't really an option either. By the time a craft encounters Pluto, the problem is that it's going too slow. It then has to match the (higher) orbital speed, and Pluto's gravity is not helping much.

You bring up a good point regarding the relative speeds.

Pluto on average travels 4.7 km/sec or 16,800 km/hr.

New Horizons, on a 9 yr mission to arrive, was traveling 50,700 km/hr as it passed Pluto. So it was going much faster than Pluto, however it was passing almost orthogonal to Pluto's trajectory.

It seems an orbiter would want to arrive using some complex slingshot mission around various solar system objects, and taking a catch up trajectory that approaches Pluto more tangentially to its orbit rather than orthogonally.

That seems more doable than when I first balked at this. Since gravity assist/slingshots can be used to slow down as well as speed up perhaps it zooms out there fast, then slingshots to nearly meet the speed and direction of Pluto, meeting up with it at its orbital path sideways and only needing a small correction at the end to insert itself into orbit.

Matching speeds to the target object isn't all that difficult, right. The problem is the travel time becomes enormous to a distant target object, because it's going so slow comparatively.

Pluto's orbital period is 248 years. If you aim to match its velocity when you arrive, the journey takes a significant fraction of those 248 years. Compare Mars, where the orbital period is 22 months, and a Hohmann transfer orbit takes 9. A similar trajectory to Pluto would be on the order of 80 to 90 years.

It's not just about the magnitude of the speed vector, it's also the direction. To match and orbit Pluto, you need that 4.7 km/sec to be tangent to the sun. You need to start out slow enough so that your radial velocity can fall to zero by the time you get there. You can't apply more radial velocity to get there faster, either by launch or slingshot, if there isn't equivalent fuel or mass to decelerate you; you'll overshoot as New Horizons did.

We now have ion thrusters, which are great for when you need a huge velocity change but can take your sweet time to do it.

> The article mentions a possible orbiter mission to Pluto. Is it even possible?

I can't read the space.com article about the orbiter mission, but a lander concept was proposed in 2017. Their idea was to use aerobreaking with a giant inflatable balloon, using New Horizon data on atmosphere density as a guideline for the maneuver:

https://www.spaceflightinsider.com/missions/solar-system/plu...

It has been suggested that the ice shards form through sublimation, like the penitentes of the Andes.

https://science.nasa.gov/plutos-bladed-terrain

https://en.wikipedia.org/wiki/Penitente_(snow_formation)

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