15
$\begingroup$

A recent answer by PearsonArtPhoto claims:

New Horizons was launched at Solar System Escape Velocity, meaning it could have visited anywhere beyond Earth without stopping. It did visit Jupiter, however, that was to allow it to leave even faster, the Jupiter stop was purely optional.

In one hand, this operation shortened the wait time, reduced risk associated with deterioration of the probe over time, and reduced cost of upkeep of the mission (through making it shorter).

On the other hand, it made the flyby of Pluto awfully fast and short, resulting in only so much data from the brief closest approach. A slower flyby would give the probe more time to collect and send data. Slow enough, it might even allow for a capture into Pluto orbit.

Weighing costs and benefits - how much longer would skipping the Jupiter assist make the period to reach Pluto? And how much slower would the flyby be?

$\endgroup$
23
$\begingroup$

It was said in here that the time to reach Pluto was shortened by 3 years. It's also said that after the Jupiter flyby the probe gained ~ 4 km/s accelerating to the speed of 23 km/s relative to the Sun. We can use simple Keplerian estimate (ignoring all complexities of the actual orbital mechanics) to obtain the speed when the probe approaches Pluto, \begin{equation} \frac{v_1^2}{2}-\frac{\mu}{R_1}=\frac{v_2^2}{2}-\frac{\mu}{R_2} \end{equation} That gives ~8 km/s without flyby and ~15 km/s with flyby. This simple estimate doesn't tell you what the relative velocity to the Pluto would be without the Jupiter flyby but considering it's current orbital velocity of ~5.5 km/s and escape velocity ~1.2 km/s one may safely assume that even in the best case scenario it would still be ~4 km/s instead of ~13.8 km/s, so we talk about 3 times longer Pluto flyby at best.

To orbit the Pluto without the Jupiter flyby the probe would still have to perform an insertion maneveur losing even in the best case ~3 km/s whereas it had only 290 m/s delta-v budget. Of course the probe could be launched in such a way that it would approach Pluto at very low relative speed but then it's orbital period would be about half of the orbital period of Pluto, so we talk about ~50 year journey to Pluto. The orbiter should have been a specially designed for that mission, much heavier because of the booster required for insertion and therefore would require a different launch rocket.

Another point is that the Pluto orbit has rather significant inclination. Much of the delta-v at the launch would be spent to put the probe on sufficiently inclined orbit resulting in much lower speed than that naive estimate gives. The Jupiter flyby inclined its orbit for free.

The higher speed also means that the New Horizons will be able to reach other Kuiper belt objects in much shorter time. For example it's expected to pass by just 3500 km from (486958) 2014 MU$_{69}$ already in 2019.

And of course Jupiter is interesting by itself. Its flyby allowed to obtain some extra scientific data.

$\endgroup$
  • $\begingroup$ Couldn't the probe have been launched into the proper inclination? Why wouldn't it be launched that way, instead of doing a plane change later? $\endgroup$ – kim holder Oct 13 '17 at 15:36
  • 2
    $\begingroup$ @kimholder: "Launch into proper inclination" works okay for orbits around Earth (LEO, MEO etc.) but to launch into properly inclined solar orbit you'd have to change inclination of orbit of Earth ;) The velocity of Earth around the Sun contributes the lion share of velocity of the probe relative to the Sun, and you need to incline that! $\endgroup$ – SF. Oct 14 '17 at 0:03
  • $\begingroup$ @SF - ah, ok. I get that. :) $\endgroup$ – kim holder Oct 14 '17 at 0:05
  • 2
    $\begingroup$ @kimholder: You could launch into proper inclination if you launch at the moment Earth passes ascending/descending node of Pluto. Except you're unlikely to be at the point where you could get a decent encounter (Hohmann transfer like), just a very speedy flyby. And unless you depart immediately, Earth will drag you out of orbital plane of Pluto and you'll be right where you began. Alignment of Hohmann-like transfer departure point resulting in encounter and ascending/descending node is a rare thing. $\endgroup$ – SF. Oct 14 '17 at 0:12
  • 2
    $\begingroup$ Especially rare for the outer planets with their multi-decade orbits. $\endgroup$ – Russell Borogove Oct 15 '17 at 2:19
11
$\begingroup$

Depending on the time of launch, there were 4 different major plans for the NH mission profile. The first 3 involved Jupiter flybys, and would have an arrival date of 2015, 2016, or 2017. The last was a direct flyby, which I believe would have been 2019-2020, per this article (.

As for the scientific value, going slower through the system would have allowed for a few more photographs, at more risk to the spacecraft and missing the Jupiter flyby. But the biggest thing is something that actually proved to not be a worry, but only after the mission launched. The thought when NH launched was that Pluto's atmosphere would freeze. Pluto is actually near its closest approach to the Sun, which was in 1999, and it was thought when it launched that the atmosphere would freeze as it got further from the sun. The earlier timeline had more of a chance of seeing that atmosphere, along with a clearer landscape (Not covered in frozen nitrogen). Further modeling determined this wasn't likely, but only in 2014.

Bottom line is, the Jupiter flyby approach path was the best path. The benefit from a slower approach would have been negligible, and the scientific bounty was much higher to do the Jupiter flyby.

$\endgroup$
  • 2
    $\begingroup$ AFAIK it's not even so much "allowed a few more photos" as it would've been "allow a few more of the photos taken to be made nearer to closest approach". NH's memory was filled up during the flyby, so all you'd get were a few more zoomed in images at the expense of fewer wide angle shots. $\endgroup$ – Dan Neely Oct 13 '17 at 16:05
  • $\begingroup$ Could you clarify why Pluto's atmosphere freezes when it is closest to the sun? $\endgroup$ – Felix Dombek Oct 15 '17 at 1:12
  • $\begingroup$ Sorry, it's not that it freezes when closest to the sun, it's that it might only be not frozen when closest to the sun. Will clarify. $\endgroup$ – PearsonArtPhoto Oct 15 '17 at 1:22
  • $\begingroup$ I don't follow how you got to "the direct approach path was the best path" - it seems to me the paths past Jupiter yielded more, and more diverse, data which increases their value. +1 for the frozen atmosphere point! $\endgroup$ – FKEinternet Oct 15 '17 at 5:32
  • $\begingroup$ @FKEinternet I meant the "Direct" approach as in the one taking the least time. Bleh. Edited that to be clearer. Hopefully my brain will cooperate now! $\endgroup$ – PearsonArtPhoto Oct 15 '17 at 12:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.