# Why didn't New Horizons stay in Pluto's orbit? [duplicate]

As everyone knows, New Horizons, which was sent some decades ago, reached Pluto on July 14th, 2015 and it stayed there for only 24 hours and after that it is going into the Kuiper Belt and thereafter into interstellar space like Pioneer and Voyager probes did before it. My question is, why was New Horizons not allowed to stay in Pluto's orbit to collect information on the dwarf planet, like MOM (Mars orbiter Mission) is doing around Mars? Is this due to scientists being less interested in searching for life in the Pluto system, or are there any other reasons?

• Look through the questions tagged new-horizons and you will see at least two duplicate of this question. It is a good idea to do that simple test (look through questions with same tag) before posting a new question. – Andrew Thompson Jul 19 '15 at 4:53
• It's pretty hard to stay someplace you never were. I have no idea where your 24 hour figure came from. – Loren Pechtel Jul 20 '15 at 1:52

## 1 Answer

New Horizons was launched roughly 9 and a half years ago, on January 19, 2006. So not decades ago. Pluto is far, far away. In fact, so far away, that both the Sun and the Earth, together with the rest of the inner Solar system planets (Mercury, Venus and Mars) that are closer to us, appear in that small inner ellipse on this image of the New Horizons flyby trajectory:

And while we managed to launch New Horizons at record setting speed of 16.26 kilometers per second (58,536 km/h or 36,373 miles per hour) on its way to Pluto and setting it free from the Earth's gravitational grip that keeps you well seated on your chair, all of its way there it was still pulled by the immense Sun's gravity towards it and was slowed down by it on its way to Pluto, so it also used a Jupiter gravity assist to gain an extra boost, trading some of Jupiter's angular momentum in its orbit around the Sun for higher exit velocity towards Pluto. A tiny amount of this momentum for the super-massive gas giant Jupiter, but a big push forward for in comparison tiny New Horizons (nearly 1/4 of a quadrillionth, or nearly 4,000,000,000,000,000,000,000,000 of New Horizons would equal the mass of Jupiter). And its velocity gain with respect to the Sun looked like this:

So now that the New Horizons probe is going real fast, so vastly hugely mind-boggling fast that your trip to the chemist seems just peanuts in comparison, it managed to get to Pluto at a relative velocity of 13.78 km/s (49,600 km/h or 30,800 miles per hour). And even for a measly 478 kg probe, halting it to a stop would require a tremendous amount of energy and some form of reaction mass to accelerate in the opposite direction, and quite a bit of it. And that's something that New Horizons doesn't have available to it, and if it did, it would increase its mass, it would then be launched at much slower speed towards Pluto, and really take decades to get there.

So, in other words, New Horizons was going too fast to have halted to a near stop and remained at Pluto for longer. It wasn't a bus trip with stops along the way, but a speeding bullet (actually about 50-60 times faster than that) moving through the vast nothingness of the vacuum of space. And unless it hits something along its way, it will continue to do so for many millennia to come and exit our Solar system entirely.

For more information, see Requirements to orbit Pluto and other answered questions tagged as . All images in my answer are from New Horizons Mission Design, Yanping Guo and Robert W. Farquhar, Johns Hopkins University Applied Physics Laboratory (PDF).

• One thing I don't quite get (perhaps the answer could be expanded to include it) - logically, shouldn't it take nearly the same amount of fuel to slow down as it does to speed up? So you take something roughly the size of the booster that we used to launch NH (although that had to fight Earth's gravity, so maybe you need even less) and you just launch NH + that. Wouldn't that give enough fuel to slow down? Basically, how big would the launcher need to be to launch all that? Sure we'd need a TON of fuel, but still, couldn't we do it with a few million \$? – Jez Jul 19 '15 at 10:41
• @Jez That's already discussed in my answer to the question I link (and it looks this question will be closed as a duplicate of). In theory, assuming our rockets have dry mass of nothing (no rocket body, no payload), which is a fair first order approximation because dry-to-wet mass ratio really is kept as low as possible, you'd need 1/e of the total launched mass of propellants to decelerate. See Tsiolkovsky rocket equation and Google "The tyranny of the rocket equation" for more info. – TildalWave Jul 19 '15 at 10:49