Today the New Horizons spacecraft flew by Pluto. It didn’t slow down. Because it really couldn’t. This is one of the sad facts of space travel. If you want to actually visit a planet, you have change speeds once you get there. And sometimes that is really, really, hard.
Most of the time, when we want to get to another planet, such as Mars, which is our most often visited planet, we send the spacecraft on a special route to get there. This is called a Hohmann Transfer, which is a trajectory that goes from one orbital altitude to another orbital altitude. So, when we want to send a spacecraft to Mars, we do the following:
- The spacecraft could go into low-Earth orbit if we wanted it too.
- Then it would escape Earth’s gravity, which means that it needs to get going at a speed of about 11.2 km/s in a direction roughly away from the Earth. This is so fast that gravity won’t slow the spacecraft down enough to return it back to Earth.
- Once the spacecraft has escaped Earth’s gravity, it would be orbiting around the sun. An interesting fact is that the USSR tried to hit the moon with the Luna-1 spacecraft, but missed, creating the first artificial “planet” that is orbiting the sun. The Earth is orbiting the sun. Mars is orbiting the sun. They are at different distance from the sun. So, in order to get from the Earth to Mars, the spacecraft has to have an elliptical orbit, which has one end at Earth’s orbit and one end at Mars’s orbit. So, at Earth, the spacecraft accelerates a bit, and gets going fast enough that the high end of the orbit (the apogee) will be at Mars’s distance from the sun.
- Once the spacecraft reaches Mars, it will fall back towards the Earth unless it speeds up again to get on a(n almost) circular orbit at Mars’s distance from the sun. Hopefully NASA or whoever has timed it just right so that Mars is there when the spacecraft get there. If they have not timed it right, you are basically screwed.
- The spacecraft is then close to Mars, moving roughly the same speed as Mars around the sun. The tricky bit here is that the spacecraft has to either get into orbit around Mars or land on the surface. More on that in a different post.
Ok, so that was Mars. It takes us about 6 months or so to get something to Mars. If we did the same thing to get to Pluto (i.e., using a Hohmann Transfer), it would take a REALLY long time. Considering that the satellite would have to approach Pluto in nearly the same orbit as Pluto has around the sun, it would take some fraction of a Pluto year, which is about 250 years. Even if it was a fifth of this, it would still be 50 years, which is totally unreasonable. We just don’t have the patience for that.
So, the way that we fly a spacecraft to Pluto is on a much more direct route. None of this elliptical orbit crap. Just fly straight out there. We (by we, I mean the human race in the form of the robot entitled “New Horizons”) ended up getting there in a decade instead of 50 years. But, the problem is that, instead of flying at close to the same speed as Pluto in the same orbital plane as Pluto, New Horizons is flying radially away from the Sun, which is perpendicular to Pluto’s orbital plane. So, in order for New Horizons to “catch” Pluto and orbit it, it would have to change its velocity vector by about 90 degrees, which is really hard to do. Essentially, it would have to stop going in one direction and start going in the other direction. Since it is going about 20 km/s at this point (one of the fastest man-made objects ever), it would be hard to stop. In essence, you would need a gigantic rocket to stop it. That means we would have had to add a rocket to New Horizons, which would have caused it to be REALLY big, which would mean that the rocket that it was launched on would have had to have been the biggest rocket ever created by man.
Long story short: we could have stopped at Pluto, but it would have taken the spacecraft too long and we would all have been dead by the time we got there. Or, we could zoom past at some insanely fast speed and take some pictures as we are zooming by. Kind of like visiting grandma by driving by her house at 60 MPH, yelling “Hi Grandma!” and taking a couple of snapshots out the window. But, it was the first time you have seen your grandma as anything more than a couple of pixels across, and the car you were driving in was one of the greatest technological feats of mankind. (Well, sort of – we did the same thing with Neptune and Uranus with spacecraft built in the 1970s.)