A New Visitor for Jupiter – If it Can Slow Down Enough

UPDATE: Juno successfully entered orbit around Jupiter at 11:53 PM EDT on July 4th.

Jupiter is about to have a new friend. After a five year journey, the Juno spacecraft will enter Jupiter’s orbit late on the 4th of July. There, it will begin a series of observations about our solar system’s largest planet. To begin, however, the spacecraft has to slow down to allow it to be captured by Jupiter’s gravity. At 11:18 PM EDT on the 4th, Juno’s main engine will begin a 35 minute engine burn to decelerate it to the necessary speed.  “If we miss this flyby, we’re assuming the mission’s over,” said Guy Beutelschies of Lockheed-Martin. If the engine doesn’t execute its burn correctly, the mission is at an end – too long, and it slows too much and smashes into the planet – too short, and it won’t get captured, and will fly off to somewhere in the solar system.

Just How Risky is the Engine Burn?

Leros 4 Engine – Credit: Lolan Naiker

Any time a spacecraft has spent that long getting to its destination, there’s always worry about the “final landing.” The good news for Juno fans is that the rocket is one of the simplest. It is essentially a scaled up version of the reaction control thrusters used on the Space Shuttle and many other spacecraft. The reason it’s so simple is that there are very few moving parts. It involves taking a fuel (hydrazine) and mixing it with an oxidizer (nitrogen tetroxide). These are called hypergols, and when the two liquids come in contact with each other, they spontaneously combust. When that happens near a nozzle, the resulting force acts as an engine.

35 Minutes Seems Like a Long Time to Run a Rocket

Leros 1c Engine

The rocket engine on the Juno spacecraft is not like the massive rockets that lift ships into orbit from the Earth. It is equipped with something called a Leros 1b engine. That produces 645 Newtons of thrust. What is a Newton? It’s a measure of force – in this case, enough force to propel the 3625 kg spacecraft at about 18% of a g – slightly more than the gravity on the moon. To slow down enough at such a low thrust, Juno needs to burn for a long time.

After the orbital insertion burn, Juno still isn’t quite ready to go. It has to wait until October, when it will do one final burn to get into its operational orbit, a highly elliptical one that will drop it to as low as 3000 miles above the Jovian surface. After circling the planet 14 times, it will be intentionally crashed into it in 2018.

Image credits: NASA