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Where Is the Centaur Rocket?

Some of you have been asking what became of our Atlas' Centaur stage. As background, our Atlas first stage and its solid rocket booster never were intended to make it into Earth orbit, so they are resting at 1 au, deep under the Atlantic Ocean; and our uppermost, STAR-48 stage that sent us on our way to Jupiter and Pluto, is headed to Jupiter and the Kuiper Belt, just like New Horizons. But the Centaur, which propelled us into Earth orbit and then out of it, isn't on an escape trajectory from the Sun. Instead, it's on an orbit that takes it from about 1 AU out just over halfway to Jupiter, The figure below shows the Centaur's path for the over its first two years of flight.

The path of our spent Atlas-Centaur upper stage from launch to early 2008.

For you orbit mechanics aficionados, the orbital elements of our Centaur have been carefully calculated by Lockheed Martin's Brian Lathrop, the lead flight designer of our Atlas launch team. Here they are:

Semi-major axis = 3.0046694E+08 kilometers
Eccentricity = 0.51053830E+00
Inclination = 0.57429941E+01 degrees
Argument of perihelion = 1.1910526E+02 degrees
Longitude of the ascending node = 5.0774401E+01 degrees
Mean anomaly = 0.27115934E+00 degrees
True angle = 0.97319138E+00 degrees

For those interested only in the basics, the Centaur's orbit is essentially in the plane of the Sun's equator, like the nearby planets, and stretches from 1 AU to 3 au, with an orbital period of 2.8469 years. When New Horizons reaches Pluto in July 2015, the Centaur will be on its fourth orbit of the Sun, outbound, just beyond the orbit of Mars.

A Trojan Course?

And while we're on trajectory matters, it's worth noting that we have just realized that New Horizons itself will be traversing through one of the Trojan regions of Neptune in 2014. For a long time, astronomers wondered if there were asteroids trapped in Neptune's Trojan regions, but in recent years a few have been discovered. These fascinating bodies probably represent a sample of the most primitive bodies in the solar system, like comets and Kuiper Belt objects.

New Horizons will cross through Neptune's trailing Trojan cloud in 2014.
(Image created by John Spencer using Starry Night.)

Only a handful of Neptune Trojans are currently known, but more will no doubt be found in coming years. If any of those come close enough to New Horizons to be usefully studied, we want to plan observations.

To see if we can help that exciting prospect along, we've alerted our professional colleagues in the planetary astronomy community and asked for their help searching for new Neptunian Trojans in the region of space where New Horizons will fly as it crosses Neptune's orbit in the summer of 2014.

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