EPOXI Passing
Published on Dec 30, 2007 at 6:56 pm.
2 Comments.
Filed under NASA, space exploration.
Tomorrow a spacecraft will pass Earth. But, not to worry, it is one of our spacecraft. The passing craft is the EPOXI spacecraft.
EPOXI is the former Deep Impact craft that got so much press attention on July 4, 2005, when it plunged an impactor into Comet Tempel 1 (9P/Tempel). The impactor created a crater in the comet. The mother ship carrying the impactor sailed past the comet observing the material thrown out by the impact. The Deep Impact mission was over soon thereafter. However, the flyby spacecraft was still operating just fine and had plenty of life left in it. So, a decision was made to recycle the spacecraft, using it to perform other missions rather than decommissioning it. The extension of the Deep Impact mission was designated DIXI (Deep Impact Extended Investigation). The initial plan was to send Deep Impact past Comet Boethin (85P/Boethin) in 2008. However, Comet Boethin turned out to be a bit elusive, and its orbit is too uncertain to guarantee that the spacecraft would actually be able to intercept it, so the plan was changed to allow Deep Impact to fly past Comet Hartley 2 (103P/Hartley). The flyby will occur in three years (December, 2010).
However, getting to the new cometary target is a bit involved. Unlike in science fiction, where a spacecraft can just fly off to another target, Deep Impact did not have a large reserve supply of fuel nor a large rocket engine designed for flying around the Solar System. The spacecraft was designed just to fly from Earth to intercept Comet Tempel 1. However, the orbit that flew past the comet eventually came back towards Earth. But, that turns out to be the key to extending the mission. By passing near Earth, the Earth’s gravity shifts the orbit of the spacecraft. Very small adjustments to the spacecraft’s orbit leading up to the encounter with Earth were all that were needed to cause the effect of Earth’s gravity to shift the original trajectory into one that would intercept another comet. The point of closet approach will be on December 31, 2007 (tomorrow!). That will put the spacecraft on the approximate trajectory to intercept Comet Hartley 2. But, there will be a need to do a bit of fine tuning to the orbit. Remember, though, that Deep Impact does not have a large reserve supply of fuel, so those course adjustments will need to be done in two further encounters with Earth in 2008 and 2009.
Along the way, though, the Deep Impact spacecraft will conduct yet more observations, though not of comets. The cameras will monitor stars known to have extrasolar planets. Careful studies of these stars may observe tiny shifts indicating additional planets. The measurements may even detect tiny variations of light from these stars as one of those extrasolar planets passes directly in front of its star. This investigation is called EPOCh (Extrasolar Planet Observation and Characterization).
The name Deep Impact refers to the initial mission to slam an impactor in Comet Tempel 1. That part of the mission is over, and apparently NASA feels that it is wrong to continue to call the flyby spacecraft Deep Impact, so a new name was chosen. The name EPOXI comes from a combination of the joint missions being conducted, EPOCh and DIXI. Actually, I think that the name is sort of cute, since it glues the two missions together.
There’s another interesting think that I’d like to say about EPOXI, though. That is that is may be visible as it passes, if you are in the right place and have the right sort of equipment. Already it has been spotted by the Astronomical Observatory of Farra d’Isonzo, as reported by Emily Lakdawalla of the Planetary Society Weblog. That this is even possible shows how far we have come in the past few years. When I started doing astronomy, it would have been an amazing feat to detect an interplanetary space probe while it was in flight. It was tough enough trying to spot the Apollo spacecraft on the way to the Moon! And, Deep Impact (or EPOXI) is small. It measures only 3.2 meters by 2.3 meters by 1.7 meters. But, though it has been seen, it is still going to be a challenging target for most amateur astronomers, even at closet approach. It will likely be no brighter than about 12th magnitude. That means that you will need a telescope of about 10cm diameter to even see it. But, that assumes perfect seeing conditions. You’d probably need something of 20cm or larger diameter for it to really be a reasonable target visually, and then only if you know just where to point the telescope. Of course, you’ll also need dark clear skies (so don’t try this from light polluted or hazy skies unless you know what you are doing and have the right equipment). You also have to be in the right place. For most of us in North America, that means that we are unlikely to get a good view of EPOXI. It will not be visible at night when at its closest approach. The ideal place to be to see the spacecraft at its best is Australia, New Zealand, and Southeast Asia.
-Astroprof
Images courtesy of NASA
Elizabeth on December 30, 2007 at 10:48 pm: 1
Hi AstroProf,
Actually, the spacecraft is still called the Deep Impact flyby spacecraft. It has been reassigned to complete the EPOXI mission. Confusing I know!
Elizabeth
EPOXI webmaster
Astroprof on December 31, 2007 at 6:20 pm: 2
Elizabeth,
Thanks for the clarification. Reading the NASA press release, it sounded as if the spacecraft itself was now being called EPOXI. It wouldn’t have been the first time that a spacecraft had been renamed, so I mistakenly assumed that was to be the new name.