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EPOXI: Extended Exoplanet Mission Begins

When you have assets in space, the thing to do is redeploy them as needed. That creates what’s called an ‘extended mission,’ and the latest spacecraft to get one is Deep Impact, the vehicle whose impactor made such a splash when it was driven into comet Tempel 1 in the summer of 2005. That July 4 explosion was memorable enough, but under the name EPOXI the doughty craft leaves its vaporized impactor behind and moves on to two other missions, one of which has direct extrasolar applications.

For one of EPOXI’s twin goals is to observe five nearby stars known to have transiting exoplanets. Observations began on January 22. The ‘hot Jupiters’ around the five stars have been confirmed previously, but EPOXI’s mission is to see whether any of these transiting gas giants is accompanied by other worlds in the same stellar system. Perhaps the most intriguing aspect of the investigation is summed up by Drake Deming (NASA GSFC): “We’re on the hunt for planets down to the size of Earth, orbiting some of our closest neighboring stars” (italics mine).

Such worlds might be making transits of their own, but transit timing of the known gas giants will tell the tale, the gravity of the unseen world perturbing the transit of the larger. Deep Impact may not have been designed for this work, but the prospects are exciting. What stands out about recent exoplanet findings from space is the success of COROT, once thought to be a relatively modest mission compared to efforts like Kepler and the much more ambitious Darwin, yet a spacecraft that is producing results far beyond what many of us had expected. Can Deep Impact, under its new name, make a successful transition and surprise us again?

Nor is exoplanet hunting the only goal of the extended mission. EPOXI is actually a conflation of two missons: Extrasolar Planet Observation and Characterization, as just discussed, and the Deep Impact Extended Investigation, aimed at taking a close look at comet Hartley 2 with a flyby on October 11, 2010. So the cometary challenge remains, but wouldn’t it be extraordinary if we turn what had been Deep Impact into the platform that flags the first Earth-sized exoplanet? We’ll also learn much about those hot Jupiters, including information about their atmospheres and the possibility that one or more may have moons or rings.

Keep your own eyes on EPOXI’s blurry vision. As discussed in these pages back in July, the Deep Impact telescope is out of focus, which actually makes for better photometry, allowing the system’s CCD to collect more photons before it becomes saturated. Drake Deming explained this puzzling point to Emily Lakdawalla last summer: “With a defocused image, we have about 75 pixels collecting light for us, so we can collect lots of photons in each exposure without saturating, and that gives us the high signal-to-noise ratio that we need.”

A blurry view, then, may be just the ticket as we go hunting for planets of Earth size and above. A final EPOXI aim: To observe the Earth in visible and infrared wavelengths. Looking back at our own terrestrial world provides useful data points when we begin to collect information about such worlds around other stars. All this from a mission that’s clearly a long way from exhausting its useful life, making the case that if you build the hardware right, the mission possibilities continue to grow.

Comments on this entry are closed.

  • jh2001 February 8, 2008, 16:11

    If we are going to the trouble and expense of putting one probe in a neighboring solar system we might was well do a little more work and redeploy enough probes in that system to discover everything that’s there. Interestingly, tasks could be divided. Many daughter ships could collect data and transmit it to a mother ship which could have enough power to beam that information back to Earth. One could really get carried away and imagine a whole variety of different types of missions being done in a neighboring solar system such as impacting/observation, orbiting reconnaisance/rovers, nanotech replication/site prep, etc.

    The alternative would be to sequentially launch many ships from our solar system and let them collect and transmit information one-by-one as they arrive in the other system.

  • Hans Bausewein February 8, 2008, 16:53

    Good idea to trade resolution for signal-to-noise ratio. Most pixels are unused in a traditional camera, when you’re looking at a point light source.

  • David February 9, 2008, 9:11

    Defocusing is a common technique when one wants to measure the flux of a bright point source and may be the only way to avoid saturating the CCD camera. Saturation is to be avoided because the camera is not then registering all the light. Its like trying to use a bucket to measure how much rain is falling in a storm when the water is overflowing the sides of the bucket.

    An advantage is also that by spreading the light among more pixels, one is getting a reading that is not so affected by the response of a pixel that may be defective, or “hot” which is the term often used.

    The technique is not without problems. If the field of view is crowded, then the defocused image may actually consist of the target star and some background star which can cause problems if the latter is variable on the time-scale of hours which is about the length of a exoplanet transit.
    So detecting the tranist is made more difficult by having another signal superimposed. This problem is called “confusion”. My guess is that the targets chosen are in areas of the sky which are not crowded.

  • daniel February 9, 2008, 10:31

    which 5 stars that already has hot jupiters the Epoxi Mission will go to search for new planets?

  • andy February 9, 2008, 15:41

    This reuse of the Deep Impact mission makes me wonder what other types of missions might be good to carry out with space probes.

    E.g. for probes to the outer solar system, much longer baselines than the size of the Earth’s orbit for parallax measurements would be possible – whether limitations of the equipment onboard would wipe out this advantage, I don’t know.

  • Hans Bausewein February 10, 2008, 12:20

    Does it make sense, if you have to wait 150 years for the probe to arrive at the other side of its orbit?

  • Hans Bausewein February 10, 2008, 13:19

    (answer my own question ;)
    Well, it makes sense when using 2 probes in different directions.

  • Administrator February 10, 2008, 13:39

    daniel, I don’t have the list of target stars for EPOXI yet — anyone who does, please post, or I will as soon as I can get it.

  • xav February 10, 2008, 18:34
  • Administrator February 10, 2008, 19:48

    Much appreciated, xav!

  • James M. Essig February 11, 2008, 2:06

    Hi Folks;

    I like the idea of our growing capability to do “extended missions” in robotic interplanetary space missions. Other examples that I find interesting are the continued and extended actual duty cycle of the Mars rovers which should have concked out years ago according to design specification. The landing of the probe on an asteriod was another interesting take on the concept. The repair of the torn solar PV fabric cell attached to the ISS is yet another cool example. Yet another example is the go ahead given by astronuats to eventually fix the Hubble Space Telescope at their pleading with the Federal authorities for permission to do such.

    It seems like whenever we read an article about a space probe in its final days of its preplanned mission or when we hear about the space shuttle on its way to visit or in docked configuration with the ISS etc., there always seem to be unforseen contingencies that if not dealt with appropriately and judiciously with creative thinking, the result could be mission failure. Although we have not yet set boots on the surface of Mars or gone to any of the gas giant planets in person yet, our versatile skill sets that we have developed among our scientists, engineers, mission control specialists and technicians, and astronaut corp has shown that we have come a long long way since Sputnick and the X-15 experimental plane. I think the fun has only just begun.


    Your Friend Jim

  • george scaglione February 12, 2008, 9:30

    jim,you are not kidding! indeed the fun has only just begun.what we need now imho is boots on the moon,mars,maybe a near earth asteroid or two and plans to head over to jupiter too!!! lol as usual,i “don’t want much”.thank you very much your friend george

  • ljk February 19, 2008, 10:33

    Detecting “Temperate” Jupiters: The Prospects of Searching for Transiting Gas Giants in Habitability Zones

    Authors: S.W. Fleming, S.R. Kane, P.R. McCullough, F.R. Chromey

    (Submitted on 18 Feb 2008)

    Abstract: This paper investigates the effects of observing windows on detecting transiting planets by calculating the fraction of planets with a given period that have zero, one (single), two (double), or $\ge$3 (multiple) transits occurring while observations are being taken. We also investigate the effects of collaboration by performing the same calculations with combined observing times from two wide-field transit survey groups.

    For a representative field of the 2004 observing season, both XO and SuperWASP experienced an increase in single and double transit events by up to 20-40% for planets with periods 14 less than P less than 150 days when collaborating by sharing data.

    For the XO Project using its data alone, between 20-40% of planets with periods 14-150 days should have been observed at least once. For the SuperWASP Project, 50-90% of planets with periods between 14-150 days should have been observed at least once. If XO and SuperWASP combined their observations, 50-100% of planets with periods less than 20 days should be observed three or more times.

    We find that in general wide-field transit surveys have selected appropriate observing strategies to observe a significant fraction of transiting giant planets with semimajor axes larger than the Hot Jupiter regime. The actual number of intermediate-period transiting planets that are detected depends upon their true semimajor axis distribution and the signal-to-noise of the data.

    Comments: 14 pages, 12 figures, 4 tables, accepted to MNRAS

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0802.2405v1 [astro-ph]

    Submission history

    From: Scott Fleming [view email]

    [v1] Mon, 18 Feb 2008 01:09:43 GMT (487kb)


  • ljk February 27, 2008, 14:26

    Homogeneous studies of transiting extrasolar planets. I. Light curve analyses

    Authors: John Southworth (University of Warwick, UK)

    (Submitted on 26 Feb 2008)

    Abstract: I present an homogeneous analysis of the transit light curves of 14 well-observed transiting extrasolar planets. The light curves are modelled using JKTEBOP, random errors are measured using Monte Carlo simulations, and the effects of correlated noise are included using a residual-permutation algorithm. The importance of stellar limb darkening (LD) on the light curve solutions and parameter uncertainties is investigated using five different LD laws. The linear LD law cannot adequately fit the HST photometry of HD 209458, but the other four laws give very similar results to each other. In most cases fixing the LD coefficients at theoretical values does not bias the results, but DOES cause the error estimates to be too small. The available theoretical LD coefficients clearly disagree with empirical values measured from the HST light curves of HD 209458; LD must be included as fitted parameters when analysing high-quality light curves. In most cases the results of my analysis agree with the values found by other authors, but the uncertainties I find are up to 3 times larger. Despite this, the analyses of sets of independent light curves for both HD 189733 and HD 209458 do not agree with each other. This discrepancy is worst for the ratio of the radii (6.7 and 3.7 sigma), which depends primarily on the depth of the transit. It is therefore not due to the analysis method but is present in the light curves. These underlying systematic errors cannot be detected from the reduced data alone. Using my results and the stellar spectroscopic orbits, I confirm the correlation between the surface gravities of transiting extrasolar planets and their orbital periods. [abridged]

    Comments: Accepted for publication in MNRAS. 25 pages with many tables and figures, plus an appendix (14 pages, all tables). The JKTEBOP and JKTLD codes are available from this http URL

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0802.3764v1 [astro-ph]

    Submission history

    From: John Southworth [view email]

    [v1] Tue, 26 Feb 2008 08:01:29 GMT (668kb)