Space Telescope Surveys

Ground-based astronomy has several limitations: The main problems concern atmospheric seeing, scattering of light in the atmosphere, and the absorption of radiation with wavelengths shorter than 290 nm by oxygen and nitrogen.

Electromagnetic radiation of all wavelengths reaches Earth’s upper atmosphere from the Universe. As different wavelengths can give information about different astrophysical processes, astronomers are interested in examining the complete spectrum. However, radiation of different wavelengths is absorbed by different amounts in the atmosphere.

O and N completely absorb all radiation with wavelengths shorter than 290 nm. Ozone (O3)

absorbs most of the ultraviolet. Electromagnetic radiation over a large range of infrared (IR) wavelengths is absorbed by water vapor and CO2. This prohibits ground-based IR observation

with the exception of the near-infrared wavelengths from 1 to 10 mm and the far infrared up to 10 µm.

A wholesale exploration of the mid/far IR regime requires a space-based platform, and the Infrared Astronomy Satellite (IRAS, Beichman et al. 1986), launched in 1983, opened a huge new area of research.

Some of the subsequent missions included the Wide-Field Infrared Survey Explorer (WISE Wright et al. 2010), launched in 2009. His all-sky survey mapped the sky at four infrared wavelength bands, 3.4, 4.6, 12 and 22 µm. WISE detected more than 250 million objects, including near-earth asteroids (NEOs), brown dwarfs, QSOs, ultra-luminous starbursts and other sources of interest. When the telescope run out of hydrogen for cooling, the mission was continued as NEOWISE, and the survey continued for an additional four months using the two shortest wavelength detectors. NEOWISE carried out measurements of asteroids and comets from images collected by the Wide- field Infrared Survey Explorer (WISE) spacecraft. NEOWISE provides a rich archive for searching for solar system objects.

Data from the original WISE as well as NEOWISE missions have already enabled research in a variety of fields. With its increased sensitivity and time-domain information, combining them to ALLWISE extends this, as well as opens avenues of study that were not possible with the individual data.

The Hubble Space Telescope (HST, with its catalog, the Hubble Source Catalog as described in Budav´ari and Lubow 2012), named in honor of astronomer Edwin Hubble, started its operation in 1990. It is observing in the near ultraviolet, visible, and near infrared, using a primary mirror with a diameter of 2.4 m. From its low Earth orbit position, being outside the influence of Earth’s atmosphere, it is able to take extremely high-resolution images with an angular resolution of 0.05 arcsec and a pointing accuracy of 0.007 arcsec. The HST has made more than 1.2 million observations since its mission began in 1990, resulting into _{∼10 TB of new archive data per year} and up to now more than 14,000 scientific papers. The HST is still operating, and could continue for decades.

The Spitzer Space Telescope (Spitzer, SST, Werner et al. 2004), named in honor of astronomer Lyman Spitzer, who had promoted the concept of space telescopes in the 1940s, is an infrared space observatory launched in 2003.

Spitzer is equipped with a 0.85 m primary mirror, that was cooled to 5.5 K, and follows a heliocentric instead of geocentric orbit. Its three instruments enable astronomical imaging and photometry from 3.6 to 160 µm, spectroscopy from 5.2 to 38 µm, and spectrophotometry from 5 to 100 µm.

The planned lifetime of the mission was 2.5 years with a possible extent of another 2.5 years until the He supply for cooling was exhausted. When this happened in May 2009, the two shortest- wavelength modules of the IRAC camera were still operable with the same sensitivity as before. Spitzer was then continued as the so-called Spitzer Warm Mission. All Spitzer data, from both the originally phase with the full waverange as well the limited warm phase, are archived at the

Infrared Science Archive (IRSA).

Among many other spectacular results like finding the youngest stars ever detected, it has directly captured light from exoplanets in 2005, namely from the “hot Jupiters” HD 209458b and TrES-1b (Deming and Seager 2005; Charbonneau et al. 2005).

HST’s scientific successor, the James Webb Space Telescope (JWST Boccaletti et al. 2015), is scheduled for launch in 2018. Its nominal mission time is five years, with a goal of ten years. Different than the HST, it will observe from long-wavelength (orange-red) visible light, through near-infrared to the mid-infrared (0.6 to 27 µm). It is currently under construction and scheduled to launch in October 2018. The JWST has a larger primary mirror than the HST (6.5 meter, segmented, resulting in a collecting area about five times as large as HST’s). The telescope will be located near the Earth–Sun L2 point, allowing it to use a single sunshield to keep the instruments below 50 K.

The design of JWST emphasizes the near to mid-infrared for three main reasons: high-redshift objects have their visible emissions shifted into the infrared, as more distant an object is, the younger it appears; cold objects such as debris disks and planets emit their radiation primarily in the infrared; infrared radiation is better able to pass freely through regions of cosmic dust that scatter radiation in the visible spectrum.

JWST’s primary mission encompasses four scientific goals: to search for light from the first stars and galaxies that formed in the Universe after the Big Bang, to study the formation and evolution of the first galaxies, to understand the formation of stars and planetary systems and to study planetary systems including direct imaging of exoplanets. Many of them are beyond the reach of current ground and space-based instruments.

Kepler (Borucki et al. 2010), named after the astronomer Johannes Kepler, is a space telescope launched in 2009 in order to discover Earth-size planets orbiting other stars. Kepler is designed to survey a portion of our region of the Milky Way to discover Earth-size exoplanets in or near habitable zones and estimate how many of the billions of stars in the Milky Way have such planets. Its photometer continually monitors the brightness of over 145,000 main sequence stars in a fixed field of view. This data is transmitted to Earth, then analyzed to detect periodic dimming caused by exoplanets that cross in front of their host star.

The initial planned operational time was 3.5 years, but greater-than-expected noise in the data, from both the stars and the spacecraft, enforced additional time was needed to fulfill all mission goals. Initially, in 2012, the mission was expected to be extended until 2016, but on July 14, 2012, one of the spacecraft’s four reaction wheels used for pointing the spacecraft stopped turning, and completing the mission would only be possible if all other reaction wheels remained reliable. Then, on May 11, 2013, a second reaction wheel failed. This meant the current mission needed to be modified to continue its search for exoplanets. Kepler was used further on in the so-called K2 mission in order to detect habitable planets around smaller, dimmer red dwarfs (Howell et al. 2014).

unconfirmed planet candidates. Further 129 planets have been confirmed through Kepler’s K2 mission, and there are 458 K2 candidate exoplanets.

The Gaia mission (Prusti 2014), launched in December 2013 will provide fundamental data for a better understanding of the structure of our Galaxy. Gaia started its scientific mission in July 2014 and has been mapping the Milky Way ever since.

Gaia is an ambitious mission to chart a three-dimensional map of the Milky Way in order to reveal its composition, formation and evolution. Gaia will provide unprecedented positional and radial velocity measurements with accuracies required to produce a positional kinematic census of about one billion stars in the Milky Way and throughout the Local Group. This amounts to about 1 percent of the Galactic stellar population.

As Gaia scans the sky, it creates a precise three-dimensional map of astronomical objects – stars, asteroids, comets and other – throughout the Milky Way and map their motions. Gaia will mon- itor each object about 70 times over a period of five years. From the observations, astrometric parameters are determined: two corresponding to the angular position of a given object on the sky, two for the derivatives of the object’s position over time, and the object’s parallax from which distance can be calculated. Gaia will determine the position, parallax, and annual proper motion of 1 billion stars with an accuracy of about 20 µas at 15 mag, and 200 µas at 20 mag. This is an accuracy 100 times better than of Hipparcos. Additionally, positions will be determined a magnitude of V = 10 down to a precision of 7 µas between 12 ,and 25 µas down to V = 15 mag, and between 100 and 300 µas to V = 20 mag. The precision depends the color of the star. Spectrophotometric measurements are carried out in order to provide the detailed physical prop- erties such as luminosity, effective temperature, chemical composition and gravity for all observed stars.

Similar to its precursor Hipparcos, Gaia is equipped with two telescopes. They provide two ob- serving fields with a fixed angle of 106◦_{.5 between them. Gaia rotates continuously around an axis}

perpendicular to the two telescopes’ lines of sight, and maintains a constant angle to the Sun. While doing so, the spin axis has a slight precession across the sky. Thus, a reference system is obtained by precisely measuring the relative positions of objects from both observing directions. The radial velocity of the brighter stars is measured by an integrated spectrometer, making use of the Doppler effect.

On September 12, 2014, Gaia discovered its first supernova in a galaxy about 500 million light- years away. On July 3, 2015, a map of the Milky Way’s star density was released. On 13 September 2016, ESA has released a 3D map based on data from the the first 14 month of the mission, con- taining over a billion stars, out of them 400 million newly found sources.