January 1 

 

1 January 1801

A view of Ceres in natural colour, pictured by the Dawn spacecraft. Images were acquired by Dawn at 04:13 UT May 4, 2015, at a distance of 13,641 km. At the time, Dawn was over Ceres' northern hemisphere. The prominent, bright crater at right is Haulani. The smaller bright spot to its left is exposed on the floor of Oxo. Ejecta from these impacts appears to have exposed high albedo material similar to deposits found on the floor of Occator Crater.
Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/Justin Cowart

On January 1, 1801, Italian astronomer Giuseppe Piazzi (1746–1826)  from the observatory in Palermo, Sicily, discovered the first known asteroid, a dwarf planet (1) Ceres. Originally considered a planet, it was reclassified as an asteroid in the 1850s after the discovery of dozens of other objects in similar orbits. In 2006, it was reclassified again as a dwarf planet.

   Piazzi spotted the object while conducting observations for his star catalogue. A few months later he wrote:

   "...on the evening of the 1st of January of the current year, together with several other stars, I sought for the 87th of the Catalogue of the Zodiacal stars of Mr la Caille. I then found it was preceded by another, which, according to my custom, I observed likewise, as it did not impede the principal observation. The light was a little faint, and of the colour of Jupiter, but similar to many others which generally are reckoned of the eighth magnitude. Therefore I had no doubt of its being any other than a fixed star. In the evening of the 2d I repeated my observations, and having found that it did not correspond either in time or in distance from the zenith with the former observation, I began to entertain some doubts of its accuracy. I conceived afterwards a great suspicion that it might be a new star. The evening of the third, my suspicion was converted into certainty, being assured it was not a fixed star. Nevertheless before I made it known, I waited 'till the evening of the 4th, when I had the satisfaction to see it had moved at the same rate as on the preceding days."

   Piazzi had measured the position of the object on a total of 24 nights between January 1 and February 11. On January 24, he had announced his discovery in letters to fellow astronomers, among them his fellow-countryman, Barnaba Oriani (1752–1832). In it he wrote:

   "I have announced this star as a comet, but since it is not accompanied by any nebulosity and, further, since its movement is so slow and rather uniform, it has occurred to me several times that it might be something better than a comet. But I have been careful not to advance this supposition to the public."

Portrait of Giuseppe Piazzi by Costanzo Angelini, circa 1825. Credit: Wikimedia Common

   Additional observations could not be taken for several months thereafter because the position of Ceres in the sky was too close to the Sun as it moved from the evening sky to the morning sky. Without an accurate orbit and ephemeris (predicted positions at specific times), attempts to reobserve the new planet in August proved unsuccessful. 

 

   To recover Ceres, the German mathematician Carl Friedrich Gauss (1777–1855), then 24 years old, developed an efficient method of orbit determination. Within a few weeks, he predicted the path of Ceres. Using an ephemeris for Ceres provided by Gauss, a Hungarian astronomer Franz Xaver von Zach (1754–1832) observed Ceres on December 7,  and, after bad weather cleared, again on December 31, 1801, and January 11, 1802. Using Gauss’s ephemeris, Wilhelm Olbers (1758–1840) also observed Ceres from Bremen on January 2, 1802.

A view of Ceres, taken by NASA's Dawn spacecraft on December 10, 2015, from low orbit, an approximate distance of 385 kilometers. It shows an area in the southern mid-latitudes of the dwarf planet, around a crater chain called Gerber Catena. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

   Ceres is the largest body in the main asteroid belt, 939 km in diameter. It follows an orbit between Mars and Jupiter, near the middle of the asteroid belt, with an orbital period of 4.60 years, at a mean distance of 2.77 au (414 million km), a perihelion 2.55 au (381 million km) and an aphelion 2.98 au (446 million km). Its inclination to the ecliptic is 10.6°. 

   Dawn spacecraft found Ceres's surface to be a mixture of water ice, and hydrated minerals such as carbonates and clay. Gravity data suggest Ceres to be partially differentiated into a muddy (ice-rock) mantle/core and a less dense but stronger crust that is at most 30% ice by volume. Although Ceres likely lacks an internal ocean of liquid water, brines still flow through the outer mantle and reach the surface, allowing cryovolcanoes such as Ahuna Mons to form. The surface composition of Ceres is homogeneous on a global scale, and is rich in carbonates and ammoniated phyllosilicates that have been altered by water, though water ice in the regolith varies from approximately 10% in polar latitudes to much drier, even ice-free, in the equatorial regions. Organic compounds were detected in Ernutet Crater, and most of the planet's near surface is rich in carbon, at approximately 20% by mass.

   Dawn revealed that Ceres has a heavily cratered surface, though with fewer large craters than expected. The largest confirmed crater on Ceres, Kerwan Basin, is 284 km across. Three large shallow basins (planitiae) with degraded rims are likely to be eroded craters. The largest, Vendimia Planitia, at 800 km across, is also the largest single geographical feature on Ceres. Ceres has one prominent mountain, Ahuna Mons, 4.1 km high; this appears to be a cryovolcano and has few craters, suggesting a maximum age of 240 million years. Hundreds of bright spots (faculae), covered by bright salt deposits, have been observed by Dawn, the brightest in the middle of 92 km Occator Crater.

   Ceres is named for the Roman goddess of corn and harvests, whose earthly home, and oldest temple, lay in Sicily.

   Giuseppe Piazzi (16 July 1746 – 22 July 1826) was an Italian priest, mathematician, and astronomer. In 1790, he established an observatory at Palermo. His most famous discovery was the first dwarf planet, (1) Ceres, in 1801. He also measured the positions of 7646 stars, discovering that the star 61 Cygni had a large proper motion. 

Occator Crater, measuring 92 kilometers across and 4 kilometers deep, contains the brightest area on Ceres. Image taken by Dawn spacecraft. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

 © 2025, Andrew Mirecki

January 1, 1925 

Edwin Hubble examining a photograph of the Andromeda Galaxy (M31).
 Credits: Edwin P. Hubble Papers, Huntington Library, San Marino, California

In a paper titled "Cepheids in Spiral Nebulae", presented in absentia at a joint meeting of the American Association for the Advancement of Science and the American Astronomical Society on January 1, 1925, Edwin P. Hubble (1889–1953) announced the observations that proved conclusively that two spiral nebulae, the Andromeda Nebula (M31) and Triangulum (M33), were in fact separate galaxies beyond the Milky Way. 

 

   Using the 100-inch Hooker Telescope, then the largest telescope in the world, and photographic glass plates at Mount Wilson Observatory, Hubble discovered Cepheid variable stars within both nebulae and used the period–luminosity relationship — discovered in 1912 by Henrietta Swan Leavitt (1868–1921) — to determine estimated distances to the galaxies. He derived a distance of about 285,000 pc for the two nebulae. The distance meant that the nebulae must lie outside our Galaxy, for Harlow Shapley (1885–1972) had already used the same calibrated period–luminosity relation to derive a maximum extent of 100,000 pc for the Milky Way.

   Hubble settled decisively the question of the nature of the galaxies, whose correct solution had previously been given using what many believed to be inconclusive arguments, by Heber Curtis (1872–1942), Knut Lundmark (1889–1958), and Ernst Öpik (1893–1985). As Walter Baade (1893–1960) showed nearly three decades later, Shapley's period–luminosity relation had been erroneously calibrated, so the correct values of distances are: about 765 kpc for the Andromeda Galaxy, 730 to 940 kpc for the Triangulum Galaxy, and about 27 kpc for the diameter of the Milky Way.

 

© 2025, Andrew Mirecki

 

1 January 2019

 

Color composite image of Arrokoth compiled from data obtained by NASA's New Horizons spacecraft. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute//Roman Tkachenko

The New Horizons spacecraft flew by the Kuiper belt object (486958) Arrokoth on January 1, 2019. The closest approach, being 3,538 km, occurred at 05:33 UTC. At 43.4 au from the Sun, it was the farthest object in the Solar System ever to be visited by a spacecraft.

   Shortly after the Pluto flyby, in August 2015, the New Horizons team chose a Kuiper Belt Object 2014 MU69 (officially named "Arrokoth" in November 2019) as the next flyby target. In early November 2015 New Horizons fired its thrusters to alter the direction of its trajectory by 1/4 of a degree, to set a course toward a close flyby of Arrokoth.

   The approach to Arrokoth began in August 2018, when the spacecraft took its first pictures of its target, which appeared as a faint dot barely visible against a crowded field of background stars. Through Fall 2018 the spacecraft continued to take regular images of Arrokoth as it got closer and brighter, using them to check that it was on the right course, and firing its thrusters to make course corrections as necessary. In early December, the spacecraft performed an intensive campaign of imaging to look for any dangerous rings or moons around Arrokoth — which it did not find.

   In the last few days of the approach, the navigation team analyzed the latest images of Arrokoth taken by New Horizons to refine estimates of the KBO's position relative to the spacecraft. The team uplinked the updated information to New Horizons, so that the spacecraft could more accurately time its observations and point its cameras.

   Intensive science observations began 24 hours before the flyby. The spacecraft took frequent grayscale, color, near-infrared and ultraviolet observations of Arrokoth as it rotated, to investigate its shape, composition, and any possible degassing, on all sides of the object. Long-exposure images of the space surrounding Arrokoth searched for rings or moons and determine their orbits. The closest approach observations, taken during the hour or so nearest closest approach, needed to account for the fact that Arrokoth's position was uncertain. Observations thus consisted of a series of long scans to obtain color and grayscale images, and infrared spectra, of all the possible places where the KBO might have been.

   After the closest approach, New Horizons pointed its ultraviolet instrument at the Sun to look for absorption of ultraviolet light by any gases being released by Arrokoth (though detection of outgassing was unlikely). It also made additional searches for rings around the KBO. Four hours after the flyby, the spacecraft turned briefly to Earth to report that the flyby was successful. A few hours after that it began downlinking the roughly seven gigabytes of data acquired during the flyby.

Artist's impression of the New Horizons spacecraft. Credit: Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

  

   Arrokoth is the first unquestionably primordial contact binary ever explored. Initial images hinted at a reddish, snowman-like shape, but further analysis of images taken near the closest approach revealed just how unusual the KBO’s shape really is. End to end, the overall shape of Arrokoth measures about 35 kilometers long. It’s about 20 kilometers wide, by 10 kilometers thick. The larger lobe was found to be "lenticular," which means it's flattened and shaped like two lenses placed back to back. It has dimensions of approximately 22 × 20 × 7 kilometers. The smaller lobe is more rounded and is approximately 14 × 14 × 10 kilometers in its dimensions.

   Because it is so well preserved, Arrokoth offered our clearest look back to the era of planetesimal accretion and the earliest stages of planetary formation. Apparently the two lobes once orbited each other, like many so-called binary worlds in the Kuiper Belt, until something brought them together in a "gentle" merger.

   In color and composition, New Horizons data revealed that Arrokoth resembles many other objects found in its region of the Kuiper Belt. Consistent with pre-flyby observations from the Hubble Telescope, Arrokoth is very red — redder even than Pluto, which New Horizons flew past on the inner edge of the Kuiper Belt in 2015 — and about the same color as many other so-called "cold classical" KBOs. ("cold" referring not to temperature but to the circular, uninclined orbits of these objects; "classical" in that their orbits have changed little since forming, and represent a sample of the primordial Kuiper Belt.)

 

 

© 2025, Andrew Mirecki 

 

 

 

1 January 2024

 

XPoSat being tested before launch. Credit: ISRO

The X-ray Polarimeter Satellite (XPoSat), an X-ray astronomy mission from India's ISRO, was launched on January 1, 2024.

   XPoSat is India’s first dedicated polarimetry mission to study various dynamics of bright astronomical X-ray sources in extreme conditions. The spacecraft carried two scientific payloads in a low earth orbit. The primary payload POLIX (Polarimeter Instrument in X-rays) measures the polarimetry parameters (degree and angle of polarization) in medium X-ray energy range of 8-30 keV photons of astronomical origin. The XSPECT (X-ray Spectroscopy and Timing) payload gives spectroscopic information in the energy range of 0.8-15 keV.

   The emission mechanism from various astronomical sources such as blackhole, neutron stars, active galactic nuclei, pulsar wind nebulae etc. originates from complex physical processes and are challenging to understand. While the spectroscopic and timing information by various space based observatories provide a wealth of information, the exact nature of the emission from such sources still poses deeper challenges to astronomers. The polarimetry measurements add two more dimension to our understanding, the degree of polarization and the angle of polarization and thus is an excellent diagnostic tool to understand the emission processes from astronomical sources. The polarimetric observations along with spectroscopic measurements are expected to break the degeneracy of various theoretical models of astronomical emission processes. This would be the major direction of research from XPoSat by Indian science community.

   XPoSAT was successfully launched from Satish Dhawan Space Centre, aboard PSLV-DL (PSLV-C58) launch vehicle, on January 1, 2024 at 3:40 UTC. The satellite entered orbit with a perigee 638 km and an apogee 653 km. The satelite's launch mass was 469 kg.

   The objectives of the mission:

1. To measure polarisation of X-rays in the energy band 8-30keV emanating from about 50 potential cosmic sources through Thomson Scattering by POLIX payload.
2. To carry out long term spectral and temporal studies of cosmic X-ray sources in the energy band 0.8-15keV by XSPECT payload.
3. To carry out polarisation and spectroscopic measurements of X-ray emissions from cosmic sources by POLIX and XSPECT payloads respectively in the common energy band.

   Scientific goals of the mission:

1. To study the distribution of magnetic field, geometric anisotropies, alignment w.r.t line of sight, nature of accelerator in galactic cosmic X-Ray sources by measuring degree of polarization and its angle.
2. Structure and geometry of magnetic field of neutron stars, mechanism of X-Ray beaming and its relation with luminosity and mass of accretion rate of powered pulsars.
3. Detailed understanding of galactic black hole binary sources.
4. To study and confirm about production of X-Rays is either from polar cap of neutron star or outer cap of pulsar magnetosphere.
5. To distinguish the synchrotron mechanism as dominant over thermal emission in supernova remnants.

   XPoSat payloads:

1. POLIX

POLIX is an X-ray Polarimeter for astronomical observations in the energy band of 8-30 keV. The payload is being developed by Ramam Research Institute (RRI), Bangalore in collaboration with U R Rao Satellite Centre (URSC). The instrument is made of a collimator, a scatterer and four X-ray proportional counter detectors that surrounds the scatterer. The scatterer is made of low atomic mass material which causes anisotropic Thomson scattering of incoming polarised X-rays. The collimator restricts the field of view to 3 degree x 3 degree so as to have only one bright source in the field of view for most observations. POLIX is expected to observer about 40 bright astronomical sources of different categories during the planned lifetime of XPoSat mission of about 5 years. This is the first payload in the medium X-ray energy band dedicated for polarimetry measurements.

2. XSPECT

XSPECT is an X-ray SPECtroscopy and Timing payload onboard XPoSat, which can provide fast timing and good spectroscopic resolution in soft X-rays. Taking advantage of the long duration observations required by POLIX to measure X-ray polarization, XSPECT can provide long-term monitoring of spectral state changes in continuum emission, changes in their line flux and profile, simultaneous long term temporal monitoring of soft X-ray emission in the X-ray energy range 0.8-15 keV. An array of Swept Charge Devices (SCDs) provide an effective area >30 cm2 at 6 keV with energy resolution better than 200 eV at 6 keV. Passive collimators are used to reduce the background by narrowing the field of view of XSPECT. XSPECT would observe several types of sources viz X-ray pulsars, blackhole binaries, low-magnetic field neutron star (NS) in LMXBs, AGNs and Magnetars.

 

 

The launch of XPoSat aboard PSLV-C58 launch vehicle from Satish Dhawan Space Centre.
 Credit: ISRO

 

 © 2025, Andrew Mirecki