C/2020 R4 (ATLAS)

Lore & Background

On 12 September 2020, James E. Robinson reported the discovery of the comet taken by the ATLAS facility of the Mauna Loa Observatory in Hawaii, noting a coma about 10 arcseconds wide. Precovery images as early as 29 August were later found. Initially predicted to become only a faint comet, an unexpected outburst in December 2020 increased its brightness a hundred-fold, from magnitude 18 to 13.

The comet reached perihelion on 1 March 2021. By 13 March, it was an 8th-magnitude object within the constellation Aquila. During its outbound flight, three outbursts were recorded between 20 April and 6 May 2021, with the first event being the strongest. Spectral and morphological analysis taken shortly after the first outburst on 22 April revealed that its nucleus had one active area producing jets of material asymmetrically to its inner coma. Its closest approach to Earth occurred on the next day, at a distance of 0.464 AU (69.4 million km).

Reader's Guide

C/2020 R4 (ATLAS) is significant as a long-period comet that exhibited multiple unexpected outbursts, challenging initial predictions of its faintness. The December 2020 outburst, which increased its brightness a hundred-fold, and the three subsequent outbursts in April–May 2021 provided opportunities for detailed study. Spectral and morphological analysis after the first April outburst revealed a nucleus with one active area producing asymmetric jets, offering insights into cometary activity. Its discovery by the ATLAS system underscores the role of automated surveys in detecting such objects. The comet's roughly 950-year orbit and its perihelion on 1 March 2021, followed by a close approach to Earth on 23 April 2021, made it a target for observation. The recorded outbursts and the analysis of its inner coma contribute to understanding the behavior of long-period comets, though the article does not discuss broader implications or legacy beyond these events.

Did You Know?

• The comet was discovered by James E. Robinson using the ATLAS facility at Mauna Loa Observatory in Hawaii.
• Its closest approach to Earth was on 23 April 2021 at a distance of 0.464 AU (69.4 million km).

Birth of a New Architecture

The VAX-11/780, internally dubbed "Star," made its public debut on October 25, 1977, at Digital Equipment Corporation's annual shareholders gathering. It was the first machine to implement the Virtual Address eXtension instruction set, a 32-bit architecture DEC had been developing since 1976. At its core sat the KA780 processor, built from Schottky transistor-transistor logic components, running at a 200-nanosecond cycle time of roughly 5 megahertz. The chip carried a 2-kilobyte cache and was microprogrammed, with control logic split between a fixed PROM store and a writable control store loaded at boot from an eight-inch floppy disk managed by a dedicated LSI-11 front-end processor. Memory was accessed through the Synchronous Backplane Interconnect; the original configuration supported up to eight megabytes via one or two MS780-C controllers, each handling between 128 kilobytes and four megabytes. The 29-bit physical address space theoretically permitted up to 512 megabytes. I/O traffic flowed over Unibus and Massbus adapters bridged to the SBI, while a proprietary Computer Interconnect link enabled disk sharing across multiple VAX machines in a VMScluster.

A Ladder of Options and Market Dominance

DEC structured the VAX-11 lineup as a descending ladder of capability and price, giving customers a clear migration path. The flagship 780 was followed by the 750, code-named "Comet," which arrived in October 1980 as a more compact bipolar gate-array implementation. Its KA750 ran at 320 nanoseconds, or 3.125 MHz, with a 0.6 VUPs rating, and memory could be expanded to 14 megabytes through the L0022 controller. Unlike the 780's floppy-based boot, the 750 used a TU58 DECtape II cassette drive for initial diagnostics, and a ruggedized rack-mount variant, the 751, served industrial deployments. In April 1982, the 730, nicknamed "Nebula," pushed further down the cost curve with a bit-slice design built around AMD Am2900 chips, delivering 0.3 VUPs at 3.70 MHz and supporting up to 5 megabytes. The 725, internally called "LCN" for Low-Cost Nebula, represented the budget floor. Rivals such as Data General and Hewlett-Packard struggled to mount effective counterattacks against DEC's rapid succession of models. The 780's performance became the industry reference point: one VAX Unit of Performance, with every subsequent model rated as a multiple of that baseline.

Pushing Beyond a Single Processor

DEC explored multi-processor configurations to extract greater throughput from the VAX-11 platform. The 782, code-named "Atlas," debuted in 1982 as a dual-processor system in which two KA780 units shared a single MA780 multiport memory bus. The arrangement was deliberately asymmetric: the primary CPU handled all I/O and process scheduling, while the secondary chip was reserved for additional compute-heavy workloads, yielding up to 1.8 times the throughput of a single 780 on multistream tasks. The design traced its origins to a graduate thesis by George H. Goble at Purdue University, where he and undergraduate assistants had hand-wired the backplanes of two 780 boards to study multi-CPU Unix kernel modifications. The rarer 784, dubbed "VAXimus," extended the concept to four processors on the same shared bus, rated at 3.5 VUPs; Avie Tevanian, a co-founder of the Mach microkernel project, later recalled running early Mach kernel versions on one. The 785, "Superstar," introduced in April 1984, was a single-processor speed bump using Fairchild Advanced Schottky TTL logic to cut the cycle time to 133 nanoseconds, delivering 1.5 VUPs. A dual-processor 787 variant was rumored, but its production status remains uncertain.

The PDP-11 Bridge and a Lasting Footprint

The very name "VAX-11" encoded a strategic promise: the new 32-bit superminicomputers could execute user-mode PDP-11 code, giving DEC's existing customer base a seamless upward migration path rather than forcing a wholesale rewrite of their software. That compatibility, combined with the raw power of the VAX instruction set, made the family one of the most successful and most widely studied computer lines in history. Development had begun in 1976, and by the time the 780 shipped in 1977, the architecture was already setting the benchmark against which every later DEC machine would be measured. The higher-end 790 and 795 models were eventually rebranded into the VAX 8600 series, while the lower end was absorbed into the MicroVAX family. In 1988, the VAX-11 line was formally discontinued, its niche fully occupied by those successor families. Yet the 780's influence persisted well beyond its production run: it defined the VUPs performance metric, shaped the VAX ISA that underpinned an entire generation of systems, and left a design legacy that continues to be examined by computer architects and historians.